Proceedings of the American Society for Composites—Thirty-Sixth Technical Conference on Composite Materials
Manufacturing Method of the Morphing Wing Structure for UAV by CFRP with Applying the Electroformed Resin Molding Method
Aircraft flight control usually requires driving flaps and ailerons. However, the air drags increase significantly due to the corners of flaps and aileron. Especially, the gap between mother wing and flap / aileron causes a drag increase. Therefore, studies are being conducted on morphing wings that smoothly and greatly deform the wing surface. For aircraft wing, it is needless to say that strength is important to sustain lift and drag for the aircraft during the flight. For morphing wings, in addition, actuators must be mounted inside the wing to enable the morphing deformation. Moreover, for the aircraft wing, weight is quite important. Therefore, carbon fiber reinforced plastic (CFRP) is currently most suitable for aircraft wing structural materials. However, it is difficult to mold CFRP so that it has sufficient strength and can be morphed. In this study, by using CFRP, the morphing wing structure was prototyped with targeting a small unmanned aerial vehicle (UAV) weighing 3 kg. The CFRP lattice structure that enables morphing deformation was designed and manufactured by applying the electrodeposition resin molding (ERM) method which was developed by the authors. In the ERM method, firstly, the carbon fiber was fixed with a jig according to the designed morphing wing structure, and immersed in the electrodeposition solution. Secondly, the epoxy polymer particle in the solution were electrophoresed and impregnated between carbon fibers. After thermal curing, the morphing wing structure was fabricated. Further, the loading-unloading torsion and bending tests of the morphing wing structure were carried out. Smooth morphing deformation and sufficient strength were confirmed.
Heating Rate Prediction for Induction Welding Magnetic Susceptors
Induction welding involves generating heat by applying an oscillating magnetic field, which produces eddy currents and Joule losses in an electrically-conductive material or hysteresis losses in a magnetic material. Most applications rely on eddy currents generation as composites are often made of electrically-conductive carbon fibres. However, in other applications, heat can be produced by a magnetic susceptor located at the weld interface of the parts to be joined. Composite films of magnetic particles dispersed in a thermoplastic matrix can serve as magnetic susceptors. Magnetic particles selection relies on various parameters that must be thoroughly defined beforehand. Firstly, the applied magnetic field amplitude and frequency is calculated, based on the generated current and the induction coil geometry. Secondly, the thermoplastic matrix is characterized, mainly with DSC measurements, to define its processing window. Finally, the magnetic properties of the particles are measured – for instance using a vibrating sample magnetometer (VSM) – to obtain the hysteresis curve for the applied field. The enclosed surface area of the hysteresis curve (i.e. absorbed energy density) is critical, as low hysteresis materials (i.e. soft magnets) will not dissipate enough heat, while high hysteresis materials (i.e. hard magnets) cannot be fully exploited as the saturation hysteresis is not reached within the used field amplitude. A methodology to approximate the hysteresis enclosed surface area with limited data is proposed, helping to anticipate the heating rate of a susceptor candidate material. Based on these parameters, the theoretical heating rates of three magnetic susceptor materials (magnetic particles of iron, nickel and magnetite) for induction welding are calculated. They are verified experimentally by comparing with the hysteresis analysis and by measuring the temperature evolution of samples made of polypropylene containing the magnetic particles.
A Novel, Scalable Production of Multifunctional Nanocomposite Polymer Filaments for Additive Manufacturing
Though a revolutionary process, additive manufacturing (AM) has left more to be desired from printed parts, specifically, improved interlayer strength and minimal defects such as porosity. To overcome these common issues, nanocomposites have become one of the most popular materials used in AM, with various nanoparticles used to achieve a variety of characteristics. The use of these technologies together allows for both to synergistically enhance the final printed parts by improving the process and products simultaneously. Here, we introduce a novel, scalable technique to coat ABS pellets with cellulose nanocrystal (CNC) bonded carbon nanotubes (CNT), to improve the adhesion between layers as well as the mechanical properties of printed parts. An aqueous suspension of CNT-CNC is used to coat ABS pellets before they are dried and extruded into filament for printing. The filament produced using this manufacturing method showed an increase in tensile and interlayer strength as well as improved thermal conductivity. This process uses water as solvent and pristine nanoparticles without the need for any functionalization or surfactants, promoting its scalability. This process has the potential to be used with various polymers and nanoparticles, which allows the materials to be specifically tailored to the end application, (i.e. strength, conductivity, antibacterial, etc.). These nanocomposite filaments have the potential to revolutionize the way that additive manufacturing is utilized in a variety of industries.
Bond Strength Degradation of Adhesive- Bonded CFRP Composite Lap Joints After Lightning Strike
Adhesive bonding to join fiber reinforced polymer matrix composites holds great promise to replace conventional mechanical attachment techniques for joining composite components. Understanding the behavior of these adhesive joints when subjected to various environmental loads, such as lightning strike, represents an important concern in the safe design of adhesively bonded composite aircraft and spacecraft structures. In the current work, simulated lightning strike tests are performed at four elevated discharge impulse current levels (71.4, 100.2, 141, and 217.8 kA) to evaluate the effects of lightning strike on the mechanical behavior of single lap joints. After documentation of the visually observed lightning strike induced damage, single lap shear tests are conducted to determine the residual bond strength. Post-test visual observation and cross-sectional microscopy are conducted to document the failure modes of the adhesive region. Although the current work was performed on a limited number of specimens, it identified important trends and directions for future more comprehensive studies on lightning strike effects in adhesively bonded composites. It is found that the lightning strike induced damage (extent of the surface vaporization area and the delamination depth) increases as the lightning current increases. The stiffness of the adhesive joints and shear bond strength did not show a clear correlation with the lightning current levels, which could be due to many competing factors, including the temperature rise caused by the lightning strike and the surface conditions of the adherends prior to bonding. The failure modes of the adhesive regions for all specimens demonstrate a mixed mode of adhesive and cohesive failure, which may be due to inconsistent surface characteristics of the adherends before bonding. The energy absorbed during the lap shear tests generally increases as the lightning current increases.
Experimental Study of Lightning Damage Resistance of Unprotected and Protected Stitched Warp-Knit Carbon-Epoxy Composites
In-flight lightning strike damage to aircraft composite structures may compromise aircraft airworthiness. Hence, it is crucial to incorporate adequate protection systems to mitigate the lightning current. Most lightning strike protection (LSP) techniques involve bonding a metallic conductive layer to the cured laminate exterior. In this study, a novel LSP integration technique was used to develop unitized panels with three different protection layers: pitch carbon fiber paper (PCFP), graphene paper (GP), and copper mesh (CM). Each LSP layer was overlaid on through-the-thickness VectranHT stitched warp-knit multiaxial dry carbon fabric stacks, resin-infused, and oven-cured. A series of lightning strike tests to protected and unprotected stitched carbonepoxy laminates were conducted at a nominal peak current of 150 kA. Visual inspection was used to investigate each panel’s lightning damage resistance, understand the damage mechanisms, and evaluate the surface morphology at the strike locations. The size and severity of the damaged area depended on several factors: the outermost ply fiber direction, the strike location relative to VectranHT and polyester knitting treads, the lightning peak current, and the conductivity of the protection layers. The CM and GP protection layers effectively dissipated the lightning current in-plane and showed no damage to the underlying composite. The degree of lightning damage on an unprotected laminate was significantly lower than for a similar panel with PCFP protection. The presence of VectranHT structural stitches and polyester warp-knitting threads profoundly reduced the size and severity of lightning damage. These threads appeared to promote close contact between adjacent carbon fiber tows, resulting in better in-plane and through-thickness electrical/thermal conductivities and reduced lightning damage.
Thermal Ablation Modelling of C/SiC for Hypersonic Applications
Hypersonic vehicles are designed to operate at speeds above Mach 5. These vehicles are optimized to have low drag and have thin, slender bodies. The leading edges (nose and wings) are subjected to very high temperatures (above 3000 °C) due to the high heat fluxes. Carbon fiber reinforced Silicon Carbide matrix (C/SiC) composite is a ceramic matrix composite (CMC) that shows great potential for hypersonic applications as it has a low specific weight, high specific strength, and high specificity specific modulus, good thermal stability, and oxidation resistance. C/SiC can be used in leading edges, acreage, hot structures, and the propulsion system. The primary challenge of C/SiC is environmental durability caused by the oxidation and ablation of the material when subjected to extreme heat fluxes. Coatings must be added to the C/SiC substrate to withstand harsh environments at hypersonic speed. These coatings consist of an ultra- high temperature ceramic, an environmental barrier coating (EBC), and a bond coat (BC). This project aims to develop a computational model that will predict the thermal ablation of UHTC coatings when subjected to large heat fluxes. The finite element software used was ABAQUS 2020. Two different models in 2D were created, one for the ablation and one for the stress distribution through the coating. Ablative heat flux was applied at the surface on one side while the other side remains insulated. Preliminary results have shown that as the material is ablated, the temperature across the model started to rise due to the heat flux.
Investigating the Influence of Environmental Conditioning on Epoxy Resins at Quasistatic and High Strain Rates for Material Operating Limit
Epoxy resin-based composite panels used for armors may be subjected to a wide range of operating temperatures (-55°C to 76°C) and high strain rates on the order of 103-104 s-1. Over the life cycle, various environmental factors also affect the resin properties and hence influence the performance of the composites. Therefore, it is critical to determine the stress-strain behavior of the epoxy resin over a wide range of strain rates and temperatures for accurate multi-scale modeling of composites and to investigate the influence of environmental aging on the resin properties. Additionally, the characterization of key mechanical properties such as yield stress, modulus, and energy absorption (i.e. area under the stress-strain curve) at varying temperatures and moisture can provide critical data to calculate the material operating limits. In this study, we characterize mechanical properties of neat epoxy resin, SC-15 (currently used in structural armor) and RDL-RDC using uniaxial compression testing. RDL-RDC, developed by Huntsman Corporation, has a glass transition temperature of ~ 120°C, compared to ~ 85°C of SC-15. A split Hopkinson pressure bar is used for high strain rate testing. Quasistatic testing is conducted using a screw-driven testing machine (Instron 4484) at 10-3 s-1 and 10-1 s-1 strain rates and varying temperatures. The yield stress is fit to a modified Eyring model over the varying strain rates at room temperature. For rapid investigation of resistance to environmental aging, accelerated aging tests are conducted by immersing the specimens in 100°C water for 48 hours. Specimens are conditioned in an environmental chamber at 76 °C and 88% RH until they reach equilibrium. Tests are then conducted at five different temperatures from 0°C to 95°C, and key mechanical properties are then plotted vs. temperature. The results presented are an important step towards developing a methodology to identify environmental operating conditions for composite ground vehicle applications.
What if Spiders Made Metamaterial Webs Using Materials with Mechanical Size-Effects?
Spider’s webs are elegant examples of natural composites that can absorb outof- plane impact energy to capture prey. Different spiders have different methods and structure of webs, and these variations in topologies have a significant effect on the prey catching abilities of the web. Taking inspiration from the spiders, metamaterials that have architectured topology can be fabricated according to end applications such as energy absorbers or impact tolerant materials. In this investigation, we theoretically examined impact loading on various orb-spider webs modeled with metamaterial architecture using materials that show size-dependent behavior. Using the size-dependent properties of nano-reinforced polymer-derived ceramics (PDCs), various metamaterial topologies were evaluated for out-of-plane impact due using ANSYS Ls-Dyna. The material properties capture the size dependency of the ceramics where smaller elements have higher strength due to reduced flaw intensity; the mechanical strength of these elements does not follow the conventional Griffith Theory. In this study, spider web geometries fabricated with PDCs with varying size elements were examined.
Dynamic Responses of Architectural Kerf Structures
Kerfing is a subtractive manufacturing approach to create flexible freeform surfaces from stiff planar materials. The kerf structures are used in both indoor and outdoor architectures for wall paneling, outdoor façade and pavilion. In addition to their physical appeal, these structures have potential applications in tuning the dynamics responses in buildings, e.g., indoor acoustic, vibration suppression, etc. To exploit these novel applications of kerf structures, this paper presents a study on the dynamic responses of kerf structures made up of Medium Density Fiberboard (MDF). MDF is a viscoelastic composite material comprising of wood fiber networks and epoxy. The influence of the material behavior, i.e. viscoelasticity of MDF is considered in determining the dynamic response of the kerf panels. Two kerf panels with similar kerfing pattern but different cut density and arrangement are studied for their modal responses. A 3D beam element is used to model the mechanical responses of the kerf panels. With the understanding of the dynamic response of these kerf panels, their applications in altering the indoor acoustics and the wind responses of the buildings can be better comprehended.
Cross-Longitudinal Reinforcement Structure Inspired by Dragonfly Wing
The aim of this study is to establish a novel aircraft design approach replacing the conventional airframe by utilizing biomimetics. This design approach particularly focused on the dragonfly wing, whose reinforcement structures are composed of cross- veins and longitudinal veins. The cross-veins have been emulated by weighted Centroidal Voronoi Tessellation (WCVT) following the out-of-plane displacement on the skin, while the longitudinal veins have been emulated by extracting a centerline from the topology optimization result on the skin to be reinforced, through image analysis of binarization and skeletonization. The longitudinal layout can reduce the compliance distributing the inner load with only essential reinforcement on the skin without increasing the mass. The weighted CVT layout can improve the effectiveness of the reinforced skin against buckling drastically. Thus, the skin reinforced along the cross- longitudinal layout by the topology optimization and weighted CVT pattern increased buckling load 2.7 times higher even with less mass than the conventional layout.
Viscoelastic Responses of MDF Kerf Structures
Medium-density fiberboard (MDF), comprised of chopped wood fibers and epoxy resin is commonly used in building construction. Given the current trend in freeform architecture, there is a need to generate complex geometric structures out of common construction materials. In this study, kerfing (relief cutting) method is used to create flexible and moldable shapes out of relatively rigid wood panels. The kerf panels can be easily formed into various complex shapes for outdoor facades and/or indoor paneling. The natural wood fibers and epoxy resin in MDF combine to produce a viscoelastic composite material that exhibits time-dependent changes in mechanical behavior, i.e., material stiffness/compliance. The work described herein comes from a study designed to develop a better understanding of the viscoelastic response of kerf unit cells and panels. Experimental creep tests were performed on dog-bone specimens under uniaxial loading conditions1in order to determine the viscoelastic response of the MDF. The experimental results were used to develop a model to be used in simulations. The simulations were carried out on a kerf unit cell, e.g., a square interlocked kerf, and kerf structures of complex geometries in order to investigate the time-dependent changes in the deformations of the kerf structures. From this study, a systematic design of kerf panels with complex shapes will be developed in order to minimize the time-dependent changes of kerf structures.
Development of a Frangible Design of Small Fixed-Wing Unmanned Aerial System
Existing studies show that small fixed-wing unmanned aircraft systems’ (FWUASs) mid-air collisions with aircraft can cause substantial damage. Upon a 250 knots impact, a ~1.8 kg “tractorâ€ configuration of FW-UAS can perforate aircraft skin, thereby damaging the internal structures such as ribs, frames, etc., posing severe threat to manned air fleet. Significant damage is primarily caused by FW-UAS’s heavy and rigid components such as motor, battery, and payload especially due to their roughly in-line arrangement and proximity with one another. In this work, a modified FW-UAS finite element (FE) model was developed that included a “pusherâ€ engine (i.e., motor in the aft of the forward fuselage) configuration to reduce the impact severity during airborne collisions. A polymeric foam nosecone was attached to the front of the FW-UAS FE model to dissipate impact energy. To assess its energy absorbing capacity, a comparative study with expanded polypropylene (EPP), polyurethane (PUR), and polystyrene (IMPAXX700) foams was performed. Conical and semi-spherical nosecone configurations were studied as part of this research. A series of LS-Dyna impact simulations were performed with the pusher configuration of FW-UAS impacting a 1.59 mm thick aluminum 2024-T3 flat plate sandwiched between a rigid target frame. In addition, a frangible design of the FW-UAS, in which the payload is diverged from the in-line collision trajectory of battery and motor upon impact, was implemented and assessed. Force generated during the initial stage of impact is leveraged through lightweight and friable structural links to diverge the payload to avoid impact along the single axis as of the battery and motor. Damage severity is evaluated through target plate tear, and velocity of payload during impact, it being the major damage causing component.
Characterizing Dual Matrix Composite Origami Deployment
Lightweight strain-energy deployable structures are well suited for applications in spacecraft structures and deployable habitats. Dual matrix composite origami describes a composite structure that utilizes two uniquely different matrix systems in the same fiber structure. An epoxy matrix is used to form rigid sections and a silicone matrix creates bending regions where one can create a collapsible structure. The added elastic properties of the silicone matrix allow the composite to harness internal strain-energy to spring open from a folded configuration to a flat plane. This is advantageous for solar arrays as they can be folded for compact transit and then return to a flat surface after being deployed. In this work, initial ground testing is used to verify repeatable deployment prior to micro gravity flight testing. To characterize the motion of the composite structures as they deploy, OptiTrack Flex 13 cameras are used in sync with a deployment mechanism that repeatably releases and tracks the origami structures. The data collected in this study will be used to validate the use of the flight rig, deployment mechanism, and motion tracking system for future micro gravity flight testing. Once tested and characterized in a micro gravity setting, the dual matrix composite origami structures developed in this work can be applied to large solar arrays to power next generation spacecraft and satellites with lower complexity compared to other deployable technology.
Manufacturing of Aircraft Wing Stiffeners Using 4D Printing of Composites
This paper presents the procedure to make omega stiffeners using the method of 4D printing of composites. The method allows the manufacturing of complex structures without the need for a complex mold. Instead, flat layers of composite prepregs are laid on a flat mold. Due to the anisotropy of the different layers in the laminates, the stack of prepregs will change from its flat configuration into the omega shape upon curing and cooling to room temperature. The cavity is filled with foam to make the final structure.
Additive Manufacturing of High-Loading Polymer Nanocomposites with Multiscale Alignment
Polymer nanocomposites have advantages in mechanical, electrical, and optical properties compared to individual components. These unique properties of the nanocomposites have attracted attention in many applications, including electronics, robotics, biomedical fields, automotive industries. To achieve their high performance, it is crucial to control the orientation of nanomaterials within the polymer matrix. For example, the electric conductivity will be maximized in the ordered direction of conductive nanomaterials such as graphene and carbon nanotubes (CNTs). Conventional fabrication methods are commonly used to obtain polymer nanocomposites with the controlled alignment of nanomaterials using electric or magnetic fields, fluid flow, and shear forces. Such approaches may be complex in preparing a manufacturing system, have low fabrication rate, and even limited structure scalability and complexity required for customized functional products. Recently, additive manufacturing (AM), also called 3D printing, has been developed as a major fabrication technology for nanocomposites with aligned reinforcements. AM has the ability to control the orientation of nanoparticles and offers a great way to produce the composites with cost-efficiency, high productivity, scalability, and design flexibility. Herein, we propose a manufacturing process using AM for the architected structure of polymer nanocomposites with oriented nanomaterials using a polylactic acid polymer as the matrix and graphite and CNTs as fillers. AM can achieve the aligned orientation of the nanofillers along the printing direction. Thus, it enables the fabrication of multifunctional nanocomposites with complex shapes and higher precision, from micron to macro scale. This method will offer great opportunities in the advanced applications that require complex multiscale structures such as energy storage devices (e.g., batteries and supercapacitors) and structural electronic devices (e.g., circuits and sensors).
Performance of Additively Manufactured Chopped Fiber Composites as a Function of Porosity
Additive manufacturing (AM) of short-fiber reinforced composites are actively being considered for construction of low-cost, weight reducing alternatives to non-structural metal components. In addition AM parts are being used for rapid manufacturing of composite tooling. AM is notorious for generating parts with higher porosity than more traditional manufacturing technologies. Due to the aerospace industry’s low risk tolerance, novel material systems require comprehensive characterization of their associated manufacturing defects and the impact of defects on performance in order to receive certification. AM short fiber composites still require this analysis. Although traditional composite manufacturing methods, such as an autoclave or VARTM can produce porosity, the origin and transport of porosity has been thoroughly studied and acceptable limits for part qualification have been established to minimize the effect on performance. In AM components studies establishing the origination of porosity origin and processing defect related minimization strategies are lacking. A preliminary study on the impacts of porosity caused by print parameters such print speed, layer height, first layer height, and step-over distance has been undertaken. Direct Ink Writing (DIW) was selected for this study using an epoxy-based ink filled with clay and chopped carbon fiber. To understand performance, fracture and shear specimens were fabricated with different test parameters based on the assumed levels of porosity from screening tests. Samples were then mechanically tested using a novel in-plane shear test method and a single edge notch tension test. This study explores the processing parameter’s contribution to porosity and establishes general trends to understand the influence of porosity on performance for additively manufactured chopped fiber composites.
3D Printed Continuous Fibre Composite Research at Deakin University: Design and Analysis Methods for UD, Hybrid and Pseudo-Woven Ply Architectures
At Deakin University we have been researching the performance of continuous fibre 3D printed composite structures and a summary of three research activities related to this research theme are provided herein. 3D printed continuous fibre composites can be used to realise significant gains in stiffness and strength compared to an equivalent component fabricated using a neat thermoplastic. To investigate the performance of these materials both commercially available and customised printers were used to fabricate composite laminates and the behaviour of these laminates evaluated experimentally. Finite element and analytical models were used to predict the mechanical response. These approaches were originally developed for thermoset matrices, however, the models have shown to be capable of predicting the behaviour of 3D printed carbon fibre and hybrid carbon-fibreglass thermoplastic composites. These validated models can be used to generate design charts to identify feasible UD and semi-woven textile architectures, thereby, allowing designers to tailor the ply architecture and stacking sequence to meet specific design requirements.
Three-Dimensional Heat Transfer Analysis of Hot Gas Torch (HGT)-Assisted Automated Fiber Placement of Thermoplastic Composites
In-situ manufacturing of thermoplastic composites using the Automated Fiber Placement (AFP) process consists of heating, consolidation, and solidification steps. During the heating step using Hot Gas Torch (HGT) as a moving heat source, the incoming tape and the substrate are heated up to a temperature above the melting point of the thermoplastic matrix. The convective heat transfer occurs between the hot gas flow and the composites in which the convective heat transfer coefficient h plays an important role in the heat transfer mechanism, which in turn significantly affects temperature distribution along the length, width, and through the thickness of the deposited layers. Temperature is the most important process parameter in AFP in-situ consolidation that affects bonding quality, crystallization, and consolidation. Although it is well known that the convective heat transfer coefficient h is not constant and has a distribution, most studies have assumed a constant value for h for heat transfer analysis, which leads to discrepancies between numerical and experimental results. It has already been shown by the authors that, unlike other studies assuming constant h value, using a distribution function to approximate the convective heat transfer coefficient h in a three-dimensional finite element transient heat transfer analysis the temperature distribution can be well predicted in thermoplastic composite parts and matches experimental data. In this study, the use of the proposed h distribution function is analysed and validated by several measuring points. Furthermore, experimental trials are carried out to validate the results from the simulation.
Modelling Compaction Behavior of Toughened Prepreg During Automated Fibre Placement
One of the most widely used automated manufacturing processes for composite parts is automated fibre placement (AFP). The deposition process involves the simultaneous warming, lay-up and consolidation of prepreg consisting of multitude of process parameters. Currently, AFP process parameters that ensure part conformance are derived by expensive and time-consuming trial-and-error approaches. The aim of this study is to demonstrate how physics-based finite element simulations that can predict the as manufactured geometry of a preform deposited by AFP can help reduce some of the empiricism associated with current industry practices. Here we particularly focus on the consolidation behaviour of toughened prepregs during the deposition process. An isothermal roller compaction model with thermal properties derived from an independent simplified thermo-mechanical model of the AFP head is used. Additionally, a fully characterised viscoelastic material definition is used for the prepreg tape along with a hyperelastic material for the compaction roller to accurately represent the physical parts. Various lay-up speeds, heater powers and compaction forces are simulated. To reduce the empiricism present in the manufacturing process, the viability of incorporating the numerical models into existing statistical relationships between process parameters and manufactured geometry is examined.
Modeling and Calibration of Uncertainty in Material Properties of Additively Manufactured Composites
Simulations quantifying the uncertainty in structural response and damage evolution require accurate representation of the randomness of the underlying material stiffness and strength behaviors. In this paper, the mean and variance descriptions of variability of strength and stiffness of additively manufactured composite specimens are augmented with random field correlation descriptors that represent the process dependence on the property heterogeneity through microstructure variations. Two correlation lengths and a rotation parameter are introduced into randomized stiffness and strength distribution fields to capture the local heterogeneities in the microstructure of Additively Manufactured (AM) composites. We formulated a simulation and Artificial Intelligence (AI)-based technique to calibrate the correlation length and rotation parameter measures from relatively few samples of experimentally obtained strain field observations using Digital Image Correlation (DIC). The neural networks used for calibrating the correlation lengths of Karhunen-Loève Expansion (KL expansion) from the DIC images are trained using simulated stiffness and strength fields that have known correlation coefficients. A virtual DIC filter is used to add the noise and artifacts from typical DIC analysis to the simulated strain fields. A Deep Neural Network (DNN), whose architecture is optimized using Efficient Neural Architecture Search (ENAS), is trained on 150,000 simulated DIC images. The trained DNN is then used for calibration of KL expansion correlation lengths for additively manufactured composite specimens. The AM composites are loaded in tension and DIC images of the strain fields are generated and presented to the DNNs, which produce the correlation coefficients for the random fields as outputs. Compared to classical optimization methods to calibrate model parameters iteratively, neural networks, once trained, efficiently and quickly predict parameters without the need for a robust simulator and optimization methods.
Reactive Extrusion Additive Manufacturing of a Short Fiber Reinforced Thermoset Composite
Additive manufacturing (AM) of high-performance composites has gained increasing interest over the last few years. Commercially available AM technologies often use thermoplastics as they are easy to process, i.e., to melt and re-solidify. However, thermosetting polymers generally achieve superior mechanical properties and thermostability. This study investigates reactive extrusion additive manufacturing (REAM) of a thermosetting polymer reinforced with carbon fibers. The process utilizes highly exothermic and fast curing resin/catalyst systems, eliminating the need for post-curing. The rheological properties of the liquid resin are first tuned for REAM using ~2wt.% fumed silica and ~10vol.% milled carbon fibers. Then, a robotic arm is used to print the composite samples. The coupons’ longitudinal and transverse tensile properties are measured and correlated with the degree of cure, porosity, fiber length distribution, and fiber orientation distribution. The incorporation of milled carbon fibers, 50-200 ï­m long, primarily affects the stiffness. Compared to neat polymer parts, carbon fiber reinforced composites are 51% stiffer and 8% stronger. In addition, polymeric crosslinking between part layers resulted in strong inter-layer bonding. Short fibers were also randomly oriented within parts due to the nozzle size and shape, resulting in nearly isotropic parts. The results presented here pave the road for fast and low-energy AM of high-performance composites.
Direct-Ink-Writing (DIW) of Higher Weight Concentration of Milled Carbon Fiber (MCF) Reinforced Epoxy Composite
Additive manufacturing or 3D printing of short carbon fiber composites is highly inclined toward thermoplastic material. Recently, there is some interest in developing 3D printing technology for thermosetting polymers and composites. Direct-ink-writing (DIW) of short fiber epoxy composites containing milled carbon fibers (MCF) exhibits the potential to replace the traditional composite material as it showcases the inherent capability of fabricating complex geometries, facile material processing, continuous printing capability coupled with low cost. Previous research in 3D printing of short carbon fiber epoxy composites suffered heavily from the limitation of printing low carbon fiber weight concentrations. This study developed a straightforward technique of designing the epoxy ink containing high weight concentration MCF (~40wt%) and a small concentration of thixotropic filler (3-wt%) for DIW using a pneumatic pressure-driven extruder having micron size tapered nozzle. This paper also intends to contribute to a better understanding of the effect of printing parameters on DIW. Furthermore, the printing parameters were optimized using non-dimensionalized constant C. Optimized printing condition was further used to print different geometry ranging from dog bone with 100% infill to 4-layer hollow circle and 3-layer hollow square with 0% infill. Lastly, ink exhibits the capability to self-support multi-layer geometrical structures during printing.
Analysis of Printing Parameters in 3D Printed Carbon Fiber Composites Using Design of Experiments
Additive manufacturing of carbon fiber reinforced polymers (CFRP) provides the advantage of quick prototyping of complex geometries, while maintaining light-weight characteristics and keeping structural integrity. This paper presents flexural strength data from 3D printing of an onyx and carbon fiber composites. A MarkForged Mark 2 3D printer, with a dual printer head, is used to 3D print several configurations of CFRP composites. The configurations are set to examine the extreme parameters of the rectangular fill pattern, including fill density, roof and floor layers, and wall layers. The print angle along with the fill pattern stays consistent. Each sample comprises twenty- four layers of CFRP and undergoes a three-point bend test. Test data of load, deflection, and maximum stress are compared among the different configurations. Results show that the roof/floor layer has the largest impact on the flexural strength for each configuration. The configurations with less roof/floor layers are able to take on a larger load, because there are more CFRP layers present; while specimens with more roof/floor layers withstand less load and behave more ductile. These results show that the amount of roof/floor layers can change the CFRP composite from its usual brittle characteristic to a ductile behavior.
Influence of Carbon in Lead Matrix for Lead Acid Battery Application: A Density Functional Theory Approach
Recent research in developing novel lead electrodes composed of carbon and lead composites have shown potential of increasing the ability of lead-acid batteries to store energy in grid scale applications but many technical challenges persist. One challenging obstacle is that influence of carbon on lead matrix in these new battery systems are obscure and the mechanism taking place at the atomic level is difficult to investigate experimentally. However, a better understanding of the working mechanisms of battery materials is crucial for developing new advanced batteries toward industrial applications. In this paper the density functional theory (DFT) calculation are carried out using the Vienna Ab-initio Simulation Package (VASP) with spin-polarized generalized gradient approximation (GGA) functional parametrized by Perdew, Burke, and Enrzerhof (PBE) is used to describe the exchange and correlation energies of the electrons. Herein, a systematic increase in system site with decrease in C concentration was investigated in Pb using DFT. This study provides vital fundamental data on Pb-C structure such as density of states, band structures, defect formation energies.
Non-Destructive Evaluation of Mechanical Damage of Adhesives Using Magneto-Electric Nanoparticles
Adhesive bonding has been shown to successfully address some of the main problems with traditional fasteners, such as the reduction of the overall weight and a more uniformly distributed stress state. However, due to the unpredictability of failure of adhesive bonds, their use is not widely accepted in the aerospace industry. Unlike traditional fastening methods, it is difficult to inspect the health of an adhesive joint once it has been cured. For adhesive bonding to be widely accepted and implemented, there must be a better understanding of the fracture mechanism of the adhesive joints, as well as a way to monitor the health of the bonds nondestructively. Therefore, in-field structural health monitoring is an important tool to ensure optimal condition of the bond is present during its lifetime. This project focuses on the advancement of a non-invasive field instrument for evaluation of the health of the adhesive joints. The tool developed is based on a B-H looper system where coils are arranged into a noise-cancellation configuration to measure the magnetic susceptibility of the samples with a lock-in amplifier. The B-H looper system can evaluate the state of damage in an adhesive bond by detecting changes in surface charge density at the molecular level of an epoxy-based adhesive doped with magneto-electric nanoparticles (MENs). Epoxy-based adhesive samples were doped with MENs and then scanned using the B-H looper system. To evaluate the health of the adhesive joint, microindentation and tensile tests were performed on MENs-doped adhesive samples to understand the relationship between mechanical damage and magnetic signal. Correlations between magnetic signatures and mechanical damage were minimally observed, thus future studies will focus on refining the procedure and damaging methodology.
Energy-Based Multifunctional Efficiency Metric for Multifunctional Composite Anodes in Structural Batteries
A multifunctional efficiency metric is developed using mean-field micromechanics solutions to quantify the multifunctionality of the multifunctional composite anodes. Multifunctional efficiency metrics evaluate the volume and/or mass savings or performance increase when structural and functional materials are replaced by multifunctional materials [1]. The proposed methodology compares the total energy associated with different functionalities, such as elastic strain energy and electric charge energy of the multifunctional materials with the total energy of the single function structural and functional material. To achieve volume and mass savings, the energy of different functionalities is set to be the same between the multifunctional and traditional single- functional materials, and, at the same time, the volume and/or mass of the multifunctional composite needs to be smaller than that of the combination of single- functional materials. The volumes and/or mass savings can be expressed using the properties of multifunctional and traditional single-functional materials. In this work, structural anodes made from silicon nanoparticles, reduced graphene oxide, and aramid nanofibers are used as an example to calculate the mass savings compared to a traditional anode with structural support. The existing multifunctionality metrics are based on the rule of mixtures method, which is adequate for certain geometries and loading conditions, such as in-plane directions for laminate composites. However, if multifunctional composite materials involve multiple phases, material property variation during the charging process, and complex geometries or orientations of the structural and functional phases, a more comprehensive method is required to accurately capture the multifunctional efficiency. The multifunctional efficiency varies significantly during the charging and discharging process. This new metric can provide both upper and lower bounds of multifunctional efficiency. This new multifunctional efficiency metric will help optimize the selection and arrangement of different phases in the multifunctional and quantify the optimization results.
Bridging of Carbon Fibers in CF/Epoxy Composites Using Electrostatically Induced CNT Alignment
Carbon fiber reinforced polymer (CFRP) composites are lightweight materials with superior strength but are expensive due to the increased cost of carbon fibers (CFs). The addition of carbon nanotubes (CNTs) to polymer nanocomposites are becoming an excellent alternative to CF due to their unique combination of electrical, thermal, and mechanical properties. With the application of an electric field across the CNT/polymer mixture before curing, CNTs will not only be aligned along the electric field direction, but also form networks after reaching to a certain degree of alignment. In this study, an alternating current (AC) electric field was applied continuously to CNT/CF/Epoxy hybrid composites before curing. By cutting off the applied voltage when the monitored electric current increased, the degree of networking of CNTs between two CF tows was controlled. The relative electric field strength around the end of conductive carbon fiber tows in the epoxy matrix was modeled using COMSOL Multiphysics. It increased after applying AC electric field parallel to the CF tows, thereby increasing the alignment degree of CNTs and building a network to bridge the CF tows. The preliminary results indicate that the microhardness and tensile modulus between two CF tows are increased due to the networking of CNTs in this area. The fracture surface of the specimens after tensile tests were characterized to reveal more details of the microstructure.
Design and Analysis of an Integrated Three- Bay Thermoplastic Composite Wingbox
The design of a multi-part aerospace structural component, such as a wingbox, is a challenging process because of the complexity arising from assembly and integration, and their associated limitations and considerations. In this study, a design process of a stiffeners-integrated variable stiffness three-bay wingbox is presented. The wingbox has been designed for a prescribed buckling and post-buckling performance (a prescribed real testing scenario) and made from thermoplastic composite material system (Carbon-PEEK) with the total length of three meters. The stiffeners and spars are integrated into the top and bottom panels of the wingbox resulting a single-piece blended structure with no fasteners or joints. The bottom skin also has an elliptical cut-out for access purposes. The composite tows are steered around this cutout for strain concentration reduction purposes. The fiber/tow steering in the top skin bays (compression side) has also been considered for improved compression-induced buckling load carrying capacity. The proposed design has been virtually verified via high fidelity finite element analysis.
Classification of Buckling Modes in Stiffened Functionally Graded Composite Tubes Subject to Bending
Bamboo poles, and other one-dimensional thin-walled structures are usually loaded under compression, which may also be subject to bending arising from eccentric loading. Many of these structures contain diaphragms or circumferential stiffeners to prevent cross-sectional distortions and so enhance overall load-carrying response. Such hierarchical structures can compartmentalize buckling to local regions in addition to withstanding global buckling phenomena. Predicting the buckling mode shapes of such structures for a range of geometric parameters is challenging due to the interaction of these global and local modes. Abaqus finite element software is used to model thousands of circular hollow tubes with random geometric parameters such that the ratios of radius to periodic length range from 1/3-1/7, the ratio of wall thickness to radius varies from 1/4-1/10. The material used in this study is a type of bamboo, where the Young’s and shear moduli are point-wise orthotropic and gradually increase in magnitude in the radial direction. Under eccentric loads with varying eccentricity, the structures can buckle into a global mode or local modes within an internode, i.e. periodic unit. Moreover, the local modes may contain only one wave or multiple waves in the circumferential direction. As expected, numerical results show that the global mode is more likely to occur in small and thick tubes, whereas the local modes are observed in larger tubes with a smaller number of circumferential waves present in thicker walls. Also, greater eccentricity pushes the local mode domains towards smaller tubes. An efficient classification method is developed herein to identify the domains of each mode shape in terms of radius, wall thickness and eccentricity. Based on linear discriminant analysis, explicit boundary surfaces for the three domains are defined for the obtained data, which can help designers in predicting the mode shapes of tubular structures under axial bending.
Design of a Composite Panel with Continuous Tow Steering Around an Elliptical Cut-Out
Cut-outs are inevitable in many structural components such as in aircrafts to accommodate windows or openings for access purposes or fasteners. Engineers usually view cut-outs, especially in primary structures, with disfavour as they result in stress/strain concentration and consequently reduced load carrying capability. Local reinforcements usually increase cost and weight to the overall design which is not favourable in aerospace applications. In case of composite panels, emerging advanced manufacturing methods such as 3D printing of automated fiber placement made it possible to continuously steer fibers/tows around a cut-out to potentially alleviate stress/strain concentration problem. Another advantage of tow steering in this case is maintaining the continuity of fiber/tow paths without any fiber cut which precludes ply-level, 3D stress/strain concentration which could otherwise lead to delaminationinduced damage. In this study, potential capability of tow steering around an elliptical cut-out (manhole) in reducing stress/strain concentration in a composite wingbox is investigated Buckling response under compression loading together with stress and strain concentrations under both tensile and compression loads are examined. Under tensile loading, the maximum stress and strain concentration factors around the cut-out in the straight fiber design are shown to be approximately 29% and 32% larger than its counterpart with steered tows around the cut-out. For the compression loading condition, the direct strain of the panel with straight fiber orientations was found to be three times that of steered fiber trajectories in the vicinity of the cut-out.
Scaling Methodology for Buckling of Composite Conical Shells in Axial Compression
Conical shells are commonly used as structural components for launch vehicles. The axial compression experienced during launch is one of the sizing load cases, because it can lead to loss of structural stability. Because experimentally testing these full-scale structures is cumbersome and expensive, it is expedient to understand how reduced-scale shells can be designed such that their buckling behavior is representative of the full-scale shell behavior. An analytical, sequential scaling methodology is developed based on the nondimensional governing equations for composite conical shells with a symmetric, balanced layup and negligible flexural anisotropy. Linear and nonlinear finite element analyses characterizing the buckling behavior of the different size shells yielded comparable results in terms of buckling load, meridional displacement, and buckling mode. The inclusion of geometric imperfections affects the prediction accuracy, but not to the extent that the methodology is no longer valid.
Experimental Evaluation of Compressive Strength of PAN-Based Carbon Monofilament by Poisson’s Deformation in Cruciform Specimen
Carbon fiber has relatively high anisotropy in tensile and compressive strength. Since, the diameter of carbon fiber is 5~10[μm], it is difficult to directly evaluate the compressive strength of monofilament. In this study, the compressive strength of carbon single fiber was evaluated using Poisson's deformation of Cruciform specimen. Using the Cruciform test method, the compressive strengths of monofilaments of PAN-based carbon fibers, T300B and T700SC, were experimentally and analytically evaluated. As a result, the compressive strengths of carbon single fibers was 5.12 [GPa] for T300/PA6 and 5.54 [GPa] for T700/PA6 in this study.
Design and Analysis of Thermoplastic Welded Stiffened Panels in Post-Buckling
The Clean Sky 2 “SmarT mUlti-fuNctioNal and INtegrated TP fuselaGe" STUNNING project focusses on the next generation composite fuselage with emphasis on manufacturing techniques such as thermoplastic welding. Welded multi-stringer panels are investigated in this paper, with emphasis on the buckling and skin-stringer separation behavior. The multi-stringer panels are designed to approximate the structural behavior of the lower half of the MultiFunctional Fuselage Demonstrator of the STUNNING project. In particular, a section of the fuselage is analyzed using Abaqus with a dynamic implicit analysis, and the results of this analysis are used for the design of the multi-stringer panels, taking manufacturing considerations also into account. The panels have three omega stringers and a length of 500 mm. The three stringer configuration allows to study the middle stringer in pristine and damaged configuration with minimal influence of the free edges and boundary conditions. It is seen that the multi-stringer test panels show very similar buckling and skin-stringer separation behavior compared to the fuselage section. The first panels have been manufactured by project partners NLR and GKN Aerospace Fokker and will be tested at the Faculty of Aerospace Engineering at Delft University of Technology.
Experimental Study on Post-Buckled Composite Single-Stringer Specimens with Initial Delamination under Fatigue Loads
The fatigue damage tolerance of a composite stiffened structure in post-buckling conditions is experimentally investigated in this work. Single-stringer specimens with an initial delamination, artificially created during the manufacturing process, are tested under cyclic compressive load. Six nominally identical specimens are manufactured: two tested under quasi-static load to understand the compressive behavior of the structure and four under fatigue load cycling between pre- and post-buckling conditions at two different maximum loads. During the tests, digital image correlation system and ultrasonic C-scan are adopted to follow the evolution of the out-of-plane displacements and the propagation of the delamination. Depending on the load level, the delamination starts to grow already in the first cycle or after a few thousand cycles, but in both cases the propagation is fast at the beginning, then slows down gradually. The fatigue tests are interrupted after 150,000 cycles and the specimens are subjected to quasi-static compressive load to evaluate the residual strength of the structure.
Progressive Damage Failure Analysis of Post-Buckled Composite Single-Stringer Panel with Teflon Inserts
The high strength-to-weight and stiffness-to-weight ratio materials, such as laminated composites, are advantageous for modern aircraft. Laminated composites with initial flaws are susceptible to delamination under buckling loads. PDA tools help enhance the industry’s understanding of the mechanisms for damage initiation and growth in composite structures while assisting in the design, analysis, and sustainment methods of these composite structures. The global-local modeling approach for the single-stringer post-buckled panel was evaluated through this effort, using Teflon inserts to simulate the defect of damage during manufacturing. This understanding is essential for designing the post-buckled structure, reducing weight while predicting damage initiation location, and addressing a potential design review for future aircraft repairs. In this work, the initial damage was captured with Teflon inserts as the starting configuration; and any reference to the damage initiation refers to any damage beyond the “initial unbonded region.â€ The effort aims to develop, evaluate, and enhance methods to predict damage initiation and progression and the failure of post-buckled hat-stiffened panels using multiple Abaqus FEA Virtual Crack Closure Technique (VCCT) definitions. Validation of the PDA using the VCCT material model was performed on a large single-stringer panel subjected to compressive loading. The compressive loading of the panel caused the skin to buckle before any damage began to occur locally. In addition, comparisons are made for critical aspects of the damage morphology, such as a growth pattern that included delamination from the skin-stiffener interface to the skin and ply interfaces. When compared against the experimental data produced through the NASA Advanced Composites Project (ACP), the present model captured damage migration from one surface to another, and model validations were ~5% of the experimental data.
Aero-Thermo-Dynamic Study of Ultra-High- Temperature Ceramic Composites for Thermal Protection Systems and Rocket Nozzles
The most extreme aero-thermo-dynamic conditions encountered in aerospace applications include those of atmospheric re-entry, characterized by hypersonic Mach numbers, high temperatures and a chemically reacting environment, and of rocket propulsion, in which a combusting, high-pressure, supersonic flow can severely attack the surfaces of the motor internal components (particularly nozzle throats), leading to thermo-chemical erosion and consequent thrust decrease. For these applications, Ultra-High-Temperature Ceramics (UHTC), namely transition metal borides and carbides, are regarded as promising candidates, due to their excellent high-temperature properties, including oxidation and ablation resistance, which are boosted by the introduction of secondary phases, such as silicon carbide and carbon fibers reinforcement (in the so-called Ultra-High- Temperature Ceramic Matrix Composites, UHTCMC). The recent European H2020 C3HARME research project was devoted to development and characterization of new-class UHTCMCs for near-zero ablation thermal protection systems for re-entry vehicles and near-zero erosion rocket nozzles. Within the frame of the project and in collaboration with several research institutions and private companies, research activities at the University of Naples “Federico IIâ€ (UNINA) focused on requirements definition, prototypes design and test conditions identification, with the aim to increase the Technology Readiness Level (TRL) of UHTCMC up to 6. Experimental tests were performed with two facilities: an arc-jet plasma wind tunnel, where small specimens were characterized in a relevant atmospheric re-entry environment (Fig.1a), and a lab-scale hybrid rocket engine, where material testing was performed with different setups, up to complete nozzle tests, in conditions representative of real propulsive applications (Fig.1b). The characterization of the aero-thermo-chemical response and ablation resistance of different UHTCMC formulations was supported by numerical computations of fluiddynamic flowfields and materials thermal behavior. The UNINA activities provided a large database supporting the achievement of the project objectives, with development and testing of full-scale TPS assemblies and a large-size solid rocket nozzle.
Processing of Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) Through RF Enhanced Chemical Vapour Infiltration (RF-CVI)
Carbon fibre (Cf) reinforced Ultra High Temperature Ceramic (UHTC) Matrix Composites (UHTCMCs) have proven to be excellent materials that can survive nearly 3000°C in highly oxidizing environments along with a good specific strength. Consequently, they have excellent potential for use in aerospace applications such as rocket nozzle throats and thermal protection systems (TPS). Due to the presence of the carbon fibres, UHTCMCs offer high strength and modulus combined with excellent thermal shock behaviour whilst the presence of the ultra-high temperature ceramic phase protects the carbon fibres at the application temperatures. High temperature oxidation, thermal ablation behaviour and mechanical properties of the UHTCMC’s relies heavily on the bonding between the carbon fibre and matrices especially the oxides formed to avoid any progressive failure and predict the life of the components. In the present investigation, a radio frequency assisted chemical vapor infiltration (RF-CVI) technique has been used to make the 2.5D Cf reinforced ZrB2, ZrB2/carbon matrices composites with various interphase materials. The advantage of RF heating is that it creates an inverse temperature profile in the sample, which means that the infiltration starts from inside and progresses outwards. This allows the time needed for processing to be reduced very significantly compared to the conventional CVI process. This presentation will report on the latest results from the research that has been undertaken at the University of Birmingham, including the results from a wide range of testing that has been undertaken at both DLR in Germany and the University of Naples in Italy.
Fabrication and Characterization of UHTCMCs
The materials currently used in aerospace and aviation, such as C/C and C/SiC composites, possess excellent mechanical properties but are limited to a maximum operational temperature of 1600°C (C/SiC) and poorly oxidizing environments (C/C). For more demanding applications, new materials able to withstand extreme temperatures without recession are required. In the framework of the C3harme project, a new class of materials labelled UHTCMCs, consisting of a UHTC matrix reinforced with carbon fibers, has been developed and characterized in order to overcome these challenges. Different fiber reinforcements and sintering parameters have been investigated from the microstructural point of view. The composites were fabricated via slurry infiltration of fiber, using a powder mixture of ZrB2 and SiC; the green pellets were then sintered via hot pressing. Extensive microstructural analysis was carried out on the sintered samples, showing how the sintering parameters and the choice of the fibers are crucial to obtain full densification without jeopardizing the fibers integrity and permit adequate load transfer.
High-speed aviation brings many challenges, one being the materials used ensure the aircraft and rockets travelling at hypersonic speed arrive at their destination safely. Control surfaces and thermal protection systems for vehicles flying at Mach 5 or above must withstand extremely hot temperatures and intense mechanical vibrations at launch, during cruising and re-entry into the Earth’s atmosphere. UHTCMCs (Ultra-High Temperature Ceramic Matrix Composites) belong to a new subclass of ceramic matrix composites (CMCs) with superior properties in terms of structural and chemical stability at elevated temperature and erosion/ablation resistance keeping excellent strength-to-weight ratio, thermal shock resistance and adequate damage tolerance. They are the latest potential candidates for thermal protection systems (TPSs), able to outperform bulk ultra-high temperature ceramics (UHTCs). C3HARME is a 4-years EU funded program involving 12 European partners from 6 countries focused on the design, fabrication and testing of UHTCMCs for nearzero erosion nozzles and near-zero ablation TPS tiles. C3harme will look at different technologies coming from the science of bulk ceramics and CMCs and combine them to find out new approaches for their manufacturing. Novel theoretical models and testing methodologies are necessary to characterize properly these materials. This talk will summarize some of the findings and advances of the program, with special emphasis on the innovative approaches that we have implemented.
Effect of Curing Temperature of Epoxy Resin on the Electrical Response of Carbon Nanotube Yarn Monofilament Composites
The cyclic thermoresistive response of individual carbon nanotube yarns (CNTYs) embedded into epoxy resins is investigated. The influence of the temperature at which the epoxy resin cures on the thermoresistive response is investigated by using two epoxy resins, one that cures at room temperature and the other one that cures at 130 °C. Heating-cooling cycles ranging from room temperature (RT, 25 °C) to 80 °C, incremental cycles (RT to 40 °C, RT to 60 °C and RT to 80 °C) and incremental heating-dwell cycles are applied to monofilament composites, while their electrical resistance is simultaneously recorded. The monofilament composites showed a negative temperature coefficient of resistance during the heating-cooling cycles of -7.07x10-4 °C-1 for specimens cured at high temperature, and -5.93x10-4 °C-1 for specimens cured at room temperature. The hysteresis after the different heating-cooling cycles was slightly smaller for specimens cured at 130 °C, in comparison to specimens cured at room temperature. Several factors including the intrinsic thermoresistivity of CNTY, level of infiltration and the effect of curing temperature may explain the thermoresistive sensitivity of the monofilament composites.
Thermal Stresses for Functionally Graded Materials (FGMs) Subject to Heat Flux
In this paper, the thermal stress due to heat flux at the far field is derived for an infinitely extended elastic medium which contains a spherical inclusion made of functionally graded materials (FGMs). The 3-D heat conduction equation subject to uniform heat flux at the far field is solved analytically to derive the temperature distribution. Based on the temperature solution, the thermal stress field due to heat flux is obtained by solving a set of two ordinary differential equations using the method of weighted residuals. Unlike the two-phase homogeneous medium, the von Mises stress distribution is continuous at the interface of the FGM-matrix medium.
Hierarchical Electrospun Veils as Potential Toughening Materials for Structural Composite Laminates
The use of fiber reinforced polymers (FRPs) as a replacement of traditional homogeneous materials is still hindered by their brittle behavior and poor interlaminar resistance. Interleaving veils in polymer matrix, especially with fibers at the nanoscale dimension, is considered as one of the most promising toughening methods. By considering the hierarchical nature of the resulting laminated composites, their properties are intrinsically dependent on the interaction between the nanofibrous veils and the thermosetting resin and, in an attempt to tailor the interfacial adhesion between the electrospun fibers and matrix, surface modification of the fibers with the integration of inorganic nanostructures could be a solution. This work is an investigation on the use of commercially available electrospun nylon nanofibers decorated with ZnO nanorods obtained by three-step chemical synthesis. The modified veils were interleaved in carbon/epoxy prepreg composites and their mechanical properties were evaluated under Low Velocity Impact (LVI) tests at different energy levels (5 J and 7.5 J). Although the presence of ZnO nanorods did not limit the extension of the delaminated area in case of high energy level test (7.5 J), nanomaterials contributed positively to reduce the extent of the damaged area when a low energy impact was adopted (5 J). A beneficial effect of ZnO-functionalized commercial electrospun veils was observed in the flexural strength of laminated composites. After LVI tests at 5 J and 7.5 J, the flexural strength resulted higher compared to that observed in the same tests on specimens with non-decorated veils (NY4M), thus suggesting a positive role played by ZnO nanorods in hindering delamination propagation.
A Size Effect Study on the Splitting Crack Initiation and Propagation in Off-Axis Layers of Composite Laminates
This work proposes an investigation on size effects in micro-scale splitting crack initiation and propagation and their consequences on the macro-scale structural behavior carbon-fiber reinforced polymers under transverse tension. Towards this goal, size effect tests were experimentally conducted on both notch-free [90]n composites and specimens with different notch types under three-point bending. The mechanical tests were followed by morphological studies to identify the micro-scale damage mechanisms and their evolution. The results clearly show that splitting crack initiation in the transverse direction of composites not only happens at the fiber/matrix interface but also in the matrix. Moreover, the subsequent development of these damage mechanisms can depend on the structure size. This interesting phenomenon inherently leads to size-dependent structural behavior which can be described through Baznt’s Size Effect Laws. This study on the splitting crack initiation and propagation can provide unprecedented information for the calibration and validation of micromechanical models for the damage behavior of fiber composites at the microscale.
Mode-I Fatigue Crack Healing in CFRP Composites Using Thermoplastic Healant
Mode-I fatigue crack healing in carbon fiber-reinforced polymer (CFRP) composites subjected to fatigue loading is investigated in this study. Laminated composites are highly susceptible to delamination, and delamination due to fatigue loading is one of the most critical damage modes in composite structures that may lead to a catastrophic failure. Hence, it is paramount to investigate and quantify the delamination crack growth behavior due to fatigue loading and explore methods to heal the delamination. Therefore, double cantilever beam (DCB) specimens of a carbon fiber-reinforced thermoset polymer (CFRP) composite containing thermoplastic healants were manufactured. Mode- I fatigue delamination experiments were carried out for virgin (initial case) and up to seven repeated healing cycles. The main objective of using thermoplastic healants, i.e., polycaprolactone (PCL) and shape memory polymer (SMP), was to close and then heal the cracks formed during fatigue loading and retain the fatigue life of the DCB specimen. The in-situ healing was achieved by activating macro fiber composite (MFC) actuators bonded to the DCB specimen, where the high frequency vibration of the actuator provides the heat necessary to close the cracks using thermoplastic healants. The insitu healing was triggered using MFCs after 5000 cycles of initial loading to allow initial crack extension. The DCB specimen was then loaded up to half a million cycles to study the effect of healing on fatigue life. From the experimental data of the virgin and healed specimens, the Paris law parameters were extracted, and the results obtained were repeatable. Significant increase in maximum Mode-I strain energy release rate (G ) observed after in-situ healing is likely due to the increase in the bond stiffness of the DCB specimen material of the healed zone. More research is needed to investigate the exact mechanism for the increase of G . Mode-I fatigue life improvement of up to a factor of 2 was observed after in-situ healing for the same delamination crack growth with respect to the virgin cycle prior to healing. We envision that these findings will be helpful in extending the service life of composites and result in significant repair cost savings.
Data-Driven Discovery of Material States in Composites Under Fatigue Loads
Our community has a widespread knowledge on the damage tolerance and durability of the composites, developed over the past few decades by various experimental and computational efforts. Several methods have been used to understand the damage behavior and henceforth predict the material states such as residual strength (damage tolerance) and life (durability) of these material systems. Electrochemical Impedance Spectroscopy (EIS) and Broadband Dielectric Spectroscopy (BbDS) are such methods, which have been proven to identify the damage states in composites. Our previous work using BbDS method has proven to serve as precursor to identify the damage levels, indicating the beginning of end of life of the material. As a change in the material state variable is triggered by damage development, the rate of change of these states indicates the rate of damage interaction and can effectively predict impending failure. The Data-Driven Discovery of Models (D3M) [1] aims to develop model discovery systems, enabling users with domain knowledge but no data science background to create empirical models of real, complex processes. These D3M methods have been developed severely over the years in various applications and their implementation on real-time prediction for complex parameters such as material states in composites need to be trusted based on physics and domain knowledge. In this research work, we propose the use of data-driven methods combined with BbDS and progressive damage analysis to identify and hence predict material states in composites, subjected to fatigue loads.
Impact Damage Detection in Composite Aerospace Structures by Multi-Resolution NDE Inspections
Assessing the health of aerospace structures and understanding the underlying mechanics that govern composite strength constitute a main focus of research in the area of aerospace design and airworthiness certification. Impact damage is one of the major threats to composite aerospace structures for its frequency of occurrence, complexity and minimum external visibility. While non-destructive evaluation (NDE) provides a variety of solutions to inspect the subsurface and internal components of structures non-invasively, a gap exists between the mechanics of damage formation, growth and tolerance, and the inspectability of the structure. This study is focused on the quantitative correlation between impact damage mechanics and ultrasonic NDE inspections, where damage severity, mode interaction and progression are identified in real-scale composite panels of complex geometry, representative of commercial aircraft, impacted to reproduce different damage types at the skin-to-stringer interface and the stringer cap. High resolution X-ray CT scanning and conventional ultrasonic scanning (UT) have been used to map the damage state and identify relevant impact damage features. Ultrasonic guided wave (UGW) scanning was then employed as a rapid in-situ inspection technique to not only detect damage but also provide quantitative information about damage severity and mode. The correlation of multi-resolution multi-dimensional NDE data promises new insights on damage studies and solutions to damage detection and prognosis through viable NDE inspections.
Self-Healing of Woven Composite Laminates via In Situ Thermal Remending
Fiber-reinforced polymer composites are attractive structural materials due to their high specific strength/stiffness and excellent corrosion resistance. However, the lack of through-thickness reinforcement in laminated composites creates inherent susceptibility to fiber-matrix debonding, i.e., interlaminar delamination. This internal damage mode has proven difficult to detect and nearly impossible to repair via conventional methods, and therefore, remains a significant factor limiting the reliability of composite laminates in lightweight structures. Thus, novel approaches for mitigation (e.g., self-healing) of this incessant damage mode are of tremendous interest. Self-healing strategies involving sequestration of reactive liquids, i.e. microcapsule and microvascular systems, show promise for the extending service- life of laminated composites. However, limited heal cycles, long reaction times (hours/days), and variable stability of chemical agents under changing environmental conditions remain formidable research challenges. Intrinsic self- healing approaches that utilize reversible bonds in the host material circumvent many of these limitations and offer the potential for unlimited heal cycles. Here we detail the development of an intrinsic self-healing woven composite laminate based on thermally-induced dynamic bond re-association of 3D-printed polymer interlayers. In contrast to prior work, self-repair of the laminate occurs in situ and below the glass-transition temperature of the epoxy matrix, and maintains >85% of the elastic modulus during healing. This new platform has been deployed in both glass and carbon-fiber composites, demonstrating application versatility. Remarkably, up to 20 rapid (minute-scale) self-healing cycles have been achieved with healing efficiencies hovering 100% of the interlayer toughened (4-5x) composite laminate. This latest self-healing advancement exhibits unprecedented potential for perpetual in-service repair along with material multi-functionality (e.g., deicing ability) to meet modern application demands.
Exploration of Cumulative Damage Evolution Under Variable Amplitude Fatigue Loading Based on Wöhler Curve
How to predict the residual strength of polymer matrix composites (PMCs) after a fatigue cycle at multiple stress levels, based on the fatigue or Wöhler (S-N) curves, remains unsatisfactorily tackled. The Miner’s Rule is a widespread example of a simple way to account for damage accumulation under different fatigue cycles. Under certain combinations of stress levels, Miner’s Rule accurately predicts the lifetime of PMCs, but it fails in other cases. The reason is the simple assumption of linear cumulative damage, not accounting for sequence effects in the loading history. Several researchers have proposed modifications to Miner’s Rule. However, due to its simplicity, Miner’s Rule is still used by structural designers. Recent research work proposed compatibility conditions for fatigue damage functions in the S–N plane, leading to a simple model that fulfils those conditions contrary to the previous models, the Miner’s Rule and the Broutman and Sahu linear model. These models predict fatigue life at variable amplitude loading based on constant amplitude fatigue data. Forcibly, the analytical form of SıN influences the model lifetime predictions. Experimental data obtained in the literature serves to illustrate the models' predictions at different loading conditions. Although this work focused on composite materials, we foresaw extension to other materials.
Mechanical Performance of Variable Stiffness Plates Subjected to Multiscale Defects
Novel manufacturing techniques that have arisen during the last decades have permitted to improve both the manufacturing quality and performance of laminates parts. Despite these improvements, such manufactured parts are not flaw-exempt, since uncertainty in the fabrication processes and in the material properties are still present. At the same time, numerical models that allow to describe the ground truth designs have been developed. Nevertheless, some defects have not been studied yet. This work aims to analyze the influence of spatially varying microscale defects on the mechanical performance of variable stiffness plates at both microscale and macroscale level. Attention has been paid to the usage of component-wise and layer-wise modeling, based on the Carrera Unified Formulation, to study the stochastic response of the micromechanical stresses and the macroscale buckling performance, respectively.
Damage Detection in Laminated Composites by Neural Networks and High Order Finite Elements
In the aerospace industry, machine learning techniques are becoming more and more important for Structural Health Monitoring (SHM). In fact, they could be useful in giving a precise and complete mapping of damage distribution in a structure, including low-intensities or local defects, which cannot be detected via traditional tests. In this work, feedforward artificial neural networks (ANN) are employed for vibration-based damage detection in composite laminates. In the framework of Carrera Unified formulation (CUF), one-dimensional refined models in conjunction with layer-wise (LW) theory are adopted. CUF-based Monte Carlo simulations have been used for the creation of a dataset of damage scenarios for the training of the ANN. Therefore, the latter is fed with the vibrational characteristics of these structures. The trained ANN, given these dynamic parameters, is able to predict location and intensity of all damages in the laminated composite structures.
Peridynamic Unit Cell Enabled Finite Element Modeling of Fiber Steered Composites
This study presents an approach based on traditional finite elements and peridynamic unit cell (PDUC) to perform structural analysis of fiber steered composite laminates. Effective material property matrix for each ply in the plate element is computed by employing the PDUC based on the orientation of the fiber path and orthotropic ply properties. Each element defines the unit cell domain if the element shape is rectangular. Otherwise, the rectangle that circumscribes the element defines the domain of the unit cell. The element stiffness matrix is constructed through a traditional finite element implementation. This approach provides an accurate and simple modeling of variable angle tow laminates. It can be readily integrated in commercially available finite element programs.
Residual Plastic Strain State and Delamination Pattern of Composite Laminates with Automated Induced-Gap Subjected to Hemi-Spherical Impact Loading
Automated manufacturing defects are types of composite structure defects that occur during fiber deposition by advanced robots. The induced gap is the most probable type of defect in the Automated Fiber Placement (AFP) technique. This defect can affect the mechanical performance of the composite structures at both material level by inducing the material inhomogeneity and the structural level by introducing the consolidation effect in the structure during the curing process. The current study investigates the effect of induced-gaps on the damage assessment of thin composite plates under Low-Velocity Impact (LVI) loading. The paper focuses on the delamination initiation and propagation and the residual plastic strain state of the impacted plates. The primary application of this study is to understand the interaction of induced gaps on the delamination pattern of composite samples subjected to LVI. For this purpose, a series of LVI tests are performed. Ultrasonic C-scan analysis and microscopic observation are implied to evaluate the internal damage due to impact loading. Finite Element (FE) analyses are then performed to evaluate the residual strain of the composite plates under Impact Energy (IE) loading less than 15 J. Then, the residual plastic strain in the impact zone is evaluated using a meso-macro method, and the effect of the local plasticity that occurs in the gap zones on the delamination initiation and propagation is studied. Results show that the stress relaxation due to the resin plasticity at the gap areas can affect the delamination pattern of the impacted composite plates. It is also shown that the residual strain of the impacted plates at the gap areas are new sources of the damages that need to be considered in the LVI analysis of the composite plates manufactured by the AFP technique.
Efficient Numerical Analysis Based on Perturbation Method for the Effect of Waviness in CFRP
The paper proposes a numerical multiscale homogenization method for carbon fiber reinforced composites, where fiber alignment is disturbed due to unintended imperfections (fiber waviness). Imperfection is introduced as input to the calculation, and the calculation is always done using idealized perfect geometry. This has a distinct advantage in numerical calculations where the same mesh can be used for a series of imperfect geometries. The calculation is based on second-order perturbation method in order to capture the anisotropy which is the feature of CFRP. The proposed method is validated by comparing the results to those of a conventional calculation. The results demonstrate that the proposed method can accurately capture the stress distribution when the amplitude of the imperfection is small.
Variability in the Failure of Composite Tubes Subjected to Combined Axial and Torsional Loadings Due to Manufacturing Defects and Nondeterministic Material Properties
Tubes made with polymer-matrix fiber-reinforced composite materials are widely used in automobile, mechanical and aerospace engineering applications. Composite tubes are increasingly manufactured using the modern Automated Fiber Placement (AFP) technique. The ply manufacturing parameters and the tube manufacturing parameters have considerable influence on the quality of the manufactured composite tubes. Manufacturing defects and variations in the material properties are inevitable in composite tubes due to the inherent unavoidable variations in these parameters. The commonly identified manufacturing defects include voids, fiber waviness, variation in volume fraction, and fiber misalignment. These have considerable influence on the mechanical behavior and failure of the composite tube. In the present work, the effects of the fiber misalignment and the variations in the material properties on the failure behavior of uniform-diameter composite tubes subjected to combined axial and torsional loadings are determined considering the First-Ply Failure (FPF) characteristics. The first-ply failure envelopes of the composite tube are developed based on the Classical Laminate Theory and Finite Element Modeling and Analysis. Existing works in the literature are used to validate the three-dimensional finite element model of the uniform-diameter composite tube developed using the commercial software ANSYS®. The variations in the first-ply failure loading limits of the uniform-diameter composite tube made of a Carbon Fiber Reinforced Polymer (CFRP) composite material are investigated using the Monte Carlo Simulation (MCS) method, considering the random variability in the material properties and the fiber misalignment. The random variables corresponding to the material properties and the fiber misalignment are generated. For the composite tube with a sample set of simulated random variables the corresponding first-ply failure envelope is determined. The ensemble of such failure envelopes is developed based on an adequate number of simulations from which the probabilistic distributions of the first-ply failure loadings are determined. Design aspects are brought out.
The Effect of Out-of-Plane Waviness Asymmetry on Laminate Strength
The study of defects and their influence on the performance of composite structures is well documented. However, simplifications are often made regarding the morphology of these defects. As part of the CerTest project, a database of composite features and defects is being built, for which an assessment must be made of what information is relevant. This study looks specifically at out-of-plane waviness and seeks to investigate whether the commonly used severity metrics (e.g. maximum angle) are sufficient to characterize such defects when considering more complex morphologies. To assess this, previously developed numerical methods are used to investigate the tensile and compressive strength of cross-ply and quasi-isotropic laminates. These laminates are seeded with asymmetrical out-of-plane waviness. The numerical results demonstrate that for reliable prediction of the strength reduction caused by out-of-plane waviness, the asymmetry is an important consideration with relative reductions from symmetric to asymmetric wrinkle of up to 50% found within the wrinkles evaluated in this study.
Stability and Vibration Analyses of an Internally Damped Tapered Composite Driveshaft Using the Finite Element Method
The present study considers the linear vibration and stability analyses of an internally damped rotating tapered composite shaft supported on rolling bearings. The Timoshenko beam theory is utilized to model the tapered drive-shaft based on Equivalent Single Layer Theory (ESLT). The ESLT considers a laminated driveshaft that consists of several lamina with different fiber orientations. Since the bearings are considered as rolling element bearings, the bearings stiffnesses are modeled using linear translational springs and dampers. The equations of motion are derived by applying Lagrange’s equation, including the hysteretic internal damping of composite material, and then finite element formulation is utilized to solve the equations. The effects of various system parameters on the natural frequencies and instability threshold are investigated. An extensive parametric study has been carried out to determine the effects of various system parameters including hysteresis internal damping, fiber orientation, stacking sequence, taper angle, rotational velocity, and bearings stiffness and damping on the natural frequencies, critical speeds, and instability thresholds of internally damped tapered composite drive-shafts. Furthermore, Campbell and critical speed map diagrams are depicted to present the effects of rotational velocity and bearings stiffness on natural frequencies and critical speeds. It is shown that the stability of the driveshaft is enhanced by increasing the damping of the bearings, whereas increasing the internal damping of the composite driveshaft may reduce the instability threshold.
Microstructural Simulation of Superelastic Zirconia-Reinforced Metal Composite for Energy Dissipation Applications
This paper studies the mechanical properties of superelastic zirconia-reinforced aluminum-matrix composites through finite element simulations of microstructural representations. The objective of this study is to exploit the reversible phase transformation of zirconia to develop a composite with improved strength and energy dissipation capacity. Zirconia is a shape memory ceramic with promising potential in actuation, energy damping, and fatigue applications. Compared to the most commonly used shape memory alloys, zirconia is distinguished by a wider operational temperature range, higher recoverable strains, and a larger energy dissipation. The two-phase composite studied in this work consists of 16 mol% ceria-doped zirconia particles embedded in an aluminum matrix. The behavior of zirconia is simulated using the isothermal superelastic constitutive relationship proposed by Auricchio and Taylor. A series of numerical simulations is conducted to examine the effects of the matrix yield stress as well as the particles’ diameter and volume fraction on the evolution of the phase transformation, the strength, and the energy dissipation of the composite. A random generating algorithm is used to produce the locations of zirconia reinforcing particles. The results indicate that zirconia-reinforced aluminum-matrix composite exhibits enhanced strength and energy dissipation. The incorporation of 50% zirconia volume fraction improves the maximum stress by 50% while the amount of energy dissipation is increased by 24%. This paper provides insight into the potential application of zirconia-based composites for an efficient damping response.
Automatic Image Segmentation of CT Data from the Low Velocity Impact Tests of CFRP Composites
In this work, the role of image segmentation in the analysis of the micro-CT data for the low velocity damage assessment in carbon fiber reinforced polymer (CFRP) composites is discussed. A novel automatic image segmentation method based on the unsupervised learning approach and the Kullback–Leibler divergence is presented. The method has been successfully applied to identify and isolate impact damage in the CFRP composites subjected to the low velocity impact. The results show that the method enables not only an automatic image segmentation, but also delivers a statistics based rigorous damage threshold.
High Energy Wide Area Blunt Impact of Composite Aircraft Structures—Part A: Design and Analysis Methodology of Representative Substructure
Large test structures, common in the aerospace industry, offer a challenge to model, manufacture and test, with high cost associated with computational as well as materials, specimen fabrication, test planning/setup, and instrumentation resources. In this paper, a methodology is presented to demonstrate use of a smaller-sized substructure to produce equivalent response to the original, larger structure. The structure under study is a quarter barrel of typical commercial aircraft fuselage section made of carbon fiber reinforced polymer (CFRP), initially consisting of two circumferential structural members (C-frames and shear ties), and 12 stringers cocured to the skin. Through a series of finite element analyses and a modified specimen design, a substructure representing the quarter barrel was validated for loading conditions generated by high energy wide area blunt impacts (HEWABI) which are potentially caused by accidental contact from moving ground service equipment (GSE). The substructure is made of one circumferential member (C-frame and shear tie), and 6 stringers co-cured to skin and is shown to have similar stiffness and stresses in the region of interest. Finite element analysis (FEA) with progressive damage analysis demonstrates the equivalent response between the substructure and full quarter barrel. This methodology can be used in a wide range of applications, as long as the loading area is distant enough from the modified structure end and the correct boundary conditions/fixtures are defined to represent the omitted portions of the structure of interest.
Impact Response Analysis of Composite Plate with Different Fiber Orientation Angles
In this study, we established an analytical model of rectangular plate as a carbon fiber reinforced plastic (CFRP) fan blade, and simulated propagation of strain due to a foreign object collision such as a bird strike. The effect of fiber orientation angles was investigated as one method to reduce the propagating strain. In this model, large deflection caused by bird-striking is considered and nonlinear vibration equation derived from Rayleigh-Ritz method was integrated using Newmark-β method. It was shown that the strain propagates to the spanwise tip of the fan blade and causes detrimental damage there. It was also suggested that changing the fiber orientation angle of CFRP can reduce the strain transmitted to the tip of fan blade.
Dynamic Impact Resistance of Composite Sandwich Panels with 3-D Printed Polymer Syntactic Foam Cores
Polymer-based syntactic foams find use in the marine industry as primary structural materials due to their inherent lightweight nature and enhanced mechanical properties relative to pure HDPE. 3-D printing these materials circumvents the use of joining assemblies, enabling the production of complex shapes as standalone structures. Although the quasi-static response of these 3D printed foams has been well studied independently in recent years, their dynamic impact resistance and tolerance as potential core material for sandwich panels have not been the focus. Moreover, 3D printing is known to impart directionality in the printed syntactic foams, which may introduce failure mechanisms typically not observed in molded foams. It is therefore important to investigate the mechanics of 3-D printed syntactic foam core composite sandwich structures under impact loading and characterize their failure mechanisms for establishing dynamic impact resistance. To this end, 3-D printed syntactic foams have been developed using rasters of High-Density Polyethylene (HDPE) and Glass MicroBalloon (GMB) fillers by adopting the Fused Raster Fabrication (FFF) technique. The current study is performed to assess the impact performance of these composite foam cores based on the volume fraction of fillers and print orientation. The weight percentage of GMB fillers in printed specimens ranges from 0% to 60% in increments of 20%. This study presents the impact response of these composite sandwich panels at different energy levels, in compliance with ASTM D7136/D7136M - 20. Observations suggest that an increase in GMB % in HDPE matrix improves the impact performance in terms of the peak load of the material, but the failure behavior becomes brittle to an extent. Observing the failed specimens under a Micro-CT scanner captures the failure morphologies and helps characterize failure processes during impact. It is noticed that core materials with higher GMB content are prone to individual raster breakage and delamination at the back face, in addition to debonding between individual rasters. Specimens printed along the longer dimension (y-direction) impart more warping in the final sandwich structures than that of specimens printed along the shorter dimension (x-direction). Therefore, they are more susceptible to delamination at the back face. Addition of GMB fillers mitigate the tendency of the sandwich panels to warp.
Hypervelocity Impact Response of Stitched CFRP Laminates
In this study, hypervelocity impact experiments were performed on both unstitched and through-thickness Vectran™-stitched laminates. Both laminate types were fabricated from DMS-2436 class-72 warp-knit multiaxial carbon fabric, infused with API-1078 resin using a Controlled Atmospheric Pressure Resin Infusion (CAPRI) process. The laminates were impacted by 4 mm diameter, spherical, Nylon 6/6 projectiles at nominal velocities of 4 km/s using a two-stage light gas gun. The primary measures of the performance of the composite at protecting against impact were in plane hole damage areal comparisons and the comparison of the target back-face debris cloud (BFDC) velocities relative to the incoming projectile velocities. Additional post-shot forensics include characterization of damage morphology and analysis of high-speed videos. Initial inferences about the damage produced in the laminate indicate that the Vectran™ stitching can effectively arrest in-plane damage propagation; impacts at or near a stitchline resulted in no damage propagation across the stitchline boundaries.
Experimental Investigation of High Velocity Impact on CNT Reinforced Composites Employing Single Stage Gas Gun
Composite plays a significant role in the field of aerospace due to its excellent mechanical properties, nevertheless, they are highly susceptible to out-of-plane impact load. Fibre-reinforced composite fails effortlessly under impact load and absorb energy through damage mechanics rather than deformation. The present study investigates the damage behaviour of the CNT reinforced carbon fibre-epoxy composite under high velocity impact using single stage gas gun. Composite plates were fabricated with 0 to 0.6 weight percentage content of CNT as reinforcement using vacuum assisted resin transfer moulding. A series of impact test with various impact energy was carried out on carbon/epoxy composite plate to study the impact performance. From the experimentation it was observed that the 0.3 weight percentage CNT addition provides the optimum impact performance. Damage characterization was performed for various impact velocity based on the micro and macro scale damage area. Knowledge of the damage behaviour of CNT reinforced carbon fibreepoxy composite plate under high velocity impact loads is essential for both the product development and material selection in the aerospace application.
Strain Rate-Dependent Characterization of the In-Plane Shear Response of a Unidirectional Non-Crimp Fabric Carbon Fiber/Snap-Cure Epoxy Composite
The dynamic in-plane shear stress-strain response of a unidirectional non- crimp fabric carbon fiber/snap-cure epoxy composite was studied by subjecting 30° and 45° off-axis specimens to compression loading at high strain rates. Tests were performed using a compression split-Hopkinson pressure bar apparatus where an approximate axial strain rate of 305 s-1 was achieved. Images of the deformed specimen surfaces were captured with high-speed cameras and digital image correlation used to obtain a strain map. Pulse shaping was performed using a copper pulse shaper to achieve dynamic equilibrium during the high-rate tests. The results demonstrated that the in-plane shear yield stress and strength increased by 53% and 68%, respectively, when the strain rate increased from quasi-static to 305 s-1.
High Energy Wide Area Blunt Impact of Composite Aircraft Structures Part B: Testing and Internal Damage Modes
Modern aircraft structures constructed from high strength laminated composite materials present a challenge in the detection of damage resulting from impact events. Broad-area contact from large sized or blunt radius objects furthermore create conditions in which significant internal damage can result, while leaving low (or even no) externally visible indications of damage being present. Composite fuselage structures subjected to high energy wide area blunt impact (HEWABI) sources have been studied. These impact sources act over a wide area and can possibly damage multiple internal structural elements. HEWABI sources of concern are generally heavy ground service equipment (GSE) such as belt loaders, cargo loaders, catering trucks, etc., which have soft rubber/elastomeric bumper-type pads mounted at locations where the GSE could contact the aircraft. Of particular interest in this study is the formation of damage in composite frame and shear tie components during HEWABI events occurring near the passenger floor joint. Carbon/epoxy composite test specimens reflecting modern composite fuselage features were designed and fabricated having continuous shear ties and representative stiffness interaction between the floor beam and circumferential frame members. Quasi-static and slow speed tests (representing impact) conducted using rubber bumper faced indentors developed significant internal damage to internal components. Specifically, fiber failures and large-size crack formation in shear ties, frames, and stringers occurred with low/no external observability in the external skin. A clear quantitative understanding of damage modes and damage location that could result from HEWABI events is important for establishing damage size criteria in the evaluation of a structure’s residual strength and damage tolerance capability.
Dynamic Response of Photopolymer Resins Cores for Naval Applications in Extreme Environments
Due to the melting of polar ice caps, large deposits of natural resources are becoming more readily available, leading to an increase in arctic naval exploration. Naval vessels and ship hulls must be built with lightweight structures, such as sandwich composites, to increase the ship’s fuel efficiency. However, identifying new material choices that can withstand the harsh Arctic environments is crucial for the survivability and safety of personnel and structures. This study investigates the potential of photopolymer resins through additive manufacturing as a lightweight sandwich composite core material. The thermo-mechanical properties of this resin were evaluated using: tensile, flexural, and compressive tests according to the ASTM Standards D638, D695, and D790, respectively. Tests were conducted at room temperature (23 C) and arctic temperature (-60 C). The experimental data will be used as an input for high-fidelity finite element (FE) simulations with the software ABAQUS. From the performed tests, the photopolymer exhibited isotropic behavior at both room (RT) and Arctic temperatures. Preliminary quasi-static results for “Durable Resinâ€ at AT showed an increase of ~300% in its tensile, flexural, and compressive modulus and an increase of ~300% in its tensile and flexural strength and ~100% in its compressive strength when compared to the same resin at RT. The finite element analysis models showed good agreement with the experimental results. Zackery Nieto, The University of Texas at El Paso, 500 W. University Ave, El Paso,
Dynamic Facture of Hydrothermally Degraded Carbon-Epoxy Composites
Structures used in naval applications are often subjected to marine or other aqueous environments during their work-life. It has been observed that many composites absorb moisture when exposed to such environments, which can lead to material damage and degradation of mechanical properties. The addition of salt to the water solution may have additional impacts on the material degradation over time. Furthermore, many of these applications require that the material be subject to stresswave loading environments like impact that may cause dynamic fracture. In this experimental study, the effect of water absorption on the Mode-I dynamic fracture behavior of carbon-epoxy composites is investigated. Specifically, the effects of moisture uptake and the role of saline on the critical dynamic SIF are investigated. Samples were hygrothermally soaked in an elevated temperature bath (70 °C) of either ASTM standard sea water or distilled water. The elevated temperature accelerates mass absorption, making experimental observation possible on a more reasonable timeline than natural aging. Soaking durations varied between 3.5 hours to excess of 4 months to explore the role of soak time on dynamic fracture behavior. Pre-cracked specimens were impacted using a unique long-bar striker device at 4 m/s, and the resulting Mode-I (opening mode) fracture behavior investigated. Digital image correlation (DIC) was used in conjunction with ultra-high-speed imaging to track the crack tip surface displacements, and an elastodynamic solution was leveraged to extract the stress intensity factors (SIF) at fracture initiation. The dynamic fracture toughness of the soaked samples was compared with results from a group of unsoaked, ambient condition samples. The results indicated that the ambient condition samples had a consistently higher Mode-I SIF than any of the soaked conditions. The difference between the ambient condition SIF and the soaked conditions was observed to be approximately 60% for the 3.5 hour soaks, 40% for the 2 day soaks, 50% for the 2 week soaks, and 80% for the samples soaked for longer than 4 months. The SIF reduction is thought to be due primarily to matrix and interface degradation. No statistically significant difference in the SIF was observed between samples soaked in DI water versus salt water.
Effect of Impactor Mass on CFRP in Arctic Condition under Low-Velocity Impact
This study investigates the impact response and damage characterization of carbon fiber reinforced polymer (CFRP) under low-velocity impact by impactors of different masses and velocities at 62J. Low-velocity impacts are conducted at room temperature (23ºC) as well as low temperature (-70ºC) conditions in the thermal chamber of the drop tower testing machine, Instron CEAST 9350. The aim is to observe composite behavior in the cold Arctic environment due to equal energy impacts. Moreover, a 3mm thickness of ice is created on the CFRP samples at -12ºC after 24 hours of freezing and impacted at -70ºC. The goal is to elucidate the contribution of surface ice on the overall impact damage of composites. X-ray micro-computed tomography is utilized to reveal the inner damages of the composite structures. Intralaminar damage in the form of fiber breakage is found as the dominant failure mode on the CFRP samples from 62J impacts. But differences in the delamination and matrix crack formation are identified for different mass-velocity configurations and environmental conditions. Results show that low mass impactors produce a larger damage initiation force on the composites at all temperatures, whereas no specific trend is observed in the peak force values due to severe fiber failure. Although higher mass impactors show longer impact duration, lower mass impactors develop greater damage on the CFRP, as seen by a greater reduction in specimen stiffness. Furthermore, the presence of ice is observed to have a minimal effect on the damage behavior of composites. But ice layer assists to reduce the amplitude of initial load drop by the low mass impactor and as such, less permanent displacement is identified in the CFRP specimens than both room temperature and low-temperature conditions. This study explores the understanding of the dynamic behavior of composites under low-temperature icy conditions.
Experimental Analysis of Low Velocity Impact Near the BVID Limit on Carbon Fiber Reinforced Polymer Panels
As carbon fiber reinforced polymer matrix panels (CFRP) are increasingly being used in aircraft due to their advantages in strength and weight, it is necessary to understand their durability. In this study, a detailed analysis of critical events during low velocity impact (LVI) will be presented as well as initial compression after impact (CAI) results. Two CFRP layups of 16 and 24 plies are impacted with varying impact energies below and near the barely visible impact damage (BVID) limit. The panels are subsequently compressed in the transverse direction in order to quantify the strength knockdown from pristine panels to damaged ones. The sequence of failure initiation, delamination initiation, and subsequent delamination pattern was identified. The sequence is critical to predictive modeling of failure and failure progression of CFRP composites.
Fracture Analysis of Rubber Toughened Additively Manufactured Thermosets
This study explores the role of rubber toughening on the dynamic fracture behavior of additively manufactured (AM) high-performance thermosetting polymers formed through digital light processing (DLP). Using DLP to create these polymers allows for rapid, agile manufacturing of prototypes meeting the lightweight and building speed requirements of relevance to military mission applications. This method also provides flexibility in part complexity while maintaining relatively high isotropy compared to traditional AM techniques. Previous work has demonstrated a dependence of these DLP specimens on print layer orientation and loading rate, prompting further investigation into other manufacturing parameters to improve toughness [1]. This study examines the role of rubber toughening on the quasi-static and dynamic fracture behavior of bis-GMA thermosets. Current literature largely reports on quasi-static behavior of DLP specimens, although dynamic conditions are more applicable to many realistic loading scenarios and extreme environments often seen in defense applications. Dynamic experiments leverage a unique long bar striker device that impacts a specimen opposite a pre-crack, sending a stress-wave driven load to initiate a dynamic Mode-I (opening) fracture event. Full-field displacement data ahead of the propagating crack is obtained using ultra high-speed imaging combined with 2D digital image correlation (DIC). An elastodynamic solution following the principles of dynamic fracture mechanics extracts the stress intensity factor (SIF) using a least squares fit at crack initiation and a Newton-Raphson scheme for crack propagation. The rubber toughened thermosets in this study exhibited a rate dependence in fracture toughness with the quasi-static SIF being 1.20 MPa and the dynamic SIF being 0.41 MPa .
Numerical Model of Tubular Composite Sandwich Structures under Low-Velocity Impact
This study aims to investigate the dynamic impact response of tubular composite structures with honeycomb sandwich core under transverse low-velocity impact (LVI) test. We establish a finite element analysis (FEA) model of composite sandwich structures. Simulation results are compared with experimental results to verify the correctness of the model and analysis method. This model can accurately predict the impact response and damage mechanisms of a composite sandwich structure, as validated by experimental testing, specifically capturing major failure modes. LVI experiments are conducted utilizing cylindrical impact striker to enact both point and line impact. Damage mechanisms, such as matrix cracking, delamination, and fiber breakage/rupture, occur in the facesheet, as well as honeycomb crushing and breakage in the core. The combination of experimental and numerical results illustrates the effects of facesheet thickness and core thickness on failure mechanisms. A three-dimensional model can also be used to clearly visualize the effect of different core material properties on the failure mechanism and dynamic response during an impact event.
An Image Based Inertial Impact Test to Extract Viscoelastic Constitutive Parameters
Polymer matrix composites often exhibit a strong strain rate dependance in their mechanical response. In many of these materials, the viscoelastic behavior of the polymer matrix drives the rate dependence in the composite, however identifying these parameters at high strain rate presents a significant challenge. Common high-rate material characterization techniques such as the Kolsky (split-Hopkinson pressure) bar require a large test matrix across a range of strain rates. Kolsky bars also struggle to identify constitutive parameters prior to the yield due to inertial effects and the finite period of time required to reach force equilibrium. The Image Based Inertial Impact (IBII) test has been successfully used to identify linear elastic constitutive behavior of composites at high strain rates, but, to date, has only been used to extract constitutive properties at a single nominal strain rate in each test. Here, we propose an adaptation of the IBII test to identify viscoelastic parameters at high strain rates using full-field displacement data and the nonlinear virtual fields method (VFM). We validate the technique with finite element simulations of an IBII test on a model viscoelastic material that is characterized with a Prony series formulation of the generalized Maxwell model. The nonlinear VFM is then used to extract the Prony pairs for dynamic moduli and time constants from the full-field deformation data. The nonlinear viscoelastic identification allows for characterization of the evolution of mechanical response across a range of strain rates in a single experiment. The experimentally identified viscoelastic parameters of the matrix can then be used to predict the behavior of the composite at high strain rates. This approach will also be validated experimentally using a single-stage gas-gun to characterize the high-rate viscoelastic response of PMMA.
Process Modelling the Cure of Bisphenol-A Epoxy/Jeffamine System Using ICME
The prediction of thermo-mechanical properties of a thermoset resin at different stages of cure is a complex process. An Integrated Computational Material Engineering (ICME) approach is used to predict the properties of a EPON828/Jeffamine D230 system. The proposed framework integrates two length scales - nano and microscale. Molecular Dynamics (MD) is used to predict the volume shrinkage and mechanical properties of the epoxy resin as a function of the progressing crosslink density at room temperature using the Reactive Interface forcefield (IFF-R). The predicted resin properties show good agreement with the literature, proving that IFF-R can be reliably used for this purpose. Once characterized, the predicted properties are used to further predict the effects of cure shrinkage and property transformation on the bulk-level composite residual stresses.
Prediction of PEEK Resin Properties for Processing Modeling Using Molecular Dynamics
Polymer Matrix Composites (PMCs) have been the subject of many recent studies due to their outstanding characteristics. For the processing of PMCs, a wide range of elevated temperatures is typically applied to the material, leading to the development of internal residual stresses during the final cool-down step. These residual stresses may lead to net shape deformations or internal damage. Also, volumetric shrinkage, and thus additional residual stresses, could be created during crystallization of the semi-crystalline thermoplastic matrix. Furthermore, the thermomechanical properties of semi-crystalline polymers are susceptible to the crystallinity content, which is tightly controlled by the processing parameters (processing temperature, temperature holding time) and material properties (melting and crystallization temperatures). Hence, it is vital to have a precise understanding of crystallization kinetics and its impact on the final component's performance to accurately predict induced residual stresses during the processing of these materials. To enable multi-scale process modeling of thermoplastic composites, molecular-level material properties must be determined for a wide range of crystallinity levels. In this study, the thermomechanical properties and volumetric shrinkage of the thermoplastic Poly Ether Ether Ketone (PEEK) resin are predicted as a function of crystallinity content and temperature using molecular dynamics (MD) modeling. Using crystallization-kinetics models, the thermo-mechanical properties are directly related to processing time and temperature. This research can ultimately predict the residual stress evolution in PEEK composites as a function of processing parameters.
A Unique Approach to Experimental Characterization of a Thermosetting Polymer Matrix for ICME Frameworks
A novel approach for characterization of thermosetting epoxy resins as a function of the degree of cure is presented. Density, cure kinetics, tensile strength, and Young’s modulus are experimentally characterized across four mixing ratios of DGEBF/DETDA epoxy. Dynamic differential scanning calorimetry (DSC) is used to characterize parameters for a Prout-Thompkins kinetic model unique to each mixing ratio case through a data fitting procedure. Tensile strength and Young’s modulus are then characterized using stress-strain data extracted from quasi-static, uniaxial tension tests at room temperature. Strains are measured with the 2-D digital image correlation (DIC) optical strain measurement technique. Strength tends to increase as amine content use in the formulation increases. The converse trend is observed for Young’s modulus. Density measurements also reveal an inverse relationship with amine content.
Using Data Science to Evaluate Nano-Reinforced Epoxy Surfaces
Toughened composites reinforced with nanofillers show improved mechanical performance such as increased abrasion resistance, fracture toughness, and fracture energy. The degree of these improvements is influenced by the degree of dispersion of the nanofillers which can be analyzed using force microscopy (AFM), a technique that allows for mapping the local height and elastic modulus of a surface. However, current AFM apparatuses can only measure a narrow range of moduli according to the type of tip, which complicates the full-field measurement of moduli in nanocomposites with nanosilica (~72 GPa) embedded in epoxy (0.1 – 5 GPa). Moreover, height mapping can only visualize filler particles exposed at the surface. These limitations make it challenging to determine the 3D location of nanoparticles near the surface of a composite. To overcome these limitations of conventional AFM, we used a combination of data science, micromechanics, and experimental data from AFM to locate the centroidal position of nanosilica (NS) particles relative to the surrounding epoxy surface. Using finite element simulations, a theoretical dataset of modulus values as a function of particle position relative to the epoxy surface was created as a training set. Bayesian optimization determines the “bestâ€ particle position that results in minimum error between simulated and experimental modulus contours. The algorithm returns the 3D position of the fully or partially embedded NS particle relative to the epoxy surface. The algorithm has shown the ability to partially produce simulated modulus contours that resemble the experimental modulus contours.
Deep Learning Framework for Woven Composite Analysis
paper, we focus on exploring the relationship between weave patterns and their mechanical properties in woven fiber composites through Machine Learning. Specifically, we explore the interactions between woven architectures and in-plane stiffness properties through Deep Convolutional Neural Network (DCNN) and Generative Adversarial Network (GAN). Our research is important for exploring how woven composite’s pattern is related to its mechanical properties and accelerating woven composite design as well as optimization. We focus on two tasks: (1) Stiffness prediction: Predicting in-plane stiffness properties for given weave patterns. Our DCNN extracts high-level features through several convolutional and fully connected layers to determine the final predictions. (2) Weave pattern prediction: Predicting weave patterns for target stiffness properties, which can be treated as the reverse task of the first one. Due to many-to-one mapping between weave patterns and the composite properties, we utilize a Decoder Neural Network as our baseline model and compare its performance with GAN and Genetic Algorithm. We represent the weave patterns as 2D checkerboard models and use finite element analysis (FEA) to determine in-plane stiffness properties, which serve as input data for our ML framework. We show that: (1) for stiffness prediction, DCNN can predict stiffness values for a given weave pattern with relatively high accuracy (above 93%); (2) for weave pattern prediction, the GAN model gives the best prediction accuracy (above 92%) while Decoder Neural Network has the best time efficiency.
Comprehensive Property Determination for Fiber-Reinforced Polymer Composites in Extrusion Deposition Additive Manufacturing—Bayesian vs Deterministic
This work introduces both deterministic and Bayesian methodologies to simultaneously determine the elastic constants of the constituent polymer and the fiber orientation state in a short fiber-reinforced polymer (SFRP) composite based on a small number of experimental measurements of the composite properties. The ability of the Bayesian approach to calibrate uncertainties makes it a promising tool for enabling a probabilistic framework for composites manufacturing digital twins. The two methods that enable the reverse engineering of the orientation of the fibers and the in-situ polymer properties are compared. For the extrusion deposition additive manufacturing (EDAM) process and other SFRP composites processes (e.g. injection molding), extensive characterization efforts are currently required to develop composites manufacturing digital twins. To circumvent the extensive characterization required, Digimat© provides a suite of tools to reverse engineer material properties of SFRPs. However, Digimat© lacks a methodology to inversely determine the fiber orientation state and the constituent polymer properties simultaneously. To that end, this work presents both a deterministic and hierarchical Bayesian approaches to determine the polymer properties and the fiber orientation state simultaneously. The results indicate that both approaches provide a reliable framework for the reverse engineering process. The deterministic approach provides a more rapid, point estimate methodology, whereas the Bayesian approach provides a more comprehensive methodology that includes uncertainties in the reverse engineering process.
Internal Damage Prediction in CFRP Laminates Using BVID Profiles and Machine Learning
The purpose of this study is to investigate the possibility of estimating impact information only from the surface profile of impacted CFRP laminates using machine learning. The low-velocity impact test was conducted to collect the data about barely visible impact damage (BVID) under various impact conditions: stacking sequence, impactor shape, and impact energy. A 3D optical profiler and an ultrasonic C-scanning system were utilized to measure external and internal damages. After feature engineering using only the specimen surface data, the machine learning models were able to estimate impactor shape with over 80% accuracy by a classification problem and over 90% by a regression problem, and delamination area and its length with over 70% R2, without significant outliers. It demonstrated that considering both local and total deformation that is unique to the low-velocity impact test is prominent in this estimation.
Artificial Neural Network (ANN)-Based Predictive Tool for Estimating Lightning Damage in Composites
In this study, three artificial neural network (ANN) models are developed with back propagation (BP) optimization algorithms to predict various lightning damage modes in carbon/epoxy laminates. The proposed ANN models use three input variables associated with lightning waveform parameters (i.e., the peak current amplitude, rising time, and decaying time) to predict fiber damage, matrix damage, and through-thickness damage in the composites. The data used for training and testing the networks was actual lightning damage data collected from peer-reviewed published literature. Various BP training algorithms and network architecture configurations (i.e., data splitting, the number of neurons in a hidden layer, and the number of hidden layers) have been tested to improve the performance of the neural networks. Among the various BP algorithms considered, the Bayesian regularization back propagation (BRBP) showed the overall best performance in lightning damage prediction. When using the BRBP algorithm, as expected, the greater the fraction of the collected data that is allocated to the training dataset, the better the network is trained. In addition, the optimal ANN architecture was found to have a single hidden layer with 20 neurons. The ANN models proposed in this work may prove useful in preliminary assessments of lightning damage and reduce the number of expensive experimental lightning tests.
Ordinary Differential Equations with Machine Learning for Prediction of Smart Composite Fracture Toughness
This work in on the development of an ordinary differential equation (ODE) model coupled with statistical methods for the prediction of fracture toughness of a magnetostrictive, piezoelectric smart self-sensing Fiber Reinforced Polymer (FRP) composite. The smart composite with sensing properties encompasses Terfenol-D alloy nanoparticles and Single Walled Carbon NanoTubes (SWCNT). To explore various configurations the of nanoparticle constituents’ effect on fracture toughness within the FRP composite, the ODE model developed within a finite element analysis (FEA) environment is considered to attain fracture observations across the solution space. The acquired FEA data is then used to feed the machine-learning (ML) algorithms to obtain composite fracture toughness predictions. A comparison and development of artificial neural networks (ANN), decision trees and support vector machines (SVM) models for FRP smart self-sensing composite fracture toughness prediction is done. Qualitative results stating if the sample has fractured or not and quantitative data giving the fracture toughness and strain energy release rate for the smart self-sensing FRP composites is attained. A comparison of all predictions from the developed models for both fracture toughness is corroborated with literature data.
Discovering Failure Criteria of Composites by Sparse Identification and Compressed Sensing
A reliable design of a composite structure needs to consider the failure of the composites. Hashin failure criterion is one of the most popular phenomenological models in engineering practice due to its simplicity of application. Although remarkable success has been achieved from the Hashin failure criterion, it does not always fit the experimental results very well. Over the past few years, a few experimental failure data have been collected. It would be of interest to leverage the existing data to improve the prediction of failure criteria. In this paper, we proposed to apply a framework that combines sparse regression with compressed sensing to discover failure criteria from data. Following the phenomenological failure models, we divided the failure of composites into tensile and compressive fiber modes, tensile and compressive matrix modes. Two examples were studied with the proposed framework. The first example was presented to demonstrate the capability of the framework. The data was generated by the Hashin failure criterion and added various magnitudes of noise. The proposed framework was implemented to discover the failure criterion from the noised data. For the second example, the proposed method was used to discover failure criteria from the experimental data which are collected from the first world wide failure exercise (WWFE I). Both examples show that the proposed method can discover the failure criteria accurately.
Failure Prediction of Composite Materials Using Deep Neural Networks
Fiber-reinforced polymer (FRP) composite materials are increasingly used in engineering applications. However, an investigation into the precision of conventional failure criteria, known as the World-Wide Failure Exercise (WWFEI), revealed that current theories remain unable to predict failure within an acceptable degree of accuracy. Deep Neural Networks (DNN) are emerging as an alternate and time-efficient technique for predicting the failure strength of FRP composite materials. The present study examined the applicability of DNNs as a tool for creating a data-driven failure model for composite materials. The experimental failure data presented in the WWFE-I were used to develop the datadriven model. A fully connected DNN with 23 input units and 1 output unit trained with a constant learning rate (α=0.0001). The network’s inputs described the laminates and the loading conditions applied to the test specimen, whereas the output was the length of the failure vector (L=(σx+σy+τxy)0.5). The DNN’s performance was evaluated using the mean squared error on a subset of the experimental data unseen during training. Network configurations with a varying number of hidden layers and units per layer were evaluated. The DNN with 3 hidden layers and 20 units per hidden layer performed the best. In fact, the network’s predictions show good agreement with the experimental results. The failure boundaries generated by the DNN were compared to three conventional theories: the Tsai-Wu, Cuntze, and Puck theory. The DNN’s failure envelopes were found to fit the experimental data more closely than the above-mentioned theories. In sum, the DNN’s ability to fit higher-order polynomials to data separates it from conventional failure criteria. This characteristic makes DNNs an effective method for predicting the failure strength of composite laminates.
Deep Convolutional Neural Network for Segmenting ï­CT Images of Fiber Reinforcements
The greatest challenge in creating digital material twins and FE mesh from μCT images of composite reinforcements is the lack of a robust and versatile tool for training μCT images. Here, we have used deep convolutional neural networks (DCNN) for segmenting μCT images of a multi-layer plain-weave fiber reinforcement. A set of raw 2D image slices extracted from the gray-scale volume of a single-layer reinforcement was used to train a DCNN using manually annotated images. The trained network was tested against the manually segmented ground truth images and it performed exceptionally well with a global accuracy of more than 96%. The trained DCNN was then used to segment unseen images from a multilayer stack of the fabric with good accuracy. The work presented here provides a robust and efficient framework of segmenting CT scan images of fiber reinforcements for generating digital material twins and FE mesh of fiber reinforcements.
Deep Learning for Topology Optimization of Triply Periodic Minimal Surface Based Gyroid-Like Structures
Triply periodic minimal surfaces (TPMS) are non-intersecting complex geometrical surfaces that can be used in unit cell design of cellular structures. TPMS possess attractive properties like large surface area to volume ratio and mathematically controlled geometry which find them applications in catalytic converters, cocontinuous composites, thermal and permeability management, to name a few. The advent of additive manufacturing eased the manufacture of these structures which were previously challenging with traditional methods of manufacturing. Design of TPMS unit-cell based materials involves topology optimization to achieve the desired physical properties depending on the specific application of the structure. Topology optimization, in turn, involves the objective function evaluations for each iteration till converging to an optimal design and this may pose a computational burden when the function evaluations are time consuming finite element or computational fluid simulations. This can be alleviated by employing machine learning based methods for the optimization process. Deep learning using convolutional neural networks (CNN) have effectively been used for prediction of optimal topologies required for desired properties thus eliminating any objective function evaluations. In this paper, we explore the use of 3D CNN models for topology optimization of a TPMS based unit cell. The Solid Isotropic Material Penalization density method in topology optimization is employed on energy based homogenized unit cell properties. The unit cell that is obtained satisfying a desired mechanical property along with their topology parameters is then learnt to build a CNN model which can then be used to predict the optimal unit cell design for any topology parameters. The class of TPMS used in this work is Gyroids. The CNN model is tested for errors in prediction using mean square error metric and dice coefficient of the 2D slices of unit cell. The results indicate that the model can predict the ground truth accurately with few data points. This showed a promising approach in the area of TPMS based unit cell design using CNN.
Effect of Thermal Input on the Temperature Distribution of Thermoplastic Composites Made by Automated Fiber Placement (AFP)
The heat transfer analysis of thermoplastic composite manufactured using automated fiber placement with a hot gas torch can be done using numerical methods such as finite difference method. The accuracy of the theoretical results depends upon the accuracy of the thermal inputs. The hot gas/air temperature and convection coefficient distributions between the hot gas and the surface of the substrate (thermal inputs) have the significant influence on the accuracy of the resulting theoretical temperature distributions in the deposited laminate. A model predicting theoretical results which agree with experimental measurements is presented
Improved Tow-End Detection for Fiber Placement Inspection Using Machine Learning
A machine learning approach to improve the detection of tow ends for automated inspection of fiber-placed composites is presented. Automated inspection systems for automated fiber placement processes have been introduced to reduce the time it takes to inspect plies after they are laid down. The existing system uses image data from ply boundaries and a contrast-based algorithm to locate the tow ends in these images. This system fails to recognize approximately 10% of the tow ends, which are then presented to the operator for manual review, taking up precious time in the production process. An improved tow end detection algorithm based on machine learning is developed through a research project with the Boeing Advanced Research Center (BARC) at the University of Washington. This presentation shows the preprocessing, neural network and postâ€processing steps implemented in the algorithm, and the results achieved with the machine learning algorithm. The machine learning algorithm resulted in a 90% reduction in the number of undetected tows compared to the existing system.
Optimization of Injection Process Parameters of Plastic Reinforced Composites Using Response Surface Methodology and Central Composite Design
The injection molding process is among the most efficient processes for mass production of polymer products with complex geometry at optimal cost. This study investigates the effect of the injection parameters on the cavity pressure, tensile and microstructural properties of plastic-reinforced composites and optimized the process to determine the optimum injection parameters using the Response Surface Methodology and central composite design. The two polymer composite materials used for this work are low-density polyethylene reinforced with aluminium powder (LDPE/Al) and low-density polyethylene reinforced with carbon black (LDPE/CB) at 250 and 200oC injection temperature respectively. The analysis of the results obtained from both the numerical and physical experimentations were used to obtain two predictive models which correlate cavity pressure and tensile strength as a function of the independent process parameters namely, injection pressure and time. An injection pressure of 70 MPa and time of 1.00 sec was found to be optimum producing a cavity pressure of 37.658 MPa while an injection pressure of 70 MPa and time of 1.75 sec was found to be optimum producing a material with tensile strength of 7.41 MPa. The results indicate that the cavity pressure increases with an increase in the injection pressure but decreases with an increase in the injection time for the two analyzed polymer composites. The study shows that process parameters have significant effects on the cavity pressure, mechanical and microstructural properties of LDPE/Al and LDPE/CB.
Effect of Processing Parameters on the Widths and Thicknesses of Thermoplastic Composites Made by Automated Fiber Placement
The advent of Automated Fiber Placement (AFP) machine has expanded the capacities to manufacture engineering structures using thermoplastic composites. Structures of cylindrical shapes, flat and curved panels can be easily made using this technique. As more applications and more studies have been made on this technique for thermoplastic composites, many issues have come up. One issue of importance is the variation of the width and thickness of the tow as it is deposited. As the melted thermoplastic composite tow is being pressed under the compression force of the roller, the material flows. This changes the width and the thickness of the tow. The values of the width and thickness depend on many parameters such as the properties of the substrate, the temperature of the material, and the applied pressure. This variation in width and thickness of the individual tow being deposited has an influence on the development of laps and gaps between the deposited tows. This paper presents some of the results on an investigation on the above topic. Widths and thicknesses of carbon/PEEK tows processed using an Automated Fiber Placement machine with a hot gas torch were examined. Preliminary results show that there is significant variation in the width and thickness of the tows upon deposition.
Yield Point and Stress Relaxation Behavior of Ply-Ply Interaction of Thermoset Prepregs
The forming of continuous-fiber composites is an automated manufacturing process capable of high production rates. Its greatest weakness lies in the tendency to develop wrinkle defects. The formation of wrinkle defects is the effect of a combination of multiple material deformation mechanisms. This paper investigates possible improvements for current manufacturing techniques by studying the interply properties of continuous-fiber thermoset prepreg. In this research, the inter-ply slip deformation behavior is tested via double lap shear tests. The results show that pressure, deformation rate, and moisture have a strong effect on the ply-ply interaction response. The multi-stage nature of the loaddisplacement response suggests the complex nature of the deformation. To better understand each stage of deformation, the total amount of slip deformation of the double lap shear test is measured. The total slip versus displacement profile indicates the presence of a yield point in the ply-ply interface. The yield point indicates both the softening of the ply-ply interface and the initiation of plastic deformation. The plastic deformation past the yield point can be further increased if the preform is held in a deformed state for some time before removing the external load. A new manufacturing process is proposed based on these results. One can take advantage of the yield point of the ply-ply interface and conform a preform onto an intermediate tool before conforming it onto the final tool. As deformation can be preserved once the material deforms past the yield point, this sequential forming approach can reduce the risk of wrinkling by reducing the slip required to complete the forming operation.
Robotic Sequential Ultrasonic Welding of Thermoplastic Composites: process Development and Testing
Multi-spot sequential ultrasonic welding is a promising joining technique for fibre-reinforced thermoplastic composites structures (TPC). In existing research on the multi-spot sequential ultrasonic welding process, welds are produced through the use of a static table-top welding machine, at a coupon level. However, in order to apply this joining technology to large structures, the welding process needs to be up-scaled through the use of a robotic platform. At the Smart Advanced Manufacturing (SAM|XL) automation field lab and TU Delft Aerospace Engineering, a robotic sequential ultrasonic welding system has been developed. The system consists of a welding end-effector (EEF) equipped with various sensors that enable online process monitoring and control, which can be mounted on an industrial robot arm to perform sequential multi-spot welds. The goal of this study was to assess the welding performance of the ultrasonic welding EEF, which was mounted on an industrial KUKA KR210 R2700 Extra 10-axis robot arm, by comparing it to the performance of welds produced through the static table-top machine. In this study, single and multi-spot welds were produced on thermoplastic composite coupons, based on welding conditions which were defined in a preliminary study. The robot and EEF deflections observed during the welding process were analysed to assess the deviation of the robotic process from the static one. The feedback obtained from the welding equipment in terms of consumed power and tool displacement in both processes was also compared. The weld quality was assessed though single lap shear testing of the welded joints as well as fractography of the failure surface. The results of this study indicate that the developed robotic welding process is quite robust and is capable of producing high-quality sequential welded joints despite significant system deflections observed during the welding process. Slightly lower welded area and weld strength was obtained which can be attributed to the system deflections. Finally, the results indicate that the use of a stiffer robotic platform as well as a stiffer EEF construction will result in better system rigidity and weld spot positioning accuracy, and through this the welding process shows promise for large-scale industrial applications.
Modeling and Simulation of the Curing Process of Epoxy Resins and Fiber Composites
This study discusses simulations of the curing process in epoxy and fiberreinforced polymer composites incorporating changes in the thermal and mechanical properties of epoxy during curing at various temperatures. A coupled constitutive model that includes an exothermic process from the cross-linking, heat conduction across the specimen and deformations of the specimen from the thermal expansion and shrinkage effects is formulated. The model is used to capture the curing process in the epoxy resin. The coupled constitutive model is then integrated into a micromechanics model of fiber-reinforced composites and used to study the influence of epoxy curing on the formation of residual stresses in the composites. Furthermore, the micromechanics model is also used to predict the macroscopic properties, i.e., elastic moduli, of the cured composites. The model can then be used to understand the influence of processing parameters, i.e., temperatures and pressure, on the formation of residual stresses and their consequences on the overall properties of cured composites.
Representation, Characterization and Simulation of Tool-Part Interaction and Its Effects on Process-Induced Deformations in Composites
The interaction between a tool and part during composites processing contributes to the formation of residual stresses and dimensional changes. A resultant mismatch of part geometries during assembly can cause a potential loss of mechanical performance in aerospace structures. Costly shimming steps are needed to compensate for processinduced deformations and satisfy specifications on mechanical performance. Due to difficulties associated with accurate measurement of interfacial shear stresses, current analysis methods fail to represent the interaction between a tool and part throughout processing. A combined approach to represent, characterize, and simulate tool-part interaction and its effects on dimensional changes is proposed. First, a characterization method was established using a custom Dynamic Mechanical Analysis (DMA) shear test setup to measure tool-part interfacial stress development in a simulated autoclave curing environment. Tool-part interfacial stresses were characterized for Toray T800S/3900-2 UD prepreg as a function of temperature, degree of cure, strain rate, and tool surface condition. Then, a previously developed numerical model was modified to include the effects of tool-part interaction in predicting dimensional changes of L-shape parts. For validation, composite parts were fabricated on tools with different surface conditions and successfully compared to simulation results. This paper demonstrates that tool-part interaction significantly impacts the spring-in of angled composite parts. The proposed method is a comprehensive and practical approach to study and simulate the effects of tool-part interaction. The results of this paper can be used to understand the complex interaction between a tool and part throughout processing and potentially mitigate processinduced deformations.
Dynamic Performance of Adhesively Bonded Single Lap Joints with Different Fiber Angle Orientations of Adherends
This work aims to investigate the dynamic response of the adhesive bonding of Single Lap Joints (SLJs) using a free vibration technique. For this purpose, the joints with fixed-end conditions were subjected to the vibration test, and the results were compared with the numerical ones which were obtained from the Finite Element Method (FEM) via the ANSYS package program. The materials used in this study are an adhesive film, AF163 2K produced by 3M, and adherends, manufactured from a glass reinforced polymer matrix composite, produced by Hexcel. While four different adherends with different fiber orientations were used, the thickness of the adhesive layer in bonded region was kept constant, 0.2 mm. In doing so, the main concentration was given to the adherends as the energy dissipation was believed to come mainly from them. The main objective was to get high damping values without compromising any decrease in the structural performance of the joints. The experimental natural frequency, flexural rigidity and damping values of the joints were obtained as a parameter of the different adherend types. The results were also validated using numerical modal analysis.
Mechanical Behavior of Adhesively-Bonded Joints under High Loading Rates
This work aims to investigate the dynamic behavior of adhesively-bonded Single Lap Joints (SLJs) under ballistic conditions. For this purpose, the joints with clamped-clamped boundary conditions were modelled using a Finite Element Method (FEM) via ABAQUS package program. The numerical model is based on the joint subjected to a projectile with a mass of 1.25 gr, a density of 11.3 gr/cm3, and an impacting velocity of 100 m/s. The experimental tests conducted in a specially designed set-up were performed via an air-pressurized gun. 6061 aluminum adherends and an adhesive film were used to manufacture the bonded structure. Curves of the velocity and dynamic load against time were predicted for the joint under the impacting projectile. Failure and stress distributions in the adherend as well as in the adhesive layer were predicted that was validated via the experimental results. The prediction was made according to the worst case scenario that accounts the input data obtained from the quasi-static conditions.
Enhanced Fracture Toughness of Adhesive Joints with Doping Epoxy by Graphene Nanoplatelets
It is necessary to enhance the mechanical properties of adhesives to replace conventional joint methods with adhesive bonding. Epoxy in its pure state often suffers catastrophic damage due to its obvious brittleness and low fracture toughness. In this study, the double cantilever beam (DCB - Mode I) was used to characterize the fracture toughness of graphene/DGEBA-epoxy nanocomposite adhesive in bonded aluminium alloy joints and bonded composite joints. Adhesives based on an epoxy adhesive DGEBA (Bisphenol A diglycidyl ether) reinforced with two percentages (1wt.% and 2wt.%) of graphene nanoplatelets (GNP) were prepared. In this study, one shows that the fracture toughness of adhesive nanocomposites was significantly better than neat epoxy-bonded adhesives. Both types of joints contain graphene resulting in increased fracture toughness. Therefore, the maximum fracture toughness was observed until the GNP reached 1wt.%, and then it began to decrease, but it is still higher than that of the pure adhesive joint. On the other hand, this work aims to determine the influence of interfacial interactions on the behavior of enhanced bonded joints and how graphene nanoplatelets can enhance the rigidity of the interface between the substrate and the adhesive. In addition, a numerical study using ABAQUS was performed and compared with the experiments performed on DCB. For the modeling of the damage in an assembly joint, the Cohesive Zone Model (CZM) was used for the fracture behavior of the adhesive.
Effects of Surface Characteristics on Mechanical and Dielectric Properties of Adhesively Bonded Carbon Fiber Composites
The rapid rise of fiber reinforced composite usage in aircraft, spacecraft and automobile industries made the proper comprehension of repair and joining of these materials a crucial aspect. Adhesive bonding is one of the most advantageous and desirable joining and repair technique for fiber reinforced composites. However, the heterogeneity of fiber reinforced composites and the complex interfacial nature of the adhesive bonds, makes most non-destructive evaluation and assessment techniques ineffective to assess the state of the bond. Different manufacturing and surface preparation techniques impart different surface characteristics to the adherends, hence proper understanding of the state of bonds is dependent upon the characteristics of the adherend surfaces. In this approach, carbon fiber composite adherends with controlled surface modifications were made into lap-shear test specimens using film adhesive. The effects of surface characteristics such as surface roughness and surface free energy of the adherend surfaces, on bond performance are studied. These surface characteristics are found to be an indicator of bond performance and can even be used to explain failure modes. Moreover, the performance of these bonds with varying surface characteristics are analyzed non-destructively by dielectric spectroscopy and compared with the mechanical performance of the bonds.
Crush Characteristics of Adhesively Bonded Composite-Aluminum Tubes
Tubular members are used in the automotive industry for body, chassis, and powertrain components such as front rails, underbody frames or sub frames, driveshaft structures and space frames. They are also extensively used in buses and other heavy vehicle structures. With focus on light-weighting, there is increasing use of multimaterial structures with aluminum and high-performance composites. Joining a variety of materials with different characteristics and compositions is a major challenge for the design of such structures. Hence, adhesive bonding is emerging as one of the key joining technique for multi-material structures due to their compatibility with commonly used lightweight materials. Since tubular joints in automotive structures may experience crush type load, this study considers the crush characteristics of composite-aluminum tubular adhesive joints using finite element analysis.
Effects of As-Received Defects on Ceramic Matrix Composites Properties Using High- Fidelity Microstructures with Periodic Boundary Conditions
The presence of microstructural defects in as-received specimens of ceramic matrix composites (CMCs) significantly influences their constitutive response and damage, highlighting the importance of characterization and quantification of these defects for accurate assessment of damage and failure in the service environment. In a recent effort, the authors developed an algorithm to generate stochastic representative volume elements (SRVEs) of Carbon fiber Silicon-Carbide-Nitride matrix (C/SiNC) CMCs based on extensive multiscale material and defect characterization data. This paper implements this algorithm within a commercial finite element solver with periodic boundary conditions (PBCs) for high-fidelity micromechanics analysis and investigation of macroscopic material behavior of C/SiNC composites. Different loading directions are used to predict the global mechanical properties, and the results are in excellent agreement with theoretical (rule of mixture) predictions. Subsequently, the effects of as-received defects on the global and local responses are investigated. The results show that intratow porosity has pronounced degradation effects on the global elastic properties and results in complex stress localization patterns, which can be attributed to potential damage initiation sites.
Micromechanical Finite Element Modeling of Unidirectional Composites in Three Dimensions: Prediction of Transverse Tensile & Compressive, Transverse Shear & In-Plane Shear Progressive Damage Behavior
Predicting the rate-dependent non-linear progressing damage behavior of unidirectional composites from the rate dependent properties of the constituents will enable computational materials-by-design and provide the fundamental understanding of the energy dissipating damage mechanisms. In this study, micromechanical finite element models of unidirectional glass-epoxy composites have been developed with fiber volume fractions, FVF = 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, & 0.70; respectively with zero thickness fiber-matrix cohesive interfaces between the fibers and the surrounding matrix. Experimentally determined rate dependent non-linear stress-strain behavior of DER353 epoxy resin [1] (Tamrakar 2019) has been used to model the large deformation matrix behavior in conjunction with a rate dependent fiber-matrix interface traction law obtained from S-2 Glass/DER353 micro-droplet experiments & simulations [2] (Tamrakar 2019). Transverse tension, compression, in-plane shear, and transverse shear loads have been applied in predicting the progressive damage behavior of unidirectional S-2 Glass/DER353 epoxy composites.
Effect of Manufacturing on the Transverse Response of Thermoset Composites
This study presents a finite element (FE) based virtual test procedure to investigate the effect of the manufacturing process on the transverse composite response. Computational models of the composite microstructures are generated and analyzed in commercial FE solver Abaqus supplemented by user-written subroutines. Several realizations of the composite microstructure with random fiber arrangement are analyzed assuming appropriate initial stress state and material definitions. The virtual test procedure is established to define the evolution of process-induced in-situ matrix properties through direct and inverse process modeling approaches. Subsequently, the composite microstructures are virtually tested in transverse tension to predict the transverse composite properties by implementing progressive damage models. In order to quantify the effect of manufacturing on the transverse composite response, predictions from the two approaches are compared to a third case which assumes an initial stress-free state and neglects the effect of processing conditions on the in-situ matrix properties. Variations of ±5% in average strength and 18% in standard deviations are observed with respect to ideally cured RVEs. It is established that process modeling in necessary to optimize the residual stress state and improve composite performance.
Study on Cooling Rate-Dependent Mechanical Properties of Thermoplastic Composites
This study investigates the effect of solidification conditions on the crystallization behaviors and mechanical properties of thermoplastic resin and carbon fiber reinforced thermoplastics (CFRTP). In particular, the crystallinity, elastic modulus, plastic behavior, strength, and fracture toughness were investigated in Polyphenylene Sulfide (PPS) and CF/PPS manufactured by different cooling rates. Based on experimental results, the cooling-rate-dependent elasto-plastic constitutive law of resin was developed empirically. Finally, the homogenized simulations of CF/PPS were conducted using the developed empirical model, and predicted results were compared with experiments.
Optimization of Process Parameters During Co-Cure of Honeycomb Sandwich Structures
Honeycomb sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive layer to both sides of the honeycomb core under a pre-defined pressure and temperature cycle in an autoclave. High dependency of the co-cure process on the materials and process parameters makes it susceptible to defect such as poorly consolidated facesheet and highly porous bondline which can cause premature failure of the structure. The temperature and pressure in the autoclave and pressure in the vacuum bag are the parameters that describe the cure cycle of the process. In this work, an optimization of the process cycle for the co-cure process of sandwich structures that maximizes the fiber volume fraction within the prepreg and reduces the porosity is presented. The objective function is constructed to reflect the quality of both the facesheet consolidation and bond-line porosity. The Surrogate Optimization Algorithm is employed to find the cure cycle resulting in the highest facesheet consolidation level and the lowest porosity within the bond-line.
Predicting Processing Induced Residual- Stresses in Carbon Fiber-Thermoplastic Micro-Composites
A computational model of residual stress is developed for AS4/polypropylene composites and implemented via user material subroutine (UMAT) in ABAQUS. The main factors included in the model are the cooling-rate dependent crystallinity, temperature-dependent elastic modulus, and temperature-dependent coefficient of thermal expansion (CTE) of the matrix, and the temperature-independent transversely isotropic properties of the carbon fiber. Numerical results are generated for the case of a single fiber embedded in a thin film of polypropylene sample to replicate the process history and test configuration. During single fiber composite processing, a precalculated weight (tensile preload) is applied at the fiber ends to eliminate buckling/waviness of the carbon fiber induced by matrix shrinkage in the axial direction of the fiber. Experiments and Finite element (FE) analysis have been conducted with different preloads (1g, 4g, and 8g) at 25°C. Micro-Raman spectroscopy is utilized to validate the residual strain with different preloads at the bulk. The measured strain values show a good correlation with the predicted residual strain for various preload conditions.
Bending Simulation of Pre-Gelled Composites Using an Explicit Cosserat Continuum Model
Forming simulation of uncured pre-preg can be made more efficient with the use of dedicated finite elements tailored for soft, layered media. These elements are based on the Cosserat continuum theory that introduces a rotational degree of freedom at each node within standard solid elements. In this study, a Cosserat element is developed within a 2D non-linear explicit finite element framework that uses the Carrera Unified Formulation for its spatial discretization. Two benchmark case studies involving bending deformations are presented as the verification of the developed model. It is demonstrated that similar accuracy of predictions can be achieved with much coarser meshes of Cosserat elements than the equivalent classical finite element models consisting of multi-layer stacks of solid elements.
Effects of Stoichiometry on Properties of DGEBF/DETDA Epoxy Using Molecular Dynamics
This article details the molecular modeling of full and off-stoichiometry models of the DGEBF/DETDA epoxy system using Molecular Dynamics to predict the mechanical properties as a function of the crosslinking density. The Reactive Interface Force Field (IFF-R) is implemented in this work to simulate mechanical deformation. The “fix bond/reactâ€ command in LAMMPS is used to simulate crosslinking between epoxy monomers. The results show that the predicted mass density, volumetric shrinkage, and bulk modulus have a strong dependence on the stoichiometry of the epoxy.
Computational Modeling of Epoxy-Based Hybrid Composites Reinforced with Carbon Fibers and Functionalized Graphene Nanoplatelets
The impact on the mechanical properties of unidirectional carbon fiber (CF)/epoxy composites reinforced with pristine graphene nanoplatelets (GNP), highly concentrated graphene oxide (GO), and Functionalized Graphene Oxide (FGO) are investigated in this study. The localized reinforcing effect of each of the graphene nanoplatelet types on the epoxy matrix is predicted at the nanoscale-level by molecular dynamics. The bulk-level mechanical properties of unidirectional CF/epoxy hybrid composites are predicted using micromechanics techniques considering the reinforcing function, content, and aspect ratios for each of the graphene nanoplatelets. In addition, the effect of nanoplatelets dispersion level is also investigated for the pristine graphene nanoplatelets considering a lower dispersion level with four layers of graphene nanoplatelets (4GNP). The results indicate that the shear and transverse properties are significantly affected by the nanoplatelet type, loading and aspect ratio. The results of this study can be used in the design of hybrid composites to tailor specific laminate properties by adjusting nanoplatelet parameters.
Molecular Dynamics Simulations of Furan Resin (Polyfurfuryl Alcohol): Predicting Mechanical Properties
Furan resins can be used as precursor resin for Carbon-Carbon Composites but has also been used in adhesives, acid/corrosion resistant materials, and as an alternative fuel precursor [15]. This paper contains the most current understanding of the structure of furan resin and a Molecular Dynamics workflow for computationally simulating its polymerization with the 'fix bond/react' command implemented in LAMMPS. The predicted mechanical properties of the polymerized resin are in good agreement with the literature values.
Effect of Microstructure on Initiation of Delamination in Cross Ply Laminates from a Transverse Crack
Transverse crack propagating towards a cross-ply interface is investigated in this study. The non-uniform fiber distribution near ply interface is modelled explicitly in order to study the effect of microstructure on crack path and initiation of delamination. The growth of fiber/matrix interfacial debond and debond kinking out of interface are analyzed based on a combination of energy and stress-based approach, which is convenient in predicting matrix crack path. Kinking of transverse crack tip when it approaches ply interface is investigated using an energy-based approach. It is found that predicted matrix crack path and crack tip kinking behavior near interface is strongly influenced by the local microstructure. The obtained results indicate that an induced symmetrical delamination, i.e., interface cracks of equal length on either side of the transverse ply crack, as often assumed in modeling studies, is not always a favorable damage mode.
Molecular Dynamics Simulation of Polybenzoxazine Resin for Process Modeling
In this work, Molecular Dynamics (MD) simulations are performed to predict the physical properties (gelation point, mass density, volumetric shrinkage) and mechanical properties (Bulk modulus, Shear modulus, Young’s Modulus, Poisson’s ratio) of a PolyBenzoxazine (PBZ) resin system as a function of crosslinking density. The molecular models are developed using the Reactive Interface Force Field (IFF-R). The results obtained from MD are in good agreement with the experimental data.
Mechanical Properties Prediction of Phenolic Resin: A Molecular Dynamics Study
Carbon-carbon composites (CCCs) widely used in the aerospace and automotive industries due to their excellent mechanical and thermal properties. Phenolic resins have a relatively high carbon yield, which makes them a suitable candidate for CCCs manufacturing. Molecular Dynamics (MD) can further reduce costs by predicting properties of a material before manufacturing and testing. In the present work, a Molecular Dynamics (MD) model of a crosslinked phenolic resin was developed to predict mechanical properties by implementing the fix bond/react algorithm in LAMMPS. The predicted mass density (Ï) and Young’s Modulus (E) agree well with experimental values and highlights the validity of the topologybased approach to building stable molecular models of phenolic resins.
Wetting Simulations of High-Performance Polymer Resins on Carbon Nanotube Surfaces Using Molecular Dynamics
There is a wide application of carbon nanotube (CNT) based composite materials for structural applications in the aerospace industry. CNT composites are often manufactured with high performance polymer resins as a matrix. Resin wettability with specific reinforcement types is a key parameter in manufacturing CNT composites. Wettability of a liquid resin and reinforcement combination is often measured and quantified by the contact angle. Various experimental methods have been developed to determine the contact angle which can be expensive while working with high-performance resins and CNT materials such as CNT yarns, bundles, or forests. Fortunately, computational simulations can greatly facilitate CNT composite material design by efficiently predicting the contact angle for a wide range of resins. In this study, a molecular dynamics (MD) framework is developed to determine the contact angle value of high-performance polymer resins on aromatic and aliphatic carbon surfaces (Figure 1). It is determined that monomer length and functional groups have a significant impact on the contact angle. Further, based on these results, qualitative deductions of contact angle values of highperformance resins on CNT materials with amorphous carbon content are made.
Multiscale Damage in Co-Cured Composites—Perspectives from Experiments and Modelling
Bonded and co-cured composites are popular alternatives to structures joined with mechanical fasteners in aircraft but the complex and coupled damage mechanisms in the co-cured/bonded region are poorly understood, thus making the evaluation of their strength and durability difficult with current modelling strategies. This study explores the potential of interleaf inclusion in failure-prone, critical regions of co-cured composite specimens in improving the joint strength and interface fracture toughness and strives to advance the understanding of damage initiation in the co-cured region using an atomistic model. A two-pronged approach is pursued here with bench-scale experimental testing and molecular modelling in this study. Experiments are performed for mode I fracture toughness with double cantilever beam (DCB) on composite laminates with an epoxy interleaf layer. Two epoxy resins and three methods for interleaf inclusion are explored in this study; we supplement the results from DCB testing with insights from confocal microscopy on the crack tip and the interleaf layer pre- and post-testing. Molecular dynamic (MD) simulations capture the cohesive interactions at the threephase interface containing the carbon fiber, the prepreg epoxy, and the interleaf epoxy. Results highlight that an interleaf layer made from partially-cured and filmed epoxy, further consolidated in the composite lay-up is the most effective way to suppress void formation, improve dispersion, and maximize cohesive interactions at the interface of co-cured composites.
Effect of Boundary Conditions on Process- Induced Stresses in a Plain Weave Unit Cell
Woven polymer matrix composites (PMCs) are leveraged in aerospace applications for their desirable specific properties, yet they are vulnerable to high residual stresses during manufacturing and their complex geometry makes experimental results difficult to observe. Process modeling is needed to characterize the effects of the curing and predict end stress states. Finite element software can be used to model woven architectures, however accurate representation of processing conditions remains a challenge when it comes to selecting boundary conditions. The effect of BCs on process-induced stress within woven PMCs is studied. The commercial Finite Element Analysis (FEA) software Abaqus is coupled with user-written subroutines in a process modeling framework. A two-dimensionally (2D) woven PMC repeating unit cell (RUC) is modeled with TexGen and Abaqus. Virtual curing is imposed on the bulk matrix. The BC study is conducted with Free, Periodic, Flat, and Flat-Free configurations. Results show that the end stress state is sensitive to the boundary condition assumptions. Flat BC results show great agreement with Periodic BCs. Residual stress results from process modeling are then compared with a linear-elastic thermal cooldown analysis in Abaqus. Cooldown results indicate an overestimation in matrix stresses compared with process modeling.
Carbon Nanocomposite Coated Textile-Based Sensor: Sensing Mechanism and Durability
Carbon nanotube (CNT) composite films are deposited onto stretchable knit fabrics using electrophoretic deposition (EPD) and dip-coating techniques, which are industrially scalable processes for producing future wearable sensors. The deposited CNTs create an electrically conductive nanocomposite film on the surface of the fibers. These nanocomposite coated fabrics exhibit piezoresistive properties; under mechanical deformation/stretching, a large change in the electrical resistance is observed. Polyethyleneimine (PEI) functionalized carbon nanotubes deposited using EPD create a uniform, extremely thin porous coating on the fiber. Initial results show ultrahigh sensitivity of the carbon nanotube coated fabric when tested on elbow/knee to detect range of motion. The sensitivity of these sensors is exceptionally high when compared to a typical carbon nanotube-based polymer nanocomposite. The nanocomposite coating does not affect fabric's breathability or flexibility, making the sensor comfortable to wear. Because of these unique properties, tremendous potential exists for their use in functional/smart garments. Changes in electrical resistance for these fabrics are influenced by a combination of electron tunneling between the carbon nanotubes and the microstructure of the fabric. To investigate and characterize the unique sensing mechanism, the nanotube coated knit fabric's electromechanical response is studied at different length scales, from individual yarns to fabric levels. For applications in wearable sensors, the durability of the nanotube coating on the fabric is critical for repeatable and reliable sensing response. Durability testing of the sensing fabric for washing loads was conducted to study the nanotube coating's robustness. CNT coating's adhesion quality is evaluated based on the weight loss in the specimen and loss in electrical conductivity in each wash cycle. This research addresses the potential of these sensors for functional/smart garments by examining the underlying mechanism of the sensor response and the durability of the carbon nanotube coating.
Stabilization of TuFF Material by Electrospinning of Low Areal Weight TPU Veil Material
The Tailored Universal Feedstock for Forming (TuFF) material is an aligned, discontinuous carbon fiber material with high fiber volume fraction up to 63% and mechanical performance equivalent to continuous fiber, unidirectional composites. The short fiber material allows at least 40% in-plane extension during processing enabling metal-like forming approaches simplifying composites manufacturing significantly. Traditionally, TuFF preforms are produced at areal weight (AW) of ~8 grams per square meter (gsm), stacked and impregnated with thermoset or thermoplastic polymer to create prepreg followed by curing/consolidation in an autoclave or stamp forming process resulting in high-performance structural parts. Here, the impregnated TuFF prepreg can be handled the same way as traditional continuous fiber prepreg. In contrast, to enable liquid composite molding (LCM) processes with TuFF material, the unimpregnated (dry) short fiber TuFF preforms must be stabilized for handling and preforming purposes. This paper details an electrospun veil approach as shown in Figure 1 to stabilize the individual TuFF sheets while maintaining the in-plane extensibility for complex geometry parts. Electrospun TPU fibers are applied onto the TuFF surface and then consolidated via a combination of heating and pressure, formingtrials were carried out using the stabilized preforms and composites werefabricated using LCM. Tensile tests show ~90-95% property retention versus theunstabilized baseline. The approach allows fabrication of stabilized TuFF fabricsfor the first time enabling the use of LCM processes for complex geometry parts.
Enhancing Single-Lap Composite Joints Life Estimation by Graphene Addition: An Hybrid Approach Analysis
The hybrid formulation based on experimental data and the finite element procedure seems to be able to predict crack initiation under fatigue conditions for single lap joints aged by UV exposure. The UV exposure decreased stiffness and strength. The addition of graphene seems to postpone such ageing. However, by considering that cracks initiates when the (TS/σVM)~1. “Softâ€ adhesives can reach longer fatigue life at low loadings, while “stiffâ€ adhesives have its fatigue life drastically decreased but at higher loadings. Stiff adhesives are more suitable for low cycle fatigue conditions, while for high cycle fatigue the option is a soft adhesive.
Fracture Simulation in Polymer Nanocomposites Using Molecular Dynamics
The objective of this paper is to (a) investigate the validity of application of continuum-based linear elastic fracture mechanics (LEFM) methodology, which is often employed by researchers to model fracture processes at the “discreteâ€ atomic scale, and (b) to study the effect of nanographene platelet size on the rupture strength of an edge-cracked polymer block. The material selected for this study is EPON 862 epoxy polymer with 85% cross-link density. Further, an atomistic J-integral is implemented as a nano-scale fracture metric to investigate flaw-tolerance at the nanoscale reported by many researchers, and to develop a methodology to predict the initiation fracture toughness of the material. For this purpose, a bond-order based potential (ReaxFF) available in LAMMPS , a molecular dynamics (MD) software, is utilized. Predictions obtained using the atomistic J-integral are compared with LEFM predictions for the case of a cross-linked epoxy polymer block with a center-crack under uniform far-field loading. Significant deviations from LEFM for crack-lengths below a certain critical crack-length threshold are observed. Further, far-field stress vs. strain plots are obtained for an edge-cracked epoxy polymer block with a single 14 nm graphene nanoplatelet embedded ahead of the crack tip and it is compared with stress vs. strain plot obtained for the same epoxy block with two 7 nm graphene nanoplatelets embedded ahead of the crack tip to study platelet size effect. Significant size effect was observed as shown in the results.
Parameterization of ReaxFF Potential of Mg/Al/Si/O Interaction and Investigation of Mechanical Properties for S-Glass
New ReaxFF parameters are developed for the description of Mg/Al/Si/O interaction for the Magnesium Aluminosilicate (MAS) glass structure. The training set contains energy curves from equation of state for various Mg/Al/Si/O crystals, valence angle and bond distance scan, and heat of formation for the Mg/Al/Si/O interactions. A semi-automated Genetic Algorithm assisted by Artificial Neural Network is applied for this parametrization. Validation efforts showed the current ReaxFF parameter set can describe the atomistic structure and property of tectosilicate MAS glass including S-glass. Estimated quasi-static modulus of S-glass structure matches well with experimental value. Analysis shows the key of high modulus of S-glass is numerous Mg-BO (Bridge Oxygen) interactions across the Mg-O-AlSi structure. In addition, atomistic origin of high ductility and progressive failure of S-glass is derived from the reconstruction of the atomic structure, forming Mg-BO-Si interactions that delays fracture formation.
Effects of Strain Rate, Transverse Pressure and Fiber Surface Roughness on Traction Laws of Glass Fiber Epoxy Interphases
In this study, molecular dynamic simulations (MD) are used to study the effects of strain rate, and transverse pressure (i.e. residual stress and impact loading) on the traction separation response of a glass fiber epoxy interphase where the fiber has nanoscale surface roughness. The interphase model is prepared with and without monolayer glycidoxypropyltrimethoxy silane (GPS). The glass fiber surface roughness profile is created from a surface map measured experimentally using atomic force microscopy (AFM). To develop the atomistic interphase model, first we deposit GPS molecules (in case of interphase with silane) on the silica (fiber) surface and react them with the silica surface through a condensation reaction. A mixture of Epon828-Jeffamine® D-230 is then placed on the silica surface and equilibrated using the general AMBER force field. The interphase structure is finally created through the epoxide-amine curing reaction among the epoxy, silane and amine using a cross-linking algorithm. The model is then subjected to Mode-I and Mode-II loading with the reactive force field ReaxFF to predict the interphase traction-separation responses and failure loci within the interphase. To investigate the effects of strain rate and transverse pressure (in case of Mode-II), interphase loading is carried out at 109/s to 1011/s strain rates and 1 atm to 4 GPa pressure that is applied normal to the fiber surface. Simulation results suggest that the Mode-II interphase traction-separation response improves with the increase in strain rate and transverse pressure. Strain-rate and pressure dependent MD traction data can be used to bridge length scales as input into micromechanics and meso-scale models.
A Computational Model for the Piezoresistive Response of Hybrid Carbon Nanostructured Networks
Carbon nanotubes (CNTs) and graphenic sheets (GSs) are commonly used fillers for polymer nanocomposites. These nanocomposites can be used as selfsensing materials (strain and damage sensors) due to their piezoresistive response. CNT/GS hybrid fillers could be used to tune the nanocomposite’s piezoresistive response. The piezoresistive response of polymers filled with hybrid carbon nanofillers is a novel topic being studied recently experimentally, and very few computational works are available. Thus, a computational model is developed to study the piezoresistive response of polymers filled with CNT/GS hybrid fillers, reproducing geometries and conditions similar to those used in experiments. This computational model generates a network of three dimensional (3D) representations of carbon nanostructures inside a cube, which represents the polymer matrix. The network of nanostructures is turned into a network of resistors to obtain the electrical conductivity of the cube, and thus the polymer nanocomposite. Mechanical strain is applied via coupling with a finite element software. To reduce computational time, embedded elements are used in the finite element simulations. Capabilities and limitations of the proposed computational model are explored.
Computational Description of the Geometry of Aligned Carbon Nanotubes in Polymer Nanocomposites
The paper considers nanocomposites, reinforced with aligned carbon nanotubes (A- CNTs). Nominally aligned, the CNTs in the forest are wavy, which has important consequences in downgraded mechanical properties, and influences electric and thermal performance. The most detailed geometrical model of A-CNTs was proposed by Stein and Wardle (Nanotechnology, 27:035701, 2015). It creates a centerline trajectory of a CNT in steps, each step defining a section of the CNT, growing in the alignment direction with certain deviations. The paper, starting from this framework, formulates a model of the CNT geometry, which is based on the concept of correlation length of the CNT waviness and maximum admissible CNT curvature and torsion. The value of the maximum curvature can be linked to the buckling criteria for CNTs, or derived from ab initio and finite element modelling. It is used as a limiting factor for the growth, defining the waviness and tortuosity of the CNTs. The CNTs in the forest are placed in a random non-regular way, using Voronoi tessellation. The full paper includes investigation of the proposed algorithm for several values of the CNT volume fraction (in the range 0.5%…8%), the dependency of the modelled geometry on the curvature, and the apparent twist of the CNT centerlines. The modelling results are compared with experimental observations in 3D TEM imaging.
Investigating Stress Transfer and Failure Mechanisms in Graphene Oxide-Cellulose Nanocrystals Films
Graphene oxide (GO) films have great potential for aerospace, electronics, and renewable energy applications. GO sheets are low-cost and water-soluble and retain some of Graphene’s exceptional properties once reduced. GO or reduced GO (rGO) sheets within a film interact with each other via secondary bonds and cross-linkers. These interfacial interactions include non-covalent bonds such as hydrogen bonding, ionic bonding, and π-π stacking. Stress transfer and failure mechanisms in GO and rGO films, specifically how linkers affect them, are not well understood. The present study investigates the influence of inter-particle interactions and film structures, focusing on hydrogen bonds introduced via cellulose nanocrystals (CNC), on failure and stress-transfer of the GO and rGO films. To this end, GO films with CNC crosslinkers were made, followed by a chemical reduction. The few-micron thick films were characterized using tensile testing. All tested films exhibited a brittle failure and achieved tensile strengths and modulus in the ~40-85 MPa and ~3.5-9 GPa ranges, respectively. To reveal stress transfer mechanisms in each sample, tensile in-situ Raman spectroscopy testing was carried out. By monitoring the changes in bandwidth and position of Raman bands while stretching the film, useful information such as sheet slippage and cross-linker interactions were gathered. The addition of CNC enhanced modulus but degraded strength for both GO and rGO films. Interestingly, the Raman G-peak shift at failure, indicative of stress transfer to individual GO/rGO particles, is commensurate with the films’ strengths. Correlating these results with the structure and composition of different films reveals new understanding of stress transfer between GO/rGO particles, paving the way for the scalable manufacturing of strong and stiff GO-based films.
Designing Multifunctional Interfaces to Bridge Heterogeneous Materials to Reduce Thermal Fatigue
Composite interfaces between heterogeneous materials exist in many applications which includes electronics packaging. The interface will affect properties such as mechanical integrity during thermal cycling, heat transport and electrical transport due to the inherent disparate properties such as coefficient of thermal expansion (CTE), atomic structure, interface bonding and fabrication processes. Novel interface engineering will be vital for electronics packaging utilized in extreme environment temperatures beyond the standard ranges such as -55 °C to 150 °C. The failure of standard electronics packaging materials with heterogeneous interfaces under thermal cycle fatigue is investigated. Based on the failure analysis, several multifunctional interfaces are developed to bridge the heterogeneous interface and to provide the desired properties and functionality. Approaches to achieve these heterogeneous structures, the materials choices to preserve the multifunctional properties and the modeling predictions are considered. Test structures are prepared of the candidate interfaces, the morphologies are investigated and testing of the thermal cycle fatigue properties over the range -55 °C to 300 °C is performed and will be discussed.
Structural and Electronic Properties of BCN Nanoflakes via Graph Theory
Boron-carbon-nitride (BCN), a ternary system, enables us to compose a wide variety of novel materials due to their unique mechanical, thermal, and electrical properties. We study two-dimensional structures called nanoflakes made of boron, carbon, and nitrogen atoms arranged in hexagonal lattice structures. The physical properties of these nanostructures, in general, are functions of the overall shape, stoichiometric ratio of boron carbon and nitrogen atoms, and their distribution. In this study, we utilize graph theory to randomly generate these structures, forming three different phases: hexagonal graphene, hexagonal boron nitride, and hexagonal BCN in various proportions. We perform density functional theory (DFT) simulations to obtain the optimized nanoflake structures and analyze the electronic structure. Our results have important implications for future studies of novel materials based on BCN nanoflakes and their experimental realizations.
A Case Study of Biocomposite Material Use in Automotive Applications
Interest in biocomposites is growing worldwide as companies that manufacture high-performance products seek out more sustainable material options. Although there is significant research on biocomposite material options and processing found in the literature from at least the last two decades, there are few experimentally based case studies published to help guide product designers and engineers when considering these materials. This paper discusses the use of biocomposites in the seat of an electric bus. Although it is clear that biocomposite material options are quite limited, the authors eventually settled on three natural reinforcements (cellulose, hemp, flax), two epoxies (one low and the other high viscosity) with high biobased carbon content, and one flax precoated with bioepoxy for consideration. Laminate plates with a 4mm nominal thickness are manufactured using VARTM (low viscosity epoxy only), hand layup as a surrogate for prepregging (high viscosity epoxy only), compression molding, and an out-of-autoclave process called the Pressure Focusing Layer (PFL) method. Permeability of the three reinforcements infused with the high viscosity epoxy and fiber volume fractions are determined experimentally to provide insight into VARTM processing and mechanical performance. The tensile modulus, maximum tensile stress, flexural modulus, and maximum flexural stress are measured for all combinations of reinforcement, resin, and processing using tension testing and three-point bending based on ASTM standards. Basic conclusions are drawn about the specific application and more generally about the process of using biocomposites in commercial products.
Improvement of Flexural Strength and Fatigue Properties of Glass Fiber/Epoxy Composites by Grafting Cellulose Nanofibers onto the Reinforcing Fibers
Recently, cellulose nanofibers (CNFs) have been added as a modifier to fiber- reinforced polymer composites (FRPs) to improve their mechanical performance. In this study, vacuum impregnation of CNFs onto glass fibers (GF) was proposed to avoid processing high viscosity resin or the formation of aggregations in the matrix. Therefore, different CNF concentrations (from 0 to 0.1 wt%) were prepared and used to investigate the effect of CNFs on flexural properties of GF-CNF/epoxy composites by three-point bending tests. A field-emission scanning electron microscope was used to characterize the strengthening mechanisms. Fatigue tests were conducted at a stress ratio of 0.1 and a frequency of 5 Hz. The results indicated an improvement in flexural properties of glass fiber-reinforced polymer with increasing CNF concentrations. The flexural strength of CNF-modified GF/Epoxy composites (GF-CNF/EP) increased slightly up to 6 %. However, the fatigue life was significantly improved by 5 times in comparison with the neat GF/epoxy composite. This suggested that the use of CNFs onto the GF surface can contribute to improving the flexural fatigue of GFRP. Mouhamadou
Optimization of Retting and Extraction Through Constitutive Material Modelling of Plant Stems for Variability Reduction in Extracted Natural Fibers
Natural plant fibers compared to glass fibers can provide a cost effective, lightweight and carbon negative reinforcement for polymer composites. However, the current commercial fiber extraction process induces defects including middle lamellae weakening during retting and kink bands during mechanical working. This leads to high variability in mechanical properties, making these fibers less favorable for structural applications at industrial scale. The aim of current research is to reduce this variability by studying the underlying mechanisms of natural fiber extraction to minimize fiber damage occurring at various steps in the process. In this study, flax stems were retted using the conventional dew/field and lab scale controlled enzymatic retting. The hand decorticated fibers from both methods were compared and enzymatic retting showed promising results in producing fine and uniform fibers as compared to fibers extracted by dew retting. To establish the constitutive parameters of the fibers for Finite Element Modeling (FEM), single retted flax stems were compression tested using a Texture Analyzer. This data can serve as the basis for modeling the mechanical deformation of plant stems passing through breaking rollers which is the first step in extraction after retting. The goal is to optimize the roller design and process conditions required to extract fibers with minimal damage and variability.
Nanoparticle Modification of Natural Fibers for Structural Composites
Natural fibers are a lightweight, carbon negative alternative to synthetic reinforcing agents in polymer composites. However, natural fibers typically exhibit lower mechanical performance than glass fibers due to weak interfacial adhesion between plant cells in the fiber and damage to the fibers during extraction from a plant stem. However, improvement of natural fiber mechanical performance could enable their wide-scale incorporation in structural composite applications, significantly reducing composite weight and carbon footprint. This study seeks to develop a novel, cost-effective method to significantly improve natural fiber stiffness via repair of damage caused by extraction and/ or stiffening of the weak cellular interfaces within a natural fiber. Supercritical fluids have been shown to be capable of swelling and plasticizing amorphous polymers, increasing additive absorption. In this work. supercritical-carbon dioxide (scCO2) was used as a solvent to assist with infusion of nanoparticles into flax fibers at pressures ranging from 1200-4000psi. Fiber analysis with Plasma Focused Ion Beam-Scanning Electron Microscopy (PFIB-SEM) showed that nanoparticles were capable of penetrating and bridging openings between cells, suggesting the ability for nanoparticle treatment to assist with crack repair. Additionally, treated fibers contained uniform surface coatings of nanoparticles, potentially reducing fiber porosity and modifying interfacial properties when embedded in a polymer matrix. Overall, this method of nanoparticle reinforcement of natural fibers could enable development of high-performance lightweight, low-carbon footprint composites for transportation or industrial applications.
ONR Review: Architected Composites for Damage Tolerance in Extreme Conditions
The long-term goal of this ONR funded project is to facilitate the design of architected composites that play a key role in damage tolerant and resilient structures. The main emphasis is on developing new composite structures with improved performance and durability as compared to conventional structural composites. To that end, we will present our work in detail on the following within the realm of sandwich composites along with a novel Machine Learning framework for stress prediction in composites: 1) Novel recoverable sandwich composite structures: Traditional sandwich cores such as foam core or honeycomb structures are good options for enabling lightweight and stiff structures. Although, these cores are known to dissipate energy under extreme conditions such as impact loading, they experience permanent damage. Here, our goal is to design core structures that undergo substantial deformation without accumulating damage and recover their original geometric configuration after the loading is removed. In contrast to a traditional foam or honeycomb structure, we have developed a multi-layer architected core design that facilitates significant deformation beyond the initial peak load, yielding a larger energy dissipation during impact and other extreme loading scenarios. We utilize the concept of pseudo-bistability of truncated cone unit cells to achieve elastic buckling for energy dissipation and shape recovery of core structures. 2) Tailoring of sandwich composite facings: Our objective is to establish the influence of fiber architecture on moisture diffusion pathways in FRPC facings for enabling damage tolerant facing designs. To that end, we have evaluated the moisture kinetics in FRPCs by developing micromechanics based computational models within FEM. We have explained the effect of tortuous diffusion pathways that manifest within FRPCs due to internal fiber architectures. Finally, we established the relationship between tortuosity and diffusivity that can be used for studying moisture diffusion in other FRPCs.
Synergistic Effects of Plasma Polymerization and Covalent Surface Modification of Carbon Fibers
This work demonstrates the efficacy in performing an electrochemical pretreatment on carbon fibres to improve the effect of plasma polymerization of acrylic acid on these surfaces. Modified samples demonstrated improve physical properties including tensile strength and Young’s modulus, as well as an increase in composite performance as measured by the interfacial shear strength. The electrochemical pretreatment was shown to enhance the advantages observed when coating fibres using plasma polymerization.
Multifunctional Ceramic Composite System for Simultaneous Thermal Protection and Electromagnetic Interference (EMI) Shielding for Carbon Fiber Reinforced Polymer Composites (CFRP)
Achieving high electrical conductivity while maintaining good thermal insulation are often contradictory in the material design for the goal of simultaneous thermal protection and electromagnetic interference (EMI) shielding. The reason is that materials with high electrical conductivity often pertain high thermal conductivity. To address this challenge, this study reports a multifunctional ceramic composite system for carbon fiber reinforced polymer (CFRP) composites. The fabricated multifunctional ceramic composite system has a multi-layer structure. The polymer derived SiCN ceramic reinforced with yttria stabilized zirconia fibers serves as the thermal protection and impedance matching layer, while the carbon nanotubes provide the EMI shielding. Thermal conductance of the multi-layered ceramic composite is about 22.5% lower compared to that of the carbon fiber reinforced polymer composites. Thermal insulation test during the steady-state condition shows that the hybrid composite can be used up to 300oC while keeping the temperature reaching the surface of carbon fiber reinforced polymer (CFRP) composites at around 167.8oC. Flame test was used to characterize the thermal protection capability under transient condition. The hybrid composite showed a temperature difference of 72.9oC and 280.7oC during the low and high temperature settings, respectively. The average total shielding efficiency per thickness of the fabricated four-layered ceramic composite system was 21.45 dB/mm, which showed high reflection dominant EMI shielding. The average total shielding efficiency per thickness of the eight-layered composite system was 16.57 dB/mm, revealing high absorption dominant EMI shielding. Typical CFRP composites reveal reflection dominant EMI shielding. Results of this study showed that materials with good thermal insulation and EMI shielding can be obtained simultaneously by confining the electron movement inside the materials and refraining their movement at the skin surface.
Seminal Developments in the Durability and Damage Tolerance Assessment of Bonded Joints
Structural adhesives are widely used for joining composite components in many industries and crack growth in such materials is far more likely to occur when they are subjected to repeated cyclic loading than to monotonic loading. Whilst the Hartman- Schijve equation for fatigue crack growth (FCG) has been shown to hold for cohesive crack growth in adhesives under Mode I, Mode II and Mixed-Mode I/II loading, little attention has been paid to its ability to capture the effects of the thickness of the adhesive layer. The present paper examines the growth of fatigue cracks, that occurs cohesively through the adhesive layer, in two toughened epoxy adhesives typical of those used in the automotive and the aerospace industries. Firstly, it is established that when the crack growth rate, da/dN, curves are expressed as a function of Δ√G, or ΔG, where G is the energy release-rate, then the crack growth curves are a function of the thickness of the adhesive layer. It is then shown that this dependency vanishes when da/dN is expressed as a function of the crack-driving force, Δκ, as defined by the Hartman-Schijve equation. Therefore, it is suggested that the parameter Δκ appears to be a valid similitude parameter.
Strength Size Effect in Fiber Composites Failing Under Longitudinal and Transverse Compression
The size effect in the structural strength of fiber reinforced composites has been typically analyzed for tensile failures. However, this is not true for the equally important compressive failures, primarily due to the difficulties in conducting compression tests on specimens of multiple sizes. These size effects are analyzed here numerically for two important compressive failure mechanisms in composites, viz. (i) fiber kink bands forming under longitudinal compression (typically accompanied by axial splitting matrix cracks) and (ii) inclined shear cracks forming under transverse compression. The former mechanism is modeled by a semi-multiscale microplane model, while the latter by the fixed crack model. Both models are calibrated and verified using available test data on carbon fiber composites and then used to predict the failure and load bearing capacities of geometrically scaled pre-cracked specimens of different sizes. In all cases, the predicted failure is found to be of a propagating nature, accompanied by release of strain energy from the specimen causing a distinct size effect in the nominal strength. For the composite considered here, under longitudinal compression, the fracture process zone (FPZ) is found to be fairly small (<1 mm) and the strength size effect is seen to follow linear elastic fracture mechanics (LEFM). The size effect deviates from LEFM for smaller specimen sizes due to increased flaw size insensitivity but cannot be fitted by Bažant's size effect law since the geometric similarity of the failure mode is lost. On the other hand, under transverse compression the FPZ is found to be much larger (34 to 42 mm) and the size effect is found to obey Bažant's size effect law, deviating from LEFM. The failure is geometrically similar despite being inclined to the pre-crack. These findings provide evidence of the general applicability of fracture mechanics-based size effect laws to compressive failure in fiber composites, and prompt suitable experimental investigations.
Fatigue Failure of Fibrous Composite Based on Multiscale Approach
A framework was presented for a fatigue failure model of fibrous composites using a multiscale approach, which uses the fatigue data of the fiber and matrix materials, respectively. Using this model, fatigue failure of fibrous composite materials and structures can be predicted from the constituent material behaviors. To that end, fiber bundles were tested under cyclic loading to determine their residual strength and stiffness. A successful completion of the model is expected to replace many fatigue tests as the configuration of the fibrous composite is varied.
The Effects of Structural-Induced Deformation on the Resonance of Patch Antennas
Conformal Loadbearing Antenna Structures (CLAS) take advantage of a combination of structural and electromagnetic functions. CLAS have been developed as an advanced replacement for conventional antennas (such as blades, wires and dishes) to improve the structural efficiency, as well as the electromagnetic and aerodynamic performance of a platform. The CLAS concept permits the direct integration of microwave radiating elements in the structural skin of a platform. Therefore, the antenna will be subjected to structural loading and will deform accordingly. The effects of these structural-induced deformations on the resonant frequency of the antenna will be reported in this paper. This paper will investigate the performance of a carbon veil patch antenna when it is subject to static in-plane. The work presented will include the effects of in-plane loading on the resonant behavior of the patch antenna when the carbon veil is fully bonded and when it is disbonded by the parent structure. This paper will also discuss the effects of substrate delamination on the RF response of the patch antenna. The RF characteristics of the antenna will be modelled using ANSYS High Frequency Structure Simulator (HFSS).
Bond Quality Evaluation Using Adhesive Doped with Magneto-Electric Nanoparticles
Adhesive bonding for composite structures offers multiple advantages over mechanical fasteners. Although the use of adhesive bonding has increased in the aerospace industry, it has still not replaced mechanical fasteners due to it being harder to inspect for damage after being manufactured/assembled, causing unreliability. Therefore, intensive quality control is needed while manufacturing to avoid weak bonds or any type of imperfection at the adhesive-adherend interface. To ensure the reliability of an adhesive bond, this project focuses on the advancement of a non-invasive field tool for adhesive quality evaluation. The tool developed is based on a B-H looper system, which can approximate the quality of an epoxy-based adhesive containing magneto-electric nanoparticles (MENs) by detecting changes in electric fields at the molecular level. Epoxy based adhesive samples containing 5 vol. % of MENs were manufactured and then scanned using the B-H looper system to correlate their magnetic signature as a function of curing time. It was determined that the magnetic signal converged between curing hours 10 and 12, indicating proper curing. Plain adhesive dogbone samples were used to determine the maximum tensile stress of the adhesive as a function of curing time, which also started converging at around the same curing hours until reaching ~41 MPa. Additionally, the evolution of the glass transition temperature of the adhesive was evaluated during the first curing hours. Convergence began at a curing time of 10 hours until reaching ~137 â° C for fully cured samples. B-H looper magnetic signatures, tensile stresses testing, and glass transition temperatures were all correlated indicating a fully cured adhesive sample between 10 and 12 curing hours. These studies demonstrate the capabilities of the B-H looper system as a non-invasive inspection tool for adhesive quality.
Toughening of Boron Carbide Composites by Hierarchical Microstructuring
Due to a unique combination of properties including high hardness, low density, chemical and thermal stability, semi-conductivity, and high neutron absorption, boron carbide (B C) is a potential candidate for various applications involving extreme environment. However, B C’s current application is limited because of its low fracture toughness. In this study, a hierarchical microstructure design with features including TiB grains and graphite platelets was used to toughen B C by simultaneously utilizing multiple toughening mechanisms including crack deflection, bridging, and micro-crack toughening. Using field-assisted sintering technology (FAST), B C composites with dense and hierarchical microstructure were fabricated. Previously, the fracture toughness of fabricated B C composites was measured at micro-scale using micro- indentation to have up to 56% improvement. In this work, the B C composites’ fracture toughness was characterized at macro-scale using four-point bending methods and compared with previous results obtained at micro-scale. Micromechanics modeling of fracture behaviors for B C-TiB composites was also performed to evaluate the contributions from experimentally observed toughening mechanisms. From four-point bending tests, B C composites reinforced with both TiB grains (~15 vol%) and graphite platelets (~8.7 vol%) exhibited the highest fracture toughness enhancement from 2.38 to 3.65 MPa·m1/2. The measured values were lower than those obtained using micro- indentation but maintained the general trends. The discrepancy between the indentation and four-point bending test results originated from the complex deformation behaviors triggered by the high contact load during indentation tests. Through micromechanics modeling, introduced thermal residual stress due to thermal expansion mismatch between B C and TiB , and weak interphases at B C-TiB boundaries were identified as the main causes for experimentally observed toughness enhancement. These results proved the effectiveness of hierarchical microstructure designs for B4C toughening and can provide reference for the future design of B4C composites with optimized microstructures for further fracture toughness enhancement.
Topology Optimization of Plate Structures with Anisotropic Materials
This work presents a topology optimization method for the design of structures composed exclusively of rectangular plates made of a predetermined, generally anisotropic material. The geometry projection method is employed to map the highlevel geometry and material properties to a fixed grid for the analysis, thus circumventing the need to re-mesh upon each design iteration. We also impose an overlap constraint in the optimization that reduces waste material when fabricating structures by cutting and joining rectangular plates. We demonstrate our method with a numerical example comparing optimal cantilever beam designs obtained using isotropic- and orthotropic-material plates. For this example, we maximize the stiffness of the structure for a fixed amount of material, and we impose a constraint to reduce overlaps between plates. The examples demonstrate the importance of considering material anisotropy in the design of plate structures. Moreover, it is demonstrated that an optimally stiff design for plates made of an isotropic material can exhibit poor performance if the plates are naively replaced with an anisotropic material.
A Multiscale Modeling and X-Ray CT Exploration of Bearing Failure Mechanisms in a Countersunk Bolted Composite Structure
To investigate the failure mechanism in composite bolted joints, an in-situ X-Ray computed tomography (XCT) technique was developed and single shear bearing (SSB) tests were performed with quasi-isotropic layup. High-fidelity XCT was explored for the detection and characterization of bearing failure in bolted composite components without removing the fastener. A novel load frame was also introduced for in-situ XCT scan and a preliminary scan was performed. A micro-macro coupling modeling approach was proposed on the basis of continuum damage mechanics (CDM) method and a static bearing model, which was based on micromechanics analysis to consider the residual stress after fiber kinking and matrix cracking under compression in the bearing region. The SSB specimens were modified using a larger bolt diameter to avoid bolt failure and achieve extensive bearing failure. The developed modeling approach was verified using SSB test data by comparing the predicted load displacement response with experimental measurement and the failure patterns obtained from XCT scanning images.
Progress on the Development of Shape-Memory-Alloy Metacomposites
Shape memory alloys (SMA) have long been explored as a semi-passive approach to mechanical energy dissipation particularly, but not exclusively, for application to vibration control. More recently, the integration of SMAs in composite materials has opened the opportunity to synthesize tunable composite structures exhibiting significantly enhanced energy dissipation characteristics and a certain degree of adaptability to different operating conditions. Despite the significant progress in the development and manufacturing of SMAs over the past several decades, the cost of common Ni-based alloys has remained an important factor hindering their widespread engineering application. The long-term goal of this research effort is to model, design, and fabricate shape-memory-alloy (SMA) meta-composites employing lower volume fractions of a more affordable Cu-based alloy, while still enabling enhanced and tunable dynamic properties. This paper summarizes recent progress in the development of the meta-composite platform and focuses on aspects involving both numerical modeling and fabrication of SMA materials. On the modeling side, particular emphasis is given to assess the ability to tune the dynamic performance of continuous SMA structures by exploiting the different phases and transformations of the alloy. On the other side, the material development effort focuses on the identification of the optimal chemical composition, mechanical and heat treatment processes. A combination of numerical and experimental results is presented to illustrate capabilities and opportunities presented by this material platform.
Electroplating Polymer Based Additive Manufactured Parts for Enhanced Structural Performance
Additive manufacturing has been adopted in many aerospace and defense applications to reduce weight and buy-to-fly ratios of low-volume high- complexity parts. Polymer-based additive manufacturing processes such as Fused Deposition Modeling (FDM) has enabled aerospace manufactures to improve the structural efficiency of parts through generative design or topology optimization. This level of design freedom did not exist in the past due to limitations associated with traditional manufacturing processes such as subtractive machining. Improvements in the material and the maturation of the FDM process has led to the production of many non-structural flightworthy parts used in aircraft today. Polymer-based additive manufacturing can be further leveraged in aerospace applications with the addition of electroplated coatings that act as reinforcement. While many of the commonly known electroplated coating applications involve enhancing the part appearance, electroplated coatings can also improve the strength, stiffness, and durability of plastic parts. Depending on the use case, the thickness of the metallic plating material (combination of copper and nickel) can be tailored to achieve the desired composite properties (metal and polymer). In this research, the tensile and flexural mechanical properties were assessed for Ultem™ 9085 FDM printed specimens and compared to specimens with metallic coating thicknesses of approximately 75-μm, 150-μm, and 300-μm. Non- destructive inspections using x-ray computed tomography were performed prior to mechanical testing to assess the electroplated coating thickness variation and overall quality.
Low Thermal Conductivity Composite Skin Materials
Triton Systems, Inc. and our academic partner University of Dayton Research Institute (UDRI) developed and demonstrated a lightweight, affordable composite heat shield sandwich panel for aerospace applications capable of protecting an underlying Polymer Matrix Composite (PMC) sandwich panel from 500℉ external impingement. Our design outperforms the incumbent heat shield, a bolt-on metallic sheet with an air gap, in both thermal protection (15% lower skin surface temperature) and weight (40% lighter) at an equivalent thickness (about 0.3â€). Our panel has very low thermal conductivity (0.08 W/mK) but is also impact resistant, strong (~300 psi flatwise tensile strength), and tolerant to typical aerospace environmental conditions. Additionally, we demonstrated that our design could be produced as a curved panel configuration to match vehicle outer mold lines (OML’s). Now at Technology Readiness Level (TRL) 4, Triton’s panel design is ready to move to the next stage of development which we envision to be additional proof-of-concept testing including chemical and additional environmental exposure, cold exposure, thermal shock, and vibration as we scale up to a larger 4’x8’ panel.
Vertically-Aligned Carbon Nanotube (VACNT)-Reinforcement for Strength and Life Enhancement of Composite Aerostructures
Laminated composites used in lightweight airframe structures offer higher in-plane strength and stiffness to weight ratio when compared with metals but suffer from weaker out-of-plane properties. Using a building-block approach, vertically-aligned carbon nanotube (VACNT) reinforcement technology is evaluated for its efficacy in enhancing the through-thickness properties of composite laminates necessary for reducing the weight, improving the damage tolerance and extending the fatigue life of composite airframe structures. With additional insight into how VACNT reinforcement performance scales, it is anticipated that the conclusion of this evaluation will facilitate the increase in the technology readiness level (TRL) of VACNT-applications for composite airframes from three to four [1]. The content of this document includes information relevant to the planning and fabrication activities of the program, with experimental testing of articles forthcoming.
High Fidelity Building Block and Validation Protocols for NextGen Composite Methods
The High Fidelity Building Block and Validation Protocols for NextGen Composite Methods Program (N00014-21-C-1008) seeks to provide industry with the critical experimental data and supporting guidelines by which to evaluate, validate, and demonstrate the predictive capabilities of advanced computational analysis tools. Following from the successful development of a coupon level database (N00014-14-C-0128), the HiFi NextGen program focuses on the subelement and element levels of the design-test-certification building block wherein the greatest expense for design substantiation is incurred. Additionally, data is developed to provide improved understanding of fundamental material behavior for composite material models. Advanced inspection techniques such as digital image correlation and in-situ micro computed tomography are used to capture damage onset, damage propagation, and ultimate failure within representative composite structure test specimens. Where applicable, the test program includes relevant environmental conditions. Validation guidelines focus on the use of emerging progressive damage and failure analysis (PDFA) computational techniques and address modeling considerations such as length scale, material models, numeric implementation, mesh objectivity, modeling strategy, and computational scalability. All test articles are fabricated from IM7/5320-1 tape and fabric as well as FM309-1 structural adhesive to maintain a high degree of relevancy for future aircraft needs.
Impact Damage and Injection Repair Strength Restoration
Impact damage to composite structures can lead to a range of damage modes. Of interest is modest damage composed of delaminations less than 50 mm in size, and no visible impact-side fiber breakage. While resin injection is a current-practice repair technique that can be used to address these damage modes in a manner that is much less invasive than bonded scarf patch repair, the injection technique is not currently credited as one restoring strength back to laminate. Issues of quantifying the removal of any internal contamination, assessing degree of resin fill, and demonstration of how much strength is restored are being addressed within the scope of this research activity. Resin injection will be conducted and the resulting strength restoration assessed in local fracture tests (end-notch flexure). The formation of actual impact damage morphologies, namely multiple planes of delamination interconnected with matrix cracks, is a critical aspect of this problem. Three 25-ply composite panel types having varying percentage of 0o fiber content have been impacted under low velocity at a range of energy levels. Resulting force vs. time and ultrasonic mapping of damage extent. Damage produced by such impacts will be used in subsequent injection repair studies. Intentional contamination will be introduced, and then removal will be achieved via injected solvents and atmospheric plasma, with monitoring of contaminant presence achieved by in-line quantitative chemical analysis.
A Paris Law Based Mesh Independent Numerical Methodology for the Simulation of Fatigue Driven Delamination in Composites
Delamination evolution under cyclic loading is one of the most important research topics for the application of composite materials to aerospace, naval, automotive and, in general, transportation fields. Large experimental campaigns are needed to assess the fatigue behavior of Carbon Fiber Reinforced Polymers (CFRPs), which may result extremely time and cost consuming. Nevertheless, composite materials design needs to take into account the evolution of fatigue driven damage. Subsequently, the development of efficient and robust computational finite element methodologies to evaluate progression of delamination in composite structural components subjected to cyclic loading conditions has become relevant. In this paper, a numerical finite element procedure able to simulate the fatigue driven delamination growth is introduced. A Paris-law based cycle jump strategy, combined with the Virtual Crack Closure Technique (VCCT) approach, has been implemented in the commercial Finite Element Code ANSYS MECHANICAL via the Ansys Parametric Design Language (APDL). The main advantages of the proposed numerical procedure, named FT-SMXB, are related to its independence on the time step and element size in the frame of incremental analyses. The procedure has been preliminary validated, in this research study, at coupon level, by comparing the numerical results to literature experimental data on a unidirectional graphite/epoxy Double Cantilever Beam (DCB) specimen. The significant agreement between the obtained numerical results and the literature experimental benchmark data confirms the accuracy and the potential of the proposed methodology.
Global-Local Progressive Damage Analysis of Composite Laminates Using Layer-Wise Higher-Order Structural Models
The objective of the current work is to develop a global-local framework for the progressive damage analysis of composite laminated structures. The technique involves two sequential analyses—an initial low-fidelity 3D-FE based linear analysis of the global structure, followed by the local nonlinear analysis of critical regions where damage is likely to occur. The numerical models used for the local analysis are developed using higher-order layer-wise structural theories obtained via the Carrera Unified Formulation. Composite damage is modelled using the CODAM2 model based on continuum damage mechanics, and the nonlinear problem is solved using explicit time integration schemes. Preliminary assessments are carried out to validate the proposed global-local framework by considering open-hole tensile specimens of quasi-isotropic composite laminates. Both full-scale CUF models and the proposed global-local approach are used to predict the tensile strength of the specimen. It is shown that the obtained results are in good agreement with experiment data, thus validating the framework, and a multi-fold improvement in computational time is demonstrated.
Orthotropic Damage Model Using Tsai-Wu Failure Criteria
This paper presents a novel approach concerning the development of an orthotropic damage model, based on the original plane Tsai-Wu failure criteria. In its original formulation, the Tsai-Wu is a mode independent criterion only capable of acknowledging the existence of damage in a certain point of the material. It is not capable of identifying if the damage is located in the fiber, matrix or intralaminar zone. This work plans to fill this gap in knowledge by providing a simple method, based on equivalent stress and strains, that identifies the failure modes when the Tsai-Wu failure criteria is near the on-set of damage. Using this novel method, it is possible to implement classical damage evolutions constitutive laws based on the MTL formulation. At the moment the proposed damage formulation is based on plane stress space and Mode I fracture, but it is expected in the future to evolve in to a full 3D damage model. The damage model is implemented in the commercial finite element software ABAQUS using user-subroutine UMAT, and all numerical models are compared with the experimental results.
Failure Performance of Bend-Free Variable Stiffness Composite Pressure Vessels
Pressure vessels are designed to store liquids and gases and have various applications spanning from chemical plants to automotive and aerospace industries. Currently, lightweight composite pressure vessels are desirable, especially in transportation industry applications because of their subsequent benefits in fuel consumption, cost and environmental issues. Using composite materials for pressure vessels along with advanced manufacturing technologies such as automated fiber placement provides excellent scope to tailor stiffness through the structural surface using fiber steering to achieve desirable structural performance. Recently, variable angle tow (VAT) technology has been used to suppress bending in super ellipsoids of revolution composite pressure vessels, resulting in minimizing the inefficient bending stresses and deformations and increasing their load-carrying capacity. It is worth noting that such geometries can provide excellent packing efficiency. These advantages make the bend-free super ellipsoids of revolution composite pressure vessels potential candidates for the next generation of pressure vessels. Therefore, their failure performance as the most important design factor should be studied carefully due to safety reasons. In this study, the maximum allowable internal pressure for VAT bend-free ellipsoidal pressure vessels, using the first-ply failure based on both Tsai-Wu and three-dimensional invariant-based failure criteria is determined. Subsequently, VAT bend-free pressure vessels’ failure performance is compared against that obtained for conventional constant stiffness composite vessels. Among structures considered, the VAT bend-free composite vessel has the best failure performance. Moreover, the predicted failure load using the three-dimensional invariant-based failure criterion for the VAT bend-free design is 34% lower than the failure load predicted by the Tsai- Wu. Finally, the effect of various material properties on the difference in predicted failure load using these criteria is assessed. Results provide physical insight useful for designers in materials selection.
Post Mechanical Failure Fire Damage Characterization of Graphite/Epoxy Composites
Fire damage involving mechanically failed composite aircraft structures can dramatically alter their exposed surface characteristics in ways that inhibit fire forensic analyses. In this work, the effects of fire exposure on mechanically failed Cytec T40- 800/Cycom® 5215 graphite/epoxy composites were examined. Coupon level vertical fire tests were performed on mechanically failed unnotched compression and in-plane shear graphite/epoxy specimens. The fire damage was characterized by visual inspection and scanning electron microscopy. The fire damage development in the specimens involved a concurrent and sequential interaction between multiple physical, chemical, and thermal processes. This damage included melt dripping, matrix decomposition, char, soot, matrix cracking, delamination, and residual thickness increases due to explosive outgassing. The composite thermal degradation due to heat conduction, combustion, and/or thermal deformation was significantly affected by the specimen layup, ply orientation relative to the heat source, and the fracture surface morphology. Plies burned with fibers oriented parallel to the flame axis conducted heat into the interior of the composite. This resulted in melt dripping, internal pockets of matrix decomposition, and surface char deposition that, in some cases, completely obscured pertinent aspects of fiber fracture surface morphology. In contrast, plies burned with fibers oriented perpendicular to the flame axis acted like a thermal protection layer that impeded (slowed) heat transfer to the specimen’s interior. Furthermore, the thermal damage development was influenced by the specimen layup and the total available free surface area created during mechanical failure. Specimens with more free surface area promoted better airflow and oxygen availability for combustion and sustained far more thermal degradation for given fire exposure. Key fractographic features in exposed fiber bundles were destroyed due to severe thermal oxidation and thinning. A thorough understanding of these coupon-level fire tests represents a critical first step in developing a coherent strategy for the Federal Aviation Authority post-crash forensic analysis of composite aircraft structures.
Damage Evolution in Cross-Ply Laminates Under Variable Amplitude Cyclic Loadings
Predicting the initiation and propagation of multiple off-axis cracks in multidirectional laminates under cyclic loadings is essential in a stiffness-driven design approach. Even under a constant amplitude cyclic load, the multiple crack initiation represents always an inherently variable amplitude (VA) problem. Indeed, the initiation of cracks causes a stress re-distribution so that each point in a laminate is subjected to a stress state that changes continuously during the fatigue life. At present, no models or experimental evidences on the crack initiation phenomenon under VA loadings are available in the literature. Crack density prediction models usually rely on a simple linear damage accumulation rule, even if its validity has not been proved yet. In this work, two types of fatigue tests were carried out on glass/epoxy cross-ply laminates under VA two-block loadings: 1) Initially, the number of cycles in the first block was chosen low enough to prevent the initiation of transverse cracks in the first block; then the load was changed and the crack initiation phenomenon was characterized in the second block. 2) Then, two block loadings were applied on other specimens, with a high enough number of cycles in the first block to promote the initiation of multiple cracks; the crack density evolution was thus characterized in both blocks. A model recently developed by the authors was applied to the experimental data, revealing the suitability of the linear damage accumulation rule under block loadings, at least from a phenomenological point of view.
Numerical Prediction of Stiffness Degradation of Thin-Ply CFRP Laminates Under Fatigue Loading
This study aims to conduct a fatigue simulation for predicting the stiffness degradation of thin-ply composite laminates with several ply thicknesses. For the simulation, a fatigue evolution model of intra-laminar damage in thin-ply composite laminates considering the effect of ply thickness was proposed. The intra-laminar damage evolution was modeled using the continuum damage mechanics model and the static and fatigue evolution law were formulated by relating the transverse crack density to the damage variable. The finite element simulation using the proposed model was conducted to predict the stiffness degradation of the laminates as a function of the number of loading cycles. The simulation results show that the experimental data can be reproduced by using the proposed fatigue model.
Fatigue Damage Modeling in Laminated Composites by Using Rx-FEM and Strength Tracking Method
The durability and residual load carrying capacity of composite materials and structures is of critical importance for increasing their application across the industry. Regularized eXtended Finite Element Method (Rx-FEM) framework for discrete modeling of damage evolution and interaction in laminated composite materials under fatigue loading has been extended to include residual Strength Tracking (ST) method in the Mesh Independent Crack (MIC) insertion constitutive modeling as well as in the initiation phase of the fatigue Cohesive Zone Model (CZM). The ST method was initially proposed for semi empirical analysis of IM7/8852 open-hole specimen test S-N data under spectrum loading and variable R -ratios (R = 0.1, 5, and -1). In the present work, the ST method is implemented as a component of high-fidelity progressive damage analysis framework and is combined with damage variable incremental update technique for the propagation phase within the CZM formulation. 3D validation examples investigating the solution stability under constant amplitude fatigue with respect to the cycle increment size are presented and demonstrate the solution stability within a feasible range of cycles per step.
Computational Investigation of Energy Dissipation Through Progressive Failure in Tailored Composite Structures via Explicit Finite Element Analysis
A composite material tailoring concept for progressive failure under tensile loading has been previously developed, modeled, and experimentally validated by the second author and his collaborators. The concept relies upon a sequential failure process induced in a structure of series connection of parallel redundant load path elements of tailored length and strength. The resulting yield-type response under tensile loading is characterized by an increased energy dissipation compared to a reference conventional structural element of nominally identical length and crosssectional area, and of the same composite material. In this work, this composite tailoring concept is computationally investigated for IM7-8552 composite material using a dynamic, explicit finite element analysis in Abaqus. The approach offers the advantage of capturing the stress wave propagation within the model throughout the dynamic failure sequence, thereby providing a better understanding of the failure progression and the energy dissipation mechanisms at work. In this study, progressive failure of the tailored composite structure is modeled and analyzed for different configurations of lengths and widths. Model predictions are illustrated for and compared with selected tailoring configurations from the literature. Developing an explicit finite element approach for analyzing the tailoring concept opens the door to characterizing a wide variety of related, more complex configurations for which analytical solutions do not yet exist or may not even be feasible, and/or for which experimental results may be difficult or overly expensive to obtain.
Cure Behavior of Nanostructured Hierarchical Composites with Functionalized Carbon Nanotubes
Electrophoretic deposition is a promising technique to hybridize nanomaterials with conventional reinforcing materials for multifunctional applications. It utilizes the principle of electrophoresis, where electric potential drives charged particles dispersed in a liquid towards counter-charged substrates. Polymer matrix can be infused into the hybridized fibers to produce hierarchical polymer composites with reinforcements spanning several orders of magnitude in scale. This research addresses a key challenge associated with nanostructured composites produced by first dispersing the nanoscale reinforcement into the polymer matrix and then infusing into the fiber reinforcement (direct mix/infusion method). The key limitation is on the volume fraction of the nanoscale particles due to the drastic increase of the resin viscosity and the potential filtering effect of the particles during resin infusion. Our model system consists of an aqueous dispersion of carbon nanotubes (CNT) functionalized with a cationic polymer, polyethyleneimine (PEI), non-conductive glass fabric and epoxy resin. Amine functional groups of PEI are protonated under mildly acidic conditions, producing positively charged CNTs. A stable dispersion is formed through repulsive electrostatic forces among charged CNTs, which also facilitates deposition under applied electric fields. CNT-PEI films uniformly deposited via EPD on each filament throughout the fabric form a unique interface between reinforcing fiber and epoxy matrix. Concentrated amines from CNT-PEI coatings possibly alter the curing mechanism of infused epoxy resin, thereby creating the graded mechanical properties at the interface. In this study, curing kinetics and thermomechanical properties of epoxy resin are investigated with added PEI which provides stoichiometrically excessive amines. It is expected that the curing temperature profile can be designed to optimize the interfacial properties of electrophoretically processed CNT-PEI multiscale composites.
Evaluation of Effect of Surface Modification on Correlation Between Permeability of Glass Fiber/Resin and Capillary Number
In this study, we experimentally evaluated the correlation between the microscopic resin flow and permeability of a glass cloth, which surface was modified by silane coupling agent. We focused on the capillary number, which is a parameter determining the microscopic resin impregnation behavior within and between fiber bundles. The capillary numbers were classified into different parameters based on their dependency on temperatures and pressures. First, we obtained the temperature condition for each resin, to make the ratio between resin viscosity and (surface tension ・contact angle), constant be Under these temperature conditions, the pressure conditions were determined to be the resin impregnation rate constant. The permeability was evaluated with three types of resins under three conditions of capillary numbers. As a result, the permeabilities of the different resin systems were approximately equal. Therefore, it was found that the macroscopic permeability was not significantly influenced by the capillary number representing microscopic resin flow.
Nanoengineered Glass Fiber Reinforced Composite Laminates with Integrated Multifunctionality
Combining one or more functional capabilities of subsystems within a structure can provide system-level savings, particularly for weight-critical applications such as air and space vehicles. Nanoengineering presents a significant opportunity for additional functionalities on the nanoscale without the necessity to modify shape, design, or load carrying capacity of the structure. Here, an integrated-multifunctional nano-engineered system was preliminarily studied in composite laminate structures. The study would support the exploration of a system designed to serve independent yet synergistic functionalities in life-cycle enhancements, energy savings during manufacturing, in-situ cure (manufacturing) monitoring, and in-service damage sensing. For the preliminary study, an integrated multifunctional composite (IMC) laminate was created via aligned nanofiber introduction into the composite interlaminar region and the laminate surfaces of Hexcel E-glass/913 unidirectional glass fiber prepreg. Various heights ranging from 10 - 40 μm-tall vertically aligned carbon nanotube (VA-CNT) arrays, as well as patterned and buckled VA-CNT architectures, were used to reinforce the weak interlaminar regions within the laminates showing a ~ 4 - 5% increase in short beam strength of VA-CNT reinforced specimens hence demonstrating interlaminar enhancement for life-cycle advancements. The same layers, being electrically conductive, can provide several additional multifunctionalities.
A Multifunctional and Reconfigurable Microvascular Composite
Fiber-reinforced polymer (FRP) composites, consisting of stiff/strong fibers embedded within a continuous matrix, are a lightweight structural platform supporting an array of modern applications. Bioinspired vascularization of fiber-composites can augment existing performance with dynamic functionalities via liquid infiltration of the internal micro-fluidic network. Some vascular-enabled capabilities include self-healing to repair delamination damage and active-cooling to prevent thermal degradation. While such attributes have been demonstrated in separate platforms, research investigations that combine functionalities within a single composite have been limited. Here we provide a recent study that highlights a promising pathway for achieving both multifunctional, and reconfigurable behavior in microvascular FRP composites. Specifically, we detail the ability to regulate temperature and modulate electromagnetic signature via fluid substitution within the same serpentine vasculature. Varying microchannel density alters both active-cooling efficiency by water circulation and polarized radio-frequency wave reflection by liquid metal infiltration. We control these bulk property pluralities by widespread vascularization, while minimizing impact on structural performance, and decode the effects of micro-vascular topology on macromechanical behavior. Our in-depth experimental and computational investigation provides a new benchmark for future design optimization and real-world translation of multifunctional and adaptive microvascular composites.
In Situ Characterization of Fiber-Matrix Interface Debonding via Full-Field Measurements
Macroscopic mechanical and failure properties of fiber-reinforced composites depend strongly on the properties of the fiber-matrix interface. For example, transverse cracking behavior and interlaminar shear strength of composites can be highly sensitive to the characteristics of the fiber-matrix interface. Despite its importance, experimental characterization of the mechanical behavior of the fibermatrix interface under normal loading conditions has been limited. This work reports on an experimental approach that uses in situ full-field digital image correlation (DIC) measurements to quantify the mechanical and failure behaviors at the fiber-matrix interface. Single fiber model composite samples are fabricated from a proprietary epoxy embedding a single glass rod. These samples are then tested under transverse tension. DIC is used to measure the deformation and strain fields in the glass rod, epoxy, and their interface vicinity. Initiation and propagation of the fiber-matrix debond are discussed. Full-field measurements are shown to facilitate the quantitative analysis of the traction-separation laws at the fiber-matrix interface subjected to transverse tension.
Cyclic Thermoresistivity of Carbon Nanotube Yarn/Silicone Rubber Matrix Monofilament Composites
Thermoresistive characterization of CNTY monofilament composites was investigated by using the electrical response of a single carbon nanotube yarn (CNTY) embedded in a silicone polymer forming monofilament composites. Two room temperature vulcanizing (RTV) silicone rubbers with different polymerization mechanisms (OOMOO and Ecoflex) were used as the polymeric matrices. Continuous heating-cooling thermal cycling ranging from room temperature (RT~25 °C) to 80 °C was performed in order to determine the thermoresistive sensitivity, hysteresis and residual fractional change in electrical resistance after each cycle. The thermoresistive response was nearly linear, with negative temperature coefficient of resistance at the heating and cooling zones for CNTY/ OOMOO and CNTY/Ecoflex specimens. The average value of this coefficient at the heating and cooling sections was - 6.65×10-4 °C-1 for CNTY/OOMOO and -7.35×10-4 °C-1 for CNTY/Ecoflex. Both monofilament composites showed a negligible negative residual electrical resistance with an average value of ~ -0.08% for CNTY/OOMOO and ~ -0.20% for CNTY/Ecoflex after each cycle. The hysteresis yielded ~19.3% for CNTY/OOMOO and ~29.2% in CNTY/Ecoflex after each cycle. Therefore, the curing kinetics and viscosity play a paramount role in the electrical response of the CNTY immersed into these rubbery matrices.
Benchmarking Virtual Permeability Predictions of Real Fibrous Microstructure
ABSTRACT For fast and complete impregnation in Liquid Composite Molding, knowledge about the permeability of the fibrous reinforcement is required. While development of experimental methods continues, a parallel benchmark effort to numerically characterize permeability is being pursued. The approach was to send out the images of a real fibrous microstructure to a number of participants, in order for them to apply their methods for virtual permeability prediction. Via resin transfer molding a plate was manufactured, using the glass woven fabric Hexcel 01102 (295 g/m²) at a fiber volume content of 54% and a thermoset resin. From this plate, a specimen was scanned using a 3D x-ray microscope at a scan size of 1000 x 1000 x 1000 μm³ and a resolution of 0.521 μm³ per voxel. The sample extracted for the simulations with a size of 523 x 65 x 507 μm³ contains about 400 fibers of a single tow. It revealed a variation of filament diameters between 7.5-9.3 μm and a fiber volume content in average of 56.46% with a variation of 54 - 59% in the individual 2D-slices transverse to the fiber direction. The image segmentation was performed by 2D-slices, to which a Hough transform was applied to detect fiber centers and cross-sections. Then fiber paths were tracked through-out the slices by the closest neighbor algorithm. Finally, fiber paths were smoothened by means of the local regression using weighted linear least squares and a 1st degree polynomial model. The participants received a stack of 973 segmented (binary) 2D-images and a corresponding segmented 3D volume raw-file. They were asked to calculate the full permeability tensor components and fill out a detailed questionnaire including questions e.g. on applied flow models and conditions, numerical discretization and approximation methods, fluid properties etc. The received results scatter considerably over two orders of magnitude, although the participants were provided an already segmented image structure, thus eliminating from the beginning a significant source of variation that could have come from image processing. Model size, meshing and many other sources of variation were identified, allowing further specification of the guidelines for the next step.
Sub-Microscale Speckle Pattern Creation on Single Carbon Fibers for In-Situ DIC Experiments
High performance carbon and glass fibers are widely used as reinforcements in composite material systems for aerospace, automotive, and defense applications. Modifications to fiber surface treatment (sizing) is one of the ways to improve the strength of fibers and hence the overall longitudinal tensile strength of the composite. Single fiber tensile tests at the millimeter scale are typically used to characterize the effect of sizing on fiber strength. However, the characteristic length-scale governing the composite failure due to a cluster of fiber breaks is in the micro-scales. To access such micro-scale gage-lengths, we aim to employ indenters of varying radii to transversely load fibers and use scanning electron microscope (SEM) with digital image correlation (DIC) to measure strains at these lengthscales. The use of DIC technique requires creation of a uniform, random, and high contrast speckle pattern on the fiber surface such as that shown in Figure 1. In this work, we investigate the formation of sub-microscale speckle pattern on carbon fiber surface via sputter deposition and pulsed laser deposition techniques (PLD) using Gold-Palladium (Au-Pd) and Niobium-doped SrTiO3 (Nb:STO) targets respectively. Different processing conditions are investigated for both sputter deposition: sputtering current and coating duration, and PLD: number of pulses respectively to create sub-micron scale patterns viable for micro-DIC on both sized and unsized carbon fibers. By varying the deposition conditions and SEM-imaging the deposited patterns on fibers, successful pattern formation at sub-micron scale is demonstrated for both as-received sized and unsized IM7 carbon fibers of average diameter 5.2 μm via sputter deposition and PLD respectively.
Plasma Polymerization on Unsized Basalt Fibres for Improving the Interfacial Strength with Polymer Matrices
Basalt fibres are becoming a promising alternative to synthetic fibres as a green reinforcement phase in polymeric matrix composites, showing excellent mechanical, chemical and thermal properties. In this work we synthetized tetravinylsilane (TVS) or a mixture formed by tetravinylsilane and different percentages of oxygen on the surface of unsized basalt fibres through the Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique for improving the fibre/matrix adhesion. Single fibre tensile test proved the effectiveness of the process, without any degradation of the mechanical properties of modified basalt fibres. Finally, through pull out tests, the interfacial properties of basalt fibres were studied, measuring increases up to 80% of the IFSS for modified fibres compared to neat fibres. This result is the consequence of a greater chemical compatibility between the fibres and the matrix, thanks to the presence of a higher number of Si-O-C groups, and of a mechanical interlocking effect promoted by the increased surface roughness of the plasma-modified fibres.
Analyzing the Mechanical Properties of Thermoplastic Reinforced with Natural Fibers
Synthetic fibers such as glass, carbon, etc., are used as reinforcement in polymer composites due to their high strength and modulus. However, synthetic fibers contribute to high costs and have a significant environmental impact. To overcome this challenge, various natural fibers, including banana, kenaf, coir, bamboo, hemp, and sisal fiber, as reinforced in a polymer matrix are investigated for mechanical properties. In this study, biocomposites with natural fibers as reinforced are developed and characterized. Treated and untreated natural fibers such as flax, maple, and pine as reinforced in thermoplastic, in this study, polypropylene (PP), are investigated for the mechanical properties, including tensile, flexural, and impact test. Mechanical test results exhibited that adding the natural fibers enhanced the tensile, flexural, and impact properties. It can be inferred that these biocomposites can be used as potential materials for the automobile industry.
Dispersion of Cobalt Ferrite Functionalized Graphene Nanoplatelets in PLA for EMI Shielding Applications
The proliferation of wireless technology calls for the development of cost-effective Electromagnetic Interference (EMI) shielding materials that reduce the susceptibility of high-speed electronic circuits to undesired incoming radiation. Ideally, such materials offer protection over wide frequency ranges and are insensitive to the polarization or angle of incidence of the impinging fields. Here, next-generation EMI shielding materials composed of polymer composites with conductive and magnetic fillers are introduced. It is shown that careful control of the concentration and dispersion of the polymers’ conductive and magnetic constituents permits tuning of the composites’ intrinsic electrical and magnetic properties. The resulting EMI shields are lightweight, cheap and offer greater protection than traditional metal gaskets and foams. In this work, cobalt ferrite magnetic nanoparticles (CoFe2O4) decorated on graphene-based material were dispersed in polylactic acid (PLA) matrix for high EM absorption level in X-band (8-12 GHz). The decoration of the magnetic particles was performed on the as-prepared conductive graphene nanoplatelets (GNP) and reduced graphene oxide (rGO). GNP composites exhibited higher DC conductivity, and permittivity than rGO composites. This is attributed to issues associated with the reduction process, including a lack of conductivity due to the insulated oxygen functional groups and the reduction in the lateral size. Compared with rGOs, the lack of out-plane functional groups causes the cobalt ferrite nanoparticles to agglomerate and not cover the entire surface of the GNPs. These morphological differences improve the magnetization and EM absorption of the composite system. The compatibilizer (pyrene-PLA-OH) was added to the composites to enhance dispersion of the GNPs in the polymer matrix which benefits in higher absorption of the shield. The influence of the compatibilizer on parameter, the reflection loss (RL) of the composite were determined from the characterized intrinsic properties
A Complex Potential Approach to Micromechanics of Unidirectional Composites
Complex potential methods for the solution of two-dimensional boundary value problems in linear elasticity are used to perform micromechanics analysis of unidirectional composites. The composite microstructure is idealized as a square array of unit cells subjected to prescribed strains. Unit cell analyses are performed for plane strain and anti-plane strain conditions to study the interaction of the fiber and matrix materials. The unit cell solutions are used to derive predictions for the ply moduli, Poisson’s ratios and coefficients of thermal expansion, which are shown to agree with the results of other methods. Applications to strength prediction are briefly discussed, and the detailed stress field results that can be generated using the approach are illustrated.
In-Situ Measurement of Permeability between Carbon Fiber/Resin
Carbon fiber reinforced plastic (CFRP) is a composite material in which carbon fibers are impregnated with resin to achieve both high strength and high rigidity. CFRP is an excellent material, but it is expensive in terms of materials, manufacturing costs, and capital investment, and it takes a lot of time to complete a product. In order to solve these problems, the demand for de-autoclaving has been increasing in recent years. If molding can be performed without autoclaving, it will be possible to reduce costs and improve productivity in terms of materials and capital investment costs.
Effects of Number of Plies on Lightning Strike Protection of Electrically Conductive Layer-Wise Hybrid Laminates
With the development of composite technologies, aircraft become lighter and more fuel efficiency. The composite aircraft, however, become susceptible to lightning strike. Developing lightning strike protection (LSP) system need to couple with composite technologies. The authors present a concept of LSP using layer-wise hybrid laminates (CF/Hybrid) in this study. The aim of the study is to validate the effectiveness of layer-wise hybrid laminates structure for lightning strike application by using conventional epoxy-resin CFRP for main structure and electrically conductive layer as a cover layer. The composite laminates include two different types of resin in each layer: conductive polyaniline-based matrix (CF/PANI) and conventional epoxy resin (CF/epoxy). CF/PANI layers varied from 1, 2, and 4 layers with corresponding 7, 6, and 4 layers of CF/epoxy to find out the least effective number of CF/PANI that can prevent lightning strike damage. The specimens were characterized for their mechanical properties and underwent simulated lightning strike test to realize their effectiveness. The result of simulated lightning strike has shown that a layer of conductive CF/PANI can help to avoid catastrophic damage on CF/epoxy. With a greater number of CF/PANI, the less detectable damage in CF/PANI layer became. In the case of CF/Hybrid with 4 layers of CF/PANI shows 70% residual bending strength after the lightning strike. With the aid of nondestructive inspection tools, i.e., thermography and ultrasonic test, the mechanism of damage on the composite panels were observed and analyzed. From this study, CF/Hybrid with 4 layers shows the optimal properties for lightning strike protection.
Influence of Negative Poisson’s Ratio on Fracture Morphologies in CFRP Laminates
The Poisson's ratio and Young's modulus of CFRP laminates can be varied by changing their stacking sequences. CFRP with the specific stacking sequence can theoretically perform the “negativeâ€ Poisson’s ratio, which causes extension in the direction perpendicular to the loading direction, for example. However, the relationship between Poisson's ratio and fracture morphology has not yet been elucidated. In this study, tensile tests were performed for CFRP laminates with negative, zero and positive Poisson’s ratios, and failure morphologies were experimentally investigated.
Micro-CT Image-Based Reconstruction Model for Failure Analysis of Sheet Molding Compound (SMC) Composites
This research provides a multiscale framework of failure analysis for carbon-fiber sheet molding compound (CF-SMC) through a bundle packing reconstruction algorithm and material constitutive modeling, respectively. SMC composites have attracted the attention of many industries due to their high strength-to-weight ratio. However, depending on the speed and pressure in the manufacturing process, the spatially different flow patterns of fiber bundles lead to show inhomogeneous mechanical performance. This is because the location and orientation of fiber bundles in SMC composites have a significant role in mechanical behavior. Therefore, it is crucial to understand the effects of these parameters on mechanical behavior. Therefore, in this study, micro-CT imaging is performed to accurately determine SMC composites' internal geometry. The orientation and location of fiber bundles are statistically expressed as cumulative distribution function (CDF). Using these distributions as input, a mesoscale representative volume element (RVE) is constructed through a bundle packing reconstruction algorithm based on the random sequential adsorption (RSA) method. Furthermore, to demonstrate failure analysis of the SMC composites, constitutive modeling of each constituent is established. Consequently, the change of mechanical behavior is estimated under different manufacturing conditions.
Modeling CFRP Open-Hole Compression Strength Using Artificial Neural-Network with Manually Derived Layer
This paper models the open-hole compression (OHC) strength of aerospace-grade unidirectional CFRP laminates with standard and non-standard ply angles using deep learning methods. Specifically, an artificial neural-network (ANN) model is developed from experimental test results to predict laminate OHC strength from ply orientation and stacking sequence. This ANN model innovates from conventional models by adding a layer containing an interply compatibility-index that was derived manually from interply failure modes to improve the prediction accuracy. A final root mean square error (RMSE, or standard deviation of prediction errors) of 6.7% of the average failure strength (21.1 MPa) and R2a value of 0.83 is achieved using K-fold cross validation on 16 laminates studied. This paper models all angles from -90° to 90°, not just the standard angles (0°, +/-45°, 90°). This paper shows that interply failure modes are important predictors for laminate OHC strength.
Stress Relaxation Behavior of Thermoset Polymers at Large Strains
Unlike composites used in traditional structural applications, thin ply high strain composites (HSCs) common to deployable space structures often experience long term packing deformations at strains up to, and sometimes exceeding, 2%. Extended stowage of these structures can result in incomplete deployment and often permanent shape change, which is directly related to the stress relaxation of the thermoset matrix materials typical to HSCs. Traditionally, viscoelastic characteristics of polymers are measured at small strains on the order of 0.1% and very little data exists at the much larger 2% strains of HSCs. To address this lack of data, the stress relaxation response of two relevant thermoset matrix materials was characterized, over a range of strains and temperatures, to inform and validate future constitutive models. A large strain test, presented here as the Stepped Strain Extended Recovery (SSER) method, was developed to characterize non-linear viscoelastic and potential plastic polymer behavior. The method was implemented using a dynamic mechanical analyzer (DMA) at a variety of operational strains and temperatures. Specimens were prepared from cured thin films of cyanate ester resin, PMT-F6, and toughened epoxy resin, PMT-F7, for testing in a tensile configuration. Four strains, 0.5, 1.0, 1.5 and 2.0% were sequentially applied to each specimen for 100-minute durations, each separated by a 1000-minute recovery period. Tests were performed isothermally at 30°C, 50°C and 70°C. Despite the test temperatures being well below the glass transition temperature (Tg) of the crosslinked polymers, load decay was measured under constant specimen deformation. Trends in the data indicate a consistent increase in stress relaxation with increasing strain and temperature. Overall, the test protocol offers a consistent, rapid, and relatively automated method for generating stress relaxation data over a range of conditions, a necessity for the development and validation of future predictive engineering tools.
Multiscale Analysis of Thermo-Mechanical Behavior of Boron Nitride-Reinforced Epoxy Nanocomposites
The effect of size, shape, morphology, and arrangement of micro constituents of Boron Nitrite (BN) nanoplatelet/epoxy composites on their properties were investigated using a multi scale approach that includes Molecular Dynamics (MD) and micromechanics. The thermo-mechanical properties of the composites were evaluated using molecular theory and the analysis showed that the elastic constants of BN/epoxy composites were not severely affected by temperature. Also, the micromechanical analysis of Generalized Method of Cells (GMC) was utilized at higher length scale to evaluate elastic properties of the composites, for different geometries and arrangements of micro constituents. The predicted results of the analysis showed that the size, aspect ratios, morphology, and the arrangements of inclusions in BN/epoxy nanocomposites all have remarkable effect on the mechanical performances of the material systems.
Assessing the Multiaxial Deformation Response of Unidirectional Non-Crimp Fabrics
In this study, the mixed-mode deformation response of a unidirectional non- crimp fabric (UD-NCF) was investigated. Multiaxial in-plane shear-biaxial tension tests were performed using a new multi-branched fabric specimen on a custom multi-axial loading device. Tests were performed with various ratios of deformation along three loading directions to impose combined tension and shear deformation on the fabric specimens. The different loading cases revealed a strong inter-dependency between shear and tensile deformation modes. Observation and measurement of local deformations provided important quantitative and qualitative information to deeply understand the interaction of typical meso- and macro-scale deformations, which can be leveraged during the forming process of liquid composite molded components to reduce shear-induced defects such as wrinkling.
Mechanical Properties of the Woven Natural Fiber Reinforced Sheet Stocks Used for the Laminated Object Manufacturing (LOM) Rapid Prototyping Process
This paper investigates the mechanical properties of potential sheet stocks of a Laminated Object Manufacturing (LOM) 3D printer made using woven jute fabrics infused with two types of bioresin. The combinations of bioresins and the reinforcements would make green sheet stocks that are expected to be environmentally friendly comparing to traditional synthetic fibers infused with regular resins. Pure resin samples are also involved for comparison purposes. Both tensile and flexural properties are measured following ASTM D638 and D3039 standards (for tensile tests) as well as ASTM D790 and D7264 standards (for flexural tests). Detailed processes of specimen preparation followed by test procedures are introduced. Tensile strengths and moduli as well as flexural strengths and moduli are obtained for comparison. Based on the study of the mechanical properties of both types of pure resin and woven jute fiber-reinforced composites, the research team concluded a few important findings that could be used as guidelines in the sheet stock selection and preparation for the LOM 3D printer that is currently under the building process.
Influence of GNP on the Tensile Response of Stitched Composites
Through-the-thickness stitching of layered composites provides through-the- thickness reinforcement to enhance the interlaminar tensile and shear strengths while maintaining structural continuity. However, under planar mechanical loads, stitched composites develop strain concentrations in the resin rich areas around the stitch seam causing a local reduction in mechanical properties. In this study, nanographene toughened epoxy is used to reduce strain concentrations around stitch seams and increase the global tensile performance in stitched composites. Stitched carbon fiber preforms ([+45/-45] ), infused with an unmodified epoxy resin were used as baseline laminates and compared to specimens infused with an epoxy resin containing a dispersion of 9 nm nanographene platelets. Specimens with two different periodic stitching patterns (0o and 90o) were fabricated and tested under uniaxial loading. The surface strain fields were obtained using digital image correlation (DIC). Noticeable differences were seen in the strain distributions and tensile properties of these test articles. Specimens with the nanographene-modified matrix showed reductions in the strain concentrations around the stitch seams, thereby increasing the local modulus of elasticity. This study presents the influence of nanographene-modified matrix on the tensile response of stitched composites.
Asphalt can be considered a particulate composite with almost no tensile strength, that is, the only physical link between the matrix (bitumen) and the particles (gravel) is the cohesive strength of the bond itself and the aggregate simply breaks away from the binder under any number of tension-based loads such as earth shifts, heavy loads, and even moisture. Over the course of a few months, these breaks lead to larger cracks, potholes, and damaged entire road sections that require significant investment much earlier than the expected 15-year lifecycle. Increasing the strength and modulus of asphalt can improve its durability, extend its lifespan, and reduce its maintenance costs. However, as most asphalt is usually recycled during rehabilitation, improving strength cannot come at the expense of the existing infrastructure support system, i.e., materials and technologies should be compatible with road resurfacing equipment and practices. Short composite fibers have high modulus and strength but are easily broken up by road milling machines, making them ideal candidates to mix into the asphalt during rehabilitation. Additionally, by deliberately limiting the fiber size, this will have a major ancillary benefit for the environment: allowing the use of off-fall composite scraps from the manufacturing sectors that are often chopped and relegated to landfills. This investigation examines the material behavior from both experimental and numerical perspective on the inclusion of short fibers for reinforcing asphalt, creating a dual fiber and particle composite material system. Asphalt by its very nature is a relatively soft material with high strains until failure under some conditions, and brittle under others, making this a complex material system combining both hyperelastic and elastic-brittle response. Validation studies are examined for this unique material under various quasi-static to dynamic loading rates to create a material system for extended finite element analysis in improved infrastructure designs.
Prediction of Number of Cycles from Fatigue Test Conditions and Transverse Crack Density of CFRP Cross-Ply Laminates
In the present paper, we proposed a methodology that can predict the number of applied load cycles in tension-tension fatigue test of CFRP laminates from microscopic damages and test conditions. It is difficult to predict the fracture of CFRP laminates and to estimate the remaining life of CFRP laminates for ensuring the long-term reliability of the CFRP components because the fracture process of CFRP laminates is quite complex. The damage process of CFRP consists of various microscopic damage such as matrix cracks, fiber/matrix interfacial debondings, delamination and so on. In order to quantitatively estimate the remaining life of CFRPs, we focused on the degree of the microscopic damages and relate that to the remaining life of them. The tension-tension fatigue tests of CFRP cross-ply laminates were carried out, and we suspended the tests at arbitrary cycles. When the tests were suspended, we counted the number of transverse cracks occurred on the specimens by a replica method, and measured the stiffness degradation of the specimens. We formulated an equation that can predict the stiffness degradation using fatigue test conditions. The predicted stiffness degradation to the number of cycles using the formula agreed well with the experimental results. The result demonstrated that the formula can predict the number of subjected cycles from fatigue test conditions and transverse crack density.
A Fast Method for Evaluating Effects of Process Parameters on Morphology of Semi-Crystalline Thermoplastic Composites
Semi-crystalline thermoplastics such as PEEK have microstructures that are influenced by process parameters like temperature cycle, humidity, and oxygen levels. Inclusions such as carbon fibers lead to heterogenous crystal nucleation. Further, manufacturing uncertainties involved with techniques such as automated fiber placement, compression molding, or induction welding influence the microstructure of thermoplastic composites. These contributing factors impact type (e.g., spherulitic, cross-linked, transcrystalline, and needle-like), size and distribution of morphologies in the material. Even with similar degrees of crystallinities, these differences affect mechanical properties and overall performance of composite parts. In this study, an experimental method has been developed that allows for fast evaluation of morphology as a function of process parameters in semi-crystalline thermoplastic composites. A compression fixture in a Dynamic Mechanical Analyzer (DMA) is used to process thin films of thermoplastics with embedded carbon fibers, sandwiched between thin glass covers, while carefully controlling processing conditions including temperature, pressure, and strain rate. The sample morphology is then analyzed using through transmission Polarizing Light Microscopy (PLM). Samples can be reprocessed using DMA several times to analyze changes in microstructure. This experimental approach allows for fast exploration of timetemperature- transformation relationships and their effects on morphology. This can be used to enhance our understanding of the material microstructure and develop more accurate process simulation tools, leading to optimization of processing parameters.
Microstructural Correlations in Short-Aligned Glass Fiber Composites
Research shows that in unidirectional composites, fiber breaks and clusters of fiber breaks play a role in the failure onset of the composites [1]. So far, we know no attempts to the date have been made to analyze the fiber break development in short-aligned fiber composites, specifically using E-glass HYBON 2026. O[1]ur challenge is to determine whether there is a similar behavior in aligned short composites and its relationship with the microstructural parameter of fiber orientation distribution. This is important as it is measurable parameter to add value to recycled fiber composites. The more aligned the composite the higher mechanical properties and the higher the added value for recycling. We focused on using synchrotron x-ray computed tomography while performing hold-at-displacement scans to evaluate the fiber microstructure. We found the fiber breaks do not have the same orientation distribution as the non-broken fibers.
Damage Evolution in Non-Crimp Fabric Carbon Fiber/Epoxy Multi-Directional Laminates Under Quasi-Static Tension
An experimental study was performed to characterize the evolution of damage in a unidirectional Non-Crimp Fabric (NCF) carbon fiber/snap-cure epoxy composite under in-plane quasi-static tensile loads. The NCF composites were manufactured using a High Pressure-Resin Transfer Molding (HP-RTM) process and comprised a fast-curing epoxy resin and heavy tow unidirectional carbon fiber NCF layers. Laminates with stacking sequences [0/±45/90] and [±45/0 ] were subjected to axial and transverse quasi-static tensile loads and an in-situ Edge replication (ER) technique was used to capture the damage evolution at predefined intervals. An imprint of the composite microstructure, as observed on the edges of a test coupon, was created on a cellulose acetate replicating tape, which was then observed under the microscope. The onset and progression of ply cracks and delamination, which were the two major damage modes present, were quantified and correlated with the stress-strain curves and changes in stiffness. The influence of stacking sequence and ply thickness are also captured.
Interfacial Properties of Hybrid Cellulose Nanocrystal/Carbonaceous Nanomaterial Composites
Cellulose nanocrystal (CNCs) assisted carbon nanotubes (CNTs) and graphene nanoplatelets (GnP) were used to modify the interfacial region of carbon fiber (CF) and polymer matrix to strengthen the properties of carbon fiber-reinforced polymer (CFRP). Before transferring CNC-CNTs and CNC-GnPs on the CF surface by an immersion coating method, the nanomaterials were dispersed in DI water homogeneously by using probe sonication technique without additives. The results showed that the addition of CNC-CNT and CNC-GnP adjusted the interfacial chemistry of CFRP with the formation of polar groups. Furthermore, according to the single fiber fragmentation test (SFFT), the interfacial shear strength (IFSS) of CNC-GnP 6:1 and CNC-CNT 10:1 added CFRP increased to 55 MPa and 64 MPa due to modified interfacial chemistry by the incorporation of the nanomaterials. This processing technique also resulted in improvement in interlaminar shear strength (ILSS) in CFRPs from 35 MPa (neat composite) to 45 (CNC-GnP 6:1) MPa and 52 MPa (CNC-CNT 10:1).
Optimization of Tab Geometry to Minimize Longitudinal Stress Concentration During Tensile Testing of Unidirectional CFRP
A uniaxial tensile test is a useful method for determination of material properties, especially longitudinal tensile strength. To accurately derive the longitudinal tensile strength, it is desired that the specimen fails in in the gauge section defined here as ‘working zone’. Unidirectional (UD) composites require use of end tabs during this tensile testing to avoid damage to the specimen due to grip serrations. The grip pressure, along with sudden geometry change at the edge of end tabs leads to longitudinal stress concentrations. The conventionally used rectangular and tapered end tabs suffer from these longitudinal stress concentrations under the edge of end tabs, causing premature failure of specimen outside of the working zone. In the present paper, a simulation study is performed for comparison of conventional end tabs with hybrid specimen geometry [1] and a novel arrow-shaped end tab geometry to determine the effect of end tab geometry on longitudinal stress concentrations. The study is focused on high modulus carbon fibre HS40/epoxy UD (0°) composite. The numerical model replicates the actual set-up for uniaxial tensile testing, including contact interactions between testing machine components. The simulation results are used to further optimise the geometry and provide recommendations to eliminate or minimise longitudinal stress concentrations.
Preliminary Experimental/Analytical Correlations of the Forming of Stretch Broken Carbon Fiber Tows and Laminates
Stretch broken carbon fiber (SBCF) is generated by breaking individual filaments in carbon fiber tows at inherent flaws in tension in a continuous process. This process results in randomly broken, collimated fiber fragments. The shorter fiber length improves forming properties while retaining mechanical strength through shear load transfer. SBCF has the potential to take advantage of low-cost manufacturing processes like those used in sheet metal forming, resulting in ordersof- magnitude cost savings and enabling conversion to composite structures across the industry. Because uncured continuous carbon fiber composites do not exhibit significant plastic deformation, they cannot be readily adapted to many common sheet metal forming techniques. SBCF composites exhibit pseudo-plastic deformation, but this deformation is due to different mechanisms. To adapt the manufacturing processes for large and complex parts, new materials testing techniques are needed to quantify the forming behavior of SBCF at the meso-scale (tow and ply). This work’s primary objective is to develop predictive models for complex shape forming and large component characterization. Tests have been developed to characterize the behavior of SBCF tows under various forming conditions. Tow forming and laminate bulge testing allowed for experimental characterization of the SBCF response. Respectively, these tests focus on developing the load-displacement material response along with variation of the strain distribution. Using design of experiment (DOE) technique, the forming response to materials properties such as resin viscosity and mean fiber length have been related. For each test, a correlated Finite Element Analysis (FEA) model was developed, allowing for progression toward understanding a wider array of properties than experimentally.
Closed-Form J-Integral and Its Applications for Measurement of Mode I Interlaminar Fracture Toughness of Composites
Due to the interlaminar properties of composites are low, delamination is one of the major failure modes. It threatens the safety of composite structure subjected to out-of-plane static and especially impact loadings. High interlaminar fracture toughness is demanded in the society where composite structures are widely used. However, for tough material, large deformation may occur in the determination of the interlaminar fracture toughness when using the double cantilever beam (DCB) test. Therefore, accurate determination of the fracture toughness of tough material and dynamic loading is very challenging under large deformation. J-integral is an important parameter in fracture mechanics. It’s equivalent to energy release rate under monotonic loading and widely used in the determination of interlaminar fracture toughness of composites. In this paper, it is used to determine the fracture toughness for composite DCB under large deformation and wedge-insert double cantilever beam (WDCB) test, which is widely used to determine the dynamic interlaminar fracture toughness. Exact and closed form nonlinear J-integrals are derived for the largely deformed DCB and WDCB. Compared with the alternative data reduction methods for determining interlaminar fracture toughness, the J- integral method is more accurate. In addition, the J-integral method is simple and promising, since it is unnecessary to measure the crack length in the tests.
Vibrations of Prestressed Variable Stiffness Composite Aerospace Structures by Ritz Approach
With the introduction of the variable stiffness concept, the design space for highperformance lightweight composite structures has expanded significantly. A larger design space, in particular, allows designers to find more effective solutions with higher overall stiffness and fundamental frequency when considering prestressed dynamically excited aerospace components. In this context, an efficient and versatile Ritz method for the transient analysis of prestressed variable stiffness laminated doubly-curved shell structures is presented. The considered theoretical framework is the first-order shear deformation theory without further assumptions on the shallowness or on the thinness of the structure. A rational Bézier surface representation is adopted for the description of the shell allowing general orthogonal surfaces to be represented. General stacking sequences are considered and the unknown displacement field is approximated by Legendre orthogonal polynomials. Stiffened variable angle tow shell structures are modelled as an assembly of shell-like domains and penalty techniques are used to enforce the displacement continuity of the assembled multidomain structure and the kinematical boundary conditions. For the transient analysis of prestressed variable stiffness structures, classical Rayleigh damping is considered and solutions are obtained through the Newmark integration scheme. The proposed approach is validated by comparison with literature and finite elements results and original solutions are presented for prestressed free and forced vibrations of VS stiffened shell structure, proving the ability of the present method in dealing with the analysis of complex aerospace structures.
Analyzing Micro-Macro Transitional Length Scale in Unidirectional Composites
Understanding the hierarchy in the mechanical behavior of heterogeneous materials requires a systematic characterization of the material response at different length scales, as well as the nature and characteristics of the transitional scales. Characterization of such transitional length scales has been carried out in the past by analytical models that calculate and compare stiffness values at micro and macro scales. The convergence of the material stiffness at the two scales has been used as the criterion for quantification of the so-called transitional length scales. These stiffness calculation approaches are based on the idea of local strain and stress distributions obtained from complex finite element models. Recent advancements in full-field experimental strain measurements have made it possible to identify the transitional length scales in fiber composites based on pure experimental measurements without the requirement of local stress analysis. In this work, we study the validity of such ‘strain-based’ approaches that are used to identify the RVE size in unidirectional fiber composites. Our modeling platform replicates the realistic conditions present in experimental measurements through the randomization of fiber locations and volume fraction within an epoxy matrix.
Experimental and Numerical Investigations of Stochastic Thickness Effects in Discontinuous Fiber Composites
Discontinuous fiber composites (DFCs) made of chopped prepreg tapes have recently drawn the attention of the aerospace and automobile industries thanks to their flexible manufacturing capability. Because of the discontinuity in a form of prepreg tapes, the fibers easily follow the mold contours while maintaining comparable stiffness and strength to continuous quasi-isotropic laminates. Furthermore, the high production rates and part complexities enabled by DFCs make them a competitive alternative to the use of metals in several applications. However, one of the current roadblocks for the use of DFCs is the lack of reliable analysis methods to predict their mechanical behavior, which depends on different parameters such as platelet sizes, aspect ratios, and spatial distribution. In this paper, we first experimentally investigated tensile elastic modulus and strength of unnotched coupons made of two different platelet aspect ratios (square and narrow) for varying coupon thicknesses. From the experiments, the square platelets showed significant thickness effects on both elastic modulus and strength. The narrow platelets also had significant thickness effects on strength but relatively constant modulus with varying thicknesses. In both modulus and strength, the narrow platelets had higher average values with larger deviations. Next, we computationally examined the relationship between the platelet distributions and the corresponding thickness effects. To get a thorough understanding of the effects of the platelet distribution on the mechanical behavior, the analysis was performed in two steps. In the first step, computational models were generated utilizing a uniform platelet distribution. In the second step, the models were generated leveraging platelet orientation tensors obtained from X-ray micro-computed tomography characterization. It was found that the assumption of a uniform orientation distribution condition was sufficient to capture the average modulus and strength with varying thicknesses for both platelet sizes. However, the associated Coefficient of Variation (CoV) of the results were significantly underpredicted, especially in the case of narrow platelets. On the other hand, numerical results using the orientation tensor obtained via micro-CT provided significantly improved predictions of the CoVs with varying thicknesses. These numerical investigations suggest that, for parts manufactured in conditions of limited platelet flow, the average mechanical performance can be accurately predicted by stochastic Finite Element models featuring a uniform platelet orientation distribution. On the other hand, the prediction of the CoV of moduli and strengths urges the use of an accurate representation of the real platelet morphology.
Skin/Stringer Interface Damage Characterization of Stiffened Composite Structures
With stiffened composite panels being widely used in aerostructures, critical failure mechanics of the skin/stringer interface need to be thoroughly understood. The interface is design critical and must demonstrate the ability to sustain design limit load throughout the service life. Buckling and out-of-plane deformations, localized in a stiffened panel can initiate separation at the interface. In addition, presence of inherent manufacturing flaws or operational impact events can lead to premature separation and result in failure. Hence, to ensure structural integrity, thorough damage characterization of skin/stringer interface must be performed. The general building block certification approach used for analysis and test validation does not contain standardized test methods beyond the coupon level. In this study, a Seven-Point Bend (7PB) based test methodology is employed to induce localized buckling-based skin separation. The 7PB methodology is utilized to evaluate panels fabricated using both co-bonding and secondary-bonding methods. The experimental work is complimented by a cohesive zone model (CZM) where the disbond initiation and progressive damage growth at the skin/stringer interface is simulated. Delamination is observed at the interface, originating centrally and progressing asymmetrically along the length of the stringer until failure occurred. The zero-thickness cohesive elements based model implemented within this work was able to capture damage initiation and predicted the final damage map. The 7PB test methodology is demonstrated to be robust and can be introduced as a standard test practice to evaluate sub-elements.
Induction Heating of CF/PEKK Laminates Using Homogenization Techniques
The primary heat generation mechanisms of carbon fiber reinforced thermoplastic laminates during induction heating are joule losses, junction heating with contact resistance and dielectric hysteresis. In relation, these mechanisms are highly affected by the laminate’s architecture and the anisotropic properties of each ply. A finite element model of the Joule heating process is studied to compare modeling the electrical and thermal properties of each individual ply with a laminate theory homogenization of the thermal and electrical properties. Three different homogenization techniques are compared, and limitations of the approaches are discussed. Experimental induction heating of CF/PEKK panels analogous to the finite element model is performed and quantitative (infrared camera) temperature measurements are compared to the finite element simulations
Growth Monitoring of Delamination and Adhesive Debonding of CFRP Structures by Rayleigh Scatting-Based Distribution Sensors
Since delamination of CFRP laminates is generated by impact or fatigue in aircraft operation, identification method of the delamination is a very important technology to ensure safety of aircraft. Recently, built-in sensors are paid attention as a real-time monitoring method of initiation and growth of delamination. Optical fiber sensors are promised as built-in sensors of FRP due to their high strength, durability and embeddability. In this paper, we applied a Rayleigh scattering-based distribution sensor to detect delamination and debonding of CFRP structures. This optical fiber sensor can measure strain distribution along a fiber with wide area range, high spatial and strain resolutions. The optical fiber sensors attached on the surface of laminates were used to detect delamination and adhesive debonding of DCB, ENF and SLJ (single lap joint) specimens. Pre-crack were formed by inserting a Teflon films between the layers or the laminate and adhesive layer during manufacturing. The experimental results of DCB tests showed that the position of delamination edge could be identified precisely from the measured sharp peak of strain distribution. From the results of ENF tests, it appeared that the strain distribution showed the maximum at the delamination edge and the detected delamination edge positions agreed very well with the observed positions. The measured strain distributions were almost same as simulated results by FEM. From the tensile test results of SLJ specimen, it appeared that strain distribution showed extremum at debonding edge. It was also shown that the measured strain distribution agreed well with simulated results by FEM. From the above results, it appeared that the open delamination and debonding could be easily identified from surface strain distribution measured by the Rayleigh scattering-based sensor.
Development of Degree-of-Cure Measurement Method by Thin-Diameter Fresnel’s Reflection Optical Fiber Sensor
Fiber-reinforced polymer (FRP) has superior mechanical properties such as lightweight, high specific strength, and high specific rigidity. Recent important issues of manufacturing FRP are cost reduction and high cycle manufacturing of high-quality products. It is expected that in-situ process monitoring using a smart sensor can be used to solve the issues. Therefore, we paid attention to the monitoring method using an optical fiber sensor because it has good accuracy and embeddability. Up to the present, we have been developing a degree-of-cure (DOC) monitoring method for large and complex-shaped FRP products by Fresnel’s reflection optical fiber sensor. This sensor was based on Fresnel’s reflection due to the mismatch of refractive-index between glass and resin. In the previous study, the effect was investigated that the optical bending loss on the DOC measurement using a ðœ‘125 μm optical fiber sensor. It was confirmed that a ðœ‘125 μm optical fiber was sometimes broken at less than a 2 mm bending radius as one of the results of previous study. However, it is needed that stable measurement of DOC at very severe embedding condition if we want more expansion of the application range of FRP in the future. Thus, we aim to develop a DOC measurement system that can be measured by severe bending conditions using ðœ‘80 μm optical fiber in this study. The optical loss property was measured by winding the fiber around the jig whose radiuses were 1, 1.5, 2, 3, 4, and 5 mm. From the result, the ðœ‘80 μm optical fiber sensor didn’t break if it was bent less than 2 mm bending radiuses. Besides, it was found that the optical loss rate ð‘‘ð¿/ð‘‘ð‘¥ was increased with the decrement of the bending radius ð‘…, and ð¿ð‘› ð‘… and ð¿ð‘› ð‘‘ð¿/ð‘‘ð‘¥ have a linear relationship. The DOC of epoxy resin was measured by using the ðœ‘80 μm optical fiber sensor. The DOC curves were calculated from the measured refractive-index curves. From the result, it was confirmed that the DOC curve of the ðœ‘80 μm optical fiber sensor agreed very well with the simulation curve by the Kamal model. Therefore, it can be estimated that appropriate the measurement result of DOC by the ðœ‘80 ðœ‡ð‘š optical fiber sensor.
Impact Damage Detection Limits of Microwave NDE Technique for Polymer Composites
Despite recent advances, the need for improved non-destructive evaluation (NDE) techniques to detect and quantify early-stage damage in polymer matrix composites remains critical. A recently developed microwave based NDE technique which capitalizes on the ubiquitous presence of moisture within a polymer matrix has yielded positive results. The chemical state of moisture directly affects dielectric properties of a polymer matrix composite. Thus, the preferential diffusion of ‘free’ water into microcracks and voids associated with physical damage allows for damage detection through spatial permittivity mapping using techniques that are sensitive to moisture content and molecular water state. While it has been demonstrated that the method can detect damage at low levels of moisture and impact damage, the specific parameters under which the technique will accurately and reliably capture damage within a composite are unknown. The three variables affecting the performance of the method to detect impact damage are moisture content, extent of damage, and resolution of the dielectric scanning technique. Here, we report on the impact of the latter as a function of the two environmental variables (moisture and damage extent). To understand limits and optimize execution of the technique, the interrelationships between each of the variables must be explored. This study investigates the relationship between moisture content and scan resolution. Two BMI/quartz laminates were impacted at 9 Joules to induce barely visible impact damage. The specimens were inspected at a variety of gravimetric moisture levels, and several variations of the spatial permittivity map were created for each moisture level. Detection standards for the technique were investigated based on moisture content and desired scan accuracy; findings showed at 0.05-0.4% moisture content (by wt.) the technique can detect damage location and size with a minimum of 88% accuracy. Pareto frontiers were generated at each moisture level to optimize scan speed and accuracy.
High Damping Values of Sandwich Structures with Polymer Composite Face Sheets and Foam Cores
Structures with high damping and reasonable stiffness values are demanded by designers of aerospace and automotive sectors as these features of the structures are remedies to the vibratory conditions that lead to malfunction of many electronics and mechanical components of aerial and land vehicles. In this study, sandwich structure beams with face sheets of woven carbon fiber-reinforced polymer plies and a foam core have been manufactured to provide high damping performance. For this purpose, first, the beams from the carbon fiber plies with different fiber orientations were subjected to the vibrating beam test with free-free boundary conditions, and their damping and flexural modulus values were obtained. It was found that the plies with 450 angle gave the highest damping value with a reasonable modulus. The same test procedure was also applied to the bare foam beams to obtain their dynamic values. Eventually, the plies with the 450 angle were selected as face sheets for the sandwich beams that yielded reasonably high damping and equivalent flexural modulus values. The data obtained from each constituent material, the laminate face sheets and the core, were used as input data for modal simulations of the sandwich beams using a Finite Element Method.
Stress and Failure Analysis of Woven Composites Using an Automated Conformal Mesh Method
This paper presents a progressive damage modeling study to investigate the damage and failure behaviors of woven composites using an automated structured conformal meshing model developed by the authors. The composite domain consists of two materials: yarns and matrix. Yarns impregnated by matrix are considered to be homogenous and transversely isotropic. Each yarn element has its local coordinate system oriented according to the yarn’s centroid path. Properties of yarn are obtained using a micromechanics-based homogenization method. The matrix is considered to be homogenous and isotropic. Maximum stress is used as the damage initiation criteria under tensile loading. Damage evolution is governed by the material property degradation method. The meshing algorithm is based on a realistic micro-geometry generated using the Digital Fabric Mechanics Analyzer (DFMA), and thus is applicable to a wide variety of woven architectures. Numerical damage predictions are discussed and compared with previous numerical studies and experimental data to support the validity of the proposed model.
Multiscale Modeling of the Effective Thermal Conductivity of 2D Woven Composites by Mechanics of Structure Genome and Neural Networks
A data-driven multiscale modeling approach is developed to predict the effective thermal conductivity of two-dimensional (2D) woven composites. First, a two-step homogenization approach based on mechanics of structure genome (MSG) is developed to predict effective thermal conductivity. The accuracy and efficiency of the MSG model are compared with the representative volume element (RVE) model based on three-dimensional (3D) finite element analysis (FEA). Then, the simulation data is generated by the MSG model to train neural network models to predict the effective thermal conductivity of three 2D woven composites. The neural network models have mixed input features: continuous input (e.g., fiber volume fraction and yarn geometries) and discrete input (e.g., weave patterns). Moreover, the neural network models are trained with the normalized features to enable reusability. The results show that the developed data-driven models provide an ultra-efficient yet accurate approach for the thermal design and analysis of 2D woven composites.
Effects of Temperature Environment on Mode II Interlaminar Fracture Toughness in Asymmetric CFRP Laminates
When delamination occurs in CFRP laminates due to impact, a portion of the laminate structure becomes asymmetric across the delamination in the thickness direction, resulting in a coupling effect of in-plane and out-of-plane deformations. Thus, it is important to evaluate the coupling effects on the interlaminar fracture toughness and fracture mechanism in CFRP laminates. In addition, it is necessary to withstand practical temperature environments. Based on Verchery's approach, the authors intentionally prepared asymmetric laminates with non-zero and zero-coupling components in the classical lamination theory to evaluate the effects of coupling components on the interlaminar fracture toughness in CFRP laminates. In this study, the Mode II interlaminar fracture toughness of CFRP laminates with and without coupling components in the stiffness matrix was measured by the end notched flexure (ENF) test in the temperature range from room temperature to 80°C. The relationship between the temperature and the interlaminar fracture toughness was quantitatively evaluated. Fracture surface observation was also conducted to clarify the fracture mechanism.
Influence of Temperature-Dependent Resin Behavior on Numerical Prediction of Effective CTEs of 3D Woven Composites
3D woven composites are well known for their high strength, dimensional stability, delamination, and impact resistance. They are often used in aerospace, energy, and automotive industries where material parts can experience harsh service conditions including substantial variations in temperature. This may lead to significant thermal deformations and thermally-induced stresses in the material. Additionally, 3D woven composites are often produced using resin transfer molding (RTM) technique which involves curing the epoxy resin at elevated temperatures leading to accumulation of the processing-induced residual stress. Thus, understanding of effective thermal behavior of 3D woven composites is essential for their successful design and service. In this paper, the effective thermal properties of 3D woven carbon-epoxy composite materials are estimated using mesoscale finite element models previously developed for evaluation of the manufacturing-induced residual stresses. We determine effective coefficients of thermal expansion (CTEs) of the composites in terms of the known thermal and mechanical properties of epoxy resin and carbon fibers. We investigate how temperature sensitivity of the thermal and mechanical properties of the epoxy influences the overall thermal properties of the composite. The simulations are performed for different composite reinforcement morphologies including ply-to-ply and orthogonal. It is shown that even linear dependence of epoxy’s stiffness and CTE on temperature results in a nonlinear dependence on temperature of the overall composite’s CTE.
Finite Element Analysis for Textile Composites Using Fiber-Bundles/Matrix-Resin Separated Mesh
This paper proposes a finite element modeling method for textile composite, in which fiber bundle and matrix resin are separately meshed, and they are connected by using discontinuous Galerkin (DG) method. The fiber bundle geometry is often complex in the textile composite. In the conventional FEM, it causes small, distorted resin elements surrounded by the fiber bundles, because the resin must be meshed along the fiber bundle geometry. These distorted elements result in the increased effort to meshing, computing cost, and degraded accuracy. In the proposed method, we apply the DG method to the 3-dimensional analysis of the textile composite. DG method is a method which can connect two separately divided meshes in the FEM. The method proposed in this paper has a distinct advantage, because matrix resin has not to be meshed along the fiber bundle geometry. Moreover, regular cubic grid mesh can be used for matrix resin. In the present paper, the formulation of the DG method is presented first. The method and results of the microscopic stress analysis for textile composite is then described. The results agree well with those of conventional FEM, and validity of the proposed method is demonstrated.
Hybrid Experimental and Numerical Characterization of the 3D Response of Woven Polymer Matrix Composites
The need for advanced material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. The purpose of this work is to characterize carbon epoxy fabrics for composite material models that rely on a large number of input parameters to define their nonlinear and 3D response; e.g. elastic continuum damage mechanics models or plasticity damage models [1, 2]. It is challenging to obtain large sets of experimental stress-strain curves, therefore, careful selection of physical experiments that exhibit nonlinear behavior is done to significantly reduce the cost of generating threedimensional material databases. For this work, plain weave carbon fabrics with 3k and 12k tows are manufactured by VARTM. Testing is done using MTS hydraulic test frames and 2D digital image correlation (DIC) to obtain experimental stress-strain curves for in-plane tension and shear as well as transverse shear. For cases where actual experimental data is either not available or difficult to obtain, the required model input is virtually generated using the NASA Glenn developed Micromechanics Analysis Method/Generalized Method of Cells (MAC/GMC) code. A viscoplastic polymer model is calibrated and utilized to model the matrix constituent within a repeating unit cell (RUC) of a plain weave carbon fiber fabric. Verification and validation of this approach is done using MAT213, a tabulated orthotropic material model in the finite element code LS-DYNA, which relies on 12 input stress-strain curves in various coordinate directions [2]. Based on the model input generated by the micromechanics analyses in combination with available experimental data, a series of coupon level verification and validation analyses are carried out using the MAT 213 composite model.
Micromechanical Finite Element Prediction of Interlaminar Traction-Separation Laws Using J-Integral Approach
Dynamic impact loading of woven composites leads to mesoscale damage such as interlaminar transverse cracks and intralaminar tow-tow delamination cracks. At the microscale, this damage may be modeled as fracture between [90/90] and [0/90] unidirectional composite laminates. Microscale finite element model (FEM) resolution of dynamic impact at structural length scale is intractable, but mesoscale FEM resolution is possible with current computational resources. However, mesoscale cohesive zone modeling of this damage requires appropriate tractionseparation laws. These laws are predicted in this work with fiber length-scaleresolved FEMs, which include residual stress, experimentally measured, ratedependent, nonlinear matrix behavior, and experimentally measured, computationally validated, rate-dependent fiber-matrix interface properties. The J-integral from elastoplastic fracture mechanics is computed under mode I and mode II loading and differentiated to determine the traction-separation laws.
A Fast, Automatic Meshing Procedure for Composites with Interpenetrating Geometry
For decades, finite element analysis (FEA) has served as a ubiquitous tool, allowing engineers and scientists to gain critical insights into the behavior of complex composite materials. Despite the wide adoption of FEA, creating a conforming mesh for the most complex composite material models, such as mesoscale models of 3D textiles or microscale models of composites with reinforcement and irregular voids, remains a significant challenge. Much of the difficulty lies in the fact that many tools that create realistic surface geometries through process simulation often result in complex interpenetrations between objects. This paper proposes a pipeline of algorithms, some adopted from the visualization community and some novel, to automatically create a conforming, high-quality tetrahedral mesh for composite materials with complex geometries that may overlap. The details of a novel algorithm used to identify volume, surface, and edge features, while avoiding the use of a tolerance, are provided. Additionally, the paper describes three different methods to remove overlaps between tows. Finally, the algorithms are applied to a simple case with two orthogonal tows that overlap where they cross, demonstrating the result at each step and revealing the advantages and disadvantages of each of the three overlap removal algorithms.
Progressive Damage Response of 3D Woven Composites via the Multiscale Recursive Micromechanics Solution with Tailored Fidelity
Progressive failure simulations have been performed for orthogonal 3D woven composites consisting of RTM6 resin matrix and AS4 carbon fibers. The Multiscale Recursive Micromechanics approach has been used, which, while being computationally efficient, captures the primary effects of the microstructure at each considered length scale. This approach also enables use of any micromechanics theory at any length scale, and herein, the fidelity of the chosen theories across the scales has been tailored to strike a balance with computational efficiency. The Mori-Tanaka method is employed at the lowest length scale, the Generalized Method of Cells is used at intermediate scales, and the High-Fidelity Generalized Method of Cells is used at the highest woven composite repeating unit cell scale. Furthermore, two different damage models, also with different levels of fidelity and efficiency, have been used for the resin material at the lowest length scale. Results for the mechanical behavior in response to loading in various directions are compared for the two damage models and with available test data.
Experimentally Validated Simulations of Blind Hole Drilling in 3D Woven Carbon/Epoxy Composite with Processing-Induced Residual Stresses
Manufacturing-induced residual stresses in carbon/epoxy 3D woven composites arise during cooling after curing due to a large difference in the coefficients of thermal expansion between the carbon fibers and the epoxy matrix. The magnitudes of these stresses appear to be higher in composites with high throughthickness reinforcement and in some cases are sufficient to lead to matrix cracking. This paper presents a numerical approach to simulation of development of manufacturing-induced residual stresses in an orthogonal 3D woven composite unit cell using finite element analysis. The proposed mesoscale modeling combines viscoelastic stress relaxation of the epoxy matrix and realistic reinforcement geometry (based on microtomography and fabric mechanics simulations) and includes imaginginformed interfacial (tow/matrix) cracks. Sensitivity of the numerical predictions to reinforcement geometry and presence of defects is discussed. To validate the predictions, blind hole drilling is simulated, and the predicted resulting surface displacements are compared to the experimentally measured values. The validated model provides an insight into the volumetric distribution of residual stresses in 3D woven composites. The presented approach can be used for studies of residual stress effects on mechanical performance of composites and strategies directed at their mitigation.
Achieving Realistic Tow Fiber Volume Fractions in Textile Composite Models by Inducing Fiber Entanglement
Modeling composites can be an effective way to understand how a part will perform without requiring the destruction of costly specimens. By combining artificial fiber entanglement with manufacturing process simulation, a method was developed to create fiber bundle models using entanglement to control the fiber volume fraction. This fiber entanglement generation uses three parameters, probability of swapping (p_(r_S )), swapping radius standard deviation (r_(σ_S )), and the swapping plane spacing (l_S), to control the amount of entanglement within the fiber bundle. A parametric study was conducted and found that the more entanglement within a fiber bundle, the more compression mold pressure required to compact the fiber bundle to the same fiber volume fraction as that required for a less entangled bundle. This artificial fiber entanglement and manufacturing process simulation method for creating fiber bundles shows the potential to be able to create bundles with controlled final volume fraction using a desired mold compression pressure.
A Radiation Sensitive Adhesive System for Functionally Graded Composite Joints
Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose.
Artificial Generation of 2-D Fiber Reinforced Composite Microstructures with Statistically Equivalent Features
Fiber reinforced composites are used widely for their high strength and low weight advantages in various aerospace and automotive applications. While their use may be sought after, modeling of these material requires increasing fidelity at the lower scales to capture accurate material behavior under loading. The first steps in creating statistically equivalent models to real life cases is developing a method of rapid evaluation and artificial microstructure generation. The outlined work is capable of tracking microscale fiber positions and determining regions of localized volume fraction extrema (high and low end). Groupings of high and low volume fraction regions are called clusters and their geometry is used to characterize the microstructure. These cluster features can be evaluated for both artificial models and actual scans, allowing correlation to be established which can ultimately be used to regenerate statistically equivalent models. The results of this work show that if one feature is to be correlated, a model can be generated which matches almost exactly. But once more features are equally taken into account, the regeneration loses accuracy.
Failure of CFRP and Titanium Tubular Adhesive Lap Joints at Extreme Temperatures
In this work, carbon fiber reinforced polymer and titanium adhesive tubular lap- joints (TLJs) at cold-temperature, room-temperature, and elevated-temperature are studied. Finite element analysis is employed to investigate the competing damage mechanisms within the TLJs. The relationship between the joint strength and adhesive bondline length is also determined. X-ray micro-computed tomography (μCT) technique is utilized to analyze composite damage characteristics; whilst statistical design of experiment (DoE) approach is used to understand the interaction between bondline length and temperature on the mechanical performance of TLJs. Results show a mixed-mode failure region at elevated-temperatures, causing the TLJ to fail by either interlaminar shear or titanium net-section fracture. Results further reveal that deformation within the titanium creates shear stresses on the composite tube, leading to the initiation, propagation, and coalescence of delamination. Employing numerical, experimental, and statistical techniques, this works provides insights into the parameters and mechanisms affecting the behavior and failure of adhesive TLJs at extreme temperature conditions.
3D Printing of Short Carbon Fiber Composites via Frontal PolymerizationMORTEZA ZIAEE
Additive manufacturing (AM) of polymer composites is a growing field in academic and industrial research environment. Majority of research in this field is focused on thermoplastic-based composites, as manufacturing of thermoset composites requires long cure cycles that make the additive manufacturing process quite challenging. Even though thermoplastic composites are easier to print, the ultimate performance of composites is limited by low fiber volume fraction, relatively high porosity, and low mechanical performance of host polymers. Recently, a novel curing strategy based on frontal polymerization (FP) has been developed that enables 3D printing of high-quality thermoset polymers. In this approach, a monomer solution with a gel-like viscosity is in-situ cured following the extrusion from printing nozzle by a self-sustaining reaction front. In the present work, we use dicyclopentadiene as a thermoset resin that can be frontally polymerized to a high-performance solid polymer. We add short carbon fiber reinforcements (L ~74 μm) to resin to fabricate mechanically robust 3D composite structures. Our results show that incorporation of short fibers substantially improves the flexural strength and flexural modulus of 3D-printed composites by ~50 % and ~410 %, respectively, compared to traditionally molded neat samples. Optical microscopy from the crosssection of flexural samples reveals that no voids was formed within deposition lines.
Rate-Dependent Compaction and Relaxation Response of Uncured Prepregs under High-Pressure Conditions
The compaction-relaxation response at different compaction rates and fiber volume fractions plays a key role in understanding the viscoelastic response of uncured prepregs. Hence, this study characterizes the time-dependent behavior of un-cured 4- layer prepregs subjected to compaction-stress relaxation test at different displacement rates i.e., 0.1 mm/min, 1.0 mm/min, and 10 mm/min, at 0.65 fiber volume fraction and allowed to relax for two hours. In this study, the complete deformation history of the Hexply M26T multilayer prepregs is measured from a stress-free state to the cured state. The effects of rate-dependent compaction-relaxation at different rates on percentages of compaction, recovery, stress change during relaxation, and permanent deformation of prepregs are computed. It was concluded that the 0.1 / displacement rate showed the lowest peak stress level and the lowest stress relaxation and permanent deformation. A viscoelastic model was used to fit the experimental data and the results showed a good agreement. The void content was determined analytically and from the XCT-aided geometrical model. It was observed that for a given test condition, the void content increases as the displacement rate increases, due to the high applied pressure. This study highlights the importance of rate-dependent compaction-relaxation behavior and the need to determine the suitable process parameters and models to manufacture high-quality aerospace composite structures.
The Effects of Debulking on the Microstructure of Carbon Fiber Reinforced Composites
Carbon fiber reinforced plastics (CFRPs) are widely used due to their high strength to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of transversely compacting a dry fibrous reinforcement prior to wet out with the matrix resin, in order to induce fiber nesting, effectively increasing the volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRPs, varying only the debulking step in the manufacturing process. The microstructural effects of debulking on three unidirectional CFRPs made from three different levels of debulking were studied. High resolution serial sections of all three samples were taken using the UES ROBO-MET at the NASA Glenn Research Center in Cleveland, Ohio. Using these scans, the fiber positions were measured and connected to make fiber paths. Statistical descriptors such as local fiber and void volume fractions, and void distribution and morphology were then generated for each sample and compared. Using these descriptors, the effects of debulking on the composite microstructure can be measured.
Quasi-Linear Viscoelastic Modelling of Uncured Prepregs under Compaction
Reinforcement compaction sometimes referred as consolidation process and is one of the key steps in various composite manufacturing processes such as autoclave and out-of-autoclave processing. The prepregs consist of semi-cured thermoset resin system impregnating the fibers. hence, the prepreg shows strong viscoelastic compaction response, which strongly depends on compaction speed and stress relaxation. modeling of time-dependent response is of utmost importance to understand the behavior of prepregs during different stages of composites manufacturing processes. The quasilinear viscoelastic (QLV) theory has been extensively used for the modeling of viscoelastic response of soft tissues in biomedical applications. In QLV approach, the stress relaxation can be expressed in terms of the nonlinear elastic function and the reduced relaxation function. The constitutive equation can be represented by a convolution integral of the nonlinear strain history, and reduced relaxation function. This study adopted a quasilinear viscoelastic modeling approach to describe the time dependent behavior of uncured-prepregs under compression. The model was modified to account for the compaction behavior of the prepreg under a compressive load. The deformation behavior of the prepreg is usually characterized by the fiber volume fraction, V . In this study, the material used was a 2/2 Twill weave glass prepreg (M26T) supplied by Hexcel® Industries USA. We performed a compaction experiment of the uncured prepreg at room temperature at different displacement rate and subsequent relaxation to describe the viscoelastic behavior of the prepreg. The model parameter calibration was performed using the trust-region-reflective algorithm in matlab to a selected number of test data. The calibrated model was then used to predict the rate dependent compaction and relaxation response of prepregs for different fiber volume fractions and strain rates.
Hydraulic Bulge Testing to Compare Formability of Continuous and Stretch Broken Carbon Fiber Prepreg Laminates
The use of carbon fiber reinforced polymer composites has increased with the increased need for high-strength, low-density materials, particularly in the aviation industry. Stretch broken carbon fiber (SBCF) is a form of carbon fiber created by the randomized breaking of aligned fibers in a tow at inherent flaw points, resulting in a material constituted of collimated fiber fragments longer than chopped fibers. While continuous carbon fibers possess desirable material properties, the limited formability prevents their wider adoption. SBCF composites exhibit pseudo-plastic deformation that can potentially enable the use of traditional metal forming techniques like stamping and press forming well established in mass production applications. To investigate the formability of SBCF composites prepared with either continuous or stretch broken Hexcel IM-7 12K fiber, impregnated with Huntsman RDM 2019-053 resin, hydraulic bulge testing was performed to explore the strain behavior under biaxial stress conditions at elevated temperature under atmospheric pressure. Initial results show better formability of SBCF compared to continuous fiber, characterized by the axisymmetric response to the applied stress.
Uncertainty Quantification for the Manufacturing of Carbon Fiber/Vinyl Ester Laminates
Carbon fiber reinforced polymer (CFRP) composites are lightweight materials with a high stiffness-to-weight ratio and high strength-to-weight ratio. CFRP composites consist of two constituents: reinforcement and matrix. The reinforcement consists of carbon fibers and the matrix generally consists of a thermoset or thermoplastic resins. Through choices made between these two constituents, lightweight composites can be custom-tailored to fit specific criteria, including, but not limited to, water resistance for naval vessels, thermal resistance to negate environmental degradation, and UV resistance to negate UV degradation. Many uncertainties can arise with such a vast capability in adaptation depending on material choice, manufacturing methods (vacuum assisted resin transfer molding VARTM, layup, and filament winding), and post-manufacturing processes. These uncertainties can build upon one another leading to less accurate theoretical applications when compared to their real-life counterpart. The purpose of this project is to create an indepth uncertainty quantification (UQ) analysis based on vinyl-ester resin as a preliminary report for a future carbon fiber/vinyl-ester composite UQ analysis. To properly ascertain the magnitude of uncertainty during the manufacturing process, the resin to hardener ratio and cure time were studied to understand their effect on the impact response of vinyl ester samples. Vinyl ester specimens were impacted with an impact energy of 3.3 J to produce barely visible damage (BVID) on the samples. Energy, force, displacement and time were collected for analysis. Using a Monte Carlo simulation, a probability distribution model was generated to understand the effects and UQ influence of the manufacturing process in the impact response of vinyl ester specimens.