Search

For the past six years, we have been benefiting from the discovery by Tsai and Melo (2014) that the trace of the plane stress stiffness matrix ( tr ( Q ) ) of an orthotropic composite is a fundamental and powerful scaling property of laminated composite materials. Algebraically, tr ( Q ) turns out to be a measure of the summation of the moduli of the material. It is, therefore, a material property. Additionally, since tr ( Q ) is an invariant of the stiffness tensor Q , independently of the coordinate system, the number of layers, layup sequence and loading condition (in-plane or flexural) in a laminate, if the material system remains the same, tr ( Q ) = tr ( A ∗ ) = tr ( D ∗ ) is still the same. Therefore, tr ( Q ) is the total stiffness that one can work with making it one of the most powerful and fundamental concepts discovered in the theory of composites recently. By reducing the number of variables, this concept shall simplify the design, analysis and optimization of composite laminates, thus enabling lighter, stronger and better parts. The reduced number of variables shall result in reducing the number and type of tests required for characterization of composite laminates, thus reducing bureaucratic certification burden. These effects shall enable a new era in the progress of composites in the future. For the above-mentioned reasons, it is proposed here to call this fundamental property, tr ( Q ) , as Tsai’s Modulus.

Hierarchical +1 composites consisting of carbon fibers with carbon nanotubes (CNTs) grown onto them and an epoxy matrix were processed, and the mode I fracture toughness of these composites was evaluated. The mode I fracture toughness of the initial batches of the hierarchical composites was lower than that of the baseline samples without CNTs. Hence, efforts to enhance the adhesion between carbon fibers and CNTs were made, resulting in enhanced adhesion. The enhanced adhesion was confirmed by Scotch tape tests and mode I fracture toughness tests followed by fractographic studies. The mode I fracture toughness of the hierarchical composites with enhanced adhesion was 51% and 89% higher than those of the baseline samples and hierarchical composites with poor adhesion, respectively. Moreover, fractographic studies of the fracture surfaces of the hierarchical composites with enhanced adhesion showed that CNTs were still attached to carbon fibers even after the mechanical tests.

The aim of this study was to investigate the effects of adding maleic anhydride grafted polypropylene (MA-PP) and graphite nanoplatelets (GNPs) into polypropylene. The tensile strength and flexural properties of the GNP/PP composite were enhanced with an increase in the MA-PP content and the GNP content. The maximum increase of mechanical properties was shown at 10 wt.% of MA-PP having a constant GNP content of 8 wt.% and at 20 wt.% of GNP having a constant MA-PP content of 5 wt.%, whereas a decrease in tensile strength and flexural properties was observed at 20 wt.% of MA-PP. The impact strength was decreased by increasing the MA-PP content and the GNP content. The electrical resistivity of the GNP/PP composite exhibited an anisotropic characteristic due to the direction of the injection molding flow. Electrical resistivity of less than 106 Ω cm regardless of the direction of the resin flow could be obtained by adding more than 20 wt.% of GNP when the MA-PP content was 5 wt.%. The differential scanning calorimetry results suggest that an increase of the GNP content makes it act as a nucleating agent and thus causes an increase in the crystallinity of the PP.

This study presents a multiscale numerical framework designed to predict the nonlinear constitutive behavior of metal-composite interfaces in titanium-graphite fiber metal laminates. Molecular-level property predictions derived in a separate analysis are used to parameterize a finite element model of the interface by means of a traction-separation constitutive law. Additional continuum-level energy dissipation and progressive failure phenomena are implemented into commercial finite element software through a user-defined material subroutine. Results obtained from this multiscale interface model are compared against experimental measurements of titanium-graphite fiber metal laminates in a short-beam shear loading configuration. The model predictive accuracy and its application to other bonded metal-composite systems are subsequently discussed.

This paper aims to develop an autonomic thermal management system of graphite fiber/polymer composite structures using their electrical property. A number of copper tapes are integrated on top and bottom surfaces of a composite plate, and through the thickness resistances between these tapes are measured on a hotplate for temperature monitoring. The resistance at warmer area successfully reflected the temperature distribution taken by an IR imaging camera. The same copper tapes are subsequently used for generating resistive heat by supplying a large amount of current. The effective heat generation is investigated by a finite element analysis, showing that the temperature of a laminate can be controlled by adjusting the conductivity in the fiber and thickness directions.

In an effort to fabricate prototypes with improved mechanical properties in the selective laser sintering process, functionalized graphite nanoplatelets were added to polyamide-12 powder to produce a nanocomposite powder. The polyamide-12 powder was chosen as the matrix polymer because it has features conducive to laser sintering, such as a low melting temperature and high mechanical properties. As fillers, the graphite nanoplatelets were oxidized through a nitric acid treatment to improve the interfacial bonding. For the syntheses of the nanocomposite solution, intercalation in an autoclave was conducted to obtain good dispersion and manufacturability. The micro/nanostructures of the synthesized nanocomposites were examined through scanning electron microscopy. The distribution of particles was also analyzed to look at the feasibility for the selective laser sintering process. The synthesized nanocomposite powder was layered and sintered by a laser. The mechanical properties for specimens made of neat polyamide-12, polyamide-12/TiO2, and polyamide-12/functionalized graphite nanoplatelet were tested and compared. The results indicate that the functionalization of nanofiller reinforces interfacial bonding force. For this reason, a small amount of nanofiller, which does not influence the thermal and rheological behavior, can improve the material properties.

Novel nanocomposite films of polycarbonate (PC) with fullerene derivatives, such as pristine fullerene C60 and polyhydroxylated-fullerenes, C60(OH)12 and C60(OH)36, were prepared. The optical, thermal, and mechanical properties of the composites were measured. Nanocomposite films of poly (vinyl alcohol) (PVA) with C60(OH)36 were prepared as a reference to show how improved dispersion of the nanofiller affects the overall transparency of the composites. Ultraviolet-visible spectroscopy showed that the addition of hydroxylated fullerenes did not affect visible light transmittance of the films significantly in the range of 400–800 nm. Differential scanning calorimetry (DSC) and thermo–gravimetric analysis (TGA) measurements showed the increased thermal stability of PC/C60(OH)12 film as compared to pristine PC film. This phenomenon was explained by the rigid polymer interphase regions formed around C60(OH)12 due to the plausible hydrogen bonding and hydrophobic interaction. On the other hand, the lower thermal stability of PC–C60(OH)36 was assumed to be caused by large agglomeration of the C60(OH)36 particles and the partial hydrolysis of the polycarbonate matrix. Tensile testing of the composites showed reduction in elongation at break and yield tensile strength. These results may be caused by the particle agglomerations which act as the initiation points for cracks.

This article investigates the dependency of electrical resistance on temperature for graphite/polymer composites. The electrical resistance change of a unidirectional laminate is measured with elevated temperature in the fiber, transverse and through-thickness directions. Because of the properties of graphite fibers, the electrical resistance decreases as temperature increases in any direction. The activation energy is calculated from an Arrhenius plot to evaluate the difference among the different directions. The activation energy is the lowest in the fiber direction due to the high electrical conductivity of graphite fibers. Since electric current flows thorough the contacts between neighboring graphite fibers, similar activation energies are required in the transverse and through-thickness directions, indicating transversely isotropic properties. These characteristics will be used to establish the relationship between electrical resistance outputs and structural conditions, and will be eventually applied to an in-service structural management system using addressable conducting network, which is a grid arrangement of conducting lines on a composite panel.

Self-healing composites using a thermally mendable polymer, bis-maleimide tetrafuran (2MEP4F), based on Diels–Alder reaction and electrical resistive heating were fabricated using a vacuum assisted injection molding method. Delaminations were induced using short span three point bending on composite coupons. The induced damage and permanent deformation of the composite coupons were observed using X-ray micro-tomography. The same procedure was repeated to confirm multiple healings after electrical resistive heating and healing efficiency of the samples was determined. The permanent deformations and induced delaminations were repaired after the heating process, which is due to both healing and shape memory effects. The multiple healing ability and shape memory effect of the fabricated composite combined with electrical resistive heating realizes a noble self-healing composite.

Copper nano-ink with a drop-on-demand (DOD) piezoelectric inkjet printing method was introduced. The printed electrodes were thermally sintered to ensure high-quality electrical and mechanical performances. To check the reliability of the printed electrodes on a polymer layer, resistance changes were measured under static loading. The electrodes with various widths and thicknesses were used to find the optimal dimensions. A multifunctional composite laminate which can harvest and store a solar energy was fabricated using printed electrodes. An amorphous silicon solar cell and a thin film solid state lithium-ion battery were adhesively joined and electrically connected to a thin flexible printed circuit board (PCB). Then, the passive components such as resistor and diode were electrically connected to the printed circuit board by silver pasting. The integrated PCB was co-cured with a carbon/epoxy composite laminate by the vacuum bag molding process in an autoclave. The structural and functional performance of the final energy harvesting/storage composite laminate was tested under mechanical loading.

The cure kinetics of a thermally mendable polymer based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions, mendomer 401, was investigated using dynamic differential scanning calorimetry (DSC). The resulting behavior was modeled using a conventional cure kinetic model for thermosetting polymers. A composite panel with two layers of carbon fabric and mendomer 401 was fabricated following the cure cycle suggested by the cure kinetics model. Micro-indentation tests were performed to investigate mechanical properties and dynamic mechanical thermal analysis (DMTA) was conducted to study thermal behavior and to find the trigger temperature for healing. Self-healing behavior of the carbon fiber/mendomer composite was demonstrated using electrical resistive heating over the glass transition temperature.

This study demonstrates the construction of a multifunctional composite structure capable of energy storage in addition to load bearing. These structures were assembled and integrated within the confines of a multifunctional structural composite in order to save weight and space. Carbon fiber reinforced plastic (CFRP) composites were laminated with energy storage all-solid-state thin-film lithium cells. The processes of physically embedding all-solid-state thin-film lithium energy cells into carbon fiber reinforced plastics (CFRPs) and the approaches used are reviewed. The effects of uniaxial tensile loading on the embedded structure are investigated. The mechanical, electrical, and physical aspects of energy harvesting and storage devices incorporated into composite structures are discussed. Embedding all-solid-state thin-film lithium energy cells into CFRPs did not significantly alter the CFRP mechanical properties (yield strength and Young's modulus). The CFRP embedded energy cells performed at baseline charge/discharge levels up to a loading of about 50% of the ultimate CFRP uniaxial tensile rupture loading.

This paper reports on the recent progress towards the development of power composite structures capable of energy harvesting and storage in addition to load bearing. The process of physically embedding all-solid-state thin-film lithium energy cells into a carbon fiber reinforced plastic (CFRP) and the performance of the resulting power composites are reported. The embedded thin-film lithium-ion energy cells did not significantly alter the mechanical properties of the composite (modulus and strength) under quasi-static uniaxial loading conditions. The embedded energy cells performed at baseline charge/discharge levels up to a loading of about 50% of the CFRP tensile strength.

Robust magnetic vinyl ester resin nanocomposites reinforced with iron oxide (Fe2O3) nanoparticles were fabricated. The particle functionalization with a bi-functional coupling agent methacryloxypropyl-trimethoxysilane (MPS) was observed to have a significant effect on the curing process and subsequent physical properties of the nanocomposites. Particle functionalization favors the composite fabrication with a lower curing temperature as compared to the as-received nanoparticles filled vinyl ester resin nanocomposites. Thermo-gravimetric analysis showed an increased thermo-stability in the functionalized nanoparticles filled vinyl ester resin nanocomposites as compared to the unmodified nanoparticle filled counterparts. The more uniform particle dispersion and the chemical bonding between nanoparticle and vinyl ester resin matrix were found to contribute to the increased thermal stability and enhanced tensile strength. The nanoparticles become magnetically harder after incorporation into the vinyl ester resin matrix.

Silicon carbide reinforced polyurethane nanocomposites were fabricated by a facile surface-initiated-polymerization (SIP) method. The particle loading was tuned to up to 35 wt% without any obvious shrinkage and breakage as compared with the conventional direct mixing method. An increased thermal stability of the composites was observed with the addition of the silicon carbide nanoparticles under thermo-gravimetric analysis (TGA). Tensile strength was observed to increase dramatically with the increase of the particle loading. Both the uniform particle dispersion and the strong chemical bonding between the nanoparticles and the polymer–matrix contributed to the enhanced thermal stability and improved mechanical properties.

Thermally remendable polymers have recently become available that are paving the way for the creation of self-healing composites. This article investigates the use of carbon fibers as a resistive heating network for future use in a self-healing composite. Various electrode methods are studied in an attempt to reduce the contact resistance that causes localized heating around the electrode. In addition to localized heating, electrical conduction through the fibers creates heat through composite resistance. It was observed that for short distances between electrodes, contact resistance dominates while over longer distances, composite resistance dominates. The balance between the two types of heating is measured and modeled using finite element modeling (FEM).

Electrospinning was used to spin nanocomposite fiber mats from polymer solutions containing nanoparticles. The base polymer was polyacrylonitrile (PAN), and the nanoparticles included iron oxide, antimony tin oxide and zinc oxide with diameters ranging from 10 to 70 nm. The electrospun nanocomposite fiber mats were characterized by scanning electron microscopy (SEM), scanning probe microscopy (SPM) and Fourier transform infrared (FTIR) spectroscopy. The average fiber diameter was found to decrease from 200 nm for neat PAN fibers to 50–150 nm for nanocomposite fibers. The porosity increased from 70% for PAN fiber mat to an average of 86% for nanocomposite fiber mats. The fiber mats were used as a sensor in conjunction with FTIR spectroscopy to detect CO2 gas. The absorbance spectra showed a higher sensitivity with a fiber mat, regardless of its type, than without, indicating gas adsorption on the fiber mat. The highest sensitivity was obtained with PAN/Fe2O3 fiber mat. As the underlying mechanism is physisorption rather than chemisorption, the response time was short and the sensor could be used repeatedly.

This study briefly reviews various existing methods to account for the effect of particle size on the mechanical properties of particulate metal matrix composites. A simple and easy method is to use a size-dependent constitutive equation for the matrix. The suggested method does not require the development of a new computational algorithm and is compatible with any standard finite element software. Finite element analyses have been carried out to show how the deformation behavior of a metal matrix composite changes as the particle size and volume fraction are varied. The strain hardenings in metal matrix composites are increased with smaller particles and larger volume fractions which could not be predicted with size-independent constitutive relation. It has been shown that the suggested method can be used in a common finite element analysis process to account for the size effect of composite strengthening.

In this paper, a wear-out model, which is based on the concept ofdeterministic strength degradation and equal ranking between static and residualstrengths, is used to assess the fatigue behaviors of several different sandwich beams.The model requires only two parameters to describe the strength degradation infatigue. One parameter represents the rate of strength degradation and the othera relative fatigue life. A new approach is used to determine these parameters fromexperimental data. The model is shown to be able to describe rather well both thestatic strength distributions and the fatigue stress-life relations of the sandwichbeams. The residual strength distribution after 5000 cycles was predicted for theEglass–vinylester–D100 balsa wood sandwich at 60% maximum stress level with thef and s obtained through the fitting method developed in this paper. There is a goodagreement between prediction and experiment.

A forecast-based thermal control system was developed to cure thick composite laminates that produced large exothermic reactions. The system used process simulation models for heat transfer and compaction to predict future cure conditions at a designated forecast time so that the system can respond to avoid approaching adverse conditions. The thermal control system derived cure cycles that eliminated exotherms and maintained uniform through-the-thickness temperature gradients for 3-cm-thick AS4/35016 laminates. The on-line results indicated that cure cycles were slightly different from off-line simulation results, mainly because of the slow response of the hot press. Nevertheless the system was able to satisfy the cure criteria under such unforeseen circumstances, which demonstrated the flexibility of the forecasting method.

The static and fatigue behavior of vacuum-assisted resin transfer molded sandwich panels has been experimentally investigated under flexural loading. Two core materials, D-100 balsa wood and H 250 PVC foam, were used to study the effect of core material on static failure in 3-point and 4-point bending. The face consisted of a quasi-isotropic E-glass non-woven fabric cured in Derakane 441-400 epoxy vinyl ester resin. The stresses in the face and core at failure were calculated from failure loads using the simple beam theory and compared with their respective strength values to identify underlying failure mechanisms. For short beams, core failure led to failure of the beam as a whole. In long beams, however, failure of the wood core could be contained until the face failed. Only D-100 end grain balsa wood was used as core for fatigue tests. A good similarity was seen between the static and fatigue failure modes for both short span and long span specimens.

Models to predict the split growth in notched AS4/3501-6 graphite/epoxy quasi-isotropic laminates under tension-dominated fatigue are presented. First, a power law model and an artificial neural network (ANN) model are developed to describe the split growth under constant-amplitude fatigue. They are then applied in conjunction with a linear damage growth model to predict the split growth under spectrum fatigue. The ANN model is found to work better than the power law model as a predictive tool for split growth.

The tape placement for thermoplastic composites involves heating, melting, and cooling steps just as do the other manufacturing processes. Consequently, development of residual stresses is unavoidable due to disparate thermal characteristics of matrix and fiber materials and also due to nonuniform cooling rates. From the product quality standpoint, such as interlaminar strength, dimensional accuracy etc., these stresses should be kept within allowable limits. In this study, a thermoviscoelastic finite element model was developed to predict residual stresses induced during the placement of thermoplastic composite tapes. The process, being continuous, was considered to be under a quasi steady state where process conditions do not change with respect to the moving roller. Relaxation of the residual stresses in previously laid layers was also allowed for. Results were obtained for both unidirectional and cross-ply laminates. They show the residual stress distributions through the thickness for a number of chosen sets of process parameters (e.g., roller velocity and heat input). Therefore, residual stresses in a laminate can be controlled by modifying these process parameters.

The subcritical damage growth in notched AS4/3501-6 graphite/epoxy quasi-isotropic laminates under compression–compression and compression-dominated spectrum fatigue was investigated using the longitudinal split length as a damage measure. A linear split growth model was successfully used to predict the split growth in spectrum fatigue using the corresponding data under constant-amplitude fatigue. However, a linear degradation assumption was found to be inappropriate as it overpredicted lifetime under spectrum fatigue. The residual compressive strength did not show any sign of gradual reduction. The damage mode near the hole changed from longitudinal splitting to transverse extension of fiber failure and delamination as the fatigue stress increased. Thus, while the longitudinal split may serve as a measure of damage in high-cycle fatigue, the extent of transverse fiber failure is a better indication of residual compressive strength. It was shown that a compression-dominated fatigue loading spectrum could be shortened substantially by omitting the two lowest load levels at which no significant damage growth was seen under constant-amplitude fatigue.

This paper explains the development of a concurrent engineering system for the design of composite structures. The concurrent engineering system is developed to meet the demand for the better quality products with lower production cost and time. In this study, to compose the architecture of concurrent engineering system, the commercial and non-commercial programs related to design and analysis of composite structures are surveyed and classified. The concurrent engineering system includes various design modules such as design/analysis of composite structures using CLPT and finite-element method (FEM), buckling and postbuckling analysis, thermo-elastic analysis of carbon–carbon composite and optimum design using expert system. For the integration and management of softwares, a graphic-based design environment that provides multi-tasking and graphic user interface capability is built.

Voids in a composite part are deleterious because they degrade its strength and modulus. In resin-transfer molding (RTM), voids result mainly from inhomogeneous fiber architecture. Such inhomogeneity leads to non-uniform permeability of the fiber preform, which in turn causes the resin velocity to vary from point to point at a micro scale. The capillary pressure, which also prevails at this length scale, exacerbates the spatial variation of the resin velocity. The combined effect of pressure gradient and capillary pressure can be described by the capillary number. The resulting microscopic perturbations in the resin-flow front allow voids to form. The present paper proposes a mathematical model to describe the mechanisms of void formation. The existing data are used to validate the assumptions introduced. The model is then used to analyze new data from one-dimensional RTM experiments.

Convection problems governed by hyperbolic equations cannot be solved by conventional finite element methods. Various upwind schemes introduced into finite element methods to solve this class of problems suffer from a numerical error called the artificial diffusion. The amount of artificial diffusion depends not only on the mesh size but also on the flow rate. To minimize artificial diffusion the method of internal nodes is introduced. In this method, which is based on the Lagrangian interpretation of the first order upwind scheme, the flow within each nodal volume can be considered as an one-dimensional flow regardless of the number of upstream or downstream nodes. The use of internal nodes can provide an accurate solution to the degree of cure convection problem on a more coarse mesh with significantly reduced CPU cost. Sample problems are solved to validate the accuracy and efficiency of the method. When incorporated with the heat transfer equations, the use of internal nodes underscores the fact that some RTM problems are very sensitive to errors caused by artificial diffusion.

A study has been made of ply cracking in quasi-isotropic AS4/3501-6 laminates under tension–tension (T–T) and tension–compression (T–C) constant-amplitude (CA) fatigue loading and two-block loading. The CA fatigue and two-block fatigue tests were performed on unnotched laminates having two different lay-ups: [0/±45/90]S3 and [0/±45/90]S4. The thinner lay-up was tested in T–T fatigue, and the thicker lay-up in T–C fatigue to preclude buckling. Load levels of 20–50% of ultimate tensile strength (UTS) were used for CA T–T fatigue tests while 20 and 30% UTS were used for CA T–C fatigue tests. Load mixes of 20/40% and 30/40% UTS were used for T–T block loading while 20/30% UTS mix was used for T–C fatigue. The ply crack densities were measured by using the edge-replication technique. The block order was changed to assess the load sequence effect. The ply cracking in T–C fatigue was much more severe than in T–T fatigue even when the maximum tensile stress was kept the same. In both T–T and T–C, low/high sequence resulted in a higher crack densities than high/low sequence. The crack-density data from constant-amplitude T–T fatigue were used in conjunction with a stiffness reduction scheme to establish in situ S–N (stress–life) curves for the 90° and −45° plies. The S–N curves were further condensed into a master S–N curve by the use of a polynomial failure function. A cumulative damage model was then established under the assumption that the stress redistribution during ply cracking takes place in such a way that the failure function remains on the master S–N curve. The cumulative damage model was applied to predict ply cracking under block fatigue, under T–C fatigue, and also under T–T fatigue in two cross-ply laminates. A concept of equivalent root-mean-square power was used to relate the damage under T–T fatigue to that under T–C fatigue. In general, the cumulative damage model was shown to provide reasonable estimates of crack densities under the test conditions studied.

This paper investigates the bearing properties of mechanically fastened glass-fiber/vinyl-ester composite joints. Two glass composites of different fabrics and lay-ups were fabricated by vacuum-assisted resin-transfer molding (VARTM) and tested by using a double-lap joint configuration. The results of this study are presented as experimental characterization and analytical prediction. The major focus of the experimental part of the paper was to characterize the bearing failure behavior of these composites. The effects of geometric parameters were evaluated and correlated with the resulting bearing strength and failure modes by the statistical method of analysis of variance. From the experimental results obtained, it is concluded that the edge-distance ratio (e/d) and thickness of the specimen strongly affect the bearing strength of the composites. On the other hand, the failure mode is determined by the width ratio (w/d). In the analytical part of the study, Chang’s strength-prediction model, which is based on a two-dimensional finite-element analysis, was utilized to predict the bearing strength of these composite joints. The predicted values were compared with the experimental data obtained from this study and the results suggest that this model can be used to give accurate predictions of the bearing strength of these composites.

Seeking to expand the varieties of joining approaches for polymer-matrix composites beyond discrete fasteners as well as adhesive and non-adhesive bonds, novel assembly joints that are integral to the design and fabrication of individual components are presented. Grouped under the title of integral fit joint technologies, prior approaches to eliminating fasteners, avoiding adhesives, or bonding components by ultrasonic or other energy-augmented methods are identified and illustrated. By building upon prior efforts to apply integral fit joints in areas such as snap fits, electronic assemblies, wooden joint structures, and plastic building blocks, potential uses of thesejoining technologies in composites can be identified and evaluated. Application of integral fit joint technologies in composites requires a fundamental understanding of the strengths and weaknesses of composite material systems, component fabrication processes, the forces acting on joints as well as methods for assembling composite components together. A composite snap joint technology developed for high voltage transmission towers serves as an example in demonstrating the potential for using integral fit joints in composite structures.

An expensive and time consuming aspect of a product development cycle is manufacturing the prototype of the design concept. The goal of current research is to devise a methodology for the rapid process development of resin transfer molding (RTM) in order to reduce product development cycle time and lower the cost of developing composite parts. As a first step towards this goal the feasibility of an alternative rapid prototyping technique, namely laminated object manufacturing (LOM), to fabricate molds has been investigated. The durability and dimensional stability of such a mold, as well as its economic practicality in the RTM manufacturing of a composite I-beam were considered. To keep tooling and fixturing simple, vacuum pressure was used to draw resin into the mold. However, injection at this low pressure proved difficult. To remedy the situation a high permeability layer (HPL) was incorporated into the process. The benefits and drawbacks of utilizing such a layer are also discussed in the paper. Furthermore, an RTM simulation program developed at UCLA was employed to determine optimum gate locations. Finally, the total hours to manufacture the first acceptable part were quantified for comparison with future studies.

The effects of variations in curing parameters on the compressive strength of graphite/epoxy laminates were studied. The evaluated parameters were cure temperature, time, and pressure. Two specific settings were chosen for each parameter to form a unique processing envelope centered on the manufacturer's recommended cure cycle. Through factor analysis, high compressive strength was noticed with high cure temperature (204'C), low pressure (0.482 MPa), and long cure time (2.5 hours). This result suggests that deviation of cure time and temperature above the manufacturer's suggested cure cycle will still produce quality laminates. On the other hand, deviation below the manufacturer' s cure cycle will result in incomplete cure, which was evident in the experiment. Furthermore, according to the analysis of variance (ANOVA), the only factor that was significant at the 90% confidence level was cure time, while the cure temperature was slightly less significant. Therefore, cure time was the most sensitive to the compressive response. The next important parameters were cure temperature and pressure.

A finite element (FEM) approach using an implicit time integration algorithm has been developed to simulate isothermal resin transfer molding process. The finite element formulation is based on the concept of partial saturation at the flow front. The discretized equation is essentially the same as is obtained by a finite element/control volume formulation. The FEM approach is easier to implement for different types of elements. Unlike the explicit time integration schemes, the proposed implicit algorithm uses fixed time steps and allows the flow to advance farther than a layer of flow front elements in a single time step. The resulting code RTMSIM has been validated by comparing its predictions with closed-form solutions for flat plates. A wicking analysis is provided to show the code's ability to deal with the effect of capillary pressure. Several other simulations are provided to demonstrate the accuracy and efficiency of the code.

The main objective of the present study is to establish the relationships between the process variables and the quality of thermoplastic composite laminates fabricated by tape placement. The quality parameters considered in the process modeling are interlaminar bond strength, weight loss through thermal degradation, and crystallinity. Stress, heat transfer, crystallization, degradation, and bonding models are developed. These models relate the process parameters (e.g., roller velocity, roller pressure, heat input) to temperature, stress, and crystallinity distributions, degradation weight loss, and degree of bonding within the composite. These relationships are used to develop a process window to ensure product quality. The process parameters are then optimized to reduce the lay-up time.

The thermomechanical behavior of a thermoplastic composite laminate during the tape placement process has been studied. Stresses induced by the compaction roller are predicted using a finite element method. These stresses control the compaction and bonding behavior of the laminate. In the analysis, the tape placement process is considered as a quasi-steady state rolling contact problem. The effect of temperature on creep compliance is included by treating the composite as a thermorheologicaliy simple material. Thermal expansion coefficients are allowed to be temperature dependent. The effect of friction at the contact surface is also taken into account. The numerical results show how the process parameters (e.g., roller velocity, roller pressure, heat input) are related to temperature and stress distributions within the composite. These relationships can be used to predict the bonding behavior of the laminate.

The crystallization behavior of thermoplastic composites during the tape placement process has been analyzed. A heat transfer analysis has been carried out using a finite element method. The heat transfer analysis is coupled with a crystallization kinetics model to predict the resulting morphology. In order to determine the heat intensity that could be applied to the laminate, a degradation kinetics model is added. The degradation kinetics model thus provides an upper bound for the heat input. The models relate the process parameters (e.g., roller velocity, roller pressure, heat input) to temperature and crystallinity distributions and degradation weight loss. These models are incorporated into a computer code to generate numerical results. By using this code, process conditions which are favorable to consolidation and also likely to produce optimum crystallinity levels are identified.

The compressive properties of a graphite/thermoplastic composite (IM6/APC-2) were experimentally determined using unidirectional and multidirectional laminates. The notch sensitivity and damage tolerance of multidirectional laminates were also investigated by comparing the behavior of notched and impacted specimens with unflawed specimens. Whitney's average stress criterion was used to characterize the reduc tion in the compressive strength due to the presence of notches, while damage tolerance of impacted panels was compared directly using strength reduction. Resistance to low velocity impact was compared using a proposed empirical damage resistance parameter, γ, which was determined experimentally. The observed failure mechanisms were typical for a tough-resin composite. In general, the damage was limited to a small region immediately surrounding the source of the stress concentration and only minor delamination was observed. The unidirectional specimens failed due to a band of fiber kinking at a large angle to the applied load, while delamina tion was the dominant failure mechanism in the multidirectional laminates. C-scans of im pacted specimens indicated minimal damage compared with other thermoplastic and un toughened graphite-epoxy composites. The compressive properties of IM6/APC-2 compare favorably with similar commer cially available materials. The compressive longitudinal modulus (146 GPa) and strength (1062 MPa) are high and the notch sensitivity as measured by the characteristic length ( ao = 2.37 mm) is good. In particular, resistance to the initial impact was excellent when compared with untoughened graphite/epoxy systems [1].

Both unmodified cupric oxide (CuO) nanoparticles and those functionalized with a bi-functional coupling agent methacryloxypropyl-trimethoxysilane (MPS) were used to fabricate vinyl-ester resin polymeric nanocomposites. The nanoparticle functionalization was observed to have a significant effect on the physical properties of the cupric oxide filled vinyl-ester resin nanocomposite. Thermal degradation study by thermo-gravimetric analysis (TGA) showed the increased thermo-stability in the functionalized-nanoparticle-filled vinyl-ester resin nanocomposites as compared with the unmodified-nanoparticle-filled counterparts. The more uniform particle dispersion and the chemical bond between nanoparticle and vinyl-ester resin were found to contribute to the increased thermal stability and enhanced tensile strength.

The paper presents results on the damage mechanisms in hard object impact and their effect on the residual tensile strength of unidirectional Boron/Aluminum and Borsic/Titanium composites. The ballistic tests were carried out with 0.177" steel spheres impacting at the center of centilever plates over a velocity range of 50 to 4000 ft/sec. The induced ballistic damage and the different failure modes are examined by radiography and SEM, and the post impact residual tensile strength and residual velocity after perforation are studied for both materials. The damage induced in Boron/Aluminum consists mainly of lateral and axial cracks causing large strength degradation, up to 50 percent of the initial strength. Borsic/Titanium, on the other hand, shows much better impact resistance and the ballistic damage consists of localized plastic de formation and formation of a sheared plug causing strength degradation of 20 percent of the initial strength, as is the case with homogeneous titan ium. An analytical model based on fracture mechanics can predict fairly well the damage size and the residual tensile strength. Ballistic limit velocity and residual velocity after perforation are shown to be predictable by a model developed for homogeneous metallic plates.

Load-displacement curves were obtained for center notch unidirectional boron/aluminum and borsic/titanium composites. Far field displacements, obtained by means of two standard clip gages, 1 and 0.25 inch long, respectively, were used to establish the variation of global compliance with crack length. The local compliance curve was obtained from the crack opening displacement (COD) measured across the crack surfaces by means of the laser interferometric technique. The Interferometric Displacement Gage (IDG), which is more sensitive to the appearance of damage at the crack tip, yielded nonlinear load displacement curves and seemed to better characterize the fracture behavior of composites. The experimental com pliance curves and the K-calibration results were found to be in good agreement with analytical predictions. The K-calibration curves of the com posites studied were found to be almost equal to that of the isotropic materials; both compliances were predictable by using the isotropic K- calibration.

In an effort to delineate the effect of storage time on strength, we have tested three types of Kevlar 49/epoxy strands up to 3.5 y after fabrication. The same fiber, Kevlar 49 (380 denier), was used with three different epoxy systems. The epoxy composition and cure cycles are listed in Table 1, and details of the test results are given in Table 2. Specimen preparation and test procedures have been reported elsewhere (for example, see Ref. 1). Note that in Table 2, failure loads were converted into fiber strengths using the fiber cross-sectional area.

The bast fibres, a subgroup of natural fibre family, have emerged as a strong competitor of widely used man-made glass fibre for use as fillers or reinforcing materials in certain types of composite materials, which do not require very high mechanical resistance. This paper investigates the manufacturing of multi-layered jute fabric-reinforced thermoplastic composite and its mechanical performance. Hessian jute fabrics in two, four and six layers without any pre-treatment were sandwiched in 0° orientation into seven layers of high-density polyethylene sheets and pressed at high temperature and pressure to form composite laminates having three different structural designs. The laminates with two, four and six layers contain approximately 6.70 wt%, 12.90 wt% and 18.50 wt% of jute fibres, respectively. Mechanical performance of the composite laminates having four and six layers of jute fabric was found to have improved significantly when compared to the pure high-density polyethylene laminates. Within a given sample thickness of 6.5 mm, the laminate with six layers of jute fabric exhibited the best mechanical performance. Optical microscopic analysis revealed that the yarn orientation of the fabrics within the composites remained stable, and there was no visible void in the laminate structure. Fracture morphology of the composite investigated by a scanning electron microscope showed good adhesion of the jute fabrics with the high-density polyethylene matrix.