Proceedings of the American Society for Composites — Thirty-fourth Technical Conference
Design of Composite Transmission Shaft and Composite-Metal Interference Fit Joint
Nowadays, energy saving is one of the most important targets in vehicle design and it can be effectively achieved by decreasing weight of automobile structures and components. Ubiquitously known, there is a direct relationship between vehicle’s weight and fuel consumption, so the trend of weight reduction in modern cars is to use high-performance fiber-reinforced composite materials (FRC). Transmission shafts, used for delivering torque from power generators to driving wheels, are the components with high potential of utilizing FRC, since they require materials possessing high specific strength and stiffness under intensive mechanical loads. However, researches on the small-dimension shaft composed of FRC have not yet been sufficiently conducted and published in the literature. This paper presents designs of composite transmission shaft and the interference fit joint with the steel component. The shaft comprises of three sections, i.e., the steel plug, steel sleeve and the composite tube. The carbon fiber-reinforced epoxy and E-glass fibre-reinforced epoxy are used for the composite tube to transfer high-torsional loads, while its end plug and sleeve are made from steel. The interference fit joint, between the shaft tube, the sleeve and the end plug, is studied in order to analyze the effect of interference, friction coefficient, and capability of torque transfer. Fiber orientations, stacking sequences, geometries and dimensions, and yield stresses in the metal parts are the concerned parameters. The considered composite transmission shafts are simulated by employing the finite element software ABAQUS™ and the resulting torque transmissibility of three joint models is illustrated. The failure modes arisen from the manufacturing and loading conditions are examined. The results provide the guideline to appropriate design composite transmission shaft, such as the orientation of fibers, dimensional constraints, and load-bearing condition.
Manufacturing Challenges of Creating Unsaturated Polyester Syntactic Foams with Glass Fiber Reinforcements
Market demand for fuel efficient automobiles is motivating the push towards lightweight autobodies. The manufacturing of unsaturated polyester resin (UPR)-glass fiber (GF) Sheet Molding Compound (SMC) composites with hollow glass sphere (HGS) fillers creates a “syntactic foamâ€. This is a step towards achieving composites with densities of less than 1.5 g/cm3, with the goal of replacing the industry standard 1.9 g/cm3 density composites. While the physical replacement in the resin paste formula of calcium carbonate, the most dense filler (2.71 g/cm3), with lightweight HGS (0.28-0.46 g/cm3), can be done with relative ease, there are manufacturing challenges that must be addressed. Rheology, differential scanning calorimetry, and optical microscopy were used to assess changes in production parameters, such as resin maturation time and compression molding pressure. These characterization techniques will allow for the examination of UPR-GF-HGS as a promising alternative to current UPR-GF composites in the automotive industry.
Design and Analysis of an Optimized Formula 3 Nosecone Structure
In order to ensure the driver safety in motorsport crashes, special crash structures are designed to absorb the race car’s kinetic energy and limit the decelerations acting on the human body. The use of Carbon fibre epoxy as a primary structural material has been evident in the motorsport industry. By utilizing monolithic structure for crash, large amount of energy can be absorbed. However, the energy absorbing capacity, unlike metals, is highly dependent on the geometry, number of layups and layup orientation angles. By optimizing the plies and the orientation along the geometric cross-section, the deceleration of the vehicle can be controlled. For the FIA crash test regulations, the deceleration was limited to 5g’s for the first 150mm of crushing and the average deceleration was limited to 25g’s. By dividing the geometry into sections, the ply orientation, and number of plies were varied. This resulted in a nosecone structure weighing around 2.1 kgs, but able to meet the above requirements. From the research1 it is evident that the Specific Energy Absorption (SEA) is not only a function of geometric cross-section (φ) but also the angle of attack (β). The angles of attack were varied from 5.5° to 32.5° and the effects on SEA were observed. The dynamic simulations were conducted in explicit solver LS-DYNA using Mat_ENHANCED_COMPOSITE_DAMAGE material model (MAT54). The simulation results were validated with crush test data for energy absorbed.
Analysis and Optimization of a Composite Monocoque for Structural Performance
Lately numerous studies have been performed to design composite monocoque’s with high strength and low weight for various student level racing contests. The objective of this paper1 is to develop an insightful methodology to design and developed a light-weight composite monocoque. The monocoque is designed to pass the mandatory static load tests laid down by the International Automobile Federation (FIA)Formula 3. These Formula 3 tests are considered the baseline of the desired structural integrity of the composite monocoque. The presented design technique emphasises on a monocoque developed for Sports Car Club of America (SCCA) races. The three standard load tests performed on the monocoque are Survival Cell Side test, Fuel Tank test and Side Intrusion test. A sandwich layup of bi-directional woven carbon/epoxy prepreg and aluminium honeycomb is optimized for minimum weight while predicting the unknown properties of layup and ensuring the monocoque doesn’t experience failure. The approach intends to achieve minimum weight with high torsional rigidity and is capable of being used for the design and analysis of any kind of formula type composite monocoque
Post-Impact Flexural Collapse of Composite Sandwich Structures in Low-Temperature Arctic Conditions
Drastic reduction of ice over the last thirty years has grown interest in increased use of marine vessels to sail through new routes in arctic regions which are efficient and economical. The major challenge of operating in the polar environment is to avoid any potential impact and subsequent secondary damage by bending in extreme lowtemperature conditions. This study analyses the post-impact bending collapse modes of Divinycell H-100 PVC foam core with woven carbon fiber reinforced polymer (CFRP) face sheets composite sandwich panel. Effects of low temperature and impact energy on collapse modes in 3-point bending have been predicted analytically and validated by experimental results. A series of low-velocity impact tests with 4 J and 8 J impact energy were performed at three different temperatures 23, -30 and -70 °C. Impacted specimens were then subjected to a three-point bending test for identifying flexural collapse modes. Bending characteristics were analyzed on both sides of the specimens (front impact face and back distal face). Analytical results portray that indentation, core shear, face wrinkling are the main competing collapse modes. Results also elucidate that the competing flexural collapse mode strongly depends on pre-bend impact, face sheet thickness, temperature, and bending configuration. Experimental results are further compared and validated with analytical predictions. Experimental results show that thick specimens were collapsed either by indentation or core shear. However, thin specimens at 23 and -30 °C revealed a new mechanism of core tensile failure while debonding was observed at -70 °C. This new observation of core tensile failure is attributed to the reduced tensile properties of the back-face sheet. This work will lead to understanding the post-impact failure modes of composite sandwich structures, which will subsequently lead to an improved design for marine structures and materials that can operate safely and effectively in the Arctic environment.
Automated Computed Tomography Segmentation of Impact- Damaged Curved Composite Structures
Carbon fiber composite structures are highly susceptible to impact damage which results in a complex 3D network of matrix cracking, delamination, and fiber failure. The difficulty of more conventional non-destructive evaluation techniques (e.g., ultrasonic pulse-echo scanning) in characterizing the full 3D damage state has prompted research interest in X-ray computed tomography (CT), which can provide a 3D map of the internal structure and damage. However, the large size of CT datasets and the complexity of damage features can complicate the interpretation of damage as being structurally relevant damage modes or not. Additionally, geometric details such as acute curvature can complicate damage interpretation. This paper presents a CT segmentation procedure that enables the segmentation of datasets of curved composite structures. This procedure will be demonstrated on stringer stiffened composite panels with the layup [45/-45/0/45/90/-45/0/90]s impacted on the stringer cap with a 25 mm radius impactor. The segmentation procedure outputs 2D maps of the damage for each ply and interface and outputs 3D datasets of color coded damage overlaid onto the original CT dataset. Results from the segmentation can also be combined into 3D visualizations of the damage state.
Low-velocity Impact Response and Compression After Impact Behavior of Non-planar Composites Sandwich Structures
Carbon fiber reinforced plastic (CFRP) sandwich facesheets with lightweight core material (such Nomex®) can provide high stiffness-to-weight ratio and strength. This advantage makes the composite sandwich a popular material choice in transport, marine, and aerospace applications. In this study, we examine the mechanical response of CFRP facesheets with Nomex® honeycomb core composite sandwich tubes with a specific focus on transverse low-velocity impact (LVI) and axial compression after impact (CAI) tests. First, specimens with and without core material are subjected to impact energy level of 25 J, using two different shape impactors: hemispherical and cylindrical. The impact behavior observed by impact force, displacement profile and absorbed energy show that honeycomb core composite sandwich tubes exhibit higher resistance and tolerance to impact damage. Subsequently, quasi-static loading in axial direction is performed to determine CAI failure modes and residual strength of the specimens. The failure mechanisms during LVI and CAI are also observed by X-ray micro-computed tomography (μCT) technique, namely matrix crack, delamination and fiber breakage/splitting in facesheets as well as honeycomb crashing and buckling in the core. The phenomenon of these failure modes agrees with the structural strength obtained from mechanical tests. These results demonstrate the response of non-planar sandwich composites structure under compression test for impacted composite specimens, which could be useful for practical designs and applications.
Ballistic Performance of E-Glass Polyester Composites Modified with Silane Treated Alumina Nanofibers
The use of light weight composites against ballistic threat is very important for survivability and mobility in defensive architecture including but not limited to carriages for mass transit, safe rooms, armed forces and law enforcement centers. However, the existing structures need to compete with wide variety of threats and require wide ranging capabilities yet affordable. This study presents the silane functionalization of alumina nanofibers and its effect on the ballistic performance of the E-Glass polyester composites. The composite panels with E-Glass/Polyester are fabricated with functionalized and non-functionalized alumina nanofibers. Alumina nanofibers are dispersed into the polyester resin using three-roll mill technique. The EGlass woven fabrics are wetted with modified polyester by hand layup followed by compression molding press which cures the panels with heat and pressure. The samples were cut to desired size, shape and are impacted using a 9mm, full metal jacket projectile. The ballistic limit increased with addition of functionalized alumina nanofibers. However, functionalization and ballistic performance dependency cannot be completely established. The optimum loading level of alumina nanofibers was determined based on the toughness results and the effect of dispersion of alumina nanofibers in polyester examined through TEM imaging. The dispersion state of silane treated alumina nanofibers in polyester complimented the flexure toughness obtained for 0.25wt% and 0.5wt% loading rates
Multiscale Modeling of Adiabatic Heating and Fiber Breakage in Polymer Matrix Composites Subjected to Impact Loading
Experimental evidence has shown significant heat generation in polymer matrix composites subjected to ballistic impact, particularly in resin rich regions where adiabatic conditions prevail. This local matrix heat generation causes subsequent thermal softening, which can have a significant effect on the deformation, progressive damage, and failure behavior under impact loading. In this work, a multiscale modeling approach is used to simulate this experimentally observed phenomenon. The methodology is used to study the impact response of a T700/Epon 862 [0°/60°/–60°] triaxially braided composite. A subcell based approach is used to approximate the heterogeneity of the braid architecture at the highest analysis length scale. The mesoscale repeating unit cell is discretized in-plane into unique mesoscale subcell regions, which are then discretized through-thickness into an approximation of unidirectional plies. The generalized method of cells micromechanics theory is used to bridge the micro (constituent) and macroscales via localization and homogenization. Unified thermo-viscoplastic constitutive equations are used to describe the nonlinear constitutive behavior of the E862 matrix material. Matrix adiabatic heating, due to the conversion of plastic work to heat, is computed via the heat energy equation at the constituent level. A technique based on shifting resin DMA data is used to determine matrix elastic properties at various rates and temperatures. Fiber failure is incorporated into the multiscale framework and is simulated using both the maximum stress criterion and the Curtin progressive fiber damage model. The simulation results indicate that inclusion of the effects of adiabatic heating could have a significant effect on the accurate prediction of the behavior of polymer matrix composite under impact loading.
Understanding Kevlar Deformation During Ballistic Impact Using an Integrated FBG Sensor
Soft body armor works by dispersing impact energy across woven fibers. This dispersed impact energy is translated though the armor to the wearer, which can cause behind armor blunt trauma (BABT). Impacts from high caliber rounds can cause complex pressure states in the body, resulting in internal injury and death [1]. Current testing methods rely on using clay-backing materials to determine peak back face deformation (BFD). However, clay testing can provide only the max deformation depth and volume. There is no information on how we get to that state, how quickly the event occurs. Understanding the live dynamics of BFD formation will aid in improving both armor evaluation and the prevention of blunt force trauma. To improve soft body armor testing methods, non-invasive sensors were developed to track the back face deformation experienced during an impact event. The sensing layer developed features an imbedded Fiber Bragg grating which is used to capture strain measurements during testing. Theses strain measurements can then be used to reconstruct the peak deformation depth as it forms during impact. Testing has shown that the silicone mat and optical fiber are able to survive impacts behind armor consistently and the transient history of the BFD can be collected during impact. The maximum deformation found while testing theses sensors correlates well with independent clay indentation measurements
Effect of Shear Thickening Fluids on the Mechanical and Impact Response Behavior of GFRP Composite for Protective Applications
Efficient crashworthiness of composite materials is one of the fundamental issues in design of lightweight vehicles to provide high amount of energy absorption, protect human injury, reducing greenhouse gas emissions, fuel cost, and to achieve consolidation of multiple components. In this paper, the effect of shear thinking fluid (STF) on mechanical behaviour of shear thickening composite laminates was investigated. The laminates used were symmetric Glass Fibre Reinforced Plastics (GFRP) intended for cost effective vehicle and personal protective system applications. The STFs were chosen due to their benefit to improve the efficiency of protective systems, in addition to increased lightness and flexibility. Beside protective applications, these smart fluids have been suggested to absorb shock waves from blast waves, to improve the vibration and damage resistance of a structure. Experimental tests were performed using universal and impact testing machines to study the mechanical behaviour of the composite materials. Impact testing was performed at various energy levels. The impact response was evaluated using optical transmission scanning (OTS) system, displacement, and acceleration. Experimental results revealed that the impact response of GFRP varied due to the presence of STF and energy levels. Initial results show that the damage resistance is affected by the unique phenomena caused by the presence of STF. A better understanding of these phenomena and their contributions is essential for design of resulting structural components.
Progressive Damage and Failure Analysis of Bonded Composite Joints at High Energy Dynamic Impacts
Both wing and fuselage structures utilize bonded composite joints for structural efficiency in modern commercial and military aircraft. To ensure compliance with certification requirements mechanical fasteners are typically used as a failsafe mechanism for appropriate strength in the event of complete stiffener disbond. However, the use of fasteners decreases the structural efficiency of the structure by adding weight. This establishes the requirement to better exploit the efficiency of bonded structures and fully understand the failure behavior of adhesively bonded composite structures, particularly when subjected to elevated loading rates due to high energy dynamic impacts (HEDI). For this reason, the NASA Advanced Composite Consortium (ACC) HEDI team developed an experimentation and numerical modeling program for high rate loading of composite joints [1] [2]. In the present work, the response of adhesively bonded composite joints subjected to elevated loading rates is studied numerically and validated against experimental results. Due to dynamic considerations of experiments, the idea of wedge insert [3] was extended to use with Split Hopkinson Pressure Bar (SHPB) testing techniques. Mode-I and Mode- II test configurations were simulated to evaluate the capability of two continuum damage material (CDM) models in LS-DYNA, namely MAT162 and MAT261 [4]. Three different levels of fidelity were considered to investigate the level of detail required to numerically predict the failure behavior and the results from high fidelity analysis are presented.
SIMULIA Abaqus Simulations to Predict Compression-after- impact Behavior
Fiber-reinforced composites are receiving increased attention due to their superior mechanical properties compared to traditional materials. Since composites are made of multiple components, their failure mechanisms are complex and involve competing failure modes. This necessitates the need for sophisticated modeling and simulation to analyze failure as experimental studies are expensive and time-consuming. The current paper presents the experimental and numerical modeling of compression-after-impact (CAI) experiments of T650-5320 composite laminates. Experimental study is conducted at the National Institute for Aviation Research (NIAR) laboratories in accordance with ASTM D7136 for impact and ASTM D7137 for CAI experiments. The numerical finite element modeling is studied using SIMULIA Abaqus software. Shell elements are used to model the composite layup to provide a global solution. This is followed by a detailed simulation involving solid elements. Results are compared with the experiments to validate the two approaches. Shell modeling is demonstrated as a simpler approach to model CAI to get coupon-level results. Solid modeling is proposed to get detailed results to determine damage levels within each ply.
Impact Response for Sensor-less Speckle-less Elastic and Viscoelastic Material Properties of Flexible Polymer Composites
Measuring the global dynamic response without sensors of flexible polymer composite can be achieved with only high-speed camera images. We apply a noncontact, speckle-less, custom Matlab script to isolate composite specimens filmed under uniaxial low velocity impacts via Vision Research Phantom UHS V2640 and measure the displacement time history. A custom fitting curve is applied to this data and two derivatives with respect to time are performed to provide the acceleration time history. Since the impactor mass is known, we can solve for the contact force without sensors. Comparison with a dynamic load sensor and using our previous efforts with extracting the hyper-elastic and visco-elastic material constants from a single impact test, we compare the dynamic impact response of the flexible polymer composites; the non-contact, speckle-less method matches the force response within 1% peak value and 2% in pulse duration and the overall finite element validation model also matches within a small error for magnitude and duration.
Micromechanics Model of the FRC Subject to Heating High- Velocity Impact Using Continuum Damage Mechanics with Adiabatic Heating
The popularities of the composites are increasing growth in manufacturing, with numerous advanced applications ranging from military and civil vehicles to decorative architectural facade. Experiments show that adiabatic heating becomes more noticeable at high strain rate deformation, causing the stress-strain slope to become negative. However, the rate-dependent micromechanics model in Z. Li et al. 2016 lacks the interpreted connection between energy dissipation and adiabatic temperature rise. This work aims to develop a thermal-mechanical model incorporating material heterogeneities to study multi-physics damage and failure of S-glass fiber reinforced epoxy composites under high-velocity impact. This study first established a rate-dependent nonlocal continuum damage model (CDM) with adiabatic heating for composite matrix, e.g., DER 353 Epoxy. A Split- Hopkinson pressure bar (SHPB) experiment from literature are rebuilt in FEM simulation to validates this material model. The research proves that thermal softening became more prominent as the strain rate increases. The new model is then integrated into a micromechanical analysis of composites representative volume elements (RVEs) to study the interaction between stress wave propagation and crack growth in composites RVE subjected to high strain rate deformation. Deformation and failure response of the RVE models exhibiting fiber and matrix damage as well as cohesive interfacial crack. This work is an extension of a broader multiscale work examining glass fiber reinforced composites under high strain rates.
Oblique Ballistic Impact Testing of Carbon/Epoxy Torsion Tubes
Experimental research has shown that the incorporation of nanosilica (NS) filler in carbon/epoxy (c/ep) cylindrical tubes decreases the damaged area and increases the residual torsional strength of following normal incidence ballistic impact. The current investigation aims to determine the effect of NS on the residual torsional strength (RTS) of c/ep cylindrical tubes subjected to a static torsional load during oblique-incidence ballistic impact, where the projectile grazes the outer radius of the tube at a 45° angle in order to remove the largest amount of material (slashing impact). A total of four tubes were evaluated—two with 15% NS as a percent of matrix weight and two without NS filler. One of the impact tests resulted in two distinct holes in the tube rather than an elliptical gouged-out area, resulting in a reduced overall damage area and less degraded RTS versus the properly impacted tubes. The RTS retention of the three tubes having the intended impact trajectory was approximately 12% of the estimated strength of undamaged tubes. Likewise, the size of the damage zone surrounding the impact site, determined by cointapping, was also similar among the properly impacted tubes. Overall, the results are comparable with previously published slashing impact data for carbon fiber composite tubes made with various matrix materials and highlight the dominant influence of fiber damage versus matrix damage on the RTS of composite tubes following slashing ballistic impact.
Transverse Impact on a Single Layer S-Glass/SC15 Epoxy Composite Strip
Insight into ballistic impact resistance of fiber reinforced polymer composite (FRPC) strip provides detailed material damage information by isolating and investigating materials from a whole composite laminate. In this work, transverse impact experiments on single-layer S-Glass fiber/SC15 epoxy composite strips by Right Circular Cylinders (RCC) were performed. The critical velocity, essentially dividing two phenomenological phases, penetration and perforation, respectively, was obtained as 274-409 m/s after multiple shootings with different velocities. The realtime mechanical behavior of each composite strip was captured by a high-speed camera. Post-fracture images were gained by a Scanning Electronic Microscope (SEM), providing further information about failure surfaces of the fiber and matrix. Experiments show that the composite strip failed in a tent-like shape with obvious matrix failure and fiber/matrix interfacial debonding under below-critical-velocity impact. When the projectile velocity exceeded the critical velocity, materials ruptured instantly at the projectile front without wave propagating to the two ends, which indicates the loss of energy dissipation capability of the composite. An obvious transition phase, when the projectile impacted the strip in the critical velocity, was observed with part of the composite strip failing locally in transverse direction and the residual part forming a tent-like shape and being tensioned until break.
Specific Energy Absorption Improvement of Rear Crash Attenuator by Numerical Modelling for Various Angles of Impact
This research* aims on developing a reliable finite element framework to investigate the Specific Energy Absorption (SEA) of the rear crash attenuator of an open-wheel type Indycar vehicle. A meshed model representing the crash structure was designed and its failure behaviour was learnt on the basis of various non-linear finite element modelling techniques to simulate a crash as per regulations from the governing body of Indycar. All the numerical analysis was performed utilizing the LS-DYNA software with the Progressive Failure Model (PFM) and Continuum Damage Model (CDM) of MAT058_LAMINATED_COMPOSITE_FABRIC card. The sandwich structure material characterization for the tuning of the material model was done by the means of a correlation with experimental data and adjusting the non-physical input parameters in the software. Post calibration, the development of the rear impact attenuator was performed with the model. A combined failure mode was observed with a gradual crushing phenomenon during the analysis on head-on impacts (0°) while in case of oblique impacts performed at 30° off axis shows the structure failing at its rear attachment points to the bulkhead. The specific energy absorption was determined at different configurations of impact of this reinforced sandwich structure by evaluating the force over a crushed displacement. The layup was adjusted, the sensitive points at the attachments were stiffened, and the core thickness was varied throughout the structure to improve the overall specific energy absorption by 27.8% with a gradual deceleration value to that of the prescribed. Finally, the results were compared to the previous Indycar structure and the rear crash attenuator was redesigned with highlights of the refreshed results.
Numerical Assessment of Compression Strength After Impact for Impact for Enhanced Aircraft Sustainment
A means by which the strength of composite aircraft structures can degrade significantly in-service is via low velocity impact induced while in maintenance (e.g. tool drop). Recently, improvements in the fidelity of non-destructive characterization and physics-based modelling have made possible significantly improved strength assessment. The post-impact residual strength was predicted using damage mapping along with the widely known Rx-FEM and the CB2ATA frameworks that permit discrete and discrete damage informed continuum damage modelling. The results showed that the complexity of the damage mechanics means that the solution is sensitive to a number of input parameters that need to be accurately obtained. This includes accurate mapping of the delamination and lamina compression strength. The studies herein in combination with the development of non-destructive evaluation methods seek to provide the foundation for a capability to predict accurately the performance of damaged and degraded composite structures for airworthiness decision making.
A Numerical Approach for Determining Peridynamic Material Parameters
In previous presented work [1], the authors developed a bond-based peridynamic composite approach with explicit incorporation of fiber and matrix material regions. This explicit definition of fiber and matrix material regions enabled the capturing of anisotropic behavior, but required careful and laborious determination of peridynamic material constants. This work presents a numerical approach for use with composite models in which no calibration is necessary. The numerical approach calculates peridynamic elastic constants of a lamina with resin and fiber regions explicitly defined. The approach is then extended to incorporate nonlinear constitutive behavior by utilizing nonlinear optimization techniques. As demonstration of the capability of the approach, a Ramberg-Osgood relationship was implemented within bond-based peridynamics. Additionally, the methodology was validated through off-axis tension test simulations and using a number of different grid refinement levels.
Peridynamic Fatigue Model for Composites Under Constant and Variable Amplitude Loads
High-fidelity Numerical Prediction of the Compressive Strength of Laminated Composites after Low Velocity Impact
Carbon fiber laminated composites are vulnerable to impact loading and the damage induced by low velocity impact (LVI) is distributed within the material without significant external indicators. The internal damage can cause significant knock down of the post-impact compressive strength and by industrial standards, compression strength after impact (CSAI) tests are usually performed to evaluate the degradation of the compressive strength. In this paper, the CSAI tests were carried out and in-situ 2D digital image correlation was done to measure the deformation. Computational predictions of the CSAI strength were performed based on the numerical results obtained from prior LVI predictions. Three commonly used stacking sequences were studied to demonstrate the versatility of the computational model. The computational results were verified with the experimental data and the computational model was validated to be sufficiently accurate and efficient.
Test/Analysis Correlation of Damage States in Post-buckled Stiffened Validation Building Block Specimens
As part of the NASA Advanced Composites Project, several progressive damage analysis methods (PDA) have been applied toward predicting the responses of increasingly large and complex validation test articles. Ultimately, the project seeks the prediction of damage onset and evolution in post-buckled multi-stringer panels subjected to static compression loads. Several smaller specimens were designed and tested to assess specific capabilities of analysis methods in less computationally intensive settings. A three-point bend (3PB) doubler-stiffened coupon and a hatstiffened seven-point bend (7PB) panel are two lower-level validation test articles with which assessments of the PDA methods were made. The 3PB and 7PB specimens were designed to exhibit stable damage growth, with damage fronts containing interacting matrix cracks and delaminations at multiple interfaces. While the 3PB damage front propagates along the length of the specimen, similar to typical interlaminar material characterization coupons, damage in the 7PB initiates away from the panel edges while the panel is in a post-buckled-like deformed shape. A comprehensive data set was collected to measure the structural response and characterize damage. Load, strain, full-field displacement, acoustic emission, and passive thermography data were collected during testing. Most tests were interrupted during loading to characterize damage evolution in the specimen. Upon detecting any indication of damage growth, loading was stopped and ultrasonic and X-ray computed tomography scans were performed before reloading. A ply-by-ply fiber-aligned meshing strategy is employed to predict matrix cracks using a continuum damage mechanics method. Delaminations are represented using layers of cohesive elements between layers of solid elements. Test/analysis correlations are made using various metrics, including load/displacement histories,
Computational Considerations in Scaling the Application of Progressive Damage Analysis Methods from Coupons to Sub-Components
The computational scaling performance for different progressive damage and failure analysis (PDFA) problems of interest is examined using Abaqus/Explicit. Three models that range from a coupon with single-interface delamination to a structural element with a complex, several-interface, delamination migration damage mode were exercised in the study. By comparing run times across different numbers of cores, hardware, and models, the computational scaling performance is quantified. A run time reduction of ten times is achieved scaling from 24 to 512 cores. The computational scaling performance is relatively consistent among the different models, suggesting that the speed up performance that is obtained is characteristic of the class of models. By analyzing and extrapolating the data collected, the current state of computational tractability is defined and discussed.
Effect of Holes on the Damage Propagation of Composite Plate Using Peridynamic Method
The ultimate goal of this research is to investigate the effect of whole orientations on the crack propagation in unidirectional composite materials under a uniform velocity. The composite materials have been extensively utilized in the aerospace industry. To assemble one part of the fuselage to another one, the composite panels should be fastened to each other. Fastening the composite panels can be possible with joint which is necessary to make holes on the panels. The number of holes and their orientations may affect the strength of composite panels. It seems necessary to quantify the strength of composite structures with different number of holes and orientations. It is substantial to obtain the load when crack starts initiating in the composite plates. To fulfill the requirement, the nonlocal theory which is called peridynamic method is employed to study the crack propagation. In this paper, crack initiations, crack propagation, and failure mode is studied using the peridynamic method. The effect of hole and number of orientations is investigated on the strength and ultimate failure of composite plates. The PD simulation results are compared to experimental study for plate with two holes for validation.
Automated Fiber Placement (AFP)—Effects of Features and Defects
Automated Fiber Placement (AFP) manufacturing methods, considered to be one of the most advanced methods of composite manufacturing, are used in fabricating complex and large scale composite structures. During AFP manufacturing, the gaps between each fiber-tow is closely controlled. However, due to machine/operational errors, gaps between these fiber-tows exceed set tolerances and are seen as a frequent feature/defect in AFP fabrication. By understanding the effect of these AFP induced gaps, we will be able to evaluate the integrity of the specimens/structures to better understand the effects due to these features/defects. Current research focuses on AFPgap configurations associated with missing tows and mainly looks into establishing methods for developing knockdowns/design allowable that can be incorporated into the design building block analysis and test validations.
Damage Mechanisms in Tapered Composite Structures under Static and Fatigue Loading
In this work an integrated computational/experimental approach was developed to validate the predictive capabilities of State-of-the-Art (SoA) Progressive Damage Analysis (PDA) methods and tools. Specifically, a tapered composite structure incorporating ply-drops typical in the aerospace industry to spatially vary structural thickness was tested under static tension and cyclic tension fatigue loads. The data acquired from these tests included quantitative metrics such as pre-peak stiffness, peak load, location of delamination damage onset, and growth of delaminations as functions of applied static and fatigue loads. It was shown that the PDA tools were able to predict the pre-peak stiffness and peak load within ±10% of experimental average, thereby meeting and exceeding the pre-defined success criteria. Additionally, it was shown that the PDA tools were able to accurately predict the location of delamination onset and satisfactorily predict delamination growth under static tension loading. Overall, good correlations were achieved between modeling and experiments.
Stochastic Modeling of Progressive Damage and Its Scaling in Polymer Composites
This study proposes a novel stochastic framework for the modeling of damage initiation and propagation at the microscale in the matrix of polymer composites. The model builds on recent experiments and computational simulations which have shown a significant energetic-statistical size effect on the strength and fracture energy of thermoset polymers. In fact, the microscale strength of thermosets was shown to be from six to ten times larger than the values estimated from macroscopic tests whereas the microscale fracture energy was found to be one fortieth of its macroscopic value. To capture this significant size effect, the computational framework proposed in this work combines a two-scale cohesive model and an autocorrelated random field of material strength. The two-scale traction separation law, featuring two distinct characteristic length scales, accounts for the energetic size effect on the average fracturing process due to the stress redistribution in the crack Fracture Process Zone (FPZ).
Multiscale Fatigue Modeling of Composite Materials
In this paper an experimental procedure for determining the in-situ matrix response in fatigue, to be used in a micro-mechanics framework, is described. The procedure utilizes only a [±45] tensile test to back out complete equivalent matrix stress-strain behavior. Quasi-static tests on interrupted fatigue tests were conducted to obtain equivalent matrix response as a function of number of cycles and stress ratio. The result of the test procedure is an easy to implement formulation of the in-situ matrix response, in a multi-scale modeling framework.
Investigating Progressive Damage via Biaxial Bending
The objective of this research is to understand the progressive damage that occurs in composite materials beyond their peak response and the interaction of multiple failure modes. Bi-axial bending is a test method that utilizes a Teflon Insert to explore damage propagation and interaction throughout the test. This work builds upon previous testing methods performed by McElroy and moves on to more traditional stacking sequences [1]. A model is created to explore damage interactions within a square plate. The square plate is loaded on edge, where a Teflon insert is located at the mid plane of the composite. This causes damage initiation and propagation to begin at the midplane and at the Teflon insert. As of now, modeling has been pursued to validate the thought that this experiment will allow the isolation and control of damage. To attempt to validate the experiments using a finite element code, a weak cohesive zone was applied to imitate the Teflon insert and act as a stress concentration. Another method where a Teflon insert is explicitly modeled with different properties at the midplane of the composite showed better results of isolation and damage control. This modeling and testing effort utilizing NDE and high-fidelity modeling will provide a better understanding of polymer matrix composite damage interaction.
Experimental and Numerical Investigation on the Three Point Bending Response of Highly Anisotropic Composite Beam
Three major damage modes exist in the low velocity impact (LVI) induced damage of carbon fiber reinforced polymer (CFRP) laminated composite materials, including fiber breakage, matrix cracking and delamination. The enriched damage mechanisms and the interaction between the damage modes makes the problem profoundly challenging to predict and simulate. The interaction between the transverse matrix cracking and delamination often cause a featured “pine treeâ€ shape for the impact damage and this paper aims at isolating this interaction phenomenon by studying the behavior of a “sandwich-likeâ€ [0/90/0/909]s laminated composite beam subjected to three point bending loading (3PB). 2D digital image correlation (DIC) was used to capture the in-situ deformation of the composite beam. A 3D finite element method (FEM) based model was built where a novel Multi Scale Damage and Failure (MSDF) modeling was implemented to capture the damage initiation and propagation. The model results are found to be very good agreement with the experimental observations.
Influence of In-situ Constraining Effect on Transverse Ply Crack Formation in Unidirectional Composite Laminates.
A micromechanical finite element model was developed to predict the initial events of failure in the constrained 90° ply of a [0/90/0] unidirectional cross-ply laminate under in-plane tensile loading. Common manufacturing induced defects, including non-uniform fiber spatial distribution and voids, were considered in the explicit representation of the full thickness multi-fiber 90° ply, while the constraining 0° plies were treated as effectively homogeneous orthotropic materials. The inelastic behavior of the polymeric matrix in the 90° ply was modeled via a pressure-dependent yield criterion, while the carbon fibers were assumed to deform linear elastically. Two metrics for representing failure were considered in the assessment, including distortional and dilatational energy densities that are respectively responsible for local yielding and cavitation in the fiberconstrained matrix. The influence of in-situ effect was studied by comparing the local stress state of the constrained and unconstrained transverse ply. It was observed that the local response of the polymer depends on the fiber spatial distribution. Furthermore, it was found that the in-situ ply constraining effect appears subsequent to the formation of local cracks and not prior. Dissimilar constraining effects were observed in different regions, e.g., resin-rich zone versus fiber-cluster zone, suggesting that inconsistent stress distributions will directly influence the evolution of cracks in the 90° ply.
Effect of Cooling Rate on Curing Induced Thermal Strains in Bonded Joints
Measurement of strains in bondline of adhesive joints poses many challenges. The measurement technique should provide accurate and continuous measurements without affecting stress-transfer. Hence, fiber optic sensors with their inherently small size are best suited for bondline measurements. Furthermore, high-definition optically distributed sensors can provide strain measurements at every 1.2 mm. along the fiber length allowing to capture precise strain profiles in the bondline. One of the challenges in bonded joints is the lack of understanding of the effects of processing parameters on resulting mechanical behavior. In this work, the effect of cooling rate on the strains developed in the adhesive were experimentally measured. Lap-joints made of glassfiber reinforced (GFRP) substrates were bonded with a thermoplastic adhesive (ABS). The fiber optic sensor was embedded along the length of the bondline in the center of the adhesive. It is experimentally cumbersome to try a large number of cooling rates with varying adhesives and substrates. Hence, three different cooling rates were selected in this work and strains developed were monitored over time. As expected, results indicate that the thermal residual strains in the adhesive increased with increase in cooling rates. At lower/gradual cooling rates, the viscoelastic nature of the adhesive allows for higher polymer chain relaxation, thus resulting in lower residual strains. In each case, the residual strains were observed to be higher at the joint free edges. The edges of the adhesive cool and solidify faster than the center leading to a ‘prestrain/ pre-stress’ condition in the adhesive as result of processing conditions.
Monitoring Residual Strains in Oven- and Induction-bonded Joints
Differential thermal expansion of bonded joint constituents results in residual stresses within the adhesive and at the bi-material interfaces, which can significantly reduce the strength of the resulting joints. In this work, experimental strains were recorded during both the heating and cooling cycles of electromagnetic (EM) induction bonded and oven bonded joints. The fiber optic sensors that use distributed sensing technology and provide strain measurements at every 1.2 mm along the length of the sensor were placed with in the adhesive during the bonding process. A parabolic strain distribution was observed between the edges and midpoint of the adhesive bondline at the edges of joints in joints manufactured from oven bonding technique. Furthermore, the magnitude of strains developed in the geometrical center of adhesive bondline through EM bonding was three times that of the oven bonded joints due to the difference in the thermal boundary conditions of the two processes. The study showed that the EM bonding results in increased thermal residual strains despite of its other processing advantages such as rapid heating and lower energy consumption. Further studies are necessary to fully quantify the residual strains developed during the processing thereby aid in better design of both the processing and structural parameters
Damage and Strength Analysis of Carbon Fiber Reinforced Polymer and Titanium Tubular Adhesive Joints
For structures that require high performance, dissimilar materials can be joined to achieve desirable strength-to-weight ratios. Unlike traditional mechanical joints, adhesive joints allow dissimilar materials to be joined by means of an adhesive. This method creates more uniform stress distribution while maintaining desired strength-toweight ratios. This work numerically studies adhesive joints between Titanium and Carbon Fiber Reinforced Plastic (CFRP). Finite Element Analysis is performed utilizing Abaqus® paired with Cohesive Zone Modeling to observe a comprehensive stress analysis and damage evolution of a Titanium and CFRP Tubular-Lap Joint (TLJ). By understanding the damage criteria, the properties of the TLJ can be altered to increase the strength of the joint. The failure mode of the TLJ is directly related to the mechanical properties and geometry of the adhesive which is validated numerically. This study verifies the importance of geometry considerations for adhesive joints and provides solutions to increase joint strength by utilizing a hybrid adhesive approach.
Tailoring Substrate Stiffness in Bi-Material Adhesive Joints
Joining composites to metals is a common challenge in the automotive and aerospace industries because of delamination and fiber breakage caused by drilling holes in the composite to accommodate mechanical fasteners. Adhesive bonding offers a compelling alternative to mechanical fastening, with additional advantages of having fewer parts, lesser weight, and a more uniform stress distribution when compared to mechanical fasteners. However, the relatively low strength of such joints is a limiting factor preventing widespread adoption in high-strength structural applications. One way to improve bi-material adhesive joint strength is to drill holes in the metallic substrate to allow the adhesive to ooze through and form a mechanical interlock, with potential to significantly increase the joint strength. Such ‘perforated’ joints have been explored in recent literature [1–5], and experimentally found to increase the strength of adhesive joints under tensile loads. However, all such studies have used an equally-spaced, rectangular array of holes, and examined only double lap joint configurations (DLJ). Such techniques cannot be directly translated to the more common single lap joint configurations (SLJ) due to the presence of bending-induced peel stress concentrations towards the axial ends of the bond area under tensile loads. The presence of perforation holes in a stress concentration zone would be detrimental to the joint performance. In this work, perforated SLJs with a rectangular-array hole pattern and a ‘tailored’ hole pattern were tested in tension-shear, and the peak loads were compared against baseline joints without perforations. Three different adhesives were used to induce different failure types and identify the specific conditions in which perforations provide an advantage in SLJs.
Ultrasonic Welding of Thermoset to Thermoplastic Based Composites: Effect of the Process Parameters on the Weld Strength
Thermoset composites can be made “weldableâ€ by co-curing a neat thermoplastic coupling layer on the surface to be welded. With its exceptionally short heating times, ultrasonic welding is a highly promising technique for joining advanced thermoplastic (TPC) to thermoset (TSC) composites, since it can potentially prevent thermal degradation of the thermoset adherend. Previous research has shown successful results on ultrasonic welding of carbon fibre (CF)/epoxy and TPC using a PEI coupling layer. However, it is still not clear what the processing window is for ultrasonic welding of TSC to TPC, i.e. how much flexibility we have in choosing the process parameters while maintaining good weld strength. Hence, knowing the mechanisms that cause a reduction in weld strength is of great importance for the optimization of ultrasonic welding of dissimilar composites. On top of that, it is important to determine how the thickness of the coupling layer affects the span of the processing window, since precious research has shown that the thickness of the coupling layer affects the welding process and the weld strength. Thus, in this study, a processing window for welding CF/epoxy and CF/PEEK composites (i.e. hybrid welds) was defined and compared to the processing window for ultrasonic welding of CF/PEEK samples (i.e. reference welds). Samples were welded at different displacements of the sonotrode, also known as travel, and the mechanical performance of each batch was determined by single-lap shear tests. Moreover, the effect of the thickness of the coupling layer on the processing window was determined by investigating three different thicknesses. It was concluded that the processing window span is increasing with an increasing coupling layer thickness. Furthermore, the processing window of the hybrid welds is narrower than the processing window of the reference welds.
Strain Energy Release Rate Calculations of Adhesively Bonded Joints Using Spring Foundation Models
This study shows two semi-analytical models for rapid joint sizing and design, where the adhesive is modeled as a foundation of continuous linear shear and normal springs, and how the virtual crack closure technique (VCCT) has been applied to both models to obtain the strain energy release rate (SERR). One method replicates the VCCT method but with a finite thickness and continuous degrees of freedom, while the other looks at energy considerations to calculate the SERR of a point based on only values evaluated at that point. The methods are compared with each other and with 2-D plane strain finite element models.
Implementation of a Simulation Tool of Strains Induced During the Co-bonding of Autoclave-cured Parts
Our on-going research focusses upon composite structural parts such as stiffened panels with co-bonded stringers. This study is aiming to developing a numerical tool based on commercial softwares (e.g. Abaqus ® and Fortran ®) enabling after-cure deformations to be predicted for co-bonded composite parts. Several simulations moduli were developed to describe i the thermochemical, ii the thermomechanical behaviours of prepreg plies and structural adhesive film and iii the mould/part interaction. An experimental work was conducted in parallel in order to get firstly simulation input parameters (i.e. material behaviour laws) and secondly experimental deformations on co-bonded composite specimen. The materials used were the Hexcel U.D. prepreg tape M21EV/IMA and the Cytec FM300M structural adhesive film. During a first step (first cure cycle), the process-induced strains of the laminated panel that will be stiffened with stringers were computed and compared to experimental ones. During the second step (second cure cycle) the process-induced strains and deformation were computed on the cured laminate but also on the stringer foot and on the structural adhesive as shown in Fig. 1. Lastly comparing experimental results to simulations shows clearly the interest of considering a 2 steps manufacturing process with 2 cure cycles instead of a single one in which all the components (i.e. panel, adhesive and stringer foot) are cured together
Effect of Total Bonded Area on Hydrodynamic Ram Resistance of a T-Joint
Hydrodynamic Ram (HRAM), which occurs due to overpressure created in an enclosed fluid by the transfer of energy and momentum from a penetrating projectile, can be quite destructive to the enclosing structure. Fuel cells are particularly susceptible to HRAM, with the critical damage occurring at the joints. Independent joint testing using a Ram Simulator or RamGun, a device capable of creating HRAM pressure, enables testing of T-joints to assess HRAM resistance. The test chamber is instrumented with a variety of pressure transducers to record the pressure applied to the joint in the chamber. In this study a baseline T-joint is tested using the V50 ballistic testing approach to establish the pressure range required to cause failure of the joint. Additional plies are then added to another joint with the same skin thickness at the skin interface to increase the total bonded area. This modified joint is tested in the RamGun and its resistance to HRAM is examined by studying the pressure required to cause failure. A coarse mesh cohesive zone failure model (CZM) is created of both configurations using Arbitrary Lagrangian Euler (ALE) Fluid Structure Interaction techniques in the commercial software LS-DYNA to predict failure and possible damage attenuation of a full scale fuel cell model that utilizes these two T-joints types. The research indicated that increasing the total bonded joint area increases HRAM resistance of a joint.
Investigation of Rheology and 3D Printability of PDMS Nanocomposites Ink
Pristine polymers, when extruded for 3D printing purposes, often lack the ability to hold their shape after deposition due to insufficient yield stress, spreading as they wet on the deposited surface almost immediately upon exiting the nozzle. However, the addition of fillers, such as silica, can alter an ink’s rheological properties, viscoelastic properties, and shear thinning behavior, while endowing the ink with the yield stress and thixotropic behavior to hold its shape upon printing. With the addition of conductive nanofillers, such as carbon nanotubes (CNTs), these polymer-based nanocomposites can be used as 3D printable, functional inks. In this work, we study the effects of silicon-based nanofillers, on rheological behavior and 3D printability of PDMS/CNT nanocomposites. The shear thinning properties and yield point of the nanocomposite at various filler content is explored via peak hold and amplitude sweep tests, respectively. To explore the effects of the fillers on the 3D printing of the functional ink, multi-layered geometries are printed and studied under scanning electron microscopy (SEM). These images show the shape of each stacked layer and provide insight on the controllability of the print and the bonding of each layer to its adjacent. Studying the effect of fillers on the 3D printability of polymer-based functional inks is valuable, as these inks allow for the quick and facile generation of custom multifunctional structures for use in a wide array of applications.
Twisted Composite Structures Made by 4D Printing Method
This paper presents the determination of the dimensions of twisted laminates made by 4D printing of composites (4DPC) method. Flat stacks of composite prepregs are laid onto a flat mold. Upon curing and cooling to room temperature, the flat stack becomes curved and twisted. The dimensions of the twisted laminate are determined using both calculated method and experimental measurement. Reasonable agreement is obtained.
Additively Manufactured Silica Matrix Composite Structures
Additive manufacturing of preceramic resin has given a way to manufacture complex shaped ceramic parts, which is not possible with the conventional manufacturing methods. In this study, the geometry and microstructure studies were conducted on silica filled photoreactive resin specimens in terms of ceramic yield and dimensional stability. It was evident from the studies that about 66% of ceramic yield can be achieved at a cost of around 34% volume shrinkage with this resin. This study paves way for designing the process parameters and material composition to additively manufacture high quality CMC structures.
Improving Properties of Additively Manufactured Carbon Fiber Composites via Post Pressing
Additive manufacturing, specifically the fused filament fabrication (FFF), has recently begun to explore its role in the composites industry, bringing the capability of engineered fiber orientation and placement, complex part geometry, and tailored internal structure to the traditionally high strength, high stiffness, and low-density material class. Despite conventional manufacturing, however, AM composites suffer from large void fractions and poor interlaminar adhesion, artifacts of the FFF, resulting in premature failure of AM parts. This study investigates the hot isostatic pressing (HIP) post-processing to improve the flexural and interlaminar properties of AM composites. Isostatic pressure and elevated temperatures were used in combination to compress internal voids in AM composite parts and induce the diffusion of molecular chains across printed layers to improve the interlaminar bond strength. This treatment can be applied to parts with complex geometries and in general is not limited by part size. The flexural strength, flexural stiffness, and interlaminar shear strength of the AM composites were found to increase by 36%, 35%, and 33%, respectively following a 1-hour treatment, accompanied by a corresponding reduction in void volume fraction. This method demonstrates a fast and robust method to post-process continuous carbon fiber reinforced polymer AM printed parts, resulting in a significant improvement in their mechanical performance.
Development of a Multivariate Statistical Material Model for Fused Deposition Modeling (FDMTM) of ULTEMTM 9085
Fused Deposition Modelling (FDM™) is an additive manufacturing technology invented by S. Scott Crump in the late 1980’s and commercialized by Stratasys in 1990. Since that time, the technology has found wide spread use in applications ranging from trinkets to tooling for composites to non-structural flight hardware on aircraft. FDM™ growth has been stymied by seemingly unpredictable material properties that have made it difficult to design structural parts. In an AmericaMakes program featuring a government-industry-academia partnership, over 1400 ULTEM™ 9085 specimens were manufactured and tested for specific mechanical properties over a range of temperatures, including 246 tensile specimens. This paper explores those material properties in depth and develops a method for smartly applying the properties in finite element analysis.
DLP-Based 3D Printing of Low-Density Composite Formulations for Unit Cells for Mechanical Metamaterials
Mechanical metamaterials with tuned and functionally graded (FG) dynamic properties have potential for control of wave propagation in structures, as they exhibit dynamic behavior not normally observed in homogenous and natural materials. Along with the structure, the performance of metamaterials depends on the base material properties, specifically density and elastic modulus. DLP-based 3D printing has been shown as a promising technique to manufacture complex composite structures with high volume fraction of reinforcements, and which would be beneficial to achieve the required density and modulus. Commercially available resins enable good printing of complex structures; however, the resins are restricted in their intrinsic properties, which limits the design space of the metamaterials. This limitation motivates the development of new resin system that expands the design space of the metamaterial structures, thereby enhancing their performance. This paper focuses on DLP-based printing of preliminary metamaterial structures with an acrylate-based, commercially available resin (PR48), incorporation of low-density reinforcements into PR48, and discusses the need for modified formulations as greater filler concentrations are incorporated.
Investigation of Needle Tip Geometry on Orientation, Fiber Morphology and Mechanical Properties of E-Spun Nanofiber
Electrospinning is the most convenient method to fabricate continuous carbon nanofiber (CNF) in the form of yarn or tow. Traditional electrospinning uses a single needle with blunt tip, however, there are many hurdles in producing continuous yarns due to a highly localized electrical field, unstable Taylor cone, discontinuous jet flow, and non-uniform fibers. In this study, we investigate three different tip geometry systems such as needle with a blunt tip, a needle with a flash tip, and protruded needle tip. A 2D finite element analysis (FEA) is employed to investigate the electric field distribution for different needle tip conditions. Electrospun (E-spun) nanofibers are fabricated from 10% polyacrylonitrile (PAN) in N, N-dimethylformamide (DMF) solution using a rotating disk collector. The alignment of nanofibers within the yarn, diameter distribution, and fiber uniformity are investigated with various needle tip configurations while keeping all the other electrospinning parameters constant. The diameter distribution and alignment are investigated by scanning electron microscope (SEM). It has been reported that about 10% reduction in fiber diameters, as well as a uniform fiber with better alignment, are observed with protruded tip configuration compared to blunt tip configuration as a result of more uniform electric field distribution near the electrode.
The Bending Properties of CFRP I-shaped Cross-Sectional Beam with Dispersing Cellulose Nanofibers on the Surface
The strength enhancement of the matrix of CFRP has been a major research subject. In particular, strength of the surface resin layer of CFRP is quite important since the bending stress is most effective on the surface but also exposed to foreign object debris (FOD). One of the methods to strengthen resin matrix of CFRP is to disperse cellulose nanofibers (CNF), therefore, many studies have been conducted. However, due to hydrophilicity of CNF, a hydrophobic treatment is indispensable. Although various hydrophobic methods have been proposed, special chemical substances are required and resulted in higher manufacturing costs. In previous study, authors showed that the mechanical properties of CFRP were enhanced by CNF without hydrophobic treatment using the electro-activated deposition resin molding (ERM) method. In the ERM method, a non-crimp carbon fiber fabric (NCF) is immersed in the electro-activated deposition solution which contains polymer with epoxy group. Then, by energization, the resin precipitates around the carbon fiber and is efficiently impregnated to the NCF in the solution. After the ERM, CNF can be applied on the surface of CFRP since the electro-activated deposition solution is water-based. After thermal curing, CNF is embedded on the surface resin layer of CFRP. In this study, the analysis to understand the effect of CNF on the mechanical properties was carried out. As a practical application, the I-shaped cross-sectional beam (I-beam) was selected for the analysis. It was experimentally revealed that the bending strength of I-beam was enhanced by embedding of CNF. With assumed the material constants based on the tensile tests of the CFRP, the bending properties with dispersing CNF on the surface were analyzed by using the finite element method. As a result, the relationship between load and deflection could be well simulated, and the effect of CNF on the bending properties was clarified.
Design and Analysis of a Tool for Automated Fiber Placement of Composite Wind Tunnel Blades
For decades, composite wind tunnel blades have been constructed using laborintensive hand layup processes. Automated Fiber Placement (AFP) is a relatively new technology that has not seen widespread use in manufacturing composite wind tunnel blades. AFP offers the potential for reduced manufacturing time, reduced cost, and improved consistency compared to traditional hand layup procedures. These manufacturing qualities are becoming increasingly important as existing wind tunnel blades are replaced due to impact damage or wear and tear from decades of use. Researchers at the NASA Langley Research Center are currently using the Integrated Structural Assembly of Advanced Composites (ISAAC) facility to investigate the feasibility of AFP for manufacturing composite wind tunnel blades. ISAAC uses manufacturing tools as surfaces on which to place fiber tows. A manufacturing tool was developed for using AFP to build a wind tunnel blade shape. This paper presents the structural design and analysis of this tool, for use with AFP, constructed from high temperature thermoplastics. The tool fabricated by this design process was successfully used for AFP and oven cure of two representative airfoil shaped composite structures.
A Novel Reinforcement Characterization Framework for Liquid Composite Molding Processes
Liquid Composite Molding (LCM) processes are widely used manufacturing techniques in many industries including, aerospace, automotive, and marine. In order to create quality parts using LCM processes, reinforcement characterization such as, permeability and compaction response need to be performed fast and accurately. In this work, we propose an experimental and numerical reinforcement compaction and permeability characterization framework which can potentially reduce manufacturing cost by lowering material consumption and labor time and effort. The framework is non-destructive in nature, which uses in-situ X-ray micro computed tomography (XCT) experimental set-up to obtain reinforcement compaction response at different fiber volume fractions. In the numerical step, the preform permeability is predicted through computer simulations using computational unit cells extracted from the reconstructed 3D models from XCT scans. The proposed approach has the capability of obtaining a plethora of information about the preform including, compaction response, statistical measurements of the internal preform geometry, and voxel models for numerical simulations. The methodology has been successfully demonstrated using a 3D angle interlock fabric. The method has been validated by comparing the computed permeability values with benchmark experimental data with excellent agreement.
Effect of Debulking Cycles on the Fiber Volume Fraction of 3D Fabrics for Vacuum Assisted Resin Infusion Process
Liquid composite molding (LCM) processes are considered one of the costeffective out-of-autoclave manufacturing techniques with the capability of producing medium to large size composite structures. The vacuum assisted resin transfer molding (VARTM) or the resin infusion process belongs to the LCM family, with the ability to produce largescale structures using only vacuum pressure. Due to high through thickness stiffness of 3D reinforcements, achieving desirable high fiber volume fraction using the VARTM process is challenging. De-bulking before resin infusion is a common process for 2D fabrics, however single de-bulking cycle may not have significant impact on improving fiber volume fraction of 3D fabrics. Multiple cycle compaction of 3D fabrics can improve fiber volume fraction significantly. In this study, we investigate the effect of cyclic loading on fiber volume fraction, internal fabric structure, and permeability of two different types of 3D fabrics. We have utilized X-ray micro computed tomography (XCT) technique to characterize the 3D reinforcements. An in-situ compaction fixture was used to compact the reinforcements to target thickness through cyclic loading. The internal reinforcement architecture changes were captured using 3D image slices obtained using the XCT machine. The permeability values were computed using the virtual voxel models generated from the XCT images. The results show that 5-10% increase in the fiber volume fraction of 3D reinforcements was achievable after multiple de-bulking cycles. The peak stresses decreased significantly after first few cycles and became repeatable after cycles greater than five. The peak stress after 15th cycle was found to be approximately half the peak stress obtained after first cycle. The de-bulking cycles improved fiber volume fraction considerably, however the permeability decreased as the fiber volume fraction was increased.
Investigation of Sample Geometry and Strain-Rate Dependence in Shear Characterization of a UHMWPE Unidirectional Cross-ply for Finite Element Simulation of Composite Processing
In-plane shearing is the dominant mode of deformation in many composite forming processes, including thermoforming. The shear-frame, or picture-frame, test is a widely accepted method to characterize the shear behavior of a material system to find the properties to use in forming simulations, and this test has been shown to be applicable for providing shear stiffness as a function of the state of shear for a variety of woven-fabrics based material systems. The current research explores the use of shear-frame testing of a non-woven material system made of Ultra High Molecular Weight Polyethylene (UHMWPE), specifically DSM Dyneema® HB210 unidirectional fiber cross-ply. The material system was characterized at an elevated temperature for processing applications. The effects of sample size and sample geometry were investigated. The load contribution from the sample arms is of particular interest for these types of materials, so an investigation of appropriate gage area and normalization methods was performed. Finite element simulations of the shear-frame test were completed to validate the characterization methodology. Also, during the processing of UHMWPE material systems into complex shapes, the rate of shear in different sections of the part will differ. Therefore, the effects of strain rate on shear characterization were explored by performing constant strain-rate shear frame testing over a range of shear rates. In a typical shear-frame test, the crosshead rate remains constant which leads to an increasing shear strain rate. The decreasing crosshead rate required to maintain a constant shear rate was obtained and implemented into the testing software. Representative shear curves were examined to determine if rate-dependence is required in a finite element simulation of the thermoforming process.
Analysis of Spring-in for Composite Plates Using ANSYS Composite Cure Simulation
Process induced dimensional changes in composite parts has been the topic of interest for many researchers. The residual stresses that develop in fiber-reinforced laminates during curing process while the laminate is confined to the process tool often leads to dimensional changes such as spring-in of angles and warpage of flat sections. Many experimental studies have put emphasis on this issue and various researches show different methods to predict these dimensional changes. The traditional trial-and-error approach can work for simple geometries, but composite parts with complex shapes require more sophisticated models. When composite laminates are subjected to thermal stresses, such as the heating and cooling processes during curing, they can become distorted as the difference between the in-plane and the through-thickness thermal expansion coefficient, as well as chemical shrinkage of the epoxy, causes the enclosed angle of curved sections and angle components to be reduced. Distorted components can cause problems during assembly, significantly increasing production costs and affecting performance. This paper focuses on predicting these shape deformations using software simulation of composite manufacturing and curing. Various factors such as resin shrinkage, degrees of cure, difference between coefficient of thermal expansion of mold and composite are taken into consideration. A cure kinetic model is presented which illustrates the matrix behavior during cure.
Multi-Scale and Multi-Physics Finite Element Modeling of Oxidation Degradation in C/C Ceramic Matrix Composites
Fiber reinforced ceramic matrix composites (FRCMCs) have been shown to retain exceptional thermal and mechanical properties at high temperatures in inert atmosphere. However, FRCMCs like carbon fiber reinforced carbon ceramic matrix composites (C/C CMCs) degrade rapidly at temperatures as low as 450oC in oxidizing environments due to the conversion of solid carbon to gaseous oxides of carbon. With a rising demand for composite material structures to perform in extreme environmental conditions involving high temperature gradients, the prediction of oxidation damage is crucial in designing such structures. In this paper, a multi-scale finite element analysis framework is presented to study the oxidation behavior of ceramic matrix composites.
Recycling of Disposed CFRP by Undergraduate Research Programs
In the present paper, we demonstrate the engagement and the achievements of the undergraduate engineering students in research through different undergraduate research programs offered by internal university fund as well as through external sponsors. The aim was to encourage the students to develop their technical and research skills through participating in this unique opportunity under the supervisor of a principal investigator. Two groups worked successfully on recycling waste CFRP scrap material produced by domestic industries in order to investigate the potential of the waste to produce helpful products through understanding the mechanical properties of the recycled material. Hence, trying to come up with innovative solutions to resolve the undesirable and the negative impact of the recycled waste on the environment and to comply with the local legislation that enforces waste producers to handle the waste of their industries.
Direction-Dependent Mechanical Properties of a Triaxially Braided Quasi-Isotropic Carbon/Epoxy Composite
Triaxially braided fabrics with equal volume fractions of fibers oriented at 60- degree intervals allow for the quick and easy hand-layup of quasi-isotropic (QI) composite laminates, in comparison to unidirectional tape and biaxial-woven fiber architectures. The material system for this study, T700S/MTM45-1, is the first material used in full-scale testing to demonstrate lightweight aircraft fuselage shielding for openrotor engine applications. The objective of the current investigation is to measure the in-plane tensile modulus, Poisson’s ratio, failure strain, and tensile strength of a triaxially braided QI T700S/MTM45-1 carbon/epoxy laminate. Straight-sided specimens were cut from panels with the axial tow of the fabric oriented at angles of 0º, 30º, 45º, 60º, and 90º relative to the loading direction. Results were compared to previously published test results for composites consisting of the same fibers and fiber architecture but with a different (PR520) epoxy resin. Test results showed a considerable variation of strength with in-plane orientation. The initial elastic modulus and ultimate strength of the T700S/MTM45-1 material were also analyzed using classical laminated-plate theory (CLPT) and ply-discount (PD) to account for matrix cracking. As a predictive model, the quasi-laminar CLPT/PD model accurately predicted modulus but over-predicted strength. Using the model to back-calculate the best fiber-direction ply-level properties in the axial and bias directions of the braid indicated that significant adjustments of the modulus and strength can be implemented to match the initial experimental modulus and strength for nearly all specimen orientations. Reductions in these properties relative to the equivalent unidirectional tape properties were greatest in the plies representing the bias tow plies, indicating a greater degree of undulation in the bias directions due to the manufacturing process.
Effects of Off-Axis Angle and Layer Thickness on the Compressive Strength of Cross-Ply CFRP Laminates
The effects of off-axis angle and layer thickness on the compressive behavior of cross-ply carbon fiber reinforced plastic (CFRP) laminates were examined through experimental and numerical approaches. Two kinds of cross-ply carbon fiber/epoxy laminates, namely [90°/0°]4s, and [90°2/0°2]2s were prepared. The layer thickness was 0.14 mm for [90°/0°]4s, and 0.28 mm for [90°2 /0°2]2s, respectively. The off-axis angles were 0°, 2°, and 4°. Compressive tests were conducted in accordance with ASTM-D6641. The compressive strength and the longitudinal elastic modulus decreased with increasing the off-axis angle. Layer thickness had a little effect on compressive strength. As numerical approach, finite element analysis (FEA) was conducted by employing the Christensen’s failure criterion. The numerical results imply that application of the Christensen’s failure criterion to FEA be able to estimate the reduction in elastic moduli and compressive strength with increasing off-axis angle. Contrary to the experimental results, compressive strength estimated by FEA increased with increasing layer thickness. This implies that the dependence of fiber-direction strength on layer thickness should be taken into consideration for FEA.
2-Dimensional Mapping of Damage in Moisture Contaminated Polymer Composites Using Dielectric Properties
Polymer composite materials are increasingly being adopted in civil infrastructure, oil & gas, marine, automotive and aerospace industries. Achieving ubiquitous adoption would require new advances in development of non-destructive examination methods which are cost effective relative to composite structures under test, simple to utilize and sensitive enough to ascertain the structural integrity of composite parts, especially in the absence of visible damage. This study proposes a relatively simple method for non-destructive testing of polymer composites that detects 2-dimensional micro to nano-scale damage by analysis of the effects of polymer-moisture interactions on microwave-frequency dielectric properties. In this study, we apply this method to mapping of internal damage in two widely used aerospace composite laminates; a 12-ply bismaleimide/quartz and a 16-ply epoxy/7781 fiber glass composite laminate. Specimens are dried and then immersed in deionized water, to simulate long term exposure to humid environments while measuring gravimetric moisture uptake. Localized damage is induced in the laminate specimens via low velocity impact damage of 5 and 9 Joules. A split post dielectric resonator coupled with a vector network analyzer is used to determine the spatial variation in relative permittivity across the composite laminate. Generally, results show significant increase in relative permittivity towards the center of impact damage compared to surrounding undamaged areas. This increase is indicative of internal damage as a result of micro-crack formation around the point of impact. This new free volume in the damaged area is primarily occupied by free water; driving a local increase in the relative permittivity in the damaged area due to the locally-higher ratio of free to bound water. Due to the tendency of polymer composites to absorb measurable absorbed moisture in almost all environments, this relatively simple non-destructive examination method using water as a type of “imaging agentâ€ demonstrates considerable promise for early detection of damage.
In-Plane Shear Response of GFRP Laminates by ±45º and 10º Off-Axis Tensile Testing Using Digital Image Correlation1
For the determination of the shear strength, strain and modulus of reinforced composites, nearly a dozen measurement techniques exist, several of which are supported by ASTM or ISO standards. One reason for so many options is the difficulty in obtaining a reasonably pure and uniform shear stress state in the test specimen. Typically, linear strain gauges and rosette strain gauges have been used to measure deformations for the determination of the elastic modulus and the strain to failure in many of these methods. The advent of optical strain measurement techniques such as digital image correlation (DIC), provide new opportunities to generate high resolution maps of the shear strain field as a function of the globally applied strain. DIC allows for the full-field mapping of the deformations (in-plane and out-of-plane for stereo DIC) and strains (normal and shear), revealing the pattern of deformation and damage throughout the specimen. In this work, it is shown that non-contact measurements of the strain field are an important enhancement to measuring the shear properties of FRP composites. This contribution describes the use of DIC for the shear strain response while performing standardized bias extension tests on ± 45° layups and non-standardized 10° off-axis tensile tests on glass fiber reinforced (GFRP) coupon specimens. Shear moduli and shear strengths obtained will be compared to results using the V-notched method.
Notched Composite Laminate Design for Improved Compression Strength Using Non-Standard Angles and Customized Stacking Sequence
This paper compares the use of four standard ply angles [0°, +45°, -45° and 90°] in composite laminate design with non-standard ply angles for reduced ply thickness and improved notched compressive strength. This paper describes strength improvement through customized stacking sequence by placing higher-angle plies on the surface and lower angle plies at the core. Open-hole compression testing shows customized stacking sequences improve open-hole compression strength of both standard and non-standard ply angle designs by up to 33%. This improvement linearly decreases with surface ply angle.
Characterization of Interface Shear Stress-Strain and Mode II Fracture Properties Using DIC in Thick Adherent Shear Tests
Progressive damage and failure analysis tools for fiber reinforced laminated composites are sensitive to material input parameters for a wide range of failure modes. Without accurate stress-strain and fracture properties, these analyses tools will not be effective. Delaminations are the critical failure mode for this study’s focus, which aims to develop a method to measure interfacial stress-strain and fracture behavior in mode II, increasing confidence in the material input data for these failure models. A direct method of measurement is proposed to observe interface behavior under loading and fracture. High-resolution Digital Image Correlation techniques are used to measure deformations along the bondline in thick adherend shear specimens, which provide the necessary data to characterize the interface behavior. By using constrained optimization techniques on overdetermined data, the material behavior may be solved without the need for closed-form solutions nor relying on expensive iterative finite element analysis. A virtual study in the finite element environment is performed to verify the method, after which the method is applied to carbon fiber skins co-cured with an adhesive layer.
A Novel Method to Measure Laminate Shear Modulus Development of Interlayer Toughened Composite Laminates During the Curing Process
There are established process models for thermoset composites to calculate residual stress development and process-induced deformations. For thermoset composites, laminate elastic constants during cure are required for the process simulation. Matrix material properties such as bulk modulus and shear modulus develop continually as cross-linking of polymer chains progresses. Elastic constants of laminates during cure are determined from material properties of fiber and matrix, using micromechanics models. Recently, interlayer toughened composites have attracted considerable interest because of their high delamination and impact resistance. A third phase, such as thermoplastic micro-particles, is incorporated in the interlayer, which increases the complexity of the laminate microstructure. Moreover, it is difficult to measure the material property evolution of the toughening particles experimentally. The complexity of the laminate microstructure and the lack of material data makes it difficult to calculate the elastic constants of laminates using standard micromechanics. Reliable methods to measure the elastic constants of these more complex laminates are required. In this paper, we present a new experimental method based on Timoshenko beam theory to determine the laminate shear modulus (G13) development during cure in a Dynamic Mechanical Analysis (DMA) equipment with a low-friction three-point bending clamp. G13 development of an interlayer toughened prepreg system, 3900- 2B/T800SC, was successfully determined by the proposed method. The influence of interlayer particles and the frequency on the laminate shear modulus development was studied. This method can be used for studying the processing behavior of more complicated laminates, validating new micromechanical models and improving the accuracy of processing simulation.
Structural Re-Use of De-Commissioned Wind Turbine Blades in Civil Engineering Applications
The production of wind energy worldwide has increased 20-fold since 2001. Composite material wind turbine blades, typically designed for a 20-year fatigue life, are beginning to come out of service in large numbers. In general, these decommission blades, composed primarily of glass fibers in a thermoset matrix, are demolished and landfilled. There is little motivation for recycling the composite materials, as the processes for reclaiming the fibers (solvolysis, pyrolysis) have not been proven to be economically viable. This research seeks to establish structural reuse applications for wind turbine blades in civil engineering infrastructure, hypothesizing that advanced composite materials may be an attractive alternative to conventional infrastructure materials (e.g. steel, reinforced concrete). This paper presents an analysis and materials characterization of a 47 meter Clipper C96 wind blade. The primarily numerical analysis is accompanied by materials characterization taken from an un-used Clipper blade donated to the project from the Wind Turbine Testing Center (WTTC). The paper presents a brief background on wind turbine blade adaptive re-use, proposing a hypothetical load bearing application of the Clipper wind blade as an electrical transmission tower structure carrying axial compression, along with flapwise and edgewise bending forces. The paper summarizes the composite laminates and cross-section geometries of the blade and establishes the axial and flexural stiffnesses of the blade at multiple sections along the blade length. From a first-order estimation of applied loads for the tower application, the resulting stresses in the composite materials are estimated and compared to the design material properties for the wind blade as originally constructed.
Strength Reduction of Unidirectional Composites Using Micromechanical Damage Analysis of Scanned Microstructure
The microstructure of composites has a direct effect on the macroscale mechanical properties, local-global responses, and damage evolution. Therefore, accurate depicting and modeling of the composite’s microstructure is of great importance. Micromechanical methods usually employ an idealized repeating unit cell (RUC) in order to capture the effective properties of the composite material. For a unidirectional composite, a square array or hexagonal array are typically used to represent the microstructure where the decision on which one to choose is guided by acquired images of the fiber assemblage. While the experimental results for the average stiffness and stress-strain response of composites can be predicted with high accuracy using these idealized RUCs, the strength-prediction of such composites is far a more challenging task. Towards this goal, this study presents an extensive inquiry of the microstructure of unidirectional composite that has been made using scanning-electron-microscopy (SEM) combined with optical methods for the IM7/977-3 carbon-epoxy material system. Crucial characteristic phenomena of the microstructure imperfections, such as out-of-plan waviness of the laminae, fiber distribution, and large inter-laminae/interlaminar resin pockets, have been identified. In addition, modeling of the accurate microstructure taken out of SEM scan has been made using the parametric high-fidelity-generalized-method-of-cells (HFGMC) micromechanical model. Using the scanned microstructure of the composite enhanced stress concentration under transverse and shear loading, especially in the matrix region between two or more close fibers. These phenomena are shown to directly affect the damage evolution at the microscale as well as the strength of the entire composite.
Characterization of Discontinuous Fiber Reinforced Sheet Molding Compounds Under Tension-Tension Fatigue Load
Sheet molding compounds (SMC) have long been successfully used for nonstructural applications, as for example in the automotive industry. However, the prediction of their fatigue behavior is still proving difficult. This is due to a variety of possible material compositions and their complex microstructure resulting in a complex damage evolution. Comprehensive experimental investigations are still necessary to understand the damage evolution and its impact on the macroscopic material behavior during cyclic loading. In this work the fatigue behavior of discontinuous glass fiber reinforced sheet molding compounds (SMC) was investigated in a load-controlled tension-tension fatigue test for different loads. The material is based on an unsaturated polyester polyurethane hybrid (UPPH) resin system without fillers. The reinforcing glass fibers feature a length of 25.4 mm (1 inch). The macroscopic material behavior shows statistical scattering in the range of two times the power of ten. Due to the manufacturing process, the fiber orientation distribution and volume contents vary within one SMC plate, which influences the fatigue life. The evolution of stiffness was determined for each test. Characteristic stages of damage can be clearly distinguished by means of stiffness reduction. Digital images were captured during fatigue loading to visualize local damage without interrupting the test by triggering the camera via the load signal. The dominant failure mechanisms were matrix cracking and interface failure. These can be observed long before final failure. The results allow a good description of the fatigue behavior throughout all stress ranges and present a good basis for further investigations
Nondestructive Evaluation of Randomly Distributed Fiber Reinforced Composite Using Optical Transmission Scanning (OTS) Technique
In this paper, the quality of randomly distributed glass fibers reinforced composite material with the presence of intentionally introduced defects was experimentally studied using novel optical transmission scanning (OTS) system. Results showed that OTS successfully identified not only existing defects but also was able to visualize the uniformity of randomly distributed fibers within the laminate. Moreover, the high lateral resolution of OTS allowed evaluating the direction and the size of separate chopped fibers. Combined tensile tests and OTS inspection revealed that depending on the locations of defects and unevenly distributed fibers, the strength of the material was varied considerably. Generally, current work could be considered as a potential solution for performance prediction in lightweight structures applications.
Determination of In-Plane Shear Properties of UHMWPE Composites for Input Into a Thermoforming Model
Thermoplastic composite materials can be molded into complex curvature shapes via thermoforming, where the material is heated to a pliable temperature and then formed into the desired configuration. For instance, a hemispherical composite part can be thermoformed by punching a male hemispherical tool into a flat stack of heated Ultra-High Molecular Weight Polyethylene (UHMWPE) composite sheets held in place by a binder ring. The quality of the as-manufactured part is dependent on many variables including the punch rate, friction between the composite and tooling, binder ring pressure and the material properties. The in-plane shear constitutive response of the composite is a critical input for the numerical simulation of this process, as it will dictate how easily the material conforms to the prescribed shape. Achieving a hemispherical shaped part with minimal wrinkling is the desired outcome. However, experimentally investigating the optimal set of processing parameters to achieve this outcome is time consuming and costly. The goal is to create a computational model that can accurately simulate the thermoforming process and predict the end product quality. This will allow for rapid evaluation of optimal process parameter combinations, greatly reducing experimentation time and cost. Inplane shear characterization of DSM Dyneema HB 210 is performed at relevant process temperatures between 20°C and 130°C, fiber rotational rates (0.1 ï‚°/s, 0.7 ï‚°/s, 7 ï‚°/s) and pre-processing states (unconsolidated [0/90]2 sheets vs. consolidated [0/90]2 sheets) to provide constitutive material response input for the computational process model. Biased extension tests show that an elevated temperature leads to an increase of the in-plane shear compliance and that rate effects are more pronounced at a lower temperature. Consolidated specimens were found to require more applied force to deform than unconsolidated material. Fiber rotation is upwards of ± 35° for all specimens, regardless of the configuration. LS-DYNA finite element simulations are conducted using select in-plane shear response inputs generated from the experimental tests to demonstrate the effect of the in-plane shear response on the thermoforming process, the as-manufactured part thickness, and in-plane shear distributions.
Predicting Decomposition Processes of Fiber-Reinforced Polymer Matrix Composites Heated in N2 and Air Environments
Fiber-reinforced polymer matrix composites (PMCs) subjected to widely varying temperatures and heating rates in service and manufacturing may undergo decomposition processes in constituent matrix and fiber materials. Prediction of the thermal responses of PMCs requires accurate input parameter assessment derived from reliable thermal property characterization and decomposition kinetics. A methodology was developed to uniquely determine unified kinetic model parameters (UKMPs) for predicting multistage thermal decomposition kinetics of PMCs. The UKMPs were used to accurately predict thermal decomposition processes of T650/5250-4 woven composites heated in both N2 and air environments over the entire range of experimentally measured temperatures and heating rates.
Bio-like Composite Microstructure Designs for Enhanced Damage Tolerance via Machine Learning
Variations of unique and tailored composite microstructures have been observed in nature and have served as templates for the development of new synthetic materials. Microstructures are studied in fish scales for their penetration resistance, in spider webs for energy absorption, and in seashells and bone for their strength and toughness. However, it has proven difficult to reproduce the properties found in natural materials due to the interaction between the intricate structures at different length scales. Rather than attempting to replicate these materials (biomimetics), the focus of this work is to use a bio-inspired pattern generation algorithm to search for new topologies that outperform traditional composite structures due to their naturelike design. The bio-inspired pattern generation algorithm employed in this research is known as the Gray-Scott model. This model was selected due to its unique ability to manufacture patterns that propagate with time, allowing the reinforcement volume fraction of the composite structure to be controlled. The model is capable of producing Turing patterns, propagating wave fronts, homogeneous oscillations, and chaos. Traditionally, Turing models have been primarily studied for their applications in morphogenesis and pattern development. However, this research extends the application of the Gray-Scott model by investigating the patterns as physical load bearing structures. A methodology was developed by which the patterns can be converted to structures, analyzed for a desired mechanical property, and optimized via Bayesian machine learning algorithms that yield an improvement of the average quality of structures produced by nearly a factor of 10.
Optimization of Flax Roving for Bio-Based Composites Reinforced by 3D Warp Interlock Fabric
Flax is a natural fiber with a growing industrial interest. Technical textiles and composites industries are increasingly fond of this type of fiber, with good mechanical properties. The aim of this study is to characterize 3D warp interlock fabric, from the raw material, here a flax roving. After a brief introduction of this type of fabric, structural and mechanical properties will be presented. Results show that properties at yarn scale is related to the results at fabric scale. It also demonstrates that structural and mechanical properties depend on the choice of binding and number of layers of the fabrics.
Bio-Industrial Materials Institute: Characterization of Natural Fiber Material Property Variability
To become a more competitive alternative to synthetic fibers, the variability of natural bast fiber material properties must be understood. Standardizing the relationship between these fibers and their associated material properties will broaden the range of applications to include more technical industries. Initial tensile testing showed variability within all bast fibers, and flax was used to investigate methods to reduce variation. Single strands of fibers were selected, tabbed, and then placed in a fiber dimensional analysis system to record their diametral geometry. Observations for various physical defects were catalogued before tensile testing the fiber. Material property data were filtered according to defects in the fiber’s observed physical characteristics. High variation of fiber mechanical properties was reduced by accounting for various physical defects including kinks, geometric inconsistencies, fraying, and transient data due grip failure or unexplained loss of load carrying capability. While fiber color was not found to significantly affect the strength or modulus of the fiber, darker hemp fibers were found to have reduced standard deviation when compared lighter or spotted fibers. Based on these results, microscopy was performed to assess fiber constituents. Comparing modulus with lignin content for bleached flax fibers, a slight increase in modulus was noted as the percent area of lignin increased. Excepting this weak trend, no other trends could be established between the lignin content and either tensile strength or modulus in any of the other natural fibers. These results conflicted with literature studying agave plant fibers, wherein cellulose-to-lignin ratios were associated with higher tensile strength values. As such, future work is anticipated to better understand the fiber constituents toward characterization as well as investigate effects of decortication to maintain structural quality and length continuity of bast fibers.
Nanocellulose Fiber Sizing for Fiberglass Composites
The transportation industry produces the most greenhouse gas emissions of any economic sector in the United States due in large part to the burning of fossil fuels in road, rail, air, and marine transportation. Industry-wide initiatives to combat these emissions have resulted in increasing use of lightweight materials in automotive production, such as fiber-reinforced polymer composites, for the purpose of improving vehicle fuel economy. However, the displacement of heavier metal materials in these manufacturing processes is limited by the failure of existing composite technologies to exhibit a favorable balance between weight, cost, and performance. Enhancing these qualities is key to developing the materials that will be used in the lightweight cars of the future. A promising approach involves the incorporation of high-performance nanomaterial additives into the composite system. Some success has been achieved in this regard using cellulose nanocrystals (CNC), a lightweight, renewable biomaterial derived from plants. However, proposed methods of CNC composite development are limited in their scalability due to demanding processing conditions, as well as the tendency of CNC to aggregate and lose their high inherent properties in polymeric matrices. A processing path that utilizes the strengths of CNC while maintaining scalability is thus highly desirable. In this study, we have developed coating solutions for glass fibers that contain CNC. Incorporation of CNC in the fiber sizing potentially improves scalability by circumventing the need to process the CNC in the epoxy matrix. Improvements in composite strength were monitored using the single fiber fragmentation test to assess the strength of the fiber-matrix interface. Observed changes were hypothesized to be the result of alterations to fiber surface characteristics such as coating thickness, morphology, and surface energy. Tests for these parameters revealed that CNC strengthen the interface, and this phenomenon is likely related to improvements in spreading behavior.
Bio-Based Mycelium Materials for Aerospace Applications
Motivated by the growing need for sustainable material options, this research focuses on exploring bio-based composite materials made from the mycelium of fungi, which can offer a zero-waste, circular materials solution. It explores how mycelium composite materials can be formed by fungi that can digest a range of carbon-containing wastes created within the aerospace industry, and convert them into useful materials for a variety of applications with the aim to significantly reduce the environmental impact of non-structural aerospace materials. By careful selection of the fungi species, control of the feedstock, growing conditions and integration with other materials, the resulting bio-based composite material can be tailored to a specific application, such as low-density insulation for use within the aircraft cabin with stiffened parts based on generative designs. In addition, a range of possible aerospace wastes and applications were considered where fungi could provide a safe and circular remediation solution, such as oils, urethane foam and other carbon-containing materials. This research aims to leverage the development and adoption of bio-based, waste-digesting mycelium materials for non-structural aerospace applications, and aid efforts to reduce the future environmental impact of aerospace industry materials.
Manufacturable Designs for Optimized Buckling Performance of Flat Panels Using Steered Fibers
Automated fiber placement (AFP) is an additive manufacturing modality. It is a robotic manufacturing technology to manufacture CFRP laminates by using slit tapes of pre-preg material. AFP is suitable for manufacturing large structural parts (wing skin panels, ribs, fuselage sections etc.) that can have curvatures. The advent of AFP technology has revolutionized composite manufacturing by improving production rate, decreasing the cost and scrap. One other notable advantage of AFP is that the designer is no more restricted to use straight fiber paths, which means it is possible to have fiber paths designed for optimizing structural performance. The scope of this work is to identify the manufacturing parameters and associated constraints imposed by these parameters on the manufacturability of steered fiber structural panels, using currently available AFP machines and to incorporate these constraints into deriving optimal paths to maximize the buckling load. A square laminated plate under compressive in-plane loads is used to illustrate the results
Buckling Analysis and Optimization of Stiffened Varying-Angle- Tow Laminates with Manufacturing Constraints
The variable-angle-tow (VAT) fiber path laminates are known to shift the maximum in-plane stress resultants away from the composite laminated panel’s center to the panel’s supported edges for improving the buckling response when compared to the traditional laminates. However, the improvement in the structural performance could be counteracted due to the fiber placement head turning radius and gaps/overlaps induced thickness variation in the VAT manufacturing process. The present paper investigates the effect of the minimal turning radius on the buckling load of a stiffened VAT laminated plate subjected to an axial in-plane end-shortening. Different laminate fiber path angles are parameterized and studied. Optimization studies show that when there is no manufacturing constraint, there is a significant increase in the buckling load, up to 20.5% and 44.0%, respectively, by using the stiffened VAT laminates with linearly and nonlinearly varying fiber path angles. The optimal linearly- and nonlinearly-varying fiber paths are found to be almost perpendicular to the stiffeners in the panel’s center and almost be parallel to the stiffeners near the panel’s free-load support edges. The present minimal fiber placement head turning radius has a significant effect on the buckling load improvement for the stiffened VAT laminated plates. There are only 6% and 7% increases in the total buckling load for the present stiffened composite plates with linearly- and nonlinearly-varying fiber path angles laminates, respectively, when considering the fiber curvature constraint.
Numerical Studies on Buckling and Post-Buckling Behavior of Stiffened Curved Composite Panel with Repairs
Composite stiffened panels are widely employed in the aircraft industry. In particular, curved panels stiffened with hat-stringers are utilized in fuselages. The panels comprise of very thin skin to which stringers are attached (cocured/ co-bonded or secondary bonded). The panels are prone to buckling phenomenon when subjected to compression, and other complex loading scenarios. The stiffened panels may be subjected damages induced by low velocity impacts, such as tool-drop. Such a scenario may yield to local reduction in the stiffness, and especially affect post-buckling response of the structure. The primary goal of this research is to develop numerical tools to study buckling and post-buckling response of stiffened composites panels containing repair. Two types of repair are considered: secondary bonded and co-bonded. It is shown that, both repair types are able to restore the panel strength, and influence the post-buckling behavior. The presented finite-element based numerical models of the repair panels are able to capture pre- and post-buckling response. The presented tool is encapsulated as a Abaqus plug-in. The presented methodology paves way
The Effects of Circular Cutout on Buckling Analysis of Laminated Composite Plates Using FEM
All work done in this paper covers the analysis on the plate scale, which will be a reference to the composite radome shell. This study presents the effects of a cutout on the buckling behavior of symmetric cross-ply (0,90)s and symmetric angle-ply (45,- 45)s types laminated composite square plates using Finite Element Method (FEM) based on Equivalent Single-Layer Theory (ESLT). These special orthotropic laminated composite plates were examined under the uniaxial compressive loading using Composite Materials Module of COMSOL Multiphysics Software. Structural analyses were performed in order to predict the effects of a circular cutout size, location of cutout, plate thickness (t), aspect ratio (proportion of hole diameter-(d) to edge length-(b) of plate), and various boundary conditions (simply supported and clamped). Convergence study has been carried out by comparing the results obtained from the numerical results of previous work available in the literature. Some general results have been obtained depending on the variables examined. In the buckling load results of three different mesh sizes, it was seen that the critical buckling load converged more as the mesh size decreased. We found that the plates which were simply supported against the reduction in the critical buckling load on the square plates containing cutouts were more sensitive than the clamped plates. In cases where the cutout diameter was constant, it was seen that the increase in the edge length of the square plate caused a decrease in the buckling load.
Design, Manufacturing and Testing of an In-situ Consolidated Variable Stiffness Thermoplastic Composite Wingbox for Bending and Torsion
Automated fiber/tow placement (AFP/ATP) techniques can provide cost-effective, reduced waste, and repeatable high rate production of laminated composite structures with some unprecedented capabilities such as in-situ consolidation and fiber/tow steering. In fiber steering, the fiber orientation angles are continuously changed in-plane to make composites with optimum path between the loading points and supports in a structure. Thermoplastic composites (TPC) have gained increasing interest from the aerospace sector for their increased formability, weldability, recyclability and their superior toughness and fatigue performance in comparison with their thermoset counterparts.. Their potential for out-of-autoclave manufacture (OOA) also makes them an excellent candidate for low cost production of large structures for aerospace applications. To investigate the potential for aerospace structures, an OOA variable stiffness, unitized, integrated-stiffener thermoplastic wingbox demonstrator was recently built at the University of Limerick. A number of different techniques were targeted: fiber steering to achieve the variable stiffness skin, in-situ consolidation to weld the stiffeners to the wingbox skin, and as manufacturing methodology a laser-assisted tape placement (LATP) in combination with winding was used. In the design process, the wingbox loads were determined by assuming its location to be at 85% of the wing semi-span of a B737/A320 size aircraft. Previously we reported on a full-scale structural test using a bespoke testing frame with representative bending moment and shear load. Indeed, the wingbox buckled elastically at a load close to that predicted numerically. Our results highlight the potential advances that become possible in primary aerospace structures by combining fiber steering and in-situ consolidation of carbon fiber thermoplastic composites together with new blended, unitized structural concepts. Herein, we report on the results of a new test that include torsion as well as bending moment and shear. An overview of the design against buckling, manufacture and structural testing for load carrying capability is given.
Modeling Complex Sub-Elements by Using Discrete Damage Modeling
Discrete Damage Modeling is applied to complex sub-elements loaded in static and fatigue. Layered 8-node brick elements called LC3D8 elements were implemented for the first time in DDM models to reduce the computational requirements for modeling large and complex parts. Three sub-elements were tested and modeled: Fastener Pull Through (FPT), Hat Pull Off (HPO), and Three Point Bend with Flange (3PB-F). All three sub-elements were tested and simulated in quasi-static loading, while the 3PB-F was simulated in constant amplitude fatigue as well. Results demonstrate that DDM is capable of capturing peak load response accurately and damage initiation locations fairly accurately even with relatively coarse meshes.
Exploring the Use of a Digital Thread for Efficient Design of Composite Structures
Damage tolerance is very difficult to quantify for composite structures early in the design process, due to the lack of mature predictive computational tools. This uncertainty adds risk and can result in overly conservative designs. In this paper, a framework for optimizing a laminate for damage tolerance is proposed as a part of the conceptual design stage given in the context of the digital thread. The framework is demonstrated with uncertainty quantification on a simple laminate using a finite element model sampled using a polynomial chaos expansion technique
Epoxy Resin (DGEBA/TETA) Under Extreme Environment
Epoxy resin (DEGBA/TETA), used as a major ingredient for manufacture of composite and sandwich composites may be subjected to different extreme environment situations such as that of cold weather conditions. In this study, samples were exposed to cold weather conditions and FTIR was done to probe changes in the molecular structure of the material which might provide an answer for the brittleness observed in the material
Mitigation of Mechanical Property Degradation of Epoxy Resin Subjected to UV with Addition of Different Nanofillers
Epoxy resin is used as binder material for various composites which are used in aerospace, naval and space vessels. Epoxy resin has excellent adhesion properties but it is well known that they are brittle in nature and show poor resistance to crack initiation and crack propagation [1]. To address this issue different nanofillers are added to observe the changes in the mechanical strength of epoxy before and after exposure to UV for an extended period of time.
Cavitation Erosion of Polyurea Composite Coatings
Polyurea (PU) was synthesized by reacting oligomeric poly (tetramethylene oxide) (PTMO) diamines (Versalink, Air Products) with aromatic 4,4’- Methylenediphenyldiisocyanate (Isonate 143L Modified MDI, Dow Chemicals). Two Versalink prepolymers were used: VP-1000, VP-650, where 1000, and 650 refer to the molecular weight of the PTMO repeating units. PU-105 and PU-605 were synthesized by mixing VP-1000 and VP-650 with Isonate 143L. It has been observed that the temperature within the polyurea coating under cavitation erosion could increase to a considerable amount, resulting in degradation of its mechanical properties and an accelerated onset of failure. To improve the relevant thermo-mechanical properties of polyurea, milled carbon fibers of 3% and 10% volume ratios were added to the PU- 605 matrix. In the present work, accelerated cavitation erosion tests based on ASTMG32 standard were performed to study the effect of substrate, coating thickness, and formulation on cavitation resistance. The substrates of interest were aluminum and carbon fiber reinforced polymer (CFRP) plates. The experimental investigations were done on two coating thicknesses, 0.5 mm and 1 mm. Surface roughness measurements were performed after regular exposure time intervals using the stylus profilometer. Results showed that the cavitation erosion of polyurea coated substrates are different depending on the chemistry of the coating and properties of the substrate. PU-105 was more susceptible to cavitation erosion. Average depth of erosion for the composite formulation with 10% volume ratio of milled carbon fibers, was among the highest after 3 hours of erosion test.
Epoxy Resin with Carbon Nanotube Additives for Lightning Strike Damage Mitigation of Carbon Fiber Composite Laminates
Carbon fiber composites are paving the way for light weight, high strength structures in the aerospace industry. While the benefits of carbon fiber composites are undeniable, they also have their drawbacks. Crippling damage due to lightning strikes is one of them. The current solutions to reducing the damage due to lightning includes adding expensive and heavy copper mesh into the laminate. A potential solution to this problem would be to add a lightweight conductive additive to the epoxy resin instead of a copper mesh. Carbon nanotubes are chosen as the additive to create an electrically conductive resin matrix and hence increase the overall electrical conductivity of the composite. Increasing the conductivity will decrease the damage owing to a faster dissipation of lightning-strike-induced Joule heating, and more importantly increase the residual strength. In this work, we quantified the increase in conductivity through measuring the electrical resistance using the four-probe method. Results showed that the electrical resistance of the sample with carbon nanotube additives is 31% lower than the one with no additives when the same resin system is used. In addition, lightning strike tests have also been carried out with both samples using an artificially generated waveform A impulse current with a peak of 100 kA. The current results showed no visible damage to both the samples with and without CNT additives in the epoxy. Time-resolved camera images taken for the lightning strike tests showed that
Ablative Carbon Nanotube/Phenolic Resin Thermal Protection System for Potential High Temperature Composite Applications
Carbon fiber reinforced polymers (CFRP) are increasingly used in aerospace applications which demand lightweight and stability at elevated temperatures. This paper discusses a method of fabricating a novel carbon nanotube (CNT)/phenolic nanocomposite and using it as a thermal protection layer (TPL) to improve the performance of CFRP. Hybrid composites of different volume fractions of TPL and bonding interfaces were evaluated. The presence of TPLs in the hybrid composites resulted in as much as a 17% decrease of through-thickness thermal conductivity. Residual flexural strength and modulus after a flame torch test were 39% and 70% respectively of the initial value, a substantial improvement over 11% and 21% of a control sample. We also discuss the effects of TPLs on the coefficient of thermal expansion (CTE) and protection mechanisms. This research could lead to scalable manufacturing of CFRPs with enhanced performance characteristic at elevated temperatures for aerospace applications.
Influence of Automated Fiber Placement (AFP) Manufacturing Signature on the Mechanical Performance of a Composite
A finite element method (FEM) based framework is developed to create the models of laminates with gap and overlap imperfections, and to predict the tensile and compressive performance of these laminates. Undulated and consolidated ply geometries are obtained by a compaction simulation, preceding the actual loading test. The FE model for the tensile and compressive tests utilizes a multi-scale progressive damage and failure model, where the pre-peak response is captured based on micromechanical formulations and the non-linear matrix response. The post-peak composite response is modeled with the crack band approach. The FE analyses are compared to experimental results.
Defects Characterization and Evaluation of Their Effects on Compressive Failure of Composite Components
In the design and certification of composite structural components for aircrafts, compression and combined compression and shear loading cases have to be considered. Various forms of defects such as ply waviness, delamination and porosity can be generated during the fabrication and service life of composite structures. Compressive strength is very susceptible to the existence of these defects. To evaluate the effects of two major defects, ply waviness and delamination, experimental and numerical studies are performed. Compression test is designed for the unidirectional (UD) composite coupons with in-plane fiber misalignment. The fiber misalignment is introduced by tilting the fiber orientation with a small angle (from 1° to 5°). A continuum damage mechanics (CDM) model is adopted in the numerical analysis. The capability of the CDM model in predicting the effects of defects is first verified using compression test results of unnotched specimens and then it is used in predicting the effects of ply waviness and delamination on the compressive strength of open-hole (OH) specimens.
An Analytical Analysis of Fiber Waviness for Laminated Curved Beam Under Bending
A closed-form analytical solution is developed for analyzing laminated composite curved beam with fiber waviness. Explicit expressions for evaluating equivalent axial and bending stiffness are formulated based upon modified lamination theory and taking into consideration the structural deformation characteristics of beam with narrow section. The maximum radial stress is computed for a laminated composite curved beam with fiber waviness under fourpoint bending at any given location. The maximum radial stress results were in a good agreement with numerical results obtained by ABAQUS. It is concluded that the present approach is an efficient method for analyzing laminated composite curved beam with fiber waviness.
Towards a Novel Defect Severity Model for Compressive Failure Analysis of Unidirectional Composites
Compressive failure of high-performance unidirectional (UD) carbon fibre polymer composites is driven by fibre kinking. Fibre kinking is known to be controlled by the shear strength of the supporting matrix material. Consequently, compressive strength is significantly reduced as the fibre waviness increases, as postulated by Argon in the 1970’s. In this paper, additional fibre architectural features affecting the compressive strength of UD composites, and to be considered in strength analyses of UD NCF reinforced carbon fibre composites, are discussed. The current work comprises development and use of a method for characterization of fibre misalignment in CFRP. Transfer of fibre misalignment measurements into finite element models and subsequent compressive strength analyses is conducted. These analyses reveal a very strong relationship between compressive strength and fibre waviness in accordance with Argon’s model. In addition, geometrical features such as the size of zones with large fibre misalignment are also found to influence the compressive strength of the studied composites.
Parasitic Effects of Including Microwire Sensor Tube on In-Plane Compressive Strength of Carbon-Fiber Reinforced Polymeric Composite Laminates
The influence of embedding a titanium alloy sensor tube in carbon fiber-reinforced polymer (CFRP) composite laminates on the in-plane compressive strength has been presented. These sensor tubes cause geometric disturbances in fiber architecture and lead to formation of resin pockets which are considered as defects and were characterized with help of an optical microscope. These Micro-Wire Sensor (MWS) cylindrical tubes were embedded at 0° and 90° orientations with respect to the fiber direction and their impact on the compressive strength was studied experimentally and numerically for three different stacking sequences and two different material systems. Digital Image Correlation (DIC) was employed for strain mapping around the sensor region and to capture the onset of failure. It was observed that inclusion of tube in fabric did not impact the in-plane compressive modulus and strength due to inherent undulation in the material. However, a reduction of 5.31% in compressive strength was observed for a cross-ply laminate manufactured using unidirectional tape. Finite element analysis was used to evaluate stress concentrations present due to sensor and to investigate the stresses at the interface of resin pocket and composite. Progressive failure analysis was also performed using maximum strain criterion to understand damage evolution and correlate results to experimental testing. The failure modes captured were similar to the experiments and ultimate failure strength was predicted to an accuracy of ±5%.
Effects of Localized Manufacturing-Induced Porosity Defects on the Fatigue of Wind Turbine Blades—Experiment and Modeling
In the absence of credible models to provide insight on the relationship between the state of porosity and its impact on the ultimate compressive strength and compressive fatigue strength, blade manufacturers, OEMs and wind farm operators are unable to make well-informed decisions for the disposition of wind turbine blades with such manufacturing-induced defects. In the current research, a lab-scale methodology to replicate the types of local porosity defects that occur during wind blade manufacturing was developed and was used to make 35-ply composite blocks for experimental investigation and modeling. Each block represents a typical section cut from the spar cap region of a blade. Coupons were cut from the blocks then sorted into high-, medium-, low-, and “baselineâ€-porosity density groups as characterized by optical microscopy, SEM and CT scan. Axial compression and compression-compression fatigue tests were conducted to evaluate the compressive strength and fatigue life as a function of porosity classification, respectively. A complimentary finite element model of a representative unit volume (RUV) of the lab-manufactured block was built from the 3D reconstruction model using CT scan images. The test data and the complementary finite element models are to be used to guide the development of simple empirical models for predicting the fatigue life for the composite material system as a function of the state of porosity.
Deterioration Modeling of Large Glass Fiber Reinforced Polymer Composite Structures/Systems
The adoption of fossil-based hydrocarbon polymer composites has been successful in both the automotive and aircraft industries, and is rapidly expanding into buildings and civil infrastructure. One challenge to broader adoption of polymer composites in buildings and civil infrastructure is a limited ability to model the synergistic effects of the combined physical/chemical processes of environmental exposure and mechanical loading. Unlike other building materials, long-term experience and field performance data of polymer composites in buildings and civil infrastructure applications does not exist. The first and largest composite building system used in a high-rise exterior in the US is the facade of the San Francisco Museum of Modern Art (SFMOMA) completed 4 years ago in 2015. Since historical, experience-based service life models for composite building applications are not available, it is crucial to build multiphysical- based models in order to predict composite service life performance on a semi-centennial or centennial time scale. This paper begins to understand the thermochemical- mechanical degradation mechanisms of composite building materials at multiple length scales and proposes a computational model for service life prediction. This effort consists of two steps; (i) development of a theory of synergistic effects between two or more degradation processes (e.g., ultraviolet exposure and moisture exposure), and (ii) validate a computational model of this theory based on published accelerated degradation experiments. This effort extends the literature on composite degradation from empirical or observation-based models to new multi-physics-based models, and looks to assess the viability of new polymer composite building materials by leveraging both experimental and computational approaches.
Effect of Hygrothermal Aging on the Mechanical and Optical Behaviors of Hybrid Randomly Distributed Fibers/Woven Glass Composites
Glass fiber reinforced polymer (GFRP) composites constitute nearly 90% of the global composites market and are extensively used in aerospace, marine, automotive and construction industries. The effect of operational environments on the long-term durability of structural components fabricated with glass reinforced composites is an ongoing concern. In this work, the effect of hygrothermal aging in different media on the mechanical behavior of a hybrid randomly distributed fibers/woven composite was experimentally investigated. The hygrothermal aging consisted of distilled water at different time intervals and temperatures. In order to follow the physico-chemical evolutions and their influences on the mechanical properties, e.g. flexural strength/stiffness and fracture toughness of the composite laminates, a comparative study of the properties was carried out for both unaged and aged specimens. An optical transmission scanning (OTS) was also used to investigate the effect of hygrothermal aging on the coloration of the composite specimen. The analysis of all obtained results shows that aging time, medium and temperature have a significant influence on the mechanical behavior and optical properties of hybrid randomly distributed glass fibers/woven composites.
Computational Analysis of Thermal Degradation of Carbon Nanotube Reinforced Nano-Engineered Composites
This paper presents an atomistically informed approach to investigate the effects of temperature on material responses of carbon nanotube (CNT) reinforced nanocomposites. The molecular model of radially grown CNT reinforced polymer composites, also known as fuzzy fiber nanocomposites, has been developed by explicitly modeling the CNT, graphene-based carbon fiber and epoxy-based polymer matrix. A polymeric functional coating for the carbon fiber surface, which serves as a substrate for the CNT growth, is also explicitly modeled. Virtual deformation tests on the molecular model are performed to investigate the variation of mechanical properties at different temperature. The bond dissociation energy (BDE) obtained from the simulations can be directly interpreted as mechanical degradation due to covalent bond breakage. The results show accelerated bond breakage at high temperature. Mechanical instability is observed beyond a specific temperature, which can be correlated with the glass transition temperature of the system. This physics-based understanding of the effects of temperature on critical mechanical properties will be highly valuable for the design optimization of CNT reinforced nano-engineered composites.
Carbon Fiber Composites Under Combined Environmental- Compressive Loading Leveraging Nonparametric Full-Field Techniques
Carbon fiber composites are often used in defense applications where structures undergo loading at high strain rates, as well as experience seawater exposure and other extreme environments. While the absorption of seawater has been shown to degrade material over time, competing effects of water absorption in the matrix and corrosion attacking the interfaces and their influence on the bulk damage response, particularly under dynamic loading conditions, are not entirely understood. As such, this study investigates the short-term exposure of both saltwater and distilled water on the compressive response of unidirectional carbon fiber epoxy matrix composite under uniform quasi-static (10−3 ð‘ −1) conditions using a load frame, and dynamic (103 ð‘ −1) conditions using a Kolsky (split-Hopkinson) bar. Environmentallyconditioned specimens were soaked for a period of three weeks. Due to the orthotropic nature of the unidirectional laminates, experiments were performed in both the longitudinal, 0o, and transverse, 90o, loading orientations with respect to the fiber direction. However, maintaining force equilibrium during Kolsky experiments in the transverse direction is particularly challenging, and often not possible without major modifications or specialized bar systems. Here we present a novel, non-parametric inverse identification technique that does not rely on the strict assumption of stress equilibrium to achieve accurate deformation mapping over the surface of the sample in real time. The information is derived from full-field high-speed digital image correlation (DIC) via surface acceleration measurements, in addition to utilizing a boundary stress measurement on the transmitted bar. Thus, using full-field metrology in a ‘stress-gauge’ approach allows for the stress-strain response along the transverse direction to be fully, and more accurately measured. For the longitudinal orientation, the ultimate stress and maximum strain at failure significantly decreased for the conditioned samples loaded quasi-statically. Under high-rate compression, the failure strain significantly increased for the conditioned samples with very small variation in ultimate stress. These results are compared with the transverse orientation where samples exposed to water showed an increase strain at failure in the low strain rate regime.
Effective Properties of Fiber Composites Using Stress Functions
In this paper, an analytical approach is presented to derive the effective properties of fiber reinforced composites based on a three-phase model employing the Airy stress function. A refined self-consistent approximation is used based on a threephase model in which a single fiber is surrounded by a matrix phase beyond which lies an unbounded medium that has the properties of the composite The classical Airy stress function method is employed to derive the elastic fields of three-phase media analytically. The obtained solution for the three-phase composite supersedes the solution by Christensen and Lo. The effective elastic properties of the fiberreinforced composites are numerically computed which provide better prediction than the conventional self-consistent approximation.
Overall Thermal Conductivity of Carbon Fiber Polymer Matrix Composites Undergoing Thermal Decomposition
The objective of this work is micromechanics-based prediction of the effective longitudinal and transverse thermal conductivity of carbon fiber reinforced polymer (CFRP) composites at high temperatures, when pyrolytic thermal decomposition of the polymer matrix takes place. Microstructure of the composite evolves with temperature and consists of polymer phase, growing pores, which represent pyrolytic gaseous phase, and continuous carbon fibers. Volume fractions of polymer and pores at different temperatures are obtained from the Arrhenius-type equation describing decomposition of the polymer matrix. The char residue produced during pyrolytic thermal decomposition is not considered in this study. Microstructure generation algorithms are developed to create densely packed, randomly distributed, poly-dispersed, particle filled microstructures consisting of circular (representing fibers) and elliptic (representing pores) inclusions. Statistical analysis of the generated microstructures is discussed to demonstrate their suitability. A two-step numerical homogenization technique is used to compute the effective directional thermal conductivities of the composite. The computational results for the effective transverse and longitudinal thermal conductivities are obtained for the AS4/3501-6 composite in a temperature range up to 700 K.
Understanding High-Modulus CFRP Compressive Strength Improvement
There has been a strong demand in using high-modulus (HM) carbon-fiber composites potentially enabling lightweight aircraft structures with significant weight savings. However, extremely low fiber-direction compressive strength has been a well-recognized weakness of the HM carbon-fiber composites, prohibiting their implementation in aircraft platforms. Fiber-direction compressive strength of PANbased continuous HM and intermediate-modulus (IM) CFRP’s targeting structural applications is presumably governed by microstructural stability. However, strong decrease in fiber-direction compressive strength of HM carbon-fiber composites, compared to their intermediate modulus (IM) counterparts, contradicts predictions from available microstructural buckling theories. This work looks into weak fibermatrix interface as a major mechanism driving the fiber-direction compressive strength decrease of the HM CFRP’s. In-situ scanning electron microscopy (SEM) based experiments pushing out carbon fibers show approximately 40% decrease in the average values of the fiber-matrix interface shear strength for a selected HM composite compared to a legacy IM composite with the same toughened epoxy matrix. And such a strong reduction correlates with decreasing fiber-direction compression strength predicted by a microstructural buckling model which properly accounts for the fiber-matrix interface shear strength behavior. The results support the idea of integrating off-the-shelf IM carbon fibers with stronger fiber-matrix interface and higher shear modulus into a HM carbon-fiber composite to improve fiberdirection compressive strength of the HM composite material system.
Modeling of the Influence of a Damaged Thermally Grown Oxide (TGO) Layer in an Environmental Barrier Coating System
Environmental barrier coatings (EBC) are an enabling technology for the successful application of ceramic matrix composites (CMCs) in air-breathing gas turbine engines. EBCs are susceptible to a number of failure modes including oxidation/delamination, recessions, chemical attack and dissolution, thermomechanical degradation, erosion, and foreign object impact damage in a combustion environment. Spallation of environmental barrier coating (EBC), induced by a thermally grown oxide (TGO) layer, is a key EBC failure mode. The TGO layer, resulting from steam oxidation, grows either from a silicon bond coat layer (if present) or from a silicon carbide (SiC) based substrate itself. The TGO layer evolves (i.e., thickness increases with time) as water vapor and oxygen gradually diffuse through the EBC, and the EBC spalls off once the TGO layer reaches some critical thickness. The critical thickness of the TGO layer for failure is in the range of 20-30 microns, but it can vary due to exposure temperature, microstructure etc. Current work at NASA Glenn Research Center, under the Revolutionary Tools and Methods (RTM) subproject is aimed at addressing associated failure modes in EBC systems and developing robust analysis tools to aid in the design/analysis of these systems. The objective of the current work is to conduct a sensitivity study to examine the influence of uniformly and non-uniformly grown oxide layers on the associated driving forces leading to spallation of the EBC when subjected to isothermal loading. The effect of damage in the TGO layer on the resulting stress states is also assessed both in uniform and non-uniform TGO layers.
Effect of Pick Spacing and Warp and Weft Volume Fraction on Intrinsic Residual Stresses in 3D Woven Composites
Three panels of 3D woven carbon fiber/RTM6 epoxy composites with a ply-to-ply weave with 12x12 (warp/weft) picks per inch (ppi), 10x12 ppi, and 10x8 ppi were fabricated by resin transfer molding. Realistic finite element models of each weave architecture were constructed using Dynamic Fabric Mechanics Analyzer. The resin properties were isotropic and linear elastic and dependent on temperature. The resininfiltrated fiber tow properties were estimated using homogenization based on Hashin and Shapery formulas. The model was considered to be at zero stress at the 165ï‚°C curing temperature. The stresses resulting from cooling the composite to 25ï‚°C were estimated using the resin temperature-dependent properties and the temperature independent properties of the tows. The displacement fields resulting from holes drilled through the middle of the top warp or weft yarn were estimated by virtually drilling a hole in the finite element model and were measured on the specimens using electronic speckle pattern interferometry. In general, the measured displacements transverse to the yarn were lower than the predicted displacements. This suggests the resin in the infiltrated yarns relieves some of the stress by permanently deforming during cooling. The measured displacements along the yarn were approximately the same for the 12x12 ppi,, lower for the 10x12 ppi, and significantly higher for the 10x8 ppi.
Micromechanical Modeling of High Rate Punch Shear Behavior of Unidirectional Composites
Micromechanical modeling of fracture behavior of unidirectional composites has been performed by predefining the statistical distribution of fracture planes in the fiber and fiber-matrix interfaces in 2D. Five fiber segments of 2, 3, 4, 5, & 6 micron length have been randomly assembled to generate a fiber of length 900 micron. Mixed mode cohesive elements have been used to define fracture planes between fiber segments and to define fiber-matrix debonding. Experimentally determined Weibull strength distribution for S-2 glass has been used to define the stochastic normal and shear tractions for fiber fracture. Rate dependent traction and mode II energy release rate for fiber-matrix debonding have been determined by microdroplet experiments and simulations, and have been used to define the fiber-matrix interfaces. Finally, experimentally determined rate dependent non-linear matrix behavior of DER353 epoxy has been used to model the rate dependent large deformation behavior of matrix resin. Using these stochastic distribution of crack planes and rate dependent properties, punch shear simulations have been conducted to understand rate depedendent micromechanical fracture behavior and predict the rate dependent punch shear strength and punch shear modulus of unidirectional S-2 glass/DER 353 composites.
Cohesive Parametric High-Fidelity-Generalized-Method-of-Cells Micromechanical Model
The Parametric High-Fidelity-Generalized-Method-of-Cells (PHFGMC) micromechanical model is extended to include refined local cohesive formulation for simulating weak discontinuities in multiphase composites. The nonlinear parametric HFGMC governing equations are obtained from a two-layered (local-global) virtual work principle and solved using a new incremental-iterative formulation. This offers computational efficiency over previously published work on HFGMC since the system of equations is symmetric. In addition, this formulation enables implementing advanced traction-separation laws available in the literature and allows maintaining the symmetric matrix structure for computational efficiency. Two cohesive zone models were integrated with the PHFGMC with unique nonlinear formulation. These were the non-potential model proposed by Camanho and Davila (CD) and the potential based PPR model. This paper presents the newly verified modeling approach with the new cohesive capabilities. New PHFGMC-Cohesive results are shown to be in good agreement with those from a finite element analysis for various configurations and loading patterns. Progressive damage in composites using the PHFGMCCohesive model is also demonstrated.
Mesoscale and Continuum Models of Wave Propagation in a Woven Composite
Continuum damage models are often used to simulate impact, damage, and perforation of woven composite materials. Continuum models can accurately predict perforation residual velocity and large-scale damage, such as interlaminar delamination. However, the continuum assumption intrinsic to these models means stress wave propagation and damage at the mesoscale—the scale of the woven fabric architecture—may not be adequately predicted. At the earliest time scale following impact on woven composites, a stress wave propagates from the projectile-target contact area through the composite thickness. The present work compares 1D stress wave propagation theory with continuum and mesoscale models of a plain weave glass/epoxy composite impacted by a flat-nosed, rigid projectile. The results show that the continuum model effectively predicts the theoretical 1D stress wave propagation. But the continuum model cannot predict stress wave interactions with interfaces nor interfacial delamination or debonding damage. A mesoscale model, with discretely modeled woven fabric architecture, is developed and shown to adequately predict the stress wave interaction at mesoscale interfaces by particle velocity compared with 1D theory, and the model is used to study interfacial debonding.
Comparison of Numerical and Experimental Study of Mode-I Interlaminar Fracture of Self-Healing Composites Using Cohesive Zone Modeling
In this paper, the comparison of numerical and experimental study of a Mode-I interlaminar fracture for double cantilever beam (DCB) composite specimens with self-healing is presented. The cohesive zone modeling (CZM) approach has been implemented using finite element analysis (FEA). The cohesive zone is defined using traction-separation law, which includes the cohesive parameters that influence the fracture process. However, these cohesive parameters are usually unknown, and hence, this study is focused on estimating the cohesive parameters by comparing FEA simulation results to that with experimental observations. The experimental approach to obtain the new CZM models based on the J-integral method is also presented due to the inability of the traditional cohesive models to represent the macro-mechanical response behavior of healed specimens. The primary purpose of using FEA in addition to the experimental studies for self-healing DCB specimens is to complement the predictive healing and determine whether to initiate the self-healing in real time, which lies within the Dynamic-Data Driven Application Systems (DDDAS) paradigm. Therefore, the present study, which includes CZM with FEA serves as a first step of the damage prognosis module.
Composite Drilling Characterization and Performance Evaluation
Hole drilling is one of the most widely used operations in order to place bolts or rivets. Drilling holes in composites leads to drilling-induced defects and damages such as delamination, burr, microcracking, swelling, splintering, and fiber pullout. A reliable and effective simulation toolkit for composite drilling process is essential to capture the drilling induced damage and perform optimization of drilling parameters to achieve desired hole quality. Under this study, a combined experimental and numerical approach is developed to extract the composite response and damage distribution caused by the drilling operation for model exploration and validation. With an advanced reaction force evaluation system and the use of non-destructive inspection techniques, specimens of different configurations are drilled and postdrilling damage quantification are evaluated for validation of the toolkit. An enhanced user-defined material model is developed for the ABAQUS/explicit to simulate the time-dependent material removal process during drilling and associated damage distribution. Accounting for the effect of the shear dominant cutting during the drilling, enhanced damage models are implemented based on the use of a fiberorientation dependent shear failure criterion coupled with a failure mechanism driven energy dissipation for the post-peak material characterization. The applicability and high-fidelity of the developed toolkit are demonstrated by comparing both the global response and local failure predictions with the testing results of quasi-isotropic coupons with a backing plate of two different configurations.
Factory-Scale Simulation and Economic Modeling for the Advancement of the Manufacturing Process of Composite Wind Turbine Blades
Despite the ongoing interest of blade makers to identify and to implement new manufacturing concepts with high probability of good ROI, there is a lack of sophisticated models for the evaluation of the ROI for these concepts. As a result, the manufacturing process for wind blades has not changed significantly since the early 1990s with the implementation of vacuum infusion processes. Thus, there is a need for an alternative to the invest-and-test approach to enable the implementation of new manufacturing concepts. In the current research, a factory-scale simulation of the operations of a wind blade manufacturing facility is developed using DELMIA, a graphical process flow simulation software that provides a virtual means to investigate the performance of a manufacturing process with analysis capabilities well-suited to highlighting inefficiencies. The simulation of the blade manufacturing facility is developed with the flexibility to be easily customized for a given manufacturing scenario and to enable the introduction and evaluation of the impact of changes to the overall wind blade manufacturing process, i.e. the insertion of automation or other process alternatives. The DELMIA factory simulation is being developed alongside a complementary Excel/VBA-based Techno-Economic model (TEM). The TEM uses detailed blade design data with baseline economic factors, manufacturing parameters and process models to generate cost estimates for a given manufacturing scenario. The TEM is formulated to include multi-year cost-predictive capabilities that account for the evolution of costs over a product lifecycle and factory changeover. The integration of the DELMIA factory flow simulation and the TEM provides a comprehensive modeling tool for the analysis of the economic feasibility of new composite manufacturing concepts for wind blade manufacturing.
Optimization of a Parametric Composite Wing Using Engineering Sketch Pad and Nastran
Metallic aircraft wings are usually not optimized to their fullest potential due to shortage of development time. This problem gets compounded when it comes to optimization of composite wings. With roughly $1000 worth of potential fuel savings per pound of weight reduction over the operational life of an aircraft, airlines are trying to minimize the weight of aircraft structures. In this study, a multi-level optimization strategy to optimize the weight of a composite transport aircraft wing is proposed. The loads on the wing are applied from a stick model as is done in the industry. A parametric software called Engineering Sketch Pad (ESP) is used for the geometry creation and Nastran is used to optimize the model. The part is assumed to initially have some arbitrary number of composite super plies. Super plies are a concept consisting of a set of plies all arranged in the same direction. The thickness and orientation angles of the super plies are optimized. Then, each ply undergoes topometry optimization to obtain the areas of each super ply taking the least load so that it could be cut and removed. Each of the super plies are then optimized for the thickness and orientation angles of the sub plies. The work presented on this paper is part of a project done for Air Force Research Laboratory (AFRL) connecting the parametric geometry modeler (ESP) with the finite element solver (Nastran).
A Thermo-Mechanically Coupled Constitutive Model for Curing of Polymers
Thermoplastics and Thermosetting polymers have found wide variety of applications these days in automotive, electronics, healthcare and aerospace industries due to their lightweight, easy formability etc. One of the major challenge in the manufacturing and processing of thermosets and thermoplastics is curing, through which the liquid polymer transitions into a state of visco-elastic and/or viscoplastic solid. The curing process is a coupled thermo-chemo-mechanical conversion process which requires a thorough understanding of the constitutive behavior to predict cure dependent mechanical behavior of the solid polymer. In this work, a thermodynamically consistent, frame indifferent, coupled chemo-mechanical continuum level constitutive framework is proposed for thermally cured glassy polymers. The constitutive framework considers the thermodynamics of chemical reactions, as well as the material behavior for a glassy polymer. This work considers a definition for the degree of cure based on the chemistry of the curing reactions. A simplified version of the proposed model has been numerically implemented, and simulations are used to understand the capabilities of the model and framework in typical composites specimen.
Modelling of the In-Plane Shear Behavior of Uncured Thermoset Prepreg
The in-plane shear behavior of prepreg material is one of the key parameters that affect defect generation in composites manufacturing processes, like Automated Fiber Placement (AFP) and Thermoforming. Recently a new characterization method for the in-plane shear of uncured prepreg under processing conditions was proposed. A strong influence of test/deformation rate and temperature was revealed. The present contribution proposes a way forward for predictive modelling of the observed behavior. A phenomenological viscoelastic model at large deformation is proposed. Each of the model components relates to assumed physical micro-mechanical mechanisms such as friction between the fibers, tensile loading of the fibers, etc. The proposed model provides enough flexibility to be able to fit the experimental results well. It was then implemented into ABAQUS as a VUMAT user defined subroutine. Initial model verification was performed by comparing the FE prediction with the experimental data, which suggests that the model will be able to make accurate predictions of the in-plane shear behavior of prepreg under processing conditions for a wide range of temperatures and deformation rates.
Defect Prediction During Forming and Consolidation of Composite Materials Using Finite Element Analysis
Forming of composite laminates is an effective manufacturing method that improves repeatability, enables tailoring of material orientations, and reduces time and cost when compared to more labour-intensive alternatives. However, this process is often prone to defects - leading to delays and ultimately scrapping of expensive parts - that are traditionally mitigated through resource intensive trial and error approaches. The risks of the forming process can be mitigated by using process simulation as a tool to develop a robust process window. In order to successfully model the forming of composite materials, the relevant deformation mechanisms and material attributes, shown in Figure 1, need to be considered. This paper presents a finite element simulation framework to predict the occurrence of defects in the forming and consolidation steps of the manufacturing process of fibre reinforced composites. The modelling approach presents a novel discretization strategy for each ply with a superposition of solid and shell elements. This solution uses a coupled pressure-displacement implicit analysis procedure that allows for the simulation of resin flow and is well suited to simulate autoclave consolidation and simple forming cases where through-thickness deformations are important. Simulation results show good agreement with experimental forming trials and other work found in literature in terms of deformation mechanisms and defects observed.
The Effect of Chirality and Strain Rate on Mechanical Properties of Carbon Nanotube (CNT) and CNT/Epoxy Composites
The effect of strain rate and chirality on mechanical behavior of single walled carbon nanotube (SWCNT) and SWCNT/epoxy composite under tensile loadings was investigated using a computational molecular dynamics simulation and the Reactive Force Field. The analysis comprises the methodology employed to develop the molecular structures of SWCNTs and SWCNT/epoxy composites, the techniques for relaxing the materials systems, and the method utilized to characterize tensile parameters of the structures. The simulated results showed that the chirality of the SWCNTs as well as rate of deformation affect the Young’s modulus of both the SWCNTs and the SWCNT/epoxy composite.
Molecular Modeling of Silica-Epoxy Interphase with Monolayer Silane
In this study, the interactions of epoxy-amine resin with the silica surface in the presence of monolayer silane is studied using molecular dynamics (MD) simulations. First we deposit glycidoxypropyltrimethoxy silane (GPS) on the silica surface at different densities and react the GPS molecules with the silica surface through a condensation reaction. A mixture of Epon828-Jeffamine® D-230 is then put on the silica surface and equilibrated to predict the epoxy-amine diffusion into the monolayer silane using the general AMBER force field. Epoxide-amine curing reaction among the epoxy, silane and amine are modeled using the cross-linking algorithm. The model is then loaded in Mode-I and Mode-II with the reactive force field ReaxFF to predict the stress-strain responses and failure loci within the interphase. Simulation results indicate that as the number density of silane increases, proximity of the epoxy and amine molecules with the silica surface decreases; this is because increased silane density leads to less open space for diffusion to the silica surface. However, an increase in the silane density increases the covalent bonding interactions in the interphase. As results, interphase strength improves with the increase of the silane density on the silica surface and approaches the bulk epoxy properties. Damage mode changes from adhesive to cohesive with the increase in silane density on the silica surface.
Molecular Dynamics (MD) Simulation of a Polymer Composite Matrix with Varying Degree of Moisture: Investigation of Secondary Bonding Interactions
Epoxy-based polymer composites are widely used in structural applications. Environmental degradation through moisture absorption considerably deteriorates the performance of these materials, particularly in the aviation and automotive industries. Absorbed moisture behaves differently in pristine and damaged composites. In a pristine structure with fewer diffusion pathways and voids, absorption is generally less and there is a higher degree of interaction between the resin and water molecules, which limits water mobility. Conversely, in a damaged structure, voids and microcracks allow accelerated diffusion and a greater proportion of “freeâ€ molecular water. The ratio of free to bound water states can be exploited to characterize damage via the dielectric properties. Structures with greater damage, and thus greater proportion of free water molecules, will exhibit a higher relative permittivity than a pristine structure, independent of water content. Without damage, water is more restricted from rotating under the influence of an applied electromagnetic field, leading to a lower relative permittivity. To simulate this phenomenon on a molecular scale, MD simulations are performed to characterize the moisture interaction in a polymer composite matrix system with the same epoxy (DGEBA) and hardener (DETA) constituents but with varying moisture concentrations. The system is constructed using the MAPS Amorphous Builder and the simulations were performed in LAMMPS using the AMBER-Cornell force field. Cross-linking is executed based on a reaction cut-off distance and the desired degree of cross-linking using the crosslinker module in MAPS. The simulation results are then analyzed and different properties like the diffusion coefficients and dipole moment fluctuations are evaluated to establish a primary understanding of how moisture interacts with the polymer matrix. To gain a deeper insight into the distinction between “freeâ€ and “boundâ€ water, the hydrogen bonds between different polar species are counted at varying moisture concentrations to understand how secondary bonding mechanisms influences diffusion and dielectric properties.
Extensive Study on Carbon Nanotube Junctions of Different Chirality, Angles Specifications and Overlap
Carbon nanotube (CNTs) junctions are not only the defect but also the required building blocks for many applications such as: switches, rectifiers, field effect transistors etc. The irregular nature of junctions invites defect sites (non-hexagonal rings) on its structure. Even the junctions topology are random but are constrained under Euler’s formula. Different types of junctions can be formed along with parameters such as chirality, diameter, fusion angle, extent of overlap etc. Junctions thus produced can be specified along with the set of non-hexagonal rings. In our extensive search, we have generated variety of carbon nanotube junctions playing with different parameters in which primal and dual meshing concept was used. Energetic stability along with different set of non-hexagonal defects were explored using molecular mechanics method. The study helps to select the best candidate structure specified under charality, diameter, and angle of fusion and extent of overlap. Furthermore, search for identification of energetically most stable candidate junction were done along with the annihilation of defect sites as well.
Ultra-Thin and Stiff Randomly-Oriented Discontinuous Composites
This paper presents experimental and numerical results for tensile strength of tow based discontinues composite (TBDC) plates manufactured using ultra-thin fibre tapes of high stiffness carbon fibres. Numerical models were used to explore the design space of these materials, in order to identify the optimal tow properties that maximise the strength of TBDCs. The experimental show good agreement with the numerical predictions and demonstrate a significant increase in the tensile strength by reducing the tow thickness and increasing the stiffness of the fibres. Strength increases of up to 100%, compared with a commercially available material system, are found, illustrating the highly promising potential of this type of material.
Thermal Energy Storage with Polymer Composites
This work focuses on the development and characterization of multifunctional continuous fiber composites for thermal energy storage (TES). Several combinations of matrices (thermoplastic, reactive thermoplastic, thermosetting), reinforcements (glass and carbon fibers) and phase change materials (PCMs, microencapsulated or shapestabilized) have been considered, and three classes of laminates have been produced and compared. It was observed that an increase in the PCM content leads to a rise in the TES capability but also to a decrease of the mechanical properties such as the elastic modulus and the mechanical strength. Moreover, the processing conditions for typical thermoplastic matrices resulted to be quite severe for the employed PCMs, which are subjected to thermal degradation. Conversely, the mild processing conditions adopted for the thermosetting and reactive thermoplastic resins is more suitable to preserve the overall thermal performance of PCMs.
Defect Properties in MoS2 Structure as 2D Material Gas Sensor
Electronic properties of 2D materials are sensitive to changes at surface, which provides an opportunity for toxic gas sensors requiring high sensitivity. Changes at surface include surface defects and gas molecule adsorbates often trapped at special sites. To understand the role and impact of such trapping sites in sensing mechanism, various point defects on molybdenum disulfide (MoS2, 2D semiconductor) are studied using density functional theory (DFT). DFT+U method is used to recover the experimental bandgap energy of MoS2, indicating strong electron correlation effect. Preliminary studies found that Sulfur vacancy shall assume negative (positive) charges near conduction (valence) band. Such charged defects compensate electron and hole carriers, lowering the carrier density. Furthermore, the charged defects will behave as sources of strong Coulomb scattering, hampering electron mobility. Therefore, sulfur vacancy itself found to degrade the efficiency of 2D gas sensor made of MoS2. The role of sulfur vacancy for gas molecule absorbate along with the electronic property of the vacancy-absorbate complex need to be further studied to comprehend the overall impacts played by sulfur vacancy in MoS2 gas sensor.
Design of a Multi-functional Energy Storage Composite Electric Skateboard
In previous work a multi-functional energy storage composite (MESC) battery has been developed by integrating Lithium-ion batteries into a carbon fiber sandwich structure. Presented here is the first complete system level design of an MESC battery application, demonstrating an improvement in specific energy over a commercially available electric skateboard. Finite element analysis of the design has been validated by mechanical testing within 3% error. The complex modeling of the interactions between the battery core and carbon fiber sandwich is discussed. Integrated piezoelectric sensors provide battery state of charge (SoC) estimation with 98% accuracy through novel ultrasonic guided wave propagation while strain gauges and temperature sensors track battery swelling and heat generation respectively to provide full battery state awareness. Details of the design process are included for completeness.
Multifunctional Interfaces for Combined Mechanical, Thermal and Electrical Properties
Interfaces between heterogeneous materials exist in many applications and 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), density and bonding strength. Multifunctional interfaces are developed to bridge the desired properties and functionality. These interfaces are comprised of an alloy of zirconium and copper which are common materials compatible with electronics industry processes. A magnetron sputtering deposition system with dual deposition is utilized to perform the functional deposition. The functionally varied layer is placed at the interface between a high coefficient of thermal expansion material and a low CTE material and bridges the material property differences. Materials selection is driven by comparing CTE differences, elastic moduli and geometric averaging. Thermal cycling is performed to investigate mechanical integrity for extreme cycle ranges of order -55 °C to 300 °C. Additionally thin buffer zones using atomic layer deposition (ALD) are investigated for adhesion improvement. Orthogonal to the mechanical properties, thermal and electronic properties need to be maintained. Both thermal and electrical transport are investigated for the effect of the alloy gradient on them. Theoretical modelling is utilized to find the proper alloy to simultaneously provide the best mechanical, thermal and electrical properties.
Highly Conductive and Lightweight Carbon Nanotube Conductor and Scalable Manufacturing for Conductive Fiber-reinforced Composite Application
Conductive metals such as copper are heavily used as conductors or lightning protection materials in aircrafts and other industrial areas. But the high bulk density and corrosion problem make non-metal conductors with low density and corrosionresistance more promising such as carbon nanotubes (CNTs). However, the small sample size of previously reported highly conductive CNT fibers (1-2 μm in diameter) or films (less than 1 μm in thickness) limits the practical engineering applications. In this work, a macroscopic continuously stretched CNT tape was prepared with excellent CNT alignment, high conductivity, and large dimensional size (~ 9 mm in width and ~ 30 μm in thickness). Through combined post-treatments of mechanical stretching, iodine doping and polymer capping layer, the macroscopic CNT tape shows high conductivity in the range of 10,000 S/cm (the highest is ~ 13,000 S/cm) and this excellent performance is stable in open air without any sign of degradation. Various characterizations were conducted including SEM observation, Raman, TGA, PPMS, electrical, and mechanical measurements. Overall, the high conductivity was ascribed to better CNT alignment after stretching and high carrier density after iodine doping. More importantly, the substantial stability of electrical property was demonstrated, benefitting from the polymer capping layer. Additionally, this process is based on commercially available raw materials and scalable for industrial production. And the proposed method here can be readily transferred to our buckypaper (CNT sheets) with roll-to-roll production capability. Furthermore, these highly conductive CNT tape and sheet can be directly incorporated into fiber-reinforced composite to improve the conductivity which could serve multiple functionalities as structural, electromagnetic interference shielding and lightning protection materials in aircrafts. This work opens a new path to optimize the electrical performance of CNT assemblies with a wide range of engineering applications including conductive composites..
Electromechanical Properties of Architected Periodic Multifunctional Foams and Composites
Understanding the mechanical and piezoelectric properties of the architected periodic multifunctional foams and composites are of great interest when used in structural applications. Therefore, there is a need to explore and analyze different designs of piezoelectric foam, metamaterials and composites with tunable properties. Current analytical models cannot be readily applied to compute electromechanical characteristics of architected periodic piezoelectric foams and composites exhibiting elastic anisotropy and piezoelectric activity. This study presents a micromechanical modeling and finite-element based computational homogenization framework to compute the electromechanical properties of architected periodic piezoelectric foam, metamaterials and composites. The intrinsic symmetry of architected periodic foams and composites was utilized and simplified mixed boundary conditions (BCs) equivalent to periodic boundary conditions (PBCs) were recognized. As an example, 3D finite element models of the 3-0-type closed foam piezoelectric structures with spherical porosity were developed and their effective elastic, piezoelectric and dielectric properties was completely characterized. We investigate the effect of porosity volume fraction on the suitability of 3-0-type closed foam piezoelectric structures in specific engineering applications such as hydrophones. Figures of merit such as hydrostatic strain coefficient ( h d ), the hydrostatic figure of merit ( .g h h d ), the acoustic impedance ( Z ), and electromechanical thickness model coupling factor ( t k ) are computed. Results showed that 3â€0 type porous piezoelectric material exhibited an enhanced response as compared to their bulk constituent. The FE results from the proposed framework found to be in good agreement with the FE results, based on the PBCs, available in the literature. Numerical results also showed that the excellent piezoelectric properties can be obtained and porous piezoelectric materials can exhibit unique combination of properties (low impedance and more sensitivity).
An ICME Approach to Performance Prediction of Composites Manufactured Using Resin Transfer Molding
The focus of this paper is to develop an integrated multi-physics and multiscale modeling framework to predict process-induced dimensional change of composite parts manufactured using the Resin Transfer Molding (RTM) technique. The Integrated Computational Materials Engineering (ICME) approach is adopted to link the constituent materials properties to the resulting composite performance through physics-based process models across multiple length scales. The proposed integrated modeling tool encompasses a Computational Fluid Dynamics (CFD) based resin infusion model, a coupled flow compaction model, and a Finite Element Analysis (FEA) based thermo-viscoelastic residual stress model at the micro- and macro-scales. A novel numerical scheme is developed to couple the CFD and FEA simulations to understand (1) how the deformation of the fabric affects the flow front during the resin infusion process and (2) how the infusion process affects the subsequent curing process and the resulting composite performance. The process-induced dimensional change of an RTM curved flange is used as a representative example to demonstrate the predictive capability of the proposed modeling toolkit. A closed loop from processing to performance evaluation established from the integrated toolkit can pave the way for process optimization to achieve specified design objectives
MD Modeling of Epoxy-base Nanocomposites Reinforced with Functionalized Graphene Nanoplatelets
The impact on the mechanical properties of aerospace epoxy materials reinforced with pristine Graphene Nanoplatelets (GNP), highly concentrated Graphene Oxide (GO), and Functionalized Graphene Oxide (FGO) has been investigated in this study. Molecular Dynamics (MD) using a reactive force field (ReaxFF) has been employed in predicting the effective mechanical properties of the interphase region of the three proposed nanocomposite materials at the nanoscale level. A systematic computational approach to simulate the reinforcing nanoplatelets and probe their influence on the mechanical properties of the epoxy matrix are included in this study. The nanoscale outcome indicates a significant degradation in the in-plane elastic Young’s (decreased by ~89%) and shear (decreased by ~72.5%) moduli of the nanocomposite when introducing large amounts of oxygen and functional groups to the robust sp2 structure of the GNP. However, the wrinkled morphology of GO and FGO improves the nanoplatelet-matrix interlocking mechanism which produces a significant improvement in the out-of-plane shear modulus (increased by ~95%). The influence of the nanoplatelet content and aspect ratio on the mechanical response of the nanocomposites has also been determined in this study. Generally, the predicted mechanical response of the bulk nanocomposite materials demonstrates an improvement with increasing nanoplatelet content and aspect ratio. The results show good agreement with experimental data available from the literature.
Prediction of Residual Stress Build-up in Polymer Matrix Composite During Cure using a Two-scale Approach
This article details the modeling of EPON862/DETDA epoxy system using Molecular Dynamics to predict the volumetric shrinkage as a function of crosslinking density. The ‘fix bond/react’ command in LAMMPS is used to virtually simulate the curing of the epoxy. The results for the shrinkage showed a good agreement with other modeling data in literature.
A Multi-scale Approach for Modelling the Cure of Thermoset Polymers within ICME
To predict the effect of the cure cycle on the thermo-mechanical properties of thermoset polymers, a two-scale method is implemented within the Integrated Computational Material Engineering (ICME) approach. Molecular Dynamics (MD) is employed to predict the resin volume shrinkage as a function of the degree of cure at the corresponding curing hold temperature using the Interface Forcefield (IFF). The predicted resin shrinkage values show good agreement with the literature, showing that MD, using IFF, can be reliably used for this purpose. Once computed, the shrinkage values can be used to predict the effects of shrinkage on bulk-level composite residual stresses induced by curing.
Maximizing Model Predictive Performance using Minimal Experimental Data with Multiscale Designer
Successfully modeling composite material behavior requires good quality experimental data to train models on, but the expense and time required to collect such data is a major bottleneck in practice. We combine Multiscale Designer’s reduced order homogenization technology with a Bayesian framework for parameter inference to fit composite models with just a handful of unnotched tension and compression tests on unidirectional and cross-ply laminates. We demonstrate the predictive ability of the fitted model by comparing model predictions to experimental results that were held out from the fitting procedure. The held-out data consists of several more unnotched and open hole tension and compression experiments on a wide variety of laminates used in practice. This framework furnishes probabilistic predictions of behavior, and accounts for the uncertainty due to natural variability in composite behavior in addition to uncertainty arising from finite replications of the experiments used for model training. Design is an iterative process which cannot begin without some initial estimates of material strength to work with. Our framework allows the design process to begin with computational estimates of material allowables based on a small number of experiments. This can help OEMs begin the design process and test many alternate materials before committing to the time and expense of full material qualification by the traditional experimental procedures.
Multiscale Analysis of Electron Transport in Filled Elastomer Materials
We present new modeling approach capable of predicting the electrical conductivity of elastomer composite filled with conductive particles with low aspect ratio. The approach is based on a combination of non-equilibrium Green’s function (DFTB-NEGF) analysis for atomic-level electron quantum transport, and effective medium theory for calculating macro scopic properties of the material. According to the suggested model, the macroscopic resistivity of well percolated composite material is defined by the statistical distribution of interparticle contact resistances, and the distribution of these contacts in the 3D space. Multiscale analysis is used for bridging the gap between molecular-level modeling of electron transport between individual particles with the macroscopic material properties. The analysis can predict macroscopic material conductivity based on the information of material microstructure received by electron photography; explain changes of conductivity in deformed material and with temperature variation, and finally help in finding the optimal material parameters such as particle shape, size distribution, volume fraction to maximize the performance
Improving Fracture Toughness at the Nanoscale using Length-scale Effect
The objective of this paper is to investigate and evaluate, through the application of molecular dynamics (MD) simulations, the validity of obtaining cohesive tractionseparation law at the nanoscale using three different approaches: (1) stress data from near the crack tip, (2) stress data from the far-field, and (3) changes in potential energy of the structure due to applied loads. The motivation for this research comes from the fact that the traction-separation law, which is a material property, can exhibit significant dependence on flaw size at the nanoscale if the correct methodology is not employed. The objective of this paper is to mathematically demonstrate that (a) the traction separation law at the nanoscale is a material property independent of flaw size only if stresses near the crack tip are employed to obtain the traction-separation law, and (b) the use of far-field data to define traction-separation law and fracture toughness makes the traction-separation law flaw-size dependent, and hence, it is no longer a material property and therefore inaccurate. Results for a single graphene sheet with and without a center crack under pure Mode I loading are presented to validate these concepts. Future work will focus on verifying these concepts under mixed-mode conditions.
Enhancing the Strength of CFRP Composites using Cellulose Nanocrystals-bonded Carbon Nanotubes
We introduce a novel processing method, in which we utilize assisting nanomaterials, i.e. cellulose nanocrystals (CNCs), to incorporate pristine carbon nanotubes (CNTs) in CFRP composites. We show that hybrid CFRPs made with CFs coated with CNC-bonded CNTs exhibit higher flexural and interlaminar shear strength (ILSS) compared to those coated with functionalized CNTs. Coating the CF layers with 0.2 wt% functionalized CNTs (fCNTs) increases the flexural strength by 8%. However, when the CF layers are coated with 0.2CNC-0.2pCNT (pristine CNT), the flexural strength enhances by 31% compared to that of the uncoated composites. The ILSS follows a similar trend to that of flexural strength, where fCNT increases 12% the ILSS, however, 0.2CNC-0.2pCNT enhances the ILSS by 42% compared to that of neat CFRP composites. Therefore, CNC-pCNT coated CFRP exhibits 23% higher flexural strength and 30% higher ILSS compared to those coated with only fCNT.
Quantitative Microstructure Study of PAN and Pitch-based Carbon Fibers Through TEM and X-Ray Scattering Analysis
Carbon fiber is widely used reinforcement material for composite applications. Modulus and strength of carbon fiber can be tailored depending on the precursor materials and processing conditions. However, microstructure-property relationships and defects on the crystal scale induced by processing still need to be explored. We studied microstructures of two typical carbon fibers from polyacrylonitrile (PAN) and pitchbased precursor and compared with its X-ray scattering properties and preliminary simulation results including crystal size, orientation and spacing. For this purpose, transmission electron microscopy (TEM) sample was prepared using focused-ion beam (FIB) for both longitudinal and transverse directions of carbon fiber. Pitch-based carbon fiber shows large graphitic structures with triangular defects, meanwhile PAN-based carbon fiber shows smaller graphitic structures with random distribution. In the case of pitch-based carbon fiber, crystalline size ~40 nm can be observed from the low magnification TEM image. X-ray scattering pattern also exhibit the differences between two fibers. The wide-angle X-ray scattering (WAXS) pattern of each carbon fiber shows graphitic peak at 26ï‚° and that is sharper for the pitch-based carbon fiber for both radial (2ï±) and azimuthal (ï†) direction. This corresponds to the large graphitic crystalline size and high alignment degree of fiber structures. Full-width half maximum (FWHM) of the azimuthal angle for pitch-based and PAN-based carbon fiber is 5.9ï‚° and 31.6ï‚°, respectively. This confirms the high alignment degree of pitch-based carbon fiber ~ 0.99. Precise measurements of the crystal size, orientation and TEM imaging of graphitic packing allows for further comprehension of associations between processing and final material properties which will enable customization of microstructures for property targets.
A Polymer-physics-based Multiscale Modeling Approach for Inhomogeneous Deformation of Polymeric Nanocomposites
This research provides a methodology that bridges macroscale and nanoscale deformation mechanisms; experiment and molecular dynamics (MD) simulation, respectively. The homogenous deformation assumption that has been extensively accepted in MD simulation researches is in fact, not correct when it comes to plastic deformation. By considering the inhomogeneous deformation scheme in plastic deformation, we suggest a postulation that the MD tensile simulation using a nanoscale RUC can represent a crazing region in a macro specimen. Under the postulation, the ductile-brittle transition is captured at MD simulation and experiment where the two different strain rates match each other in macroscale and nanoscale. If our postulation comes out to be true, it will lead to putting a new cornerstone that bridges different timescales between MD simulations and experiments.
RVE Models with Various Filler Morphology for What-if Analysis of Random Heterogeneous Materials
This paper presents development of representative volume element (RVE) models of random heterogeneous materials with various filler geometries and applications. The 3D network of filler within RVE is controlled using random distribution, probability distribution functions using image analysis, uniform grid and close packing algorithms. The 3D RVE models generated with filler geometries and viable computational models of various composite systems based on input filler morphology can be used to predict continuum properties of random heterogeneous materials for various applications. Various algorithms used in the RVE model generation rely on filler-to-filler compatibility and contact. The 3D network of fillers within RVE models and their seamless integration with analytical tools, 3D solid modeling and finite element analysis are presented with different filler geometry and morphology for what-if analysis to quantify continuum properties in different applications.
Multiscale Analysis for Interlaminar and Intralaminar Reinforcement of Composite Laminates with Carbon Nanotube Architecture
A multiscale modeling framework that bridges length scales from nanoscale to macroscale is employed to predict the interlaminar, and intralaminar enhancement in polymer matrix composite (PMC) laminates reinforced with carbon nanotube (CNT) architecture. Recently developed constitutive models, which are based on nanoscale information, are implemented using a high-fidelity micromechanics approach accounting for various material constituents and interphases. The microscale model is coupled with a finite element (FE) analysis, and the framework is used to investigate load transfer characteristics and fracture toughness in PMC laminates with CNTs dispersed in the matrix, and radially-grown on the fiber surface. It is shown that adding CNT reinforcement into the matrix or on fiber surface suggests a noteworthy increase in the transverse elastic modulus and interlaminar fracture toughness. The CNT growth height strongly influences the enhancement in fracture toughness. The developed multiscale methodology is versatile and can be used to investigate the structural response of PMCs with varying geometry and loading conditions. The model outputs can also help guide the design, development, and optimization of CNTenhanced composites with improved mechanical properties
Coarse-grained Molecular Dynamics Force Field for Carbon Black Filled Natural Rubber by Energy Renormalization Method
Since researches using molecular dynamics simulations have begun to be applied to polymer property studies, many efforts have been made to enhance simulation accuracy and speed. In order to overcome the short time scale and length scale of traditional allatom molecular dynamics simulation, the coarse-grained method has attracted attentions that groups several atoms as a single bead. To construct a coarse-grained potential with equivalent physical and thermodynamic properties with the all-atom model, Xia et al. proposed an energy renormalization method using styrene butadiene rubber (SBR). In this study, we constructed a coarse-grained potential for the natural rubber (NR) by the energy renormalization method and reproduced results in the Arrhenius temperature regime and glassy regime.
Effect of Processing and Particle Size on the Properties and Morphology of MWCNT-Polymer and SiC-Polymer Composites
The objective of this study is to determine the effect of processing on the electrical properties and microstructure of MWCNTs/PMMA nanocomposites and the effect of particle size on SiC/PMMA composites. For the MWCNT composites, the mixing methods used were mechanical, solution, and melt mixing. Impedance spectroscopy, SEM, and optical microscopy were used to characterize the samples. Results show that the mechanically mixed composites have the lowest percolation threshold of 0.05 phr (0.05 wt % MWCNT). Melt mixed composites have the highest threshold of 4 phr (3.85 wt % MWCNT). Solution mixed composites have a percolation threshold of 2 phr (1.96 wt % MWCNT). Percolation theory was used to determine the percolation threshold and shows values very close to the experimental estimations. The microstructures are segregated, agglomerated, and randomly distributed for mechanically, solution, and melt mixed, respectively. These results indicate that the segregated microstructure allows for the CNTs to form a percolated network through the composite more easily. The equivalent circuits show that above percolation the CNT dominates the electrical properties and may be represented by a RL series circuit. Below or at percolation, the presence of PMMA plays a stronger role in the circuit. Silicon carbide/PMMA composites were fabricated using a mechanical mixing method using nano-, micro-, and whisker SiC. The results show clearly that the nanosized SiC reached percolation first at 2 phr (1.96 wt % SiC) and had a very defined microstructure. SiC whiskers were the opposite, with the least defined microstructure and a percolation at 10 phr (9.09 wt % SiC). The micron-sized SiC reached percolation at 7.5 phr (6.98 wt % SiC).
TiGr Nacre: Damage Tolerance through Damage Diffusion
We present a new hybrid CFRP concept, aimed at increasing the energy absorption capacity of the material by diffusing damage through-out the laminate. To achieve this, we combine thin-ply CFRP layers, cut into interlocking tiles, with titanium interlayers. Uniaxial tension tests and bending tests on a preliminary design iteration showed that damage initiation and initial propagation were successfully achieved, but damage propagation localized during further loading. Final failure was triggered by strain concentrations in the titanium layers, which caused the titanium failure strain to be locally exceeded. The key to achieving higher laminate ultimate strain is to prevent or ameliorate these strain concentrations in the titanium layers.
Modifying Interfacial Chemistry of Cellulose-reinforced Epoxy Resin Composites using Atomic Layer Deposition (ALD)
Automotive and aerospace industries require new lightweight materials that enhance payload and improve efficiency via vehicle weight reduction. Employing composites, such as fiber-reinforced polymers, is a common approach to lightweighting. In this work, we examine methodologies to develop cellulose-reinforced polymer composites. Cellulosic reinforcements offer advantages compared to traditional reinforcing agents in sustainable materials sourcing, lower density, and lower cost. However, as produced cellulosics are hydrophilic and immiscible with most non-polar thermoset polymer resins. In this study, a variety of atomic layer deposition (ALD)-derived surface modification schemes are discussed as methods to improve resin permeation within the fibrous paper structure and interfacial adhesion between the polymer and the cellulose paper. Specifically, we consider surface modification of the cellulose paper with the ALD-precursors trimethylaluminum (TMA) and titanium tetrachloride (TiCl4), as well as with vapor-delivered carboxylic acids and silanes. Treatments are found to make the cellulose prepreg more hydrophobic. Composites treated with both atomic layer deposition and vapordelivered species are found to improve in Young’s modulus and maximum tensile strength. Furthermore, the TiCl4 – H2O surface treatment results in a composite tensile strength equal to that of a 3-aminopropyltriethoxysilane, a widely-used surface modifying agent, treatment while avoiding the disadvantages of using wet chemistry. The structural enhancements in the cellulose – resin composite, as demonstrated via optical imaging and scanning electron microscopy (SEM), inform these improvements in mechanical properties.
Alignment Induced Self-assemblage of Carbon Nanotubes for Structural Composite Applications
Carbon nanotubes (CNTs) can be produced from floating catalyst synthesis to form random networks. However, the CNTs must be aligned and closely packed to eliminate molecular and microscale defects to reach levels of mechanical performance similar to carbon fiber systems. Previous research has shown that strain-induced alignment methods using bismaleimide (BMI) infiltration can effectively modify randomly oriented CNT networks. The BMI resin lubricates the network decreasing the friction between the CNT bundles and assists with load transfer between CNT bundles during the stretching process. This process yields unique CNT collapsing, self-assembling behaviors, and graphitic crystal packing at the nanoscale. In this study, the CNTs’ alignment degree was measured by Raman spectroscopy and X-ray scattering. Alignment degree analysis results showed a considerable increase at 40% stretch ratio with a plateau at 60% stretch ratio. An 80% stretch ratio achieved the highest alignment degree of 0.93 with noticeable graphitic crystal packing. Both scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the CNT sheet surfaces and cross-sections show that as the stretch ratio increased, the CNTs started to align along the stretching axis and self-assembled into large bundles. Additionally, HRTEM analysis indicated that the CNTs exhibited collapsing and self-assemblage to form graphitic crystal packing. Tensile testing on the aligned CNT/BMI composite samples measured an increase in tensile properties. The ultimate tensile strength (UTS) and Young's modulus reached maximums of 1.58 GPa at 70% stretch ratio and 252 GPa at 80% stretch ratio, respectively. The high alignment degree and graphitic packing improved load transfer throughout the CNT network, and consequently, better mechanical performance in the CNT composites were achieved. Furthermore, this alignment process proved its scalability and potential to transcend into industrial-scale applications that utilize CNT material systems.
Influence of Through-thickness Stitching on the Fracture Behavior of Sandwich Composites
Sandwich composites are being used in primary and secondary load bearing structures due to their high stiffness and strength to weight ratios. These structures have superior flexural rigidity due to a light-weight internal core that is bonded to thin, rigid composite facesheets. However, debonding between the core and facesheets in sandwich panels can occur at relatively low out-of-plane loads due to low interfacial strength. Therefore, this study investigates the influence of through-thickness stitching to reduce the facesheet to core separation. Sandwich composites were fabricated from 110 kg/m3 perforated foam core with carbon/epoxy facesheets. Stitched dry preforms were cured using a one-sided vacuum assisted resin transfer molding (VARTM) process with an out-of-autoclave resin system. Single cantilever beam tests were performed in accordance with ASTM STD D5528-13. During each test, the crack growth was visually recorded with a digital image correlation system. After initial propagation, a 25% reduction in the crack growth and a 50% increase in the sustained load was realized for the stitched sandwich composite. The load-displacement response and fracture energy of the stitched and unstitched sandwich composites are discussed.
Anisotropic Hyperelastic Constitutive Modeling of Fiberglass/ Phenolic Hexagonal Honeycomb Core Subjected to Large In-plane Deformations
In the present study, a compressible anisotropic hyperelastic strain energy density function (SEDF) is developed to capture the in-plane nonlinear elastic responses of a commercial Fiberglass/Phenolic hexagonal cell honeycomb core under large deformations. A 3D nonlinear finite element model (FEM) with large displacements of the representative unit cell of the periodic core microstructure is employed to generate homogenized stress-strain curves of the core to assemble the hyperelastic SEDF using curve fitting methods. The hyperelastic constitutive model is evaluated using a single element model in the finite element program on which simple states of loading are imposed. Good agreement is observed between the model predictions and test results. The proposed model can be used as an effective constitutive model of the homogenized core to study the behavior of the core under complex loading and boundary conditions such as simulations of honeycomb core forming processes wherein the honeycomb core cells experiences large in-plane deformations owing to the flexural deformation.
A Systematic Experimental Approach for Damage Quantification in Sandwich Structures under Low-heat Fire Induced Damage
The amount of damage propagating into the core due to excessive heat or lowintensity fire will initiate several interacting failure modes in sandwich structures. Using several characterization techniques, this paper presents a systematic experimental approach to demonstrate the effect of low heat-intensity damage in GFRP foamsandwich structures. GFRP composite sandwich panels were fabricated using the Vacuum-Assisted Resin Transfer Molding (VARTM) process. The laminate stacking sequence included four plies on each side of the PVC foam core (Divinycell® Inc.). Each ply was constructed of knitted E-glass [+45/−45/Mat; 883g/m2] with mat facing the core. Heat was induced by localized heat fluxes (10 kW/m2) using the cone calorimeter test. The baseline sample experienced global buckling, whereas samples exposed to one sided heat exposure exhibited skin wrinkling and core shear instability. This shift in failure modes in the presence of damage must be taken into account in the design process. Techniques including the 3D optical profilometer, Dynamical Mechanical Analysis (DMA), and X-Ray CT scan, captured the morphology of the damage, the thru-thethickness thermal decomposition of the core, and the extent of damage in the foam core, respectively. Detailed mechanical tests were also performed to quantify the degraded and residual mechanical properties of all constituent materials constructing the sandwich panel. The developed methodology may well serve to predict the edge wise compression behavior and failure mechanisms of post-fire/heat damaged sandwich structures. This approach could be implemented in a material characterization code/software to support the hierarchical modeling from the macro-scale to the structural scale of deployable and in-service sandwich structures
Characterization of Bending Stiffness for Honeycomb Sandwich Plate in Three-point Bending Test Using Mechanics of Structure Genome
Motivated by the concept of Structure Gene (SG) [1] which is defined as the smallest mathematical building block of a structure, a new approach named mechanics of structure genome (MSG) is applied to characterize the bending stiffness of honeycomb sandwich structure in this paper. The constitutive modeling is conducted over a honeycomb sandwich SG by MSG to obtain the equivalent plate properties. Then, on the one side, analytical formulas are derived to directly calculate bending stiffness by the equivalent flexural rigidity and shear rigidity found in the equivalent plate properties. On the other side, the equivalent plate properties can also be applied to shell element for simulation of bending test, which can be handled by a standard finite element analysis (FEA) package such as Abaqus or ANSYS. The MSG approach including both analytical formula and FEM are compared with experiment and direct numerical simulation (DNS). The results show that MSG approach gives good agreement with experiment and DNS while being much more efficient than DNS. The main improvement of MSG approach lies in the absence of ad hoc assumptions and efficiency for both analytical and FEM prediction.
Damage Growth Analysis of Sandwich Structures Subjected to Combined Compression and Pressure Loading
Fluid-ingression phenomena in composite sandwich structures is a concern for components employed in the aerospace industry. Poor design/material selections, impact damages, thermomechanical loads occurring during ground-air-ground (GAG) cycling and changes to internal pressure within the sandwich structure all lead to localized transverse cracks, delaminations, facesheet/core disbonds and core damages which create more passageways for fluid migration. In some cases, damage growth due to latter two cases is stable and occurs over a period of flights, but may not be readily detected on the ground when the thermo-mechanical and internal pressure loads are released. Hence, the structure will continue to sustain load-carrying capabilities until the damage size reaches a critical damage threshold (CDT) which leads to catastrophic failure. Unless such damage is detected and repaired prior to reaching CDT, GAG effects will further increase the damage size and threaten the structural integrity and safety of the aircraft. Current research provides an analytical method to predict facesheet debond based on the Winkler foundation approach that agrees well with both numerical and experimental results and evaluates the capabilities of cohesive zone modeling to predict onset of damage growth. Also, a standardized procedure and test apparatus for GAG testing to simulate damage under a mixed-mode stress state caused by the pressure differential and in-plane mechanical loads is developed. Current phase of the research focusses on investigating the effects of GAG cycling on sandwich structures by subjecting them to combined pressure and in-plane mechanical loads and investigate the conditions leading to the onset of damage, and to develop a method to conduct GAG simulation based on SCB model calibration. The information gathered through this research will be instrumental in developing analytical methods and validating finite element analysis procedures to further investigate the damage growth mechanics of sandwich composite structures.
Energy Release Rate and Mode Partitioning of Face/Core Debonds under Bending and Shear Based on an Elastic Foundation Approach
Closed form expressions for the energy release rate and mode partitioning of face/core debonds in sandwich composites, which are loading in shear and bending, are derived. This is a novel solution based on an elastic foundation approach, and includes the deformation of the substrate (core and bottom face) in addition to that of the top face. In this approach, the sandwich beam is considered as having a “debondedâ€ section, where the debonded top face and the substrate (core and bottom face) are free and a “joinedâ€ section, where a series of springs (elastic foundation) exists between the top face and the substrate. The elastic foundation analysis is comprehensive and is done for a general asymmetric sandwich construction. The closed form expression for the energy release rate is derived by evaluating the Jintegral. The mode partitioning definition is based on the transverse and axial displacements at the beginning of the elastic foundation (“debond tipâ€). A specialization to the Single Cantilever Beam (SCB) specimen is also presented. The results are compared with finite element results and show very good agreement.
A Multiscale Infusion-cure Modeling Framework to Predict the Residual Stress of a Vacuum Assisted Resin Transfer Molded Composite
The purpose of this paper is to develop an experimentally-validated integrated infusion-curing model to predict the resin flow front and the residual stress inside the plain weave composite manufactured using the Vacuum Assisted Resin Transfer Molding (VARTM) technique. The composite is made of 16 plies plain weave AS4 carbon fiber fabrics, and the matrix is EPON 862/W resin system. In the infusion model, the dry fiber fabrics are treated as a homogeneous, porous material, and the resin flow front is predicted using the Volume of Fluid (VOF) method. Due to the interaction between the flow movement and fiber structural deformation, the resin flow model created in STAR-CCM+ and the fiber structural model created in Abaqus are two-way coupled. After the infusion simulation, the deformed shape of the composite is passed into the curing model to predict the residual stress development in the curing process. In the curing model, each lamina is treated as a homogeneous orthotropic material with the effective viscoelastic properties calculated from the fiber modulus and resin relaxation modulus using the correspondence principle and the micromechanics models, and these effective composite properties are included in a 3D anisotropic viscoelastic constitutive law to predict the residual stress increment. The accuracy of the proposed model is assessed by comparing the predicted flow front and the thickness of the composite structure after cure against the experimental results.
An ABAQUS Implementation of Regularized Extended Finite Element Method (Rx-FEM) for Modeling the Interaction Between Matrix Cracking and Delamination in Composites
Regularized Extended Finite Element Method (Rx-FEM) is a discrete damage modeling (DDM) method, which represents an approach to progressive failure analysis (PFA) in composites when multiple individual damage events such as matrix cracks and delamination are introduced into the model via the displacement discontinuities. Rx- FEM is a variant of the eXtended Finite Element Method (x-FEM) where a continuous approximation of Heaviside step function is introduced unlike x-FEM. The proposed implementation to represent the enriched displacement field is based on the superposition of native ABAQUS elements with UMAT and UEL which is used to connect the superimposed elements. Both user subroutines are utilized to facilitate the step function based energy conditions required for mesh independent crack (MIC) propagation within a lamina. In this paper, an interface UEL is developed to connect different unidirectional plies of the laminate for modeling the inter-ply delamination and the interaction between matrix crack and delamination. Several benchmark examples with analytical solutions including the DCB, ENF and TCT models were performed to verify the accuracy of the interface UEL for mode I, mode II fracture, and the interaction between matrix crack and delamination, respectively. The ABAQUS implementation of Rx-FEM not only demonstrates accurate predictions, but also is able to use built-in ABAQUS capabilities such as contact between plies with mesh independent Rx-FEM cracks.
Manufacturing Study of Thin-ply Composite Prepregs within Automated Fiber Placement(AFP)
Composite aerospace structures offer high specific mechanical properties as compared to traditional materials, yet composite materials tend to lack sufficient damage tolerance. In recent years there has been a focus on thin-ply pre-preg materials, which have shown an improvement with respect to micro-cracking and fatigue resistance as compared to the standard-ply-thickness materials. Large scale automated composite manufacturing processes, such as automated fiber placement (AFP), are necessary to produce large structures reliably and affordably. However, thin-ply materials require more layers to reach the same thickness, increasing the number of passes by the AFP machine. The overall production time is thereby increased, increasing the importance to maximize lay-down rate and minimize machine downtime. Thin-ply materials have not yet seen widespread adoption in AFP manufacturing in part due to a lack of study and development that leads to uncertainty associated with their manufacture. The result is that thin-ply materials lack the same repeatability, consistency and quality as standard-ply-thickness composites in the AFP process. In this work, efforts to identify key material and AFP processing parameters of thinply materials that result in high quality, consistent and repeatable feed of material through the AFP head and onto the structure are presented. A single spool, in-house designed machine is used to mimic the AFP process using a 70 gsm thin-ply material. The material-process parameters and interactions are studied using a design of experiments (DoE) approach. The factors being studied fall into three main categories: material parameters, slitting and spooling parameters, and AFP process parameters. Some of the factors being studied include out-time, backing paper vs. double-backing paper, and pre-preg tape tension. The DoE program is supplemented with finite element models (FEM) of the relevant manufacturing parameters.
Origami-inspired Foldable Textile Composites
Recently, a significant number of studies have focused on the use of origami patterns for creating novel deployable structures and devices. For example, in aerospace applications, there is a crucial need to improve the packaging efficiency of a deployable component and this can be obtained through having a low packaging volume to the deployed size ratio. Other than occupying less space, these structures are lightweight, flexible, affordable, and have a stiffness that is higher than the construction material. It is clearly desirable to find a novel fabrication method to form an origami-based structure from lightweight composite materials which can be easily manufactured and costeffective. In this study, an out-of-autoclave method is employed to develop a novel approach that is both simple and affordable. These techniques are then implemented to manufacture an origami-inspired cylindrical design based on a fiber-reinforced composite. Tests are undertaken on simple foldable structures to investigate the bending behavior of the joints within an origami structure. The simple test samples are subjected to cyclic compression tests in order to evaluate their bending stiffness. Micro computed tomography (XCT) and scanning electron microscope (SEM) testing was also conducted to understand the structure of the fold region. In summary, the fabrication method has been shown to be successful with the structure exhibiting an energy recovery due to the presence of the folds.
Thermoplastic Self-healing of Mode I and Mode II Quasi-static and Fatigue Crack Growth in CFRP Composite Using PCL/SMP Dual Phase Healant
Interlaminar fracture is one of the most notable limiting factors for fiber reinforced composite in the high-performance structural application, which makes interlaminar crack growth study an active research topic. Several methodologies have been developed over the years to improve the interlaminar fracture toughness, delamination resistance and crack growth stability such as toughening with nanomaterials (e.g., nanographene, CNT), modification of the fiber by different functionalization techniques to improve the fiber matrix bonding, and Z-pinning. However, none of these methods have the ability to improve the desired properties in case of repeated fracture. The studied self-healing methodology employs the biomimetic concept of closing a flaw before healing. In this study, a blend of PCL (polycaprolactone) -SMP (shape memory polymer) mixture was used in the DGEBA epoxy system for stabilizing the Mode I quasi-static and fatigue crack propagation and healing of Mode II interlaminar fracture. The quasi-static experimental results showed that for each double cantilever beam (DCB) specimen the fracture process in the virgin cycle is more unstable compared to subsequent healing cycles which can be attributed to the ductile nature of PCL-SMP fibrils bridging the crack, whereas DGEBA epoxy exhibited brittle fracture in the virgin cycle. The crack growth rate (da/dt) and the slope of crack driving force with respect to crack length (dGI/da) also supported the crack growth stability in healing cycles. The biphasic PCL and SMP blend also exhibited extraordinary healing performances (recovered ~146.2 % Mode I fracture toughness and ~73.8 % Mode II fracture toughness). Mode I fatigue crack growth resistance was recovered in subsequent healing cycles. The comparative slope (m) of linear fits through log(da/dN) vs. log(Gmax), for all cycles showed the fatigue crack growth rate is slowed by the presence of healants in the composite in subsequent healing cycles.
Design and Fabrication of Resilient Sandwich Composite Structures with Multi-stable Energy Absorption Core
Composite sandwich structures, while offering desirable mechanical properties such as light weight, high flexural rigidity and energy dissipation under loading, also suffer from inelastic damage accumulation under loading conditions. With a greater demand placed on composite structures to perform in extreme environmental conditions, it is necessary for composite sandwich structures to not only absorb impact, but also dissipate the impact energy without accumulating permanent damage - that is, be resilient to damage. In this paper, we present a novel sandwich core design strategy to incorporate energy absorbing core structures that rely on structural buckling to dissipate energy. These cores are printed using a stereolithography (SLA) desktop 3D printer. A parametric study to better understand the energy dissipating buckling behavior is performed using an experimentally validated finite element analysis. Using the insights provided by the computational model and experiments, recommendations are made for an optimal core design.
Modeling and Linear Analysis of Stitched Carbon Fiber Epoxy Composite Laminates for Mode I Delamination
Composites are used in assemblies more often now, requiring different joints such as mechanical fasteners, riveting, welding, and polymeric adhesives. These methods tend to trade off on the strength of the joint for increased assembly weight and cause additional constraints to be considered in the design process, complicating the design process further. An optimal candidate would be one that could provide high strength to weight ratio with minimal hassle for maintenance and high fatigue performance. One such candidate is stitch-bonded composites. Despite several experimental investigations on the material behaviors of these composites, the corresponding material response models are still far from maturity. The problem statement of this paper is to simulate and compare the delamination damage in an unstitched adhesively-bonded joint, a stitchbonded joint, and a bolted joint of a four-layer carbon fiber epoxy composite laminate using a finite element model developed for all cases. The stitching thread considered in the model is made of Vectran liquid crystal polymer spun fibers and has been treated as one-dimensional truss elements embedded into a lap joint. The delamination failure was modeled as inter-laminar failure using a cohesive zone method. The debonding between the stitching thread and the surrounding material (i.e., epoxy and carbon fiber) was not modeled. The crack opening mode or Mode I delamination was observed for the lap joint. The simulation results include the stress and strain fields, critical strain energy release rates, and the load-displacement curves for all considered joint types. The results provided us better insights into the inter-laminar interaction when delamination occurs and the mechanisms with which the stitching arrests the propagation of the bond failure.
Quasi Monte Carlo Simulations for Stochastic Failure Analysis in Composites
Due to the heterogeneity in their microstructure composite materials exhibit considerable spatial variations in properties, which strongly impact their mechanical performance, cause variability in stress and strain fields, and influence evolution of damage in the material. Recent research has focused on considering material behavior uncertainties and quantifying the impact on the structural response. In this paper eXtended Finite Element Method (XFEM) and Continuum Damage Mechanics (CDM) models are employed for damage modeling in composite materials, where variations of material strength are taken into account using spatial random fields to model the material strength parameters. Using samples of the property fields, Monte Carlo (MC) and Quasi Monte Carlo (QMC) techniques are utilized to quantify the influence of strength variability on the structural response. As MC technique can become prohibitively expensive, research using MC technique for stochastic model analysis in composite structures is sparse. QMC technique has been shown to provide faster convergence compared to MC method, hence reducing the computational expense, though application to composite analysis has not been found. The current methodology allows examination of the stochastic progressive damage behavior of composite materials. The influence of input variability on the mean crack length in open hole angle-ply tensile tests is examined using MC and QMC techniques, and the convergence behavior of the methods is compared. The results show faster convergence when using QMC.
Prediction of Sub-Element and Common Feature Test Component Failure Using Continuum Damage Modeling
Advanced structural analysis methods, known as progressive damage and failure analysis (PDFA) tools, are being developed to predict initiation and propagation of damage under repeated loading based on capturing multiple failure modes and their interaction. The Air Force Research Laboratory (AFRL) Composite Airframe Life Extension (CALE) Program, Assessing the Durability and Damage Tolerance of Advanced Composite Structural Features, has been evaluating the applicability of these methods to assess the durability and damage tolerance of composite structures within the United States Air Force fleet for service life extension. In particular, this program focused on a Common Feature Test Component (CFTC) representative of key aircraft design considerations and failure modes. CFTC advanced structural features include a composite skin made of unidirectional tape, a fabric hat stiffener, and a mechanically fastened aluminum rib. CFTC has been the most complex PDFA validation article to date. An investigation of key CFTC failure modes was performed in this work using simpler representative structural features. The subelements were modeled for finite element analysis using fiber-aligned meshing for damage modeling in bulk elements and cohesive elements for delamination modeling. CDMat – an ABAQUS plugin implementing progressive damage and failure simulation technique developed at the University of Texas Arlington Advanced Materials and Structures Lab, was used for ply-level damage progression and failure modeling under static and cyclic loads. This work presents failure and damage growth comparisons of sub-element tests and simulations by CDMat under static and fatigue loads including the comparison of fatigue delamination growth obtained for multiple maximum loads using high-resolution camera measurements. Finally, blind predictions of critical failure loads and post-critical damage growth for the CFTC specimens are compared with test results.
Comparison of Multiscale and Kernel-based Correlations for Stochastic Permeability Models in Composites Manufacturing
Computational models simulating liquid injection processes for composites manufacturing typically invoke a porous medium representation of the fabric reinforcement material. This representation is an abstraction of the real medium in which flowing, reacting, and cooling resin interacts with meandering fibers. Stochastic permeability models have traditionally been introduced in an effort to capture the effect of this heterogeneity as it varies over the specimens. The choice of a probabilistic model for this random permeability field has strong implications on the statistical predictions relevant to cycle time, cost, and quality control. A standard approach to construct such a model is to assume it has a spatially invariant probability density function that is often described by a log-normal distribution. A spatial correlation function is then appended to the probabilistic structure by optimizing a covariance kernel over some predefined model class. This optimization approach is typically limited to the porous medium abstraction and thus completely ignores subscale features that pertain to the mechanical and geometric properties of fibers, lay-up, and forming operations. We first obtain a multiscale mechanistic solution for the forming process that characterizes the fiber shearing angles in terms of all of the subscale properties. We then invoke constitutive relations to map these shearing angles onto the local principal components of the permeability tensor. By using a polynomial chaos methodology, we obtain an explicit stochastic expression between the subscale properties and the spatially varying permeability field. The resulting permeability field is non-stationary and is strongly influenced by boundary conditions, geometry, and fiber properties. Its marginal distribution functions also exhibit significant spatial fluctuations. We construe this stochastic permeability field as being a good representation of reality, and using it, we then seek optimal representations against several classes of kernel-based covariance models.
Determining a Length Scale of FRP Composite Microstructures
Carbon fiber reinforced plastics (CFRPs) have become the material of choice for many low weight, high strength applications. One problem associated with these materials is a scatter of mechanical properties. Efforts have been made to correlate these results with random fiber packing using 2-D representative volume elements (RVEs). While these efforts have shown that fiber distribution is important, scans have shown fiber position is a function of 3-D morphology. For example, fiber meandering and entanglement can only be seen when variation along the fiber direction is studied. While the optimal scale for 2-D RVEs is well researched, little has been done to find the effective length in which the 3-D behavior of real samples can be observed. As such, the construction and analysis of 3-D RVEs with a requisite length in the fiber direction is necessary. The scope of this work is to study the fibers within a CFRP sample in order to recreate the microstructure. In this work, long micro-CT scans from an aerospace-grade are used to obtain the fiber positions. In addition, scans from an automotive grade specimen with a high fiber count manufactured using VARTM were analyzed due to the high potential for entanglement. Descriptive metrics were then generated to fiber behavior within both samples. The variance of these metrics was captured using the change in Pearson’s Correlation Coefficient (R) with respect to position along the fiber length. One
Effect of CNT Waviness on the Elastic Modulus of Carbon Nanotube Reinforced Polymer Composites
Carbon nanotubes (CNTs) are the filler materials of nanoscale dimensions, which when added to the polymer matrix, form a strong yet light-weight multifunctional composite materials. Before using them effectively and confidently in the aerospace industry, it is important to completely determine their mechanical properties. Previous studies found these properties to be significantly influenced by the waviness of CNTs embedded in the epoxy matrix. In this study, we have developed a unique Python algorithm that is employed to construct the stochastic three-dimensional finite element models, which incorporates the CNT waviness as a variable. Random normal and uniform random distributions are assumed for waviness and orientation angles of the dispersed CNTs, respectively. Four separate finite element models with maximum CNT waviness angles of 0â°, 25â°, & 50â° are generated for CNT volume fraction of 0.5%. Thereafter, the elastic constants are evaluated and compared for each case. A significant decrease in the properties is observed as the maximum allowable waviness angle of the embedded CNTs in the epoxy is increased. These results suggest that CNT waviness plays a key role in deciding the elastic properties of the CNT based composites.
Experimental Validation of Multiscale Stochastic Models for Composites
In this work, we address several issues that influence the accuracy and reliability of model-based predictions for composite materials systems. During this process, we pursue data-driven stochastic modeling and simulation of complex composite materials across a hierarchy of length scales. The key feature of our approach is the utilization of polynomial chaos expansion (PCE) within a hierarchical multiscale setup to carry out stochastic data-driven model characterization, model updating, and model validation. The most prominent aspect of PCE-based modeling is coupling the key descriptors of properties and behaviors, featuring the different length scales, explicitly with respect to fundamental uncertainties across these length scales. A variety of central and consequential implications of PCE-based modeling within the context of some pressing complexities are discussed and demonstrated in this paper. Among these central features, we specifically highlight: (i) the explicit mapping among the constitutive properties of composites at different length scales with physics-constraint data models, (ii) the efficient integration of a matrix of different data sets featuring multiscale properties , (iii) the construction of explicit reduced-order surrogates of intricate dependent and coupled structure of quantities of interests, (vi) the versatility of PCE models to perform model calibration, efficient simulation of multiscale quantities of interests, efficient quantification of uncertainties in predictions, model updating as new data sets become available.
Uncertainty Quantification of the Dimensional Variations of a Curved Composite Flange
Composite manufacturing involves a large number of materials data and process parameters, inevitably resulting in performance variations of final parts. To maintain and improve product quality, characterizing the stochastic performance of composite manufacturing processes and thus identifying the key uncertainty inputs becomes critical. However, the conventional way to quantify the stochastic behavior of composite manufacturing is primarily based on Monte Carlo simulations, which are computationally prohibitive because of complex finite element (FE) process models. In this study, we employ the Gaussian process (GP) technique to investigate the variations of the spring-in behavior of a curved composite flange fabricated using the resin transfer molding (RTM). The data-based GP technique, which is commonly used in the field of machine learning and data processing, is employed to emulate the numerical sampling required in evaluating the spring-in angle of a curved composite part. The fundamental theory behind the GP is extended from the multivariate Gaussian distribution on a finite-dimensional space to a random function defined on an infinite-dimensional space. When interpreted from a Bayesian perspective, the GP technique becomes a powerful tool to emulate complex simulations, such as advanced manufacturing processes. The effectiveness of the proposed uncertainty quantification (UQ) technique is demonstrated by predicting the processing-induced variations of the spring-in angle and identifying the key material properties that affect the prediction.
Intelligent Optimisation of Aligned Discontinuous Composites
Aligned discontinuous composites offer a tailorable structural response, as their mechanical behaviour can be tailored by adjusting their microstructure. However, the stochastic nature of their microstructure, and the myriad permutations of different constituent properties makes optimisation of these materials difficult. In this work, an accurate yet efficient virtual testing framework is combined with an intelligent Bayesian optimisation routine to maximise the initial stiffness, ultimate strain, and ultimate strength of aligned discontinuous composites.
Strain Distributions in Bonded Composites Using Optical Fibers and Digital Image Correlation
Adhesive bonding is often utilized to assemble lightweight composite structures in the aeronautical industry. However, bonded composite joints are seldom exclusively used in primary load bearing structures due to the lack of adequate testing techniques to monitor the quality of the bonds during service. Continuous optical fibers can be used to monitor adhesive bonds since they are small (diameter~155 microns) and have minimal effect on the structural integrity of the bond. This work investigates the feasibility of using embedded optical fibers to monitor strain distributions within double-lap shear joints. Optical sensors were embedded in one ply from the adhesive/composite interface, and in the adhesive. Surface strain fields were recorded using a digital image correlation system. The specimens were tested in tension and the internal strain distributions were obtained from optical fibers. Optical sensors placed in one ply from the interface captured the greatest strain gradient. Experimental data was compared to a 3-D finite element model that includes the geometry of the specimens and the optical fibers. The simulations and the optical fiber measurements are in good agreement.
Investigation of Composite Bond Thickness Using Optical Fibers
The use of polymer matrix composite materials in primary load bearing applications has increased in the aerospace industry in recent years. Unlike structural metals, composites are generally joined by adhesives, which result in weight savings, reduction in part count and a more homogenous distribution of stresses in the bond. However, due to the lack of testing techniques, it is difficult to monitor the quality of bonds during service. It is important to understand the behavior of composite adhesive bonds, particularly for certification purposes. In this study, the performance of thin and thick bonded composite structures is investigated by obtaining the strain distribution from the adhesive joint using embedded optical fibers. Double lap shear specimens were fabricated in accordance with ASTM D 3528-96. Two plies of Solvay FM 209-1 film adhesive were used for thin bonds and six plies were used for thick bonds. Unmodified optical fibers were embedded at the midplane of the adhesive plies to obtain the strain distributions. Failure loads were recorded and strain readings from the adhesive layer were compared to a finite element solution.
Investigation of Damage in Composite Structures under Vibration Using Hilbert-Huang Transform
In this study, the behavior of damaged and undamaged composite structures under dynamic loading is investigated by using Fast Fourier transform (FFT) and Hilbert-Huang transform (HHT) techniques. Unidirectional composite beams with and without damage were tested in vibration in a clamped-free configuration. The damage was introduced by inserting release film of two different diameters at the midplane of the beams. Strain distributions from within the beams were obtained from optical fibers embedded one ply below the surface. The beams were also instrumented with surface-mounted dual-axis accelerometers. The beams were subjected to a frequency sweep of 1-50 Hz with the oscillations induced by a shaker unit. The vibration response using FFT and HHT of the damaged and undamaged composite beams using optical fiber sensors and accelerometers data is presented.
Cure Process Monitoring for Carbon Fiber/Epoxy Composite Laminates Using Tilted FBG Sensors
Tilted Fiber Bragg Grating (TFBG) sensors is an optical fiber sensor with a diffraction grating tilted small angle. TFBG sensors has Bragg resonance, the ghost mode resonance and the cladding mode resonance. It is known that strain and refractive index (RI) can be measured individually analyzing appropriately these resonances. The RI corresponds to degree of cure of epoxy resin, hence TFBG sensors is expected to apply to cure process monitoring of carbon/epoxy laminates and epoxy film adhesive. However, simultaneous measurement of strain and RI has not been established, and a TFBG sensor has not been applied to cure process monitoring of carbon/epoxy laminates. This study demonstrated the applicability of the TFBG sensor for cure process monitoring of carbon/epoxy laminates. Firstly, a simultaneous measurement method of strain and RI was developed by using the TFBG sensor. by using a simultaneous measurement method, RI and strain in curing process monitoring of carbon/epoxy laminates were measured. As a result, the RI due to curing of carbon/epoxy laminates were successfully measured by using the cladding mode. The strain due to thermal expansion and shrinkage could be measured by using the Bragg resonance. Thus, the TFBG sensor is useful to measure RI and strain in cure process monitoring of carbon/epoxy laminates.
Carbon Nanotube-based Flexible Sensors for Human Motion Analysis
Traditionally, carbon nanotubes (CNTs) are integrated with textiles using chemical vapor deposition. Due to the high processing temperature, the use of conventional fibers such as polyester and nylon is challenging. In this research, we use electrophoretic deposition (EPD) to create a thin electrically conductive film of a nanostructured composite consisting of carbon nanotubes functionalized with a dendritic polymer polyethyleneimine (PEI). Different types of fabrics such as cotton, wool, nylon, polyester and aramid can be coated with carbon nanotubes. EPD is inherently scalable because it is performed at room temperature without using any harsh chemicals or volatile solvents. This research is focused on the development of flexible and low-cost wearable technology that can be used to create functional fabrics and smart footwear. Typically, human motion is analyzed using instrumented treadmills and motion capture cameras. Their extremely high cost and complexity make them prohibitive for large scale commercial use. Additionally, the patient/subject can be monitored only for a limited amount of time and not in their natural work/home environment. As a result, a critical need exists for low-cost, comfortable and flexible wearable sensors for human motion analysis. For developing a flexible pressure sensor, carbon nanotubes are deposited on a non-woven aramid veil with randomly oriented fibers. The pressure sensor displays a large in-plane change in electrical conductivity with applied out-of-plane pressure. Upon compression, the number of fiber-fiber contact points between the conductive carbon nanotube coated aramid fibers increases which leads to a decrease in the electrical resistance. The pressure sensors can detect a wide range of pressures from tactile (<10 kPa) to thousands of pounds (~40 MPa). Preliminary experiments of integrating a pressure sensor in the heel of footwear have shown promising results.
An Investigation on the Mechanical Behavior of 3D Warp Interlock Carbon Fabrics
There is a challenge on the use of stiffer carbon yarns into fabric structures to be used as fibrous reinforcement of composite material and their capacity to be draped inside the mold. Thus, one of the solution to cope with this issue lies in the use of 3D warp interlock fabrics. However, it is a challenge to choose the right 3D warp interlock architecture that has optimal performance for the requested application. The aim of this research is filling this gap and explores the potential properties of the 3D warp interlock fabric as a function of its architecture and geometrical properties. The 3D warp interlock carbon fabrics were produced and their tensile properties were investigated in the research. All the fabrics were produced on the same dobby loom with same carbon yarn (6K - 400 Tex) and with similar warp and weft densities. They were differentiated by binding warp yarn parameters, weave diagram pattern and including or not stuffer warp yarns in the woven structure. The carbon yarn strength was controlled at the different fabrication scales from bobbin to fabric in order to track the effect of the weaving process on the yarn strength. No significant loss in the carbon strength is observed. 3D warp interlock fabrics are manufactured with different weave pattern and other weaving parameters. The tensile behavior of these fabrics are evaluated and analyzed in light of the measured shrinkage of the constituting yarns. High impact for the yarn shrinkage value is revealed resulting in a higher breaking strain for the fabric comparing to the breaking strain of the carbon yarn (less than 1 %)
Experimental Investigation of Multiaxial Loading Behavior of UHMWPE Yarns Using a Fragment Simulating Projectile
Ultra-high molecular weight polyethylene (UHMWPE) Dyneema® SK-76 fibers are widely used in personnel protection systems. Transverse ballistic impact onto these fibers results in complex multiaxial deformation modes such as axial tension, axial compression, transverse compression, and transverse shear. Studies on single fibers have showed reduction in tensile strength due to multiaxial deformation and projectile geometry induced stress concentration. In this work, we present the preliminary experimental results of quasi-static multiaxial loading of Dyneema® SK-76 yarns using a 0.30 caliber fragment simulating projectile (FSP) to create a foundation for a failure model. Dyneema® SK-76 yarns are loaded at different yarn starting angles using an FSP projectile and the transverse failure loads are measured. Preliminary experimental results show a little variation in the estimated axial loads with increasing starting angles. This is attributed to the ‘bluntness’ of the FSP’s radius of curvature compared to the diameter of the fibers within the yarn in contact with the projectile.
Design and Fabrication of 3-D Woven Composite Preforms with Complex Topologies
The purpose of this research is to develop a design and manufacturing algorithm of 3-D weaving processes for woven composite preforms with complex shapes, such as integrated woven composite preforms for stiffened structural panels, for tubular components, and for stiffened tubular components. Three design modes are introduced to generate fabric patterns. Warp-yarn paths are defined by two warp matrices, weft-yarn paths by two weft matrices. Fabric topology is then derived from these two sets of matrices. A link is established between fabric topology and weaving matrix, which determines the kinematics of the Jacquard weaving loom. A CAM/CAD tool is developed based on the new algorithm. There are two output files. One relates to fabric topology, which can be applied to determine fabric microgeometry; the other to the kinematics of the Jacquard weaving loom, which can be input into the controller of the 3-D weaving machine. Several detailed design examples are presented.
Interlaminar Fracture Toughness of a Quasi 3D Braided Composite
Fiber reinforced composite materials are a heavily sought after material for next generation vehicles for light-weighting components due to their high specific strength and stiffness. However, when these composite materials are manufactured into laminates, they have relatively weak interlaminar strength and are prone to delamination. This is especially the case when a delamination crack already exists. Quasi-3D (Q3D) braided composites seek to solve this issue by weaving the bias tows into the adjacent (above and below) plies. The plies are physically connected through fiber tows as opposed to being bonded simply by the epoxy, and the composite will achieve a higher interlaminar strength due to fiber failure being required for crack propagation as opposed to simply matrix failure. The [0◦/60◦/-60◦] UD and Q3D carbon composites are investigated in this study for their better in-plane isotropy. Both laminates were manufactured using the vacuum assisted resin transfer molding (VARTM) method with API SC-15 toughened epoxy. The tensile property of the Q3D composite has been found to be competitive with UD composite. Mode I and Mode II interlaminar fracture toughness tests were conducted on UD and Q3D samples. In Mode I experiments, the samples were continuously loaded to full beam split using the double cantilever beam method to obtain the fracture toughness throughout the sample. The Q3D composite shows a large increase in fracture toughness once the crack stabilized and the interlaminar tows become engaged. The Q3D composites also shows a larger energy absorption when the area under the load-extension curve is taken into account. In Mode II testing under end-notch flexural test conditions, the Q3D composite shows
Multi-scale Modeling of Non-Orthogonal Twill Weave Composites
Advantages such as ultra-lightweighting, parts consolidation, and using newly available fast-curing resin formulations are driving the need to develop improved understanding of carbon fiber composites. Towards this end, the present segment of research is focused on developing a multi-scale model to predict the performance of non-orthogonal woven fabric composites. Woven fabrics (where the fiber tows are orthogonal) are used extensively to manufacture components with complex geometries due to their excellent drapability. However, during draping, the fiber tows in the composite preform reorient themselves to conform to the part geometry (the fiber tows are sheared to become non-orthogonal), and the fiber directions may change drastically compared to the original preform, resulting in significant changes in strength and stiffness of the final composite. The ability to include these processing induced effects in predicting the structural performance of these non-orthogonal fabric composites allows us to improve the accuracy of the prediction and optimize the designs. In this paper, a multi-scale modeling approach was developed to predict the performance of composites made using non-orthogonal woven fabrics. Sample coupons molded with and without a pre-determined amount of shearing were evaluated in tensile and three-point bend experiments, and the numerical predictions were compared with the experimental results. A good correlation was observed, validating the developed model.
Effect of Unit Cell Geometry and Fiber Waviness on the Overall Mechanical Response of 3D Woven Composites
Effects of two meso-scale geometry generation approaches on finite element predictions effective elastic properties of an orthogonal 3D woven composite are studied in this paper. In particular, one model is created by simulating the weaving process in the software DFMA (Kansas State University). The second model is created by directly processing X-ray microtomography (μCT) data. Experimental measurements of transverse Young’s moduli are used to inform the accuracy of the predicted elastic results. In both cases, a unit cell with in-plane periodic boundary conditions is modeled, which has not been previously done in the case of μCT-based models. The effect of high frequency oscillations in tow element orientations imparted by a wavy centerline (artifact of μCT image processing) on the elastic properties is studied. The μCTbased model is then used to simulate tension-to-failure.
Investigation of a Twill Orthogonally Woven Textile Composite Under In-Plane Extension
A 2x2 twill orthogonal weave was investigated using detailed finite element analysis that was enabled via a high-performance computing framework. The figure below shows the tow architecture of the finite element model.
High Curvature Bending of Viscoelastic Thin-Ply Composites
This paper presents a micromechanical model for viscoelastic analysis of thin-ply composites. The thin-ply laminate is modeled as a Kirchhoff plate with viscoelastic properties described by its relaxation ABD stiffness matrix. Unit cell homogenizations are conducted at two scales to evaluate the relaxation ABD matrix. A single fiber-matrix model is first employed to evaluate the properties of a unidirectional tow. The computed tow properties are utilized in a laminate model that explicitly represent the weave geometry and ply configuration derived from micrographic analysis. Boundary conditions that enforce the Kirchhoff plate assumptions are imposed on the laminate model to determine the relaxation ABD matrix. Unit cell homogenizations are carried out via the finite element method. The model is applied to a 4-ply plain-weave thin-ply composite. The behavior of thin-ply composite at high bending curvatures and specifically studied for deployable spacecraft structure applications. The viscoelastic response is found to be dependent phase configuration of the plies. Geometrically nonlinear response is also observed at high curvatures.
Strain Rate Dependent Response of Non-Crimp Fabric Composites
The dynamic response of a unidirectional non-crimp carbon fiber/epoxy composite subjected to tension along the transverse and 45° off-axis directions was studied at different strain rates. Intermediate strain rate tests were conducted using a hydraulic intermediate strain rate test frame at approximate strain rates of 0.2/s and 5/s. High strain rate tests were performed with the Split-Hopkinson tensile bar technique at approximate strain rates of 70/s and 180/s. Pulse shaping was developed to achieve dynamic equilibrium during the Split-Hopkinson tensile bar tests, in which pure lead wire was found to be a good pulse shaper for the material tested. The results demonstrated that the applied strain rate had a significant effect on the stress-strain response for the 45° off-axis test specimens, while the transverse test specimens exhibited less strain rate dependence for the rates considered in this study.
Enhancing Load Capacity of Plastic Gears Via the Application of Woven Composite Blankets
In this work, the application of a woven composite fabric covering the surface of the fillets of the teeth of plastic gears is proposed to increase their loading capacity. Tip-relief is also applied to avoid severe edge contacts and peaks of high contact stresses as well as to achieve a smooth load transfer between consecutive pairs of contacting teeth. The investigation is carried out using the finite element method (IGD/ANSYS). A five-tooth model is applied for the gears, and nylon and carbon/nylon are adopted as materials. The evolution of the maximum contact stresses as well as maximum bending stresses is evaluated over two cycles of meshing for both the pure plastic (nylon) gears and the gears with the composite blanket (carbon/nylon). The results indicate that, for the case considered, covering the fillets of the gears with composite blankets reduces the bending stresses in the plastic material at the fillet region but does not consistently alleviate the contact stresses, and subjects the composite blankets to high level of stresses. However, in general, for the specific case studied, the set of gears safely tolerated double the torque recommended by VDI 2545 standard and consequently may be a viable method for increasing the load capacity of plastic gears.
A Model for the Equilibrated Co-Cure of Honeycomb Core Sandwich Structures in Autoclave Processing
Honeycomb core sandwich structures can be fabricated in the co-cure process by bonding partially cured thermoset prepreg sheets with an adhesive layer to either side of the honeycomb core structure under a selected pressure and temperature cycle in the autoclave. The co-cure of honeycomb core sandwich structures is strongly dependent on many materials and process parameters such as prepreg facesheet consolidation, adhesive bond-line fillet formation, porosity development within facesheets and the adhesive, and the dynamic core pressure. This paper describes the fundamental physics associated with the co-cure process of sandwich structures and present numerical models describing the facesheet consolidation, adhesive bond-line fillet formation, and bond-line porosity during the co-cure process.
Processing of Post-Industrial Unidirectional Prepreg Tapes Using SMC Equipment
The aerospace industry has an issue of what to do with waste, unused carbon fiber unidirectional prepreg tapes. One idea is to repurpose unused prepreg tapes for the manufacture of sheet-molding compound (SMC). The resulting material, per Sanzida [1], should be stronger due to their carbon fiber and high strength epoxy resin, as compared to traditional SMC which has lower strength glass fibers and epoxy resin. The cure state of the epoxy resin in the prepreg tapes, which is affected by the amount of time the material has spent at elevated temperatures or room temperature, significantly changes the ability of the material to be chopped into short fiber SMC. In addition, several equipment-related factors impact the material’s ability to be chopped into SMC. This thesis characterized each of the factors impacting the process and used statistical methods to design experiments to investigate the impact each of these material-related and equipment-related factors on the success of the cutting portion of the process. The level of cure, as defined by endothermic peak, glass-transition temperature (Tg), or heat of reaction/degree of cure, has the greatest impact on the cutting process. For material with a moderate level of cure, the sharpness of the blades in the cutting roller also has a major impact on the success of the process. The cutting pressure, roller speed, and tape tension also have an impact on the success of the process, but impact of these factors lessens for feedstock prepreg with a sufficient high level of cure. An optimal setup was determined, and general rules of thumb for selecting material and equipment settings were established.
Glass Fiber Reinforced Polyester Syntactic Foam
Glass bubbles (GB) have been used as additives in polymer composites to produce light weight syntactic foams. Developments in light weight polyesters have provided an avenue for a class of low density (<1.5 g/cm3) composites. Low density glass fiber (GF) reinforced syntactic polyester (PE) foams were manufactured by incorporating GB into the PE resin paste prior to sheet molding compound (SMC) manufacturing. This study examines the mechanical properties of a syntactic foam composite in comparison to a baseline GF reinforced PE composite. Resin formulations are similar to those used in industrial SMC practice, and are modified with GB to produce syntactic foams. The mechanical properties are characterized by a variety of techniques including tensile testing, flexural testing, and impact testing. The rheological behavior of the PE paste prior to SMC fabrication is also determined. The syntactic foam composites are expected to reduce the density of the final composite without reducing the volume fraction of GF, thereby lowering part mass while maintaining mechanical integrity.
Predicting Defect Formation at Early Stages of Manufacturing Process
Combination of wrinkles and voids are common manufacturing defects in laminated composite components and structures. In particular, composite helicopter rotor components are typically thick and often have areas with tight radii of curvature, making them especially prone to process-induced defects. Wrinkles and voids may significantly affect structural integrity and increase rejection rates in the production of rotor composite components. Recent advances in high-fidelity non-destructive inspection of composites, such as X-ray Computed Tomography (CT), have shown that reduction of strength and fatigue performance of laminates can be strongly related to shape, size, and location of critical individual defects. However, none of the existing tools available for process modeling are able to predict the formation of such individual defects, including their geometry and position in the composite parts. This work is part of the Office of Naval Research (ONR) project “Physics-Based Composite Process Simulationâ€ that seeks to fill the gaps in understanding the underlying physical principles governing the formation of manufacturing defects. In particular, there is strong evidence that improper laminate consolidation at early stages of the laminate manufacturing process, including debulking or vacuum consolidation, is one of the major root causes for both wrinkle and void formation in autoclave composites. Therefore, understanding and modeling defect formation at the early stages of the manufacturing process might be the missing link to enable development of practical engineering solutions for better control of the manufacturing process This work presents the latest developments of a simulation method for predicting defect formation during vacuum consolidation using discrete modeling of the large pockets of entrapped air at ply interface in resin-saturated autoclave tape prepregs. Experimental procedures developed for measuring critical input data used in the numerical models and the fabrication of a rotor blade grip test element for verification of applicability of the method are also presented.