Design, Manufacturing and Applications of Composites 2014
Prediction Method of Internal Structure for Designing Braided Composites
Braided fabrics are one of typical textiles along with woven fabrics and knitted fabrics. The schematic image of braided fabrics was shown in Fig.1. Braided fabrics were composed of a lot of yarns which oriented diagonally called braiding yarns (BY). And yarns called middle-endyarns (MEY) can be inserted into braiding yarns along the longitudinal direction. As materials for braiding yarns and middle-end-yarns, various kinds of fibers, such as glass fiber, carbon fiber and aramid fiber, can be used. One of the important features of braided fabrics is the capability to change the orientation angle of braiding yarns called braiding angle (at ±θ degrees to longitudinal direction). Braiding angle can be changed freely as shown in Fig.2. In addition, another unique feature of braided fabrics is the continuity of all fibers. All fibers are continuously oriented from one end to the other end and subjected to forces evenly. Therefore braided fabrics have been expected to be excellent preforms for reinforcements of composite materials with excellent mechanical properties, and it enables to produce excellent structural parts according to various kinds of requirements.
Damage Initiation and Growth in Composite Laminates of Wind Turbine Blades
The physical properties of composite materials, such as high strength-to-density and stiffness-to-density ratios, are key properties for wind turbine blade structure. These materials are however vulnerable to damage during service. The static failure of composites occurs in two stages: 1) onset of damage and 2) damage evolution that leads to final failure. The response of damaged composites depends upon a mixture of mechanisms that take place at the micro level, i.e., in the fiber and the matrix. Consequently a model is proposed for predicting ultimate strength of composite laminates based on the constituent’s properties; the fiber, the matrix, and the interface. For onset of damage, the Stassi Equivalent stress model eq,m ï³ is used for the matrix. This model take cares of different tensile and compressive strengths of the matrix. For the fiber, the failure criterion for onset of fiber breakage is related to fiber strength.
Thermal Properties of Titanium Boride Particle Reinforced Pure Aluminum Composites
20vol. % TiB2 particle dispersed pure aluminum composites were fabricated by spark plasma sintering process. Before sintering, two types mixing processes was performed in order to obtain the composites with different dispersivity of particles by dry and wet mixing process. The dispersibility was estimated by two-dimensional local number (LN2D), quantitatively. TiB2 particles in the composites fabricated from dry process shows higher clustering behavior than that from wet process. On the other hand, the density of the composites fabricated from dry and wet processes is 99.8% and 97.0 %, respectively. The electrical conductivity and the thermal conductivity was estimated. As increasing dispersibility of the particles, the electrical conductivity was depredated. This tendency was verified by theoretical model, but the experimental value was lower than theoretical value, which is affected by the plastic deformation region in matrix around the particles introduced by the difference of thermal expansion of TiB2 and Al. On the other hand, the thermal conductivity of the composites fabricated by dry process is 119.2W/mK, which is higher than that by wet process (117.2 W/mK) in spite of the degradation of relative density, which is caused by the dispersibility of particles in composites.
Experimental Investigation of the Tensile-Tensile Fatigue Behaviour of Carbon/Epoxy Plain Weave Laminates
In order to develop safe designs of aerospace composite structures it is important to study the effect of manufacturing flaws such as delamination on the fatigue behaviour of composites. The objective of the present study is to determine the fatigue behaviour of carbon/epoxy plain weave laminates containing artificial flaws under tensile loading. The standard test coupons contain a Teflon insert of pre-determined size, shape and location, which simulates a common manufacturing defect. The coupons were subjected to tensile load-controlled cyclic loading with a stress ratio R = 0.1 and frequency f = 7 Hz. A failure onset criterion was proposed to determine the delamination propagation threshold under cyclic tensile loading. The methodology used for this purpose is based on the changes in the test coupon’s compliance which was measured using a high-speed video-extensometer. The propagation of delamination was verified using the ultrasonic imaging (C-Scan) technique and validated by optical microscopy observations of the coupons’ cross-sections. A S-N curve corresponding to the delamination onset was established permitting to determine the fatigue life of the specimen versus the maximum applied stress.
Fabrication and Characterization of SWCNT Reinforced Aluminum Composites
Single-wall carbon nanotubes (SWCNTs) were successfully incorporated into pure aluminum by a modified powder metallurgy route in which the SWCNTs were very homogeneously dispersed in the aluminum matrix. Sintered densities of over 99% of its theoretical density were achieved through the optimization of the processing conditions. This composite has a potential application for the power transmission cables since the addition of SWCNTs improves the strength of aluminum while maintaining its high electrical conductivity and also makes the electrical power cables much lighter than the traditional aluminum composite cables. In addition, the electrical current carrying capacity (ampacity) is increased. The relationships among pressing pressure, sintering parameter and sintered density in the fabrication process of SWCNT reinforced aluminum composites were discussed and the electrical properties were also measured for both composites and aluminum matrix.
Integration of Carbon and Boron Nitride Nanotubes into Ceramic Materials
Carbon nanotube (CNT) can be conceptualized as a rolled-up sheet of one (or more)- atom-thick layer of graphite, i.e. graphene. Owing to their extraordinary mechanical properties, e.g., tensile strength of up to ~100 GPa, CNTs find applications as additives to various structural materials, such as ceramics. The CNT-reinforced ceramics are capable of overcoming ceramic’s intrinsic brittleness and relatively low fracture toughness by absorbing energy over a large area, and consequently increase the ballistic performance of the ceramic plate.
Integration of Nanotube Paper into UHMWPE Multilayer Structures
Ultra-high-molecular-weight polyethylene (UHMWPE) fabrics and UHMWPE composite materials are increasingly used in modern armour due to their high strength-to-weight ratios and energy absorption capacities. Commercially available sheets of UHMWPE fibre composites can be layered and compacted to form armour structures. Selection of appropriate materials and design of the structure are effective approaches to develop energy dissipation materials with high mass specific energy absorption. Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) exhibit unique mechanical properties, having excellent compression capability coupled with extreme structural flexibility and recovery. Macroscopic assemblies of CNTs with low density and excellent compression capability, with potential applications as energy absorption and protective material, have been developed (1, 2). CNT buckypaper is a free-standing nonwoven sheet, like conventional paper, composed of entangled CNT ropes. Such sheets are easy to handle and can be integrated similarly to other fiber fabrics into conventional composite manufacturing processes to fabricate nanocomposites. In this study, we explore the idea of layering CNT or BNNT buckypaper (BP) and UHMWPE fabrics to form composite panels with improved energy absorption capacity. The layered hybrid composites were fabricated by alternating UHMWPE plies and BP sheets using a hot-compression technique. To incorporate the BP into the UHMWPE panel, the BP was modified with a thermoplastic resin.
Development of Hybridized Graphene Nanoplatelets/Fiber-Reinforced Composites with Enhanced Interlaminar Fracture Toughness
In this research, the effect of inclusion of as-received graphene nanoplatelets, and chemically modified graphene nanoplatelets (i.e., graphite oxide (GO) and silane-functionalized (Si-G)), on the fracture toughness of epoxy resin and mode-I interlaminar fracture toughness of glass fiber-reinforced epoxy laminates are experimentally investigated. Raman spectroscopy and thermo-gravimetric analysis (TGA) are employed to characterize the chemical and structural changes the modified nanoplatelets. The nanocomposites containing Si-G provide the best results for the fracture toughness (82% improvement). The critical energy release rate of all laminates also increased; however, the ones bonded with Si-G reinforced epoxy gained the most. The SEM micrographs revealed that a superior dispersion status could be obtained by GNP functionalization, in turn generating more optimum fiber/matrix interaction.
Effect of Initial Notch-tip Damage on Failure Behavior of Composite Laminates .
Tensile static tests of carbon fiber reinforced composite laminates (CFRP) with a machined notch and an impact notch penetrated by a foreign object were conducted and non-destructive and destructive inspections of the damaged specimens were done to understand the failure mechanism and the effect of initial damage on the damage accumulation process and final failure behavior. The tensile tests of several damaged specimens were suspended several times until final failure to observe the internal damages by using X-ray CT scan machine. A specimens was cut and polished to observe the internal damage state. With the present careful observation, interesting behaviors of the damage growth were found, such as the strong effect of the matrix cracks in the ±45° layers on the fiber failure in not only 0° layers but also 90° layers, dependence of damage growth processes on the initial damage condition and dependence of final strength on the damage accumulation process.
Knowledge in Practice: The Next Step in Reducing Risk in Composites Manufacturing and Design
Advanced composites is an emerging technology, rapidly growing in importance. In recent years all major aerospace OEMs have invested significantly in this technology and its utilization in non-aerospace sectors is also fast increasing. Nevertheless, the insertion of this promising technology is threatened by persistent manufacturing risk. The inability to correctly identify this risk early in program development means that composite manufacturers cope inadequately with unanticipated manufacturing challenges and late engineering design changes. Over the past 20 years attempts to reduce composites manufacturing risk have focused on three key fundamental research thrusts: process simulation, integrated computational materials engineering and informatics. Although robust enabling tools have resulted from these research efforts and are capable of contributing to the reduction of manufacturing risk, overall composites manufacturing knowledge remains relatively poor and incomplete. Many manufacturing outcomes still exist that cannot currently be predicted with models. We propose that the necessary next step in reducing composites manufacturing risk is an approach we call Knowledge in Practice. This approach aims to systematically integrate and balance the creation and application of composites manufacturing knowledge, in whatever form it may exist. In this paper, we introduce this aspects of this pragmatic approach and use a thermal management problem to demonstrate how Knowledge in Practice, in a case where the knowledge base is relatively mature, can address composites manufacturing risk efficiently and effectively.
Dynamic Properties of Hybrid Composite Hollow Cylinders: Application for Mountain Bicycle Handlebars
Significant interest in natural fiber composites by industry is being predominately driven by recent environmental policies. Hybrid composites of natural and synthetic reinforcement are a growing field, with the intent to develop a composite material with the damping advantages of natural fibers and the strength characteristics of synthetic fibers. These material characteristics are advantageous in sports equipment, especially bicycle design where stiffness and damping are desired for optimum power transfer and comfort. In this investigation, a hollow cylinder cured with internal bladder pressure was studied. The static strengths of the flax and carbon fiber laminates were first determined to assess the effect of the geometry and manufacturing process on performance. The dynamic characteristics of various hybrid layups were then modeled using Finite Element Analysis (FEA) software, focusing on optimizing modal behaviour and flexural strength. FEA results were then used to select two laminate testing groups to determine the effect of flax layer placement and number of flax layers in the hybrid composite. The investigation results will be used to design a prototype mountain bicycle handlebar, which will be compared to an existing carbon fiber benchmark to evaluate dynamic behaviour design improvements.
Improvement of Fatigue Life of Plain-Woven CFRP Modified with Enhanced Matrix with Sub-Micron Sized Glass Fibers
This study discussed on the effect of the size of added glass fibers into the epoxy matrix on the fatigue life and its internal crack growth along carbon fiber bundle of the plain-woven CFRP (Carbon Fiber Reinforced Plastic). Static bending strength, fatigue lives and fracture toughness were measured after modification with added glass fibers. The resistance of debonding was characterized with double cantilever beam Mode-I fracture specimen. In addition, local Mode-I and Mode-II fracture toughness around carbon fiber bundle were also discussed with model specimen fabricated with carbon fiber bundle embedded in the modified epoxy matrix. Fatigue life of CFRP was dramatically 10-100 times increased by addition of sub-micron sized (close to nano-sized) glass fibers. Local Mode-I and Mode-II fracture toughness was 3-7 times improved when epoxy matrix was modified with sub-micron sized glass fibers compared with unmodified case. When the model specimen was modified with 500nm glass fibers, fatigue crack growth rate was much decreased than that with 11μm glass fibers. It was found by current study that sub-micron sized glass fibers should be selected as the enhancer to prevent internal crack for improving the fatigue life and other mechanical properties of plain-woven CFRP.
Moisture Effects on Mixed-Mode Delamination of Carbon/Epoxy Composites
An investigation of the effects of moisture on fracture and fatigue performance of a carbon/epoxy composite is presented, including delamination toughness and growth for a range of mode mixities from mode I to mode II loading. Experimental static and fatigue tests were carried out on composite specimens. The static test results show that exposure to moisture led to a decrease in delamination toughness under mode II and mixed-mode I/II loading while enhancing delamination toughness under mode I loading. These results are compared with existing mixed-mode fracture criterion to determine which model better predicts mixed-mode delamination toughness for both dry and moisture conditioned specimens. Fatigue tests demonstrate that moisture has an adverse effect on delamination resistance of the composite under loadings of all mode mixities. An increase in delamination growth rate for exposed specimens is also observed for all mode mixities. Existing delamination growth rate models are compared with experimental values to determine the most suitable model for predicting delamination growth rates for these specimens.
SHM of Composite Patch Repairs Using Chirped Fiber Bragg Grating Sensors
The application of health monitoring systems to reduce the penalizing safety factors in composite repairs such as joints and adhesive patches is of considerable importance in critical components where non-destructive tests are difficult to perform. In this paper, a structural health management strategy to monitor patch repairs in carbon/epoxy structures is presented. The feasibility of using Chirped Fiber Bragg Grating sensors was analyzed with gradual heat tests, while evaluating their capability of sensing the presence of strain field variations along their physical length. The presence of patch debond was obtained by the reflection spectrum from the sensor, and also its size was estimated with reasonable accuracy. The same set of sensors were placed and embedded into patches, and two methods were used based on the calibration charts and tables obtained experimentally. For evaluation and verification, a finite element analysis of the problem was carried out, supported by tensile tests for material characterization. Good results were obtained from the experiments.
Development of Carbon and Other Fiber Tow Spreading System for Thin Fiber Reinforced Composites
In order to expand the application of fiber reinforcement as light-weight material it is important to spread the fiber tow in thin geometry. There are various methodologies to spread the fiber tow, however, the spread fiber tow was sometimes not uniform and its cost was very high. In this study we have developed the fiber tow spreading system to obtain the uniform thin fiber tow, and have applied this developed system to fabricate thin fiber tow reinforced adhesive tapes. In this developed system the mechanical load was applied to the fiber tow through the plate with arc-shape edge. By using this system we tried to spread the UHMWPE fiber and aramid fiber.
Stress Analysis of a Thick Composite Laminate with a Bolted Joint
The present study concerns with the stress and strain distributions around a bolted joint and inside of a thick laminated composite plate. The high specific strength and stiffness of composite materials make them an appropriate choice in different industrial applications. Today use of thick laminates is developing at a faster rate and bolted joint is the most convenient method to connect them to other parts of an assembly. In the present research work the bolted joint effects on stress and strain distributions in a unidirectional glass epoxy laminate with a thickness of 0.72 inches have been studied using both finite element (FE) and digital image correlation (DIC) methods. A full 3-D finite element model consisting of the plate, washer and bolt is developed using ANSYS. To verify the simulation, DIC test is performed to find the strain distribution around the washer and also through the thickness of the laminate. In addition, effects of different parameters such as the bolt and washer size and stiffness, clamping torque, layer orientations and external loadings on the structural behavior of the plate are investigated.
Modelling and Optimization of Curvilinear Fiber Composites
The current study focuses on improving the structural performance of curvilinear fiber composites in plates and shells. Possible applications for these types of composite structures include wind turbine blades that exhibit low tensile and shear strength in the out-of-plane direction, low circumferential stiffness and critical production defects. These issues can be addressed by using curvilinear fibers to provide an optimal variable stiffness profile in composite laminates. The design of plate type structures as a level of abstraction of blades is optimized by using Discrete Material Optimization (DMO) technique to determine the fiber angle distribution. In the optimization process, the shape and the thicknesses of the structures are considered fixed and the maximum stiffness of blade is considered as the design driver. Numerical results illustrate that curvilinear fiber paths can increase stiffness and structural performance.
High Fidelity Numerical Simulations of Textile Preform Deformation
The complexity of dry textile preforms is inherent in their multi-scale nature, where their behaviour at the macro-scale is dependent on the yarn interactions at the meso-scale and the fibre interactions at the micro-scale. Low internal cohesion between the individual yarns and fibres allows for large relative displacements. Consequently the internal architecture of the fibrous preform is able to deform during compaction processes ultimately changing the mechanical behaviour of the material. Therefore, for better predictability, modelling of the internal architecture of the preform must not only consider the as-woven state of the fabric but must also be able to accurately simulate the structural changes which occur to the fabric during the processing performed to achieve the final composite. The Multi-chain digital element method has proven to be a promising solution for capturing this behaviour. Initially proposed by Zhou et al. (1) the method has seen substantial development in increasing accuracy, computational efficiency and use for mechanical modelling (2, 3, 4). The present paper will explore the level of detail this method is able to capture during compaction processes and its flexibility in modelling a range of standard two dimensional woven fabrics as well as more geometrically complex non-crimp fabrics.
Modeling of Out-of-Plane Stresses in C/PPS Composite L-Shape Brackets
With the increasing use of composites in civil aviation, thermoplastic composites have become an interesting material choice for repeating part numbers such as fuselage brackets. The simple L-shaped bracket geometry can be mass-produced using stamp-forming manufacturing. When subjected to opening loads, out-of-plane stresses, i.e., interlaminar tensile stresses, develop in the 90° corner radius. Those gradually increasing stresses ultimately lead to delamination failure. One approach to evaluate the interlaminar tensile strength is to test a curved beam using a four-point bending apparatus to create an opening load. This test is described in the ASTM D6415 standard. The geometry of the bend (inside/outside radii), the materials and the lay-up sequence have a strong influence on the mechanical performance of the L-shaped bracket. Analytical equations were developed to evaluate the radial interlaminar tensile stresses in the bend. However, these equations can be erroneous for non-unidirectional stacking sequences. Moreover, geometric non-linearity can occur when flexible coupons are subjected to large deformation. In those cases, modeling can be used to evaluate the interlaminar tensile strength between layers at a given applied load. In this study, a finite element model of the ASTM D6415 test was created to calculate the out-of-plane stresses in the radius. The use of parametric modeling enables the simulation of various coupon configurations. The model is compared to the analytical equations and carbon fiber/polyphenylene sulphide composites L-shaped brackets of different stacking sequences, thicknesses and bend radii.
Exact 3-D Stress and Stiffness Analysis of Functionally Graded Cylindrical Shells
The deflection and stress distributions of metal-ceramic functionally graded cylindrical shells are determined in the present work based on the Sampling Surfaces method (SaS). The material properties are assumed to vary continuously along the radial direction according to a power law distribution of the volume fraction of the constituents. In the three-dimensional formulation developed, the displacement field of not-equally-spaced surfaces parallel to the middle surface of the functionally graded shell is expressed as fundamental shell unknowns. The straindisplacement relationships, which are invariant rigid-body shell motions in the convected curvilinear coordinate system are then determined. By using sufficient number of sampling surfaces located at Chebyshev polynomial nodes, the exact 3-D elasticity solution for the functionally graded cylindrical shell is obtained. The effects of material properties and shell geometry on both the deflection and 3-D stress field are determined. The results of the present analysis are compared with the existing solutions available in the literature. The results reveal that by considering large number of sampling surfaces inside the thickness of the cylindrical shell, the accuracy of the solution can be improved significantly wherein the error will approach zero. A practical value for the number of sampling surfaces is determined and presented.
Current Challenges Toward Accurate Forming Simulation of Woven Fabric Composite Reinforcements: Roadmap for a Probabilistic-Based Design
Forming process of engineering materials is often simulated using numerical/computer tools such as Finite Element methods. Although this type of simulation has a long history in the metal forming and polymer industries, it is not as widely established when it comes to high performance composites. This is due to the fact that the mechanical behaviour of most composites is different from monolithic materials and extended research on their physical and mechanical behaviours is still under development. Moreover, among different types of composites, textile composites because of their multi-scale nature and fibrous architecture have more sources of embedded uncertainties. In this article, selected sources of uncertainties in the woven fabric type of textile reinforcements, their manufacturing and simulation process are discussed. In addition, applications of statistical techniques for their characterization as well as sensitivity analysis on the effect of uncertainties on their overall behavior are addressed. Subsequently, a combination of finite element simulation and statistical data analysis is suggested to estimate, and eventually to control, the performance variability in the formed part based on uncertainties observed in the raw material, process, and simulation itself.
Fabrication and Application of Lignin-Lignin Composites
Lignin, the second most abundant native polymer on Earth, can be a major source of renewable materials. This low cost material has potential in composites as a matrix and/or reinforcement in resin, filler, fibre, and aerogel forms. In the resin form, lignin and its derivatives can be used to make coatings, coupling agents, and composites, and can form stable mixtures. Some of the applications, for instance, can be in the wood industry, civil and electrical engineering. In the fibre form, lignin, a promising precursor candidate for manufacturing carbon fibre, can replace petroleum based precursors, such as polyacrylonitrile and pitch (petroleum or coal). The lignin-based composite nanofibres reinforced by carbon nanotube/nanocrystalline cellulose can be used for automobile and structural applications. In addition, the fibre can be functionalized by embedded nanoparticles and/or surface modification. The concept of multifunctional carbon nanofibre can be extended to a wide range of advanced applications in the energy, electronic, and environmental sectors. In the aerogel form, lignin-based carbon composites, due to their unique network structure, low density and high surface area, are promising materials as electrodes for hydrogen and electrical storage, supercapacitors, thermal and acoustic insulators, adsorbents, catalyst supports and sensors.
Development of Pultrusion Method for Fabricating FRTP and FRP with Phenolic Resin
Phenolic resin has originally advantages of heat proof, fire resistance and less smoke during burning and a good balance between its cost and mechanical properties compared with other resins using in FRPs. The composite with phenolic resin having these advantages is the one of the suitable materials for structures of vessels and railway carriages. This paper presents a pultrusion method for fabricating fiber reinforced thermoplastics (FRTPs) and fiber reinforced plastics (FRPs) using a novolak phenolic resin. The novolak resin with curing agent is solid state under the room temperature and can be changed to two states of thermoplastic state and thermosetting state according to a heating temperature. Therefore, the pultrusion method combined with an extruder for fabricating the phenolic FRTP was developed. Next, two kinds of FRPs were fabricated by using the developed FRTP. One was a subsequent pultrusion molding and another was a hot press molding, respectively. In the hot press molding, the FRTP was used as an intermediate material. In addition, their mechanical properties were compared with each other based on tensile and bending tests.
Fabrication of Conductive Microfilaments and Liquid Sensor from CNTs/PLA Nanocomposites
Electrically conductive composites (ECCs) have recently shown a very high potential to replace metals for several applications such as electromagnetic interference (EMI) shielding, sensors, antistatic coatings, and flexible electrodes. The ECCs are mainly composed of a conductive filler dispersed in a polymer matrix. In this work we demonstrate the fabrication of a nanocomposite by dispersing carbon nanotubes (CNTs) in polylactic acid (PLA) by using the ball mill mixing method. Nanocomposites with various concentrations of CNTs in PLA were fabricated and their electrical conductivities were measured. The ball mill mixing method enabled us to make CNT/PLA with a very high concentration of CNTs (up to 40 wt.%). The PLA/CNTs were extruded from a micronozzle to fabricate nanocomposite filaments with a diameter as small as ~ 70 μm and a high electrical conductivity reaching up to ~4000 S/m. Conductive porous structures (CPSs) were fabricated by hot pressing the conductive filaments. The application of these CPSs is shown here as a liquid sensor since their electrical resistivity changes when they get in contact with acetone. These CPSs are promising candidates for liquid sensor and EMI shielding applications due to their lightweight, mechanical flexibility and electrical conductivity.
Modelling of Compression Moulding Process Cycle Time for Carbon Fibre Reinforced Thermoplastic Flat Panels
In the process of developing new manufacturing technology to make aircraft parts, a cost model has been proposed to predict the manufacturing costs of two compression moulding processes for carbon fibre thermoplastic composites. This model was based on industrial and academic research data [1]. Later on, DFMA concept was used by the same authors to estimate the tooling costs for these two processes [2]. This study aimed at the prediction of the cycle time of a compression moulded carbon/PEEK composite flat panel made of short fibre randomly oriented prepreg strands. The compression moulding cycle time included the heating and cooling times of the part. Commercially software COMSOL was used to calculate the cycle time of flat panels of different sizes. The obtained results were validated by experimental data realised in a scientific research laboratory. Keeping the same cycle time, the flat panels moulding process energy can be predicted for other similar geometries in function of either area or thickness of the mould and the flat panel.
The Effect of Processing Parameters on Intra-ply Shear Property of Out-of-Autoclave Carbon/Epoxy Prepreg
During the sheet forming of composites, various deformation mechanisms must occur to reach the desired shape without flaws; such as, in-plane waviness and out-of-plane wrinkling. Understanding of these deformation mechanisms is required to reduce the failure rates of the composite parts and to optimize the processing parameters. In this study out-of-autoclave 8-Harness satin woven carbon-epoxy prepregs were investigated to determine their in-plane shear properties using the picture frame test. The load responses at various displacement rates were determined on fabrics of varying areal density. The locking angle at which adjacent yarns come in contact with each other with no further deformation which eventually leads to wrinkling has been investigated. The effect of temperature on wrinkling was determined using the sequential images taken by digital image co-relation (DIC). The study shows that angle of lock-up is prolonged with the increase in temperature.
Dispersion of Cellulose Nanocrystals in a Water-Methanol Co-solvent System
This study reports how a co-solvent system of water and methanol can be used to better disperse cellulose nanocrystals (CNC) into a polyvinylpyrrolidone (PVP) matrix through careful selection of mixing parameters and process steps. With appropriate order of mixing, a purely mechanical means of magnetic stirring and sonication provides good homogeneity without the need for chemical surface treatments of the CNC. The effect of preparation method on dispersion and solution rheology were characterized for CNC-PVP mixtures of up to 20% CNC by weight. A pre-dispersion of CNC into pure water before the addition of methanol was found to have a dramatic impact on the rate of agglomeration of CNC particles as well as the viscosity of the solutions of CNC-PVP-water-methanol. Birefringence measurements of the dried CNC-PVP mixtures demonstrate how the agglomeration is dramatically reduced through an appropriate predispersion step.
Pultrusion Process for Continuous Fiber Reinforced Thermoplastic Composites
The purpose of this study is to construct frame structure of continuous fiber reinforced thermoplastic composites with high impregnated intermediate materials and the high cycle molding. For the continuous straight pipe, pultrusion systems by using braiding technology with fibrous intermediate material were employed. Braided fabric was pulled in the pultrusion die having a cylindrical hole. While the resin fiber of the braided fabric was passing through the die, it melted and impregnated into reinforcing fiber. For the branched pipe joint structure, inner pressure molding by heating device of high frequency IH was adopted as high cycle molding for braided fabric reinforced thermoplastic composites. Braiding technique with two guide-rings was proposed to fabricate the braided fabric of pipe joint structure. Finally frame structure was obtained by connecting pipe joint product and cylindrical product
Stamp Forming of Continuous Fiber S-shape Thermoplastic Composites
Manufacturing of complex-shaped continuous fiber thermoplastic composite parts has received increase attention lately. Stamp forming is a cost effective and rapid shaping method for the thermoplastic composites that has similar mechanism with compression moulding technique. The main steps include heating up the blank to the melting point, pressing and then cooling. In this study, stamp forming was used to make an S-shape thermoplastic composite part. While most investigations are done for L- or U-shapes, the part studied has two opposite curvatures with different radii. The material was carbon fabric/PPS provided by TenCate®. Different layer sequences were considered: thin cross-ply, thin quasi isotropic and thicker quasiisotropic laminates. DMA and DSC analyses were performed to determine the Tg and degree of crystallinity. These were used to determine the processing conditions which include blank temperature before molding, tool temperature, compaction force and time. CMM was used to measure the sample dimensions after stamping. It was observed that the actual part angle and radii are not the same as those of the tool.
Numerical and Experimental Investigation of Prepreg Compaction for Defect Formation Mechanisms
Manufacturing defects in composites materials are responsible for severe reduction in mechanical properties. For example, it is reported in the literature that out-of-plane waviness can cause up to 36% knockdown on the overall strength of composites parts. The formation of fibre wrinkling is influenced by through-thickness compression and shear forces during processing and is one of the more detrimental defects that can be found in composites materials. In the present study an experimental and numerical programme is undertaken to explore the compaction behaviour of carbon fibre prepreg material under processing conditions consistent with automatic fibre deposition, hot debulking and pre-cure consolidation in an autoclave. Two main concurrent deformation mechanisms were observed in a bespoke experimental programme; shear flow, where the laminate behaves as a highly viscous incompressible fluid, and percolation flow where the pressure gradient causes resin flow relative to the fibres. An analytical model, based on micro-structural considerations and addressing the percolation/shear flow transition, was developed. A 3D hyper-viscoelastic constitutive relation was then constructed based on this analytical model and implemented in implicit formulation within the finite element package Abaqus/Standard. The model was validated against compaction experiments over a wide range of load rates, temperatures, and laminate configurations
Relationships between Degree of Skill, Dimension Stability, and Mechanical Properties of FRP in Hand Lay-up Fabrication Method
Hand lay-up fabrication has been used for forming composite structures since ancient times as it can be performed as long as the mold, skills, and materials are available. On the other hand, the hand lay-up fabrication work itself relies on human skills, which means that the finish differs according to the operator carrying out the work, the quality of the product differs among parts depending on the ease of forming, and that defects such as bubbles tend to occur easily. Hence highly specialized control technique and the tradition of skill are required to ensure the consistent stability of product quality. For this reason, the tradition of molding techniques needs to be carried on as quickly as possible. In this study, the authors thus conducted a motion analysis experiment using hand lay-up fabrication experts as subjects. The experiment, seemingly a new and only attempt in Japan, quantified techniques that are not visibly apparent and considered to be tacit knowledge. The mechanical properties and dimension stability of samples were measured, and their relationships with the motions of experts were also evaluated. It was also suggested that highly specialized control techniques, the appropriate training of nonexperts, and technical tradition are possible
In-situ Measurement of Cure Index and Strain of FRP Laminates by Fiber Optic Sensors
Recent sensing technologies allow us to monitor cure process in FRP materials at real time. Fiber optic sensors are most promising for measuring cure index, cure shrinkage and residual strain. In the present paper, we used refractive-index sensors and FBG (Fiber Bragg Grating) sensors in order to measure cure index of matrix and curing strain in FRP laminates during a hot-press molding. Refractive-index sensors, FBG sensors and thermocouples were embedded in the laminated prepregs. From the measured refractive-index variations and temperature, we estimated cure index of matrix resin from B-stage state to cure completion. The experimental results showed that the cure index curves obtained by the refractive-index sensors showed good repeatability. Comparing results of sensors embedded in 0° direction with 90° direction, it appeared that the both results agreed very well with each other. However, the cure index curves measured by the sensors embedded in resin rich region showed apparent delay from the curves by the sensors in FRP region. Strain profiles were obtained by the FBG sensors embedded in 0° and 90° of unidirectional FRP laminates. From the results, it was found that strain of laminates showed isotropy when the cure index was small
Fabrication and Mechanical Properties of Noncrimp CFRP Pipe by Braiding Technique
This study is aimed to introduce a non-crimp tubular preforming technique (MFW) by using braiding technique. Observing the fiber orientation state clarified that MFW CFRP pipe had non-crimp fiber structure. A new preforming mechanism was invented by improving MFW method from the productivity point of view. Results of calculation show that the improved MFW indicated 81% higher productivity than the braiding. It became clear that MFW method has a high potential to realize the high productivity. Furthermore, CFRP pipes with non-crimp structure were made by MFW, and the lateral compressive tests were performed in order to investigate mechanical properties comparing with braided CFRP pipes. From these results, it was clarified that non-crimp structure had the higher capability to improve bending modulus than braided structure for small winding angle such as 30°.
Effects of Distance Between Electrodes, Proximity of Electrodes to Damage, Plate Size, Carbon Nanotube (CNT) Concentrations and Damage Severity on Change in Electrical Resistance of In-situ Damage Detection, Location and Quantification in Glass Fiber/Epoxy Plates Contaning CNT
This paper explores the possibility to use carbon nanotube (CNT) networks for the development of a structural health monitoring (SHM) technique for large polymer composite structures (LPCS) made of electrically non-conductive fibers and CNT networks using electrical resistance measurement (ERM). The effects of distance between electrodes closest to damage, proximity of electrodes to the damage location, plate size, CNT concentration and damage severity on the change in electrical resistance are investigated. The experimental results demonstrate that the change in electrical resistance increases with reducing distance between electrodes closest to damage, distance between electrodes and damage location, plate size, CNT concentration and increasing hole size
Piezoelectric Zinc Oxide Woven Fabric Sensors for Dynamic Force and Damage Sensing
The objective of this work is to develop intelligent integrated composites capable of detecting the applied dynamic strain of composite structures. The composites consist of fabrics composed of interwoven bare conductive carbon fibers and orthogonal carbon fibers coated with ZnO nanowires. Nanowires with diameter range between 80 and 344 nm with average length of ~5 μm are grown on carbon fibers using hydrothermal technique. X-ray diffraction results indicate that the ZnO nanowires have a hexagonal wurtzite crystal structure that is responsible for the piezoelectric form of ZnO. The functionalized fabric was subjected to mechanical deformation and revealed piezoelectric behavior with effective piezoelectric coefficient of 2-3 pC/N. The proposed hybrid fabric can serve as the structural backbone for aircraft, automobile and many other structures allowing for integrated monitoring of structural health in real time
Highly-sensitive Fiber-optic Ultrasonic Sensing Systems for Damage Monitoring in CFRP Laminates
Optical fiber sensors (OFSs), especially fiber Bragg gratings (FBG), have been applied to ultrasonic structural health monitoring (SHM) of composite structures. However, because of their low sensitivity, they have not been able to detect acoustic emission (AE) signals. Recently, the authors developed novel OFS systems by introducing phase-shifted FBGs (PS-FBGs). The systems can achieve the compatibility of high sensitivity and broadband performance owing to the characteristic structure of PS-FBGs. First, we developed a PS-FBG balanced sensing system. This system succeeded in detection of small AE signals under tensile test of CFRP laminates. The results indicated the Kaiser Effect and were able to discriminate different damage types. Moreover, AE measurement was conducted under a three-point bending test. The positions of transverse cracks could be identified through signal processing. Next, erbium fiber ring laser (EFRL) sensing system with inbuilt PS-FBG was developed. This system was found to have high robustness due to its self-adjustment function for the environmental disturbances. In addition, this system achieved much higher sensitivity than the traditional piezoelectric ceramic sensors and ultrabroadband performance of up to 7 MHz. These systems have large potential to realize the ultrasonic SHM of composites
Micro-computed Tomographic Synchrotron Techniques for Non-destructive Testing of Out of Autoclave Composite Materials
The Canadian Light Source synchrotron uses an extraordinarily wide range of intense beams of light to investigate materials. One very powerful technique for investigating the internal structure of material in a non-destructive way is micro-tomography. In this presentation we will give examples of how this technique can used to non-destructively test and image out of Autoclave pre-preg composite materials. Examples will be shown, whereby the fundamental process that occur when carbon based composites are debulked can be followed in real time, using micro tomography, thus giving images of void evolution in out of autoclave pre-pregs
Gas Permeability Measurements: Effect of Measurement Technique
There is a desire in aerospace industry to replace traditional autoclave processing methods with lower cost out-of-autoclave (OOA) methods such as OOA processing of prepregs. However, high porosity is a challenge in this process. One of the main means of void reduction in OOA processing of prepregs is reducing the gas pressure inside voids by gas transport. It is therefore important to understand the gas permeability in a laminate throughout the process. A key problem is that when the resin is soft, sending gas through this soft porous structure alters the microstructure and thereby the gas permeability. This study investigates the effect of the measurement technique on microstructure and permeability of the test sample both in-plane and through thickness. Permeability measurements in continuous and interrupted modes are performed and the void morphology of the test samples is characterized using optical microscopy. It is shown that the porous structure and the permeability are affected by permeability measurements when taken continuously during the low viscosity region as it creates an interconnected network of inter-laminar voids. This effect can be reduced by discrete measurements of permeability, which is more representative of the situation during actual processing conditions
Comparing Compressive Strength of PAN Based Carbon Fibers by Direct Compression Test
Longitudinal direct compression testing of a single polyacrylonitrile-based carbon fiber was performed with a piezo-actuated testing machine installed in a scanning electron microscope, and the longitudinal compressive stress–compressive strain relation was measured. The variability of the compressive strength was evaluated via Weibull analysis, and was found to be smaller than the tensile strength variability. The compressive strength of intermediate tensile strength carbon fiber was almost the same to the tensile strength. By contrast, the compressive strength of high tensile strength carbon fiber was about a half of the tensile strength. The compressive strength to the tensile strength ratios of the carbon fibers were almost coincided to the ratio of the unidirectional carbon fiber reinforced plastics. The low compressive strength of a unidirectional high strength carbon fiber reinforced plastic would be attributed to the low compressive strength of the carbon fiber
A Study of the Effect of Scratch on the Mechanical Property of Injection Molded Polymer and Glass Fiber Composite
The scratch behavior of polymer materials and corresponded reinforced composites have raised significant concern and attention, due to their widely applications such as automotive interior parts and household products where surface’s high quality and durability are demanded. As well known that scratch damage of material is complex and also attributed to various parameters such as scratch conditions (scratch load, velocity, and scratch tip geometry/size) and material’s properties (modulus, strength, coefficient of friction and indented hardness), which accelerates critical fracture existence and evolution in following mechanical testing process. In this paper, various scratch patterns were introduced for injection molded neat polycarbonate (PC), and 20wt% of glass fiber (GF) additive corresponding GF/PC dumbbell sample and plates. Initially, critical scratch depth in dumbbell sample’s thickness direction for respective material was investigated. Afterwards, all kinds of plate materials with corresponded critical scratch depth were employed for plate bending test with impact striker under static loading. Finally, impact damage tolerance of scratched injection molded plates was discussed based on impact energy absorption, related observation of damage development and formed fractural appearance. The results indicates that scratch effect must be taken into consideration when estimate composite material’s mechanical property
Fatigue Damage Monitoring and Stiffness Assessment in Plain Weave Composites by Means of Acoustic Emission and Digital Image Correlation
The main objective of this study is to assess the influence of manufacturing flaws on the fatigue life of plain weave composites. The paper proposes combined and complementary approaches for delamination onset monitoring and quantification of the loss of mechanical properties during fatigue loading. Standards test coupons with an artificial delamination, created by inserting an artificial Teflon tape which is implanted in the mid-plan of the layup, were assessed in real time by using Acoustic Emission (AE) and Digital Image Correlation (DIC) under tension-tension cyclic loading. During mechanical cycling, the damage and delamination onset was supervised by detecting elastic waves emitted by cracks while the loss of stiffness of the material is quantified by DIC techniques. The fatigue life curves have been generated by exploiting AE data associated with delamination initiation and predictive fatigue model, based on log-linear straight-line assumption, was derived. Furthermore, the results of mechanical properties monitoring by DIC technique, were used to quantify the stiffness degradation of the plain weave coupons. The mechanical properties degradation has been correlated to matrix crack's development and to delamination extension during fatigue loading
Injection Molding of Wood-Plastics Composites using Powder Technology
In this work, wood plastics composites (WPC) were produced by injection molding and characterized in terms of density, hardness, morphology and mechanical properties (impact, flexural and tensile). In particular, the initial material used (linear low density polyethylene as the matrix and maple wood flour as reinforcement) was dry-blended in a powder form and no preliminary compounding step (like extrusion) was performed before molding to save on processing time/cost and to limit material degradation. As a first step, no coupling agent or surface treatment was performed and only the effect of wood content between 0 and 40% wt. was studied after optimization of the molding conditions. The results show that adding up to 40% wt. of wood flour increased the tensile modulus of LLDPE by 145% (from 210 to 515 MPa), without losing much on tensile strength (15.2 to 14.2 MPa). Similarly, a substantial improvement in flexural modulus was obtained with an increase of 281% (577 to 2204 MPa). On the other hand, density increased from 930 to 1091 kg/m3, while notched Charpy impact strength decreased from 219 to 19 J/m, as well as tensile elongation at break dropping from 1567 to 20%.
Japanese ‘Omotenashi’ Mind at Traditional Industries—A Case of Kanaami Research
In Japan, that satisfaction was the result of hospitality heart—’Omotenashi’, which has been widely applied as a keyword or standard in service industry and manufacturing for the top level services and products, which may provide affective experience and suggest a deeper part of the human consciousness. In addition, the ‘Omotenashi’ service behavior and its superiority were also analyzed for expanding and applying ‘Omotenashi’ mind to world business. In a word, this research mainly discussed the application feasibility in other countries based on the produced benefits and proposed some contributed advices for their development. ‘Kana-ami’ is a kind of metal wire network, which was employed as one kitchen instrument or fence knitting for cultural relic in Kyoto. Date back to around 50 years ago, there were about 50 handmade wire net shops in Kyoto. And it has decreased dramatic until 7 shops now, and there are less than two masters in each shop. However, Just this few people have been committed to the quantitative research of ‘Kana-ami’ technology in order to keep this culture wealth with ‘Omotenashi’ spirit. In this study, cooperating with ‘Kanaami Tsuji’ workshop, their full concentration and effort on ‘Kana-ami’ enterprise was employed as the example to present ‘Omotenashi’ mind annotated Japanese insistence, wisdom and dedication. The wire net masters, expert and nonexpert, were employed as the target investigated subject. The main difference of ‘Kana-ami’ production process and quality evaluation between expert and non-expert was investigated and listed in order to clear the production skills and product performances. In a word, the making process of ‘Kana-ami’ does not use the machine, and ‘Kanami Tsuji’ workshop always try to prepare the perfect products with a calm mood, which are liked by customers at the deepest heart. Compared with the profits, what matters more is appreciate and love from customer. That is ‘Omotenashi’ mind