Showing papers in "Journal of Composite Materials in 1995"

Failure Analysis of Woven and Braided Fabric Reinforced Composites

Abstract: A general purpose micromechanics analysis that discretely models the yarn architecture within the textile repeating unit cell was developed to predict overall, three-dimensional, thermal and mechanical properties, damage initiation and progression, and strength. This analytical technique was implemented in a user-friendly, personal computer-based, menu-driven code called Textile Composite Analysis for Design (TEXCAD). TEXCAD was used to analyze plain weave and 2 × 2, 2-D triaxial braided composites. The calculated tension, compression, and shear strengths correlated well with available test data for both woven and braided composites. Parametric studies were performed on both woven and braided architectures to investigate the effects of parameters such as yarn size, yarn spacing, yarn crimp, braid angle, and overall fiber volume fraction on the strength properties of the textile composite.

The Macroscopic Elasticity of 3D Woven Composites

Abstract: The elastic properties of graphite/epoxy composites with three-dimensional interlock weave reinforcement have been measured over length scales somewhat greater than the characteristic length of the weave pattern. Orientation averaging models similar to those developed elsewhere over the last twenty years provide estimates of elastic constants that are in fair agreement with the experimental data. However, in-plane Young's moduli are consistently too high and properties related to the through-thickness reinforcement show considerable scatter. Most of the discrepancies can be attributed to waviness and other geometrical irregularities in nominally straight tow segments. Much improved agreement with in-plane properties is obtained by measuring and accounting for the out-of-plane waviness of in-plane tows. Other observed distortions of in-plane tows and irregularity in through-thickness tows are very difficult to quantify experimentally. There results a significant and apparently unavoidable uncertainty in pr...

An accumulative damage model for tensile and shear failures of laminated composite plates

Abstract: A progressive failure model was developed for predicting the accumulated damage and the effect of such damage on the in-plane response of laminated composite plates subjected to tensile and shear loads. Internal damage in composites induced by matrix cracking under in-plane tensile and shear loads was the primary concern. Free edge-induced damage was not considered.The model assumes that the damage would accumulate uniformly in a laminate when it is subjected to uniformly distributed loads. The progressive failure model consists of two parts: constitutive modeling and damage accumulation prediction. Constitutive equations were established for relating the effective material properties of a damaged ply in a laminate to the extent of damage. Damage accumulation criteria were proposed by modifying the existing failure criteria for estimating the accumulated damage as a function of the applied loads.A finite element analysis, designated as "PDCOMP," was developed based on the model. In order to verify the mod...

Two-Dimensional Modeling of Composite Pinned-Joint Failure:

Abstract: The modeling of damage in a laminated composite pinned-joint presents many difficulties because of the inherent complexity of the failure process. The joint area is a region with stress concentrations thus a complicated stress state exists. In order to model progressive damage from initial to final failure, finite element methods are used rather than closed form stress analyses. Two approaches for the finite element technique can be used: a simple two-dimensional linear model or one that has been enhanced with non-linear assumptions. Adding non-linear material behaviour and large deformation theory are two improvements that can be made to a linear finite element model. Failure analysis must be a logical combination of suitable failure criteria and appropriate material property degradation rules. It is understood that a three-dimensional analysis may now be a better alternative, but the goal here is to fly understand the limits of two-dimensional modeling for solving the composite pinned-joint problem.

Notched Strength and Fracture Criterion in Fabric Composite Plates Containing a Circular Hole

Abstract: The effects of the hole size and the specimen width on the fracture behavior of several woven fabric composite plates are experimentally investigated in tension. It is shown in this paper that the characteristic length (do) in the Point Stress Criterion (PSC) depends on the hole size and the specimen width. A modified PSC for predicting the notched strength is proposed. An excellent agreement is found between the experimental results and the analytical predictions using the modified PSC. When the unstable fracture occurred, the equivalent critical crack length (ac) corresponding to the damage zone size is about twice the characteristic length. The characteristic length decreases with an increase in the notched strength. The critical energy release rate (Gc) corresponding to the unstable fracture criterion is independent of the hole size for the same specimen width. Gc increases with an increase in the specimen width. These results can be explained by the correlation between the Gc and the notch sensitivit...

Measurement of the Three-Dimensional Permeability of Fiber Preforms Using Embedded Fiber Optic Sensors:

Abstract: An experimental technique is described which can be used to measure simultaneously the three principal permeabilities of fiber reforms made of continuous or short fibers. The technique utilizes embedded optical fibers for detecting the position of the liquid front inside the reform. Expressions were derived for calculating the permeabilities from the three-dimensional measurement of the liquid front. Permeabilities measured by the present fiber optic and by the conventional, pressure drop and flow visualization techniques were compared. Good agreements were found between the results of these techniques.

High Strain Rate Response of Angle-Ply Glass/Epoxy Laminates

Abstract: The effects of strain rate on the mechanical behavior of Scotchply Type 1002 glass/epoxy angle-ply laminates is investigated. High strain rate tests (approximately 103 sec−1) using a direct tension...

Weibull Analysis of the Strength of Carbon Fibers Using Linear and Power Law Models for the Length Effect

Abstract: The single-filament test measurements for strength of carbon fibers obtained by Bader and Priest [5] are analyzed using a broader class of Weibull models with the scale parameter assumed to be a function of fiber gauge length. Two specific models are proposed for the fiber gauge length effect on tensile strength: The power law model and a linear model. The methods of maximum likelihood estimation and Weibull plots provide tools for the parameter estimation and assessment of the fitted models. In particular, the models provide excellent prediction of the fiber strength distributions at longer gauge lengths not tested, but illustrate that extrapolation to very short gauge lengths yield larger predicted fiber strengths than those actually observed experimentally, and extrapolation to very long gauge lengths yield smaller predicted fiber strength than observed experimentally. Analysis of the Bader-Priest data using the proposed models supports the observed "end effects" (or "clamp effects) for short gauge len...

Nonlinear Analysis of the Poisson's Ratio of Negative Poisson's Ratio Foams

Abstract: This article contains an analytic study of Poisson's ratio of re-entrant foam materials with negative Poisson's ratio. These materials get fatter when stretched and thinner when compressed. The Poisson effect is so fundamentally important to the properties of a material that a large change in the value of the ratio will have significant effects on the material's mechanical performance. Isotropic foam structures with negative Poisson's ratio have been fabricated through a permanent volumetric transformation. The cells were converted from the convex polyhedral shape of conventional foam cells to a concave or "reentrant" shape. Mechanical behavior of a re-entrant open cell foam material will differ from that of a conventional foam in ways not addressed by existing theoretical treatment. Poisson's ratio as a function of strain is obtained by modeling the three-dimensional unit cell as an idealized polyhedron unit cell. Poisson's ratio is predicted to approach the isotropic limit of −1 with increasing permanen...

Delamination-Free and High Efficiency Drilling of Carbon Fiber Reinforced Plastics

Abstract: The wear-resistant metal bonded diamond drill was applied to the drilling of carbon fiber epoxy composites in order to increase the drilling performance without fuzzing and delamination. For the first time, the helical-feed drilling method was applied to overcome the drilling depth limitation in the conventional drilling with metal bonded diamond drill. From the drilling experiments, it was found that the helical-feed method employed in this work was efficient and did not cause thermal damage in high speed drilling.

Effect of Perturbation of Fibre Architecture on Permeability Inside Fibre Tows

Abstract: The influence on the permeability inside fibre tows from fibre packing arrangement and non-uniform fibre diameter is investigated. The analysis mainly consider flow transverse to the fibres and is based on the fibre geometry. Perturbations from perfect quadratic and hexagonal arrangements of fibres at constant fibre volume fraction are considered. The importance of microgeometry on permeability is pointed out and it is indicated how to deal with this problem. The analysis shows that the fibre transverse permeability is strongly dependent on the fibre size distribution. When an arrangement of equally sized fibres is continuously changed from quadratic to hexagonal it is furthermore found that the transverse permeability is highest just before the packing gets hexagonal. The analysis is also applied to a simple FORTRAN code to simulate removal of fibers from a quadratic arrangement. The simulations shows that the transverse permeability decreases with increased number of removed fibres under constant fibre ...

Behavior of Stitched Laminates under In-Plane Tensile and Transverse Impact Loading

Abstract: Experimental results for in-plane tensile and transverse impact responses of stitched composite laminates were presented in this paper. (02/902). E-glass/epoxy laminates of 2.8 mm nominal thickness fabricated by resin transfer molding were used as the specimens. The through-the-thickness reinforcement was provided by untwisted Kevlar-29 ravings of 1,000 and 3,000 denier. In the in-plane tensile test, although the damage mechanism of the stitched laminates was much affected by the loading directions, the stiffness was not significantly affected by the addition of the stitch threads. In the impact test using a hemispherically tipped impactor, stitching was found to significantly reduce the delamination crack area, and the 3,000 denier threads provided a better resistance to the propagation of these cracks. It was also found that, although the stitch step, stitch spacing, as well as the impact location relative to the stitch threads affected the size and the shape of the delamination area during impact, the ...

Experimental and Numerical Studies of a Laminated Composite Single-Lap Adhesive Joint

Abstract: The problem of a single-lap bonded joint with laminated polymeric composite adherents and with a spew fillet, subjected to tensile loading, is investigated. Experimental and numerical analyses of this problem are presented to address the mechanics and deformation of such material and bonding configuration. Strain gages are employed to record the geometrically nonlinear deformation of the specimen. Full-field moire interferometr is used to measure the surface deformation of the adherends and adhesive (including a spew fillet). A geometrically nonlinear, two-dimensional finite element analysis is performed to check the mechanics assumptions by comparing with the experimental measurement. A good correlation between experimental and numerical solutions is obtained. It is observed that the composite single-lap joint deforms nonlinearly when subjected to tensile loading. The resulting displacement and strain fields for the adherend and adhesive layer are also presented. The moire results show that the transvers...

Composite Materials Which Exhibit High Stiffness and High Viscoelastic Damping

Abstract: Composite micro-structures are studied, which give rise to high stiffness combined with high viscoelastic loss. We demonstrate that such properties are most easily achieved if the stiff phase is as stiff as possible. Incorporation of a small amount of damping in the stiff phase has little effect on the composite damping. Experimental results are presented for laminates consisting of cadmium and tungsten and of InSn alloy and tungsten. The combination of stiffness and loss (the product E tan δ) exceeds that of well-known materials.

The Continuous Cuing Process for Thermoset Polymer Composites. Part 1: Modeling and Demonstration:

Abstract: This paper presents a new manufacturing method, continuous curing, that is specifically engineered for extremely thick parts made from thermosetting polymer matrix compounds. In this process, matrix curing takes place continuously as the precursor material is being laid down. The energy for curing is obtained from the exothermic curing reaction through controlled heat release; once started, the process is self-sustaining. Thermal spiking, an unavoidable feature of autoclave curing, can be completely eliminated in continuous curing by manipulating the process parameters. In addition, manufacturing times are reduced by an order of magnitude or more.A simple analytical model is presented first to show the inherent features of continuous curing. Next, a detailed thermo-chemical process model is developed and numerical results are obtained for a glass/polyester composite. The process is demonstrated by curing a 100 mm thick graphite/epoxy lamina. Finally, thermal spiking behavior and manufacturing times are co...

Thermo-Oxidative Aging of Composite Materials:

Abstract: A series of tests was conducted on two carbon/epoxy systems to assess key factors in the characterization of the thermo-oxidative stability of composite materials. Mechanical properties (open hole ...

Delamination Fracture Criteria for Composite Laminates

Abstract: Due to the singularity nature of delamination, the fundamental concept of fracture mechanics was applied. However, the oscillatory characteristics near the delamination tip prohibited the use of conventional definition assuming the crack embedded into a homogeneous solid. A proper definition for the stress intensity factors and energy release rates of bimaterial interface cracks was introduced in this paper to study the fracture criterion for delamination. To measure the delamination fracture toughness, the test specimen usually used for the unidirectional composites was redesigned to suit for the cases that the delamination lies between two laminae with different fiber orientations. The test results show that the double cantilever beam (DCB) test is near to pure Mode I and the end-notched flexural (ENF) test is near to pure Mode II. Therefore, like the cracks in homogeneous materials, the test methods DCB and ENF are useful for the measurement of delamination fracture toughness GC and G11,. Based upon th...

Three Dimensional Analysis and Resulting Design Recommendations for Unidirectional and Multidirectional End-Notched Flexure Tests

Abstract: Results are presented from a theoretical investigation of the effects of stacking sequence on the energy release rate in laminated composite end-notched flexure test specimens Deflections and energy release rates of unidirectional and multidirectional ENF specimens are obtained by classical laminated plate theory, shear deformable plate theory, and three dimensional finite element analyses It is shown that the distribution of energy release rate varies across the front of an initially straight delamination The percentage of mode II and mode III energy release rates for the specimen, as well as the local peak values of the mode II, mode III and total energy release rates that occur at the specimen's free edges are shown to correlate with a nondimensional ratio comprised of the specimen's flexural rigidities The results of the study are used as a basis for a proposed “ENF test design procedure” that may be used for the determination of appropriate specimen stacking sequences and test geometries for stud

Effects of Processing Temperature and Layup on Springback

Abstract: Tests were performed to assess the effects of processing conditions and laminate layup on the change in shape (springback). Twelve inch long and one inch wide coupons made of Fiberite T300/976 graphite/epoxy were tested. Coupons of different layups were cured using different types of molds and different types of vacuum bagging arrangements. The curing was performed with different temperature cycles. Springback was measured from the change in shape of initially flat coupons. The results indicated the effects of the applied temperature, applied pressure, material of the mold, arrangement of the teflon surrounding the coupon, and laminate layup on springback.

Finite Element Analysis of Effective Thermal Conductivity of Filled Polymeric Composites

Abstract: A novel method to predict the macroscopic effective thermal conductivity of filled composites, based on its microstructural characteristics is developed and validated. A finite element method which incorporates the effect of microstructural characteristics such as filler aspect ratio, interfacial thermal resistance, volume fraction, and filler and fiber dispersion to determine the effective thermal conductivity of a composite with circular and rectangular fillers is presented. Filler interactions and chain formation effects are included in the model. To overcome the laborious task of repeated finite element model generation, an algorithm which generates the positions and orientations of the fillers in the matrix is developed. The automated model development capability is used to create several microstructures for finite element analysis. The trends predicted by the finite element models are compared with existing analytical models and available experimental results. The advantage of this method over exist...

Nonlinear and First-Ply Failure Analyses of Laminated Composite Cross-Ply Plates

Abstract: A method is presented to study the nonlinear behaviors and first-ply failure strengths of centrally loaded laminated composite plates with semi-clamped edges. The method which is developed from the...

Fatigue behaviour of polymer composite sandwich beams

Abstract: The static and flexural fatigue characteristics of foam cored polymer composite sandwich beams are investigated. The skins of the beams are made from hybrid glass-aramid fibres set in epoxy resin and the core materials are linear and cross-linked polymer foams. The applied load in a ten-point configuration approximates a uniformly distributed load throughout the span of the beam which is simply supported at the ends. The testing frequencies are 0.33-0.91 Hz. Failure modes relate to both core shear and skin failure.

A non-isothermal process model for consolidation and void reduction during in-situ tow placement of thermoplastic composites

Abstract: In in-situ composites processing, heat and pressure are applied locally to composite tows to achieve consolidation. During the consolidation of thermoplastic composite tows or plies, a modest amount of flow takes place. Due to the high viscosity of thermoplastics, the fibers and the matrix move together. We propose a novel approach to model this process-approximate the medium as a compressible viscous continuum rather than use Darcy's law which has been successful in modeling thermoset composites processing. The void content of the composite changes during the consolidation process due to various mechanisms. A consolidation model for thermoplastic composite that incorporates—the relevant void growth and transport phenomena has been developed. The model is capable of predicting the final void fraction and the final thickness of a composite part as a function of the processing speed and the consolidation pressure under non-isothermal conditions.

A closed-form solution of the transverse effective thermal conductivity of woven fabric composites

Abstract: This paper examines the transverse effective thermal conductivity of plain weave fabric composites. A three-dimensional thermal resistance network model is proposed. Based upon this model, a closed...

In-Situ Observations in SEM of Degradation of Graphite/Epoxy Composite Materials due to Seawater Immersion

Abstract: The effect of immersion in seawater on the mechanical properties of three graphite/epoxy composite materials has been studied. The transverse tensile strength was found to be reduced by 17% in one ...

The Nonlinear Viscoelastic-Viscoplastic Behavior of IM7/5260 Composites Subjected to Cyclic Loading:

Abstract: A constitutive model consisting of a combination of the Schapery non-linear viscoelastic heredity integral and a nonlinear viscoplastic functional employed by Zapas and Crissman is described. Material constants associated with the constitutive models are measured for graphite-bismaleimide (IM7/5260) composites at elevated temperatures and stress levels. These results are then combined with classical lamination theory, so as to predict the response of a multi-angle laminate to cyclic thermomechanical loadings. Predictions are favorably compared with measurements obtained during a 50-hr test involving ten 5-hr loading cycles.

Stitching vs. a Toughened Matrix: Compression Strength Effects

Abstract: The compression strength of a stitched and a toughened matrix graphite/epoxy composite was determined and compared to a baseline unstitched untoughened composite. Two different layups with a variety of test lengths were tested under both ambient and hot/wet conditions. At longer gage lengths where failure was due to global buckling, no significant difference in strength was found between the different materials. For shorter specimens, a 30% reduction in strength due to stitching was found for both layups, presumably due to an increase in fiber misalignment. An observed increasing strength with decreasing gage length was found for all materials and was explained using statistics and a random distribution of flaws (misaligned fibers). The toughened material showed a small increase in strength over the baseline material, presumably due to the compensating effects of a more compliant matrix and straighter fibers. A hot/wet environment reduced the strength of the baseline and stitched material by 30% and the t...

Fracture Mechanics and Mechanisms of Impact-Induced Delamination in Laminated Composites

Abstract: The fracture mechanism of impact-induced delamination is studied in carbon fiber/PEEK (polyetheretherketone) cross-ply laminates under drop weight impact. The study is based on the energy theory of fracture mechanics and the concept of crack arrest toughness. The damaged laminate is modeled by a finite element method which simulates delaminations and transverse cracks. The numerical results are combined with test data to study the delamination behavior. It is found that the delamination occurs in a deflection-controlled condition and is a process of Mode II dominated unstable crack growth and subsequent arrest. The fracture behavior can be described by strain energy release rate and the delamination size is governed by the delamination arrest toughness of the composite.

Numerical and experimental failure analysis of composite laminates with bolted joints under bending loads

Abstract: A three-dimensional contact mechanics analysis has been successfully made on the composite laminates with bolted joints under bending loads. The AS4/3501-6 laminates of [08/908]s and [908/08]s lay-up with two types of countersunk bolted joints, MS-24UNF-3A (ψ = 80°) and NAS-24UNJF-3A (ψ = 100°), subjected to bending loads are analyzed, respectively. The effects of friction, stacking sequence and countersunk bolt head on the contact tractions around the bolted joint are studied systematically. The progressive failure analysis based on the maximum stress theory, Ye delamination criterion and the complete ply failure approach is performed for predicting the maximum bending load that the laminate can sustain before it catastrophically fails. Toward this end, experimental tests were also conducted to measure the maximum bending load for the laminates with the countersunk bolted joints under bending loads. From the results, it is shown that the joint configuration of [908/08]s laminate and NAS-24UNJF-3A (ψ = 10...

Model for Determining the Vent Locations and the Fill Time of Resin Transfer Molds

Abstract: Simple models were developed for determining the vent locations and the fill time of resin transfer molds containing thin flat preforms with isotropic permeabilities. The in-plane shape of the preform is arbitrary and the preform may contain impermeable inserts. The vent locations required to avoid trapping air bubbles are determined by geometric considerations. The time required to fill the mold is calculated by treating the resin flow inside the mold as partly radial and partly channel-like flow. Tests were performed with different shape molds with and without inserts. The vent locations required to avoid air bubbles and the time required to fill the mold were determined. The experimentally established vent locations were in good agreement with the vent locations given by the model. The measured and calculated fill times also agreed well.