bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Fiber-Matrix Bond Strength on the Performance of Nicalon/CAS-II Composite
Authors: Kahraman Ramazan , Mandell John F.
Published: April 2011

The influence of the fiber-matrix interphase on the tensile behavior of unidirectional Nicalon fiber-reinforced CAS-II glass-ceramic matrix composite has been investigated. The failure strain decreases at high temperatures (1000°C) to a value comparable to the matrix cracking strain. The embrittlement process appears to be the result of oxidation of the carbon interphase as the matrix crack encounters fibers (given sufficient strain and/or time), the interphase subsequently fuses with a high bond strength and the crack grows through the fibers. In the absence of matrix cracking (at high temperatures) the oxidation process does not penetrate more than a few fiber diameters in from exposed surfaces transverse to the fiber direction. The contribution of the degradation of the fibers to the embrittlement appears to be less important.
Shear Correction Factors for Thin-Walled Composite Boxbeam Considering Nonclassical Behaviors
Authors: Kim Seung Jo , Yoon Ki Won , Jung Sung Nam
Published: April 2011

In this study, shear correction factor for thin-walled composite boxbeam is investigated with considering the anisotropic coupling effects in its laminated walls parallel to the external shear load. We first consider a generally orthotropic laminated beam whose cross section is rectangular. The beam does in-plane shear deformation by the applied shear force, so that there arises the effect of extension-shear coupling which does not arise in the conventionally studied out-of-plane shear deformation. The shear correction factor is defined by the energy considerations with taking into account the coupling effect. Then the procedures are extended to the case of laminated thin-walled boxbeam, where the coupling plays a significant role due to the geometric characteristics of boxbeam. The shear correction factors for the representative boxbeams which are the core structural components of modern composite helicopter rotor blade are presented with the change of their laminate sequences.
Failure Behavior of Unidirectional AS4/3501-6 Carbon/Epoxy Laminates
Authors: Karayaka Metin , Sehitoglu Huseyin
Published: April 2011

The work is aimed at improved understanding of deformation and failure mechanisms of unidirectional carbon fiber reinforced plastics. Monotonic tension and compression experiments are conducted on 3501-6 epoxy resin and AS4/3501-6 carbon/epoxy composite at temperatures ranging from room temperature to 200°C. The goal is to better resolve the matrix modulus and matrix strength influence on the compressive strength and associated failure mechanisms. Two compressive failure mechanisms are observed. At temperatures below 100°C, the failure is governed by formation of longitudinal matrix cracks followed by transverse shear failure. Above this temperature, the failure is governed by localized kinking of fibers. The tensile and compressive strengths of both materials decreased with increasing temperature, the sensitivity to temperature was higher in compression versus tension. A deformation model based on Eshelby's equivalent inclusion theory is utilized to investigate the local and mean field stresses in the matrix as a function of fiber misalignment angle, fiber/matrix modulus mismatch and fiber volume fraction. Deformation maps are constructed to illustrate the link between local stresses and failure mechanisms. The results are compared to the models proposed by Argon and Budiansky.
An Overall View of Laminate Theories Based on Displacement Hypothesis
Authors: Liu Dahsin , Li Xiaoyu
Published: April 2011

With the use of a recursive technique, this paper presents an overall comparison of laminate theories based on displacement hypothesis. A generalized polynominal form is used to unify the displacement hypothesis. Both theories available in the literature and inferable from the existing theories are addressed. Firstly, Shear Deformation Theories are recognized to give good results for in-plane stresses but poor results for interlaminar stresses. However, Layerwise Theories give excellent results for both global and local distributions of displacement and stress (both in-plane and out-of-plane). A compromising theory, the Generalized Zigzag Theory, is presented. Due to its success in laminate analysis, a series of Quasi-layerwise Theories are presented. Unfortunately, a physical impossibility-coordinate dependency-takes place. It then requires a Global-Local Superposition Technique to formulate the laminate theories. By examining the results of Superposition Theories, it is concluded that the completeness of the terms is very important. Based on a Hypothesis for Double Superposition, this study presents three Double-Superposition Theories. They are verified to give excellent numerical accuracy, along with computational efficiency, when compared with elasticity solutions.
Intelligent Curing of Thick Composites Using a Knowledge-Based System
Authors: Pillai Vikram , Beris Antony N. , Dhurjati Prasad
Published: April 2011

An expert-system-based tool was developed in order to operate the autoclave cure of a thick fiber reinforced thermosetting matrix composite laminate in an optimal manner. The best temperature profile is obtained using a novel model-based optimization technique. It incorporates known heuristics about the problem to prune the search space and reduce the dimensionality of the problem so as to converge to a near-optimum in a reasonable amount of time. The objective function used has components for minimizing the total cure time, reducing the temperature excursions during an exotherm and minimization of process-induced residual stresses in the laminate. The inevitable divergence between the process and the simulation precludes the direct application of such optimal profiles. We have devised a general methodology to address this issue of process-model mismatch, which allows for the implementation of model-based optimal profiles in real situations. This methodology is directed at ensuring that the actual cure follows the same trends as predicted by the optimal cure, rather than trying to achieve an exact match. Trend analysis is used to access the underlying process trends in the profiles of different variables of the optimal solutions. Data from the autoclave is also analyzed on-line to determine the trends in real-time. A correlation between the control decisions and the trend analysis of a number of profiles from the optimization module is used to determine rules to translate the `simulation' optimal profile to one that will be used on the process. The results from application of this overall strategy for the curing of glass-polyester composites are presented. The expert system achieved the desired objectives of minimizing the cure time, while still producing a high quality part.
Analysis and Modeling of Plain Knitted Fabric Reinforced Composites
Authors: Ramakrishna S.
Published: April 2011

This paper presents the analysis and modeling of elastic properties of plain knitted fabric reinforced composites. Knitted fabric is formed by interlocking of looped yarns. Knitted fabric composite is heterogeneous with impregnated curved yarns and resin rich regions. Firstly, the three-dimensional orientation of yarns was expressed in terms of known parameters of the knitted fabric; these parameters are course density, wale density and impregnated yarn diameter. An analytical model named "cross-over model" has been proposed. This model considers the crossing over of yarns of adjacent loops at the interlocking region. The average effective elastic properties of each curved yarn were determined using the stress-strain relations for a lamina of arbitrary orientation. Using classical lamination theory, the effective elastic properites of the yarns in the cross-over model were determined. Finally, the elastic properties of the composite were determined by combining the effective elastic properties of the curved yarns and the resin rich regions. It has been shown that the cross-over model provides a rough estimate of the elastic modulus of the knitted fabric composite (volume fraction of fibers, Vf = 9.5%) by comparing theoretical predictions with the experimental results. However, it is to be noted that at higher Vf, the geometry of knit loop would be distorted thus the model accuracy would be affected.
Effects of Moisture Content on Nonlinear Deformation Behavior of CF/Epoxy Composites
Authors: Ogi Keiji , Takeda Nobuo
Published: April 2011

This paper presents the effects of moisture content on nonlinear stress-strain behavior of CFRP composites. First, a plane-stress elastic-plastic constitutive model is newly developed based on a fourth-order complementary elastic energy function and a one-parameter plastic potential in which an anisotropy parameter changes with plastic deformation. Secondly, elastic constants, the anisotropy parameter and the effective stress-effective plastic strain relation are determined at various moisture contents for both the present model and the conventional model where the anisotropy parameter is constant. Finally, the above results are utilized to predict the off-axis stress-strain response. The transverse and shear elastic moduli decrease with increasing moisture content, and the anisotropy parameter and the effective stress-effective plastic strain relation also vary with moisture content. Off-axis stress-strain curves predicted by the present model agree with experimental results better than those predicted by the conventional model.
Stresses and Deformations Induced during Manufacturing. Part I: Theoretical Analysis of Composite Cylinders and Shells
Authors: Jain Lalit K. , Mai Yiu-Wing
Published: April 2011

Composite cylinders and cylindrical shells are important structural members. It has been observed that during the manufacture of these components, residual stresses build up. In composite cylinders, the state of residual stresses may lead to defects such as micro-cracking, fibre waviness, delamination, and wrinkling, which might degrade the final mechanical performance of the cylinders. In the case of cylindrical segments, the residual stresses also lead to a reduction in the enclosed angle where the out-of-plane contraction is higher than the in-plane contraction. This phenomenon (of the reduction in the enclosed angle) is referred to as spring-in. In this paper, a simple mechanics-based model is developed using modified shell theory that predicts the degree of spring-in in anisotropic cylindrical shells and determines the residual stresses, strains and displacements in both anisotropic laminated composite cylinders and shells with arbitrary lay-ups subjected to random temperature gradients. The model accounts for the resin shrinkage occurring during curing and also includes the effect of moisture gradients. The effects of various factors such as geometry of the component, the ply stacking sequence, the part thickness and the tool radius are examined. The accuracy of the present analysis is investigated by comparing predicted results with previously published results.
Modeling the Initiation and Growth of Delaminations in Composite Structures
Authors: Reedy E. D. , Mello F. J. , Guess T. R.
Published: April 2011

A method for modeling the initiation and growth of discrete delaminations in shell-like composite structures is presented. The laminate is divided into two or more sublaminates, with each sublaminate modeled with four-noded quadrilateral shell elements. A special, eight-noded hex constraint element connects opposing sublaminate shell elements, and makes the two opposing shell elements act as a single shell element until a prescribed failure criterion is satisfied. Once the failure criterion is met, the connection is broken, and a discrete delamination initiates or grows. This approach has been implemented in a three-dimensional finite element code. This code uses explicit time integration, and can analyze shell-like structures subjected to large deformations and complex contact conditions. Tensile, compressive, and shear laminate failures are also modeled. This paper describes the eight-noded hex constraint element used to model the initiation and growth of a delamination, and discusses associated implementation issues. In addition, calculated results for double cantilever beam and end notched flexure specimens are presented and compared to measure data to assess the ability of the present approach to model a growing delamination. Results are also presented for a diametrally compressed ring to demonstrate the capability for analyzing progressive failure in a highly deformed composite structure.
Determination of Stress Gradients in Fibrous Composites by X-Ray Diffraction from Fillers
Authors: Prinz B. , Schnack E.
Published: April 2011

The method of X-ray diffraction from crystalline filler particles has been applied to fibrous composites. The stress/strain transfer behaviour between the particles and the surrounding matrix is described by the X-ray elasticity constants (XEC). These constants have been determined by experiment as well as by means of theoretical models. Above all for the first time, complete quantitative XEC are given for CdO fillers which have turned out to be the most suitable filler material.

The exact knowledge of the XEC of particle/laminate systems formed the foundation to apply the method to analyse interlaminar stress concentrations. For the first time, stress gradient measurements even on notched laminates have been successfully carried out. Free edge stress distributions obtained by experiments are in good agreement with results from finite element simulations.
Delamination and Fiber Bridging Modelling in Composite Samples
Authors: Daridon L. , Cochelin B. , Ferry M. Potier
Published: April 2011

Crack propagation in a long fiber composite material typically involves the development of a fiber bridging which connects the two crack tips, resulting in an increase of the crack-growth resistance, known as the R-curve effect. The specific contribution of this paper is to account for this effect within the framework of simple one dimensional models and to perform efficient, simple, and complete numerical simulations of crack propagation taking into account creation and transport of the fiber nesting. The composite sample is modelled by an assembly of three beams and a density of damageable springs for the fiber bridging. With this type of modelling, we can separate the global dissipation in the sample into two specific terms: one associated with crack tip propagation and one with degradation of the fiber nesting. We present results for classical Mode I and Mode II tests, using Double Cantilever Beam and End Load Split specimens.
A Micromechanical Model for the Prediction of the Lamina Longitudinal Compression Strength of Composite Laminates
Authors: De Morais Alfredo Balaco , Marques Antonió Torres
Published: April 2011

This paper describes a new numerical model for the prediction of the lamina longitudinal compression strength. The model assumes failure occurs by fibre microbuckling and takes into account initial fibre misalignments and matrix non-linear stress-strain behaviour. The results obtained with the 2D-base and the 3D-extended formulations agree quite well with those of Finite Element models presented in a previous paper. Model predictions were compared with experimental data supplied by the authors' partners in a Joint Research Project. The 3D formulation proved to be the most accurate. Agreement with the experimental data is satisfactory, particularly in view of the numerous relevant variables and inevitable simplifications implicit in the models.
Experimental Characterization of Constitutive Models for PEEK Thermoplastic Composite at Heating Stage during Forming
Authors: Wang C. , Sun C. T.
Published: April 2011

The deformation behavior of APC-2 PEEK matrix thermoplastic composite at elevated temperatures was characterized in order to increase the understanding of the mechanics in solid-phase forming of advanced thermoplastic composite structures. Off-axis tension tests were performed to determine the inelastic and rate-dependent material properties. An anisotropic elastic/viscoplastic constitutive model was proposed to describe the inelastic and rate-dependent material responses. The constitutive model was evaluated using off-axis and laminate specimens subjected to uniaxial tension.
Residual Strength Simulation of Fibre Reinforced Metal Laminates Containing a Circular Hole
Authors: Afaghi-Khatibi Akbar , Ye Lin
Published: April 2011

The main objective of this study was to evaluate the residual strength of fibre reinforced metal laminates (FRMLs) with a circular hole using an effective crack growth model (ECGM). Damage is assumed to initiate when the local normal stress at the hole edge reaches the tensile strength or yield strength of composite and metal layers, respectively. The damage in constituent materials was modelled by fictitious cracks with cohesive stress acting on the crack surfaces, and the damage growth was simulated by extension of the fictitious cracks and reduction of the cohesive stress with crack opening. The apparent fracture energy of composite layers and fracture toughness of metal layers was used to define the relationships between the tensile/yield strength and the critical crack opening. Based on the global equilibrium, an iterative technique was developed to evaluate the applied load required to produce the damage growth. The residual strength of notched fibre reinforced metal laminates was defined by instability of the applied load with damage growth. The effect of hole size on the residual strength was studied and the stress redistribution with damage growth was discussed. The residual strength simulated from ECGM correlated well with experimental data in the open literature for various ARALL laminates with a circular hole. The results indicate that damage first occurs in the composite layer, followed with stable crack growth well before ultimate failure, associated with plastic yielding of metal layer at the hole edge.
Modeling of Fatigue in Cross-Ply Ceramic Matrix Composites
Authors: Solti J. P. , Mall S. , Robertson D. D.
Published: April 2011

This study proposes a methodology for modeling the fatigue response of cross-ply ceramic matrix composites (CMCs). The micromechanics based analysis and failure criteria are formulated to model stress-strain hysteresis, strain ratchetting and S-N behavior specific to room-temperature cyclic loading environments. The damage mechanisms considered are matrix cracking in the 90° and 0° plies, fiber/matrix interfacial debonding, fiber fracture, and fiber pull-out. These damage modes are modeled by a set of failure criteria with a minimum reliance on empirical data which can be easily employed in a variety of numerical and analytical techniques. The predicted results are found to be in good agreement with the experimental data; however, it is observed that the assumed degradation in the frictional resistance along the fiber/matrix interface plays a dominant role in determining the fatigue response.
Transverse Fracture Study of a Ceramic Composite by Double Torsion Technique under Oxidizing Conditions
Authors: Kahraman Ramazan , Mandell John F.
Published: April 2011

This paper makes a critique of the double torsion (DT) test technique in studying the transverse fracture behavior of SiC/CAS-I1 glass-ceramic composite. Previous to this study, there has not been much work on high temperature transverse properties of this composite. The applicability of the DT method to elevated temperatures was demonstrated. Test rate had a considerable effect on the compliance for the DT tests performed at high temperatures. This is believed to be due to the thermal expansion/shrinkage of the metals used in the DT test system. However, this did not appear to be a problem for the purposes of this study. Transverse fracture toughness data, coupled with scanning electron microscopy and microdebonding analysis of the fracture surfaces, indicated no significant effect of high temperature oxidative environment on cracking parallel to the fibers in SiC/CAS-I composite.
A Model for Thermosetting Composite Pultrusion Process
Authors: Kim Dae-Hwan , Han Poong-Gyu , Jin Geun-Ha , Lee Woo Il
Published: April 2011

The pultrusion process of thermosetting composite was modeled. The temperature and the degree of cure distribution inside the composite were calculated numerically using the finite element method. The cure kinetics of the resin was modeled from the data obtained from the differential scanning calorimetry. The pulling force during the pultrusion process was also estimated from a model. The model assumes that the pulling force is induced from backward resin flow as well as Coulomb friction. Effect of partially cured resin inside the resin layer between the composite and the mold wall was also considered in the model. Mathematical forms of the model were derived. Effects of the pulling speed as well as the die temperature on the pulling force were modelled. Validity of the model was tested experimentally. The experiments were performed for two resin systems (vinylester and epoxy resins) with two different die geometries. The results showed close agreement between the model and experimental results.
Analysis of Thickness-Tapered Unidirectional Composite Compression Specimens
Authors: Adams Donald F. , Finley Garth A.
Published: April 2011

Four different thickness-tapered compression specimen geometries were modeled using a three-dimensional finite element analysis. Shear-, end-, and combined-loading conditions were used to simulate specimen testing in the Illinois Institute of Technology Research Institute (IITRI) and the Wyoming End-Loaded, Side-Supported (ELSS) compression test fixtures. Both carbon/epoxy and glass/epoxy unidirectional composite materials were simulated. From the results of twelve finite element analysis models, predictions were made concerning optimum thickness-tapered compression specimen geometry and the influence of loading conditions. These predictions were then correlated with experimental results for axial compressive strength, modulus, and failure modes in thickness-tapered Hercules AS4/3501-6 carbon/epoxy and CYTEC S2/5216 glass/epoxy unidirectional composite specimens
Crushing Characteristics of 3-D Braided Composite Square Tubes
Authors: Chiu C. H. , Lu C. K. , Wu C. M.
Published: April 2011

Although textile composites have been studied for some time, the crushing failure modes and energy absorption data have been limited. In this study, static crushing tests were conducted on 3-D carbon/epoxy and Kevlar/epoxy braided composite square tubes to investigate the failure modes and energy absorption capability of those materials. During crushing test the carbon/epoxy braided composite square tubes splayed first, then bent until final failure. In contrast, the Kevlar/epoxy braided composite square tubes exhibited a stable progressive folding failure. The failure modes of the crush tests of carbon/epoxy braided composite tubes included compressive shear failure, bending, local buckling of the axials, shear and tensile failure of the braiders, matrix crush, interface debonding and open mode crack propagation. The specific energy absorption capability of carbon braided composite tube is higher than Kevlar braided composite tube, but Kevlar tube has good post-crush structural integrity. In the 3-D braided composite tubes, the axial yarns are the main supplier for energy absorption whereas the braider yarns determine the crushing failure mode
Influence of In-Plane Fiber Misalignment on Moiré Interferometry Results
Authors: Hale Richard D. , Adams Daniel O'Hare
Published: April 2011

Moiré interferometry is an experimental technique that is being viewed favorably for verifying analytical predictions of mechanical performance in composite materials. However, since interferometry measures displacements at a free surface, one must separate the effects due to fiber architecture from those due to free-edge effects. This paper addresses the influence of in-plane fiber misalignment on free-edge interlaminar shear strain. Experimental results for laminates with intentional fiber misalignment are compared to analytical predictions by using the finite element method. Small degrees of fiber misalignment are shown to produce large free-edge interlaminar shear strains. Thus, moiré interferometry results may not be quantifiable for nonidealized composites which exhibit in-plane fiber misalignment.
Energy Absorption of Composite Plates and Tubes
Authors: Dubey Dean D. , Vizzini Anthony J.
Published: April 2011

A total of 28 graphite/epoxy flat-plate specimens and six graphite/epoxy tube specimens were crushed under quasi-static conditions to provide a basis for comparison of the measured energy absorbency of these two geometries. The energy absorbency of each specimen was measured, and the specific sustained crushing stress (SSCS) was determined. All plate and tube specimens were manufactured from AS4/3501-6 graphite/epoxy with the same layup and thickness, thus providing a common laminate for comparison. Flat-plate specimens of two different widths were tested to determine the effect of the testing fixture on the observed energy absorbency. The flat-plate results indicate that the specific energy absorbency per unit thickness is nearly independent of the specimen width, with a small contribution to the energy absorbency by the testing fixture. However, this calculated value of measured energy absorption due to the test fixture is within the scatter of the data. The results also indicate that specimen geometry affects specimen stability and therefore the failure modes exhibited by the specimen during crushing. Similar failure modes were observed in the tube and flat-plate specimens with the flat plates absorbing 12% less energy per unit mass. Consequently, flat-plate specimens can be used as a lower-cost alternative to tube specimens or in test programs requiring simpler geometries.
Transverse Shear Strength of Unidirectional Carbon Fiber Reinforced Aluminum Matrix Composite under Static and Dynamic Loadings
Authors: Dai L. H. , Bai Y. L.
Published: April 2011

The static and dynamic transverse shear strength of a unidirectional carbon fiber reinforced cast aluminum alloy matrix composite (C f/A356.0) and aluminum alloy were studied with Instron-1195 testing machine and a split Hopkinson pressure bar, respectively. The results indicate that the transverse shear strength of the composite is about half that of the aluminum matrix. Both the composite and the matrix show almost the same rate effect, namely a slight increase in the shear strength with increasing strain rate. This may be an indication that the rate effect of the unidirectional Cf/A356.0 composite is mainly controlled by the A356.0 aluminum alloy matrix. Moreover, the shear failure mechanisms of the composite are discussed.
Loading Rate Effect on Woven Glass Laminated Plates by Penetration Force
Authors: Wu Enboa , Chang Ling-Chen
Published: April 2011

The effect of the loading rate on 2.9 mm thick E-glass/epoxy woven composite laminates subjected to quasi-static contact and impact forces applied in the transverse direction was investigated. The quasi-static contact test was performed using a 12.7 mm hemispherically-tipped indentor. On the other hand, impact was conducted using a gas gun with projectiles of the same frontal shape weighing 8.7 g, 17.7 g and 35.5 g. The test speeds were from several m/s to the ballistic limits of the laminates. The impact response was measured using the self-developed laser Doppler anemometer (LDA) system and was subsequently converted into the force and displacement histories. Significant loading rate effects were observed when comparing the laminates loaded quasi-statically to those struck by projectiles with a four-fold difference in mass. These effects included differences in the impact force history, a drastic increase of the force peak and the absorbed energy, and a completely different delamination damage pattern. Thus, the loading rate effect for composite laminates reinforced by glass fibers cannot be overlooked. On the other hand, fiber breakage throughout the plies on the distal side of laminates was found to absorb a significant amount of impact energy. The absorbed energy versus the initial projectile energy for the dynamically loaded specimens, when plotted in a non-dimensionalized manner, was found to have a bilinear relationship regardless of the projectile mass employed. This bilinear line was also closely bounded from below by the quasi-static results.
A Study on the Effects of Clearance and Interference Fits in a Pin-Loaded Cross-Ply FGRP Laminate
Authors: Di Scalea F. Lanza , Cloud G. L. , Cappello F.
Published: April 2011

The elastic behavior of a cross-ply fiber glass-reinforced epoxy laminate loaded in tension through a pin fitted with clearance and interference is examined. A nonlinear contact analysis using the Finite Element Method and an experimental study by Speckle Interferometry are performed, showing fairly good correlation. An interesting comparison is performed between FEM results and Hertzian contact theory. Major conclusions are: (I) clearance causes high compressive stress in the bearing area, and does not influence the peak tensile stress; (2) interference has beneficial effects on the joint, as (a) it lowers the peak circumferential stress, and (b) it decreases the change of stress due to the application of the load. This last conclusion suggests that interference can improve the fatigue life of the joint.
The Fatigue Life Prediction of SiC Whisker Reinforced Al6061 Alloy Composite by Monte-Carlo Simulation
Authors: Kwon Jae-Do , Ahn Jeong-Ju , Kim Sang-Tae , Ishii Hitoshi
Published: April 2011

The Monte-Carlo simulation program is developed to predict the fatigue life of SiCw/Al composite. In order to verify the accuracy of the developed program, the predicted fatigue life of 25% SiCw/Al is compared with the data obtained from the fatigue crack growth tests. The result shows that the average value of difference between the predicted fatigue life and the experimental data is about 15%. So, the fatigue life could predict successfully by means of the Monte-Carlo simulation program. The fatigue crack growth lives of 5%, 10%, 15%, 20%, 30% and 35% SiCw/Al composites are predicted using the Monte-Carlo simulation.
Investigation of Dynamic Pressure Behavior in a Pultrusion Die
Authors: Sharma D. , McCarty T. A. , Roux J. A. , Vaughan J. G.
Published: April 2011

The fluid resin pressure rise in a pultrusion die inlet can have a significant effect on the quality of the pultruded product. An appreciable pressure rise is required to suppress void formation and promote good fiber "wet out." Most of the pressure rise in the die occurs in the short, tapered die entrance region. In this study a finite element model is developed to predict this pressure rise as a function of various process parameters for a given die entrance geometry. The fiber resin system was modeled based on the assumptions of Darcy's law for flow in porous media. The momentum equations were combined with the continuity equation to save computational time and memory. The resulting equation was solved using a Galerkin weighted residual based finite element method. This model predicts the pressure rise in the tapered entrance region of the pultrusion die as well as along the straight portion of the die. Variations in pressure rise in the die inlet as a result of changes in the pull speed, resin viscosity, and percentage fiber volume are presented. These results are important because they can lead to more appropriate process variables for obtaining a suitable pressure rise in the die.
Modeling of Composite Laminates with Multiple Delaminations under Compressive Loading
Authors: Kyoung Woo-Min , Kim Chun-Gon , Hong Chang-Sun , Jun Seung-Moon
Published: April 2011

Multiple delaminations are more realistic damage types in the laminated composite structures. In this study, buckling and postbuckling analysis was conducted for the composite laminates with multiple delaminations under compressive loading. In a nonlinear finite element formulation, the updated Lagrangian description and modified arc-length method were adopted. For a finite element modeling of composite laminates, the eight-node degenerated shell element was used. To avoid the overlapping between delaminated layers, which is a physically inadmissible buckling mode, the contact node pair was defined by use of virtual beam element. It employed constraints at the overlapped region in the load incremental procedure. Using this modeling of multiple delaminations, accurate results were obtained for the buckling and postbuckling behavior of composite laminates. Numerical results showed that multiple delaminations lower buckling loads and load carrying capacities in the postbuckling region.
Tensile Strength Testing of GRP Pipes at Elevated Temperatures in Aggressive Offshore Environments
Authors: Hale J. M. , Gibson A. G. , Speake S. D.
Published: April 2011

A programme of tests has been conducted to characterize the strength reduction of two GRP composite materials in the form of filament wound pipes as a function of temperature and testing environment. This complements previous work on the same materials using coupon specimens [1].

The GRP materials and testing environments were selected to be of maximum relevance to the offshore industry. The materials were E-glass fibre reinforcement in matrices of two epoxy resin systems. The testing environments were sea water and sea water with dissolved CO2 and H2S gasses. The temperature range tested was ambient to 150°C.

The tests were performed on specimens of commercial filament wound pipe loaded in pure axial tension. A special purpose testing facility was developed for this work, and this is described briefly.

Results are presented for the strength of both materials as a function of temperature when tested dry and a quantitative discrepancy noted with the previous results obtained using coupon specimens. Results of further tests conducted to investigate this phenomenon are presented and a suggested explanation is proposed. Further results are presented for the strengths of the materials in the test environments which are shown to be in qualitative agreement with the previous coupon tests.

It is shown that the axial strength of GRP pipes is significantly reduced at elevated temperatures, that the degree of this reduction is dependent on the matrix material and is magnified by sea water absorption and that dissolved CO2 and H2S have little effect.
Mode I Fracture Toughness of Fiber Reinforced Composite-Wood Bonded Interface
Authors: Davalos Julio F. , Qiao Pizhong , Madabhusi-Raman Prabhu , Lang Elemer M.
Published: April 2011

An experimental characterization of the opening-mode (Mode I) fracture toughness of bonded interfaces for hybrid laminates is presented. A bi-layer Contoured Double Cantilever Beam (CDCB) specimen, which is designed by the Rayleigh-Ritz method, is effectively used for the fracture toughness tests of the bonded interfaces. The bi-layer specimen consists of constant thickness adherends bonded to straight tapered sections of an easily machinable material, and it is contoured to achieve a constant rate of compliance change with respect to crack length. Using linear slope CDCB specimens, Mode I fracture tests for wood-wood and Fiber Reinforced Plastic (FRP)-wood bonded interfaces are performed to determine the critical loads for crack initiation and crack arrest, and from the critical loads measured, the critical strain energy release rates (GI,) are easily evaluated by making use of the experimentally verified constant compliance rate change over defined crack lengths. Based on finite element analyses, the predicted GI, values by the Jacobian Derivative Method correlate closely with experimental results. The FRP-wood bonded interface considered in this study exhibits a stable crack propagation behavior, and the Resorcinol Formaldehyde adhesive used offers a good potential for bonding FRP to wood sub-strates. The efficient CDCB specimen and experimental/analytical program presented in this paper can be used to evaluate the Mode I fracture toughness (Gkn) for hybrid material interface bonds, such as FRP-wood.
Repair of Composite Laminates-I: Test Results
Authors: Ahn Sung-Hoon , Springer George S.
Published: April 2011

Tests were conducted with the objective of evaluating the effectiveness of techniques used for repairing damaged fiber reinforced laminated composites. Effectiveness of the repair was assessed by the tensile strength of the repaired laminate. In the tests the failure loads of laminates repaired either by the scarf, the uniform lap, or the stepped lap technique were measured. Data were generated with the following parameters having been varied: type of material of the damaged laminate, type of repair material, scarf angle and number of external plies in scarf repair, length and number of repair plies in uniform and stepped lap repair, moisture content of the laminate prior to repair, moisture content of the laminate after repair, test temperature, roughness of grinding tool used in preparing the repair surface, and the temperature applied during the cure of the repair patch. The aforementioned parameters were varied over wide ranges resulting in systematic sets of data which shed light on the influence of each of these parameters on the effectiveness of the repair.
Fatigue Sensitivity of Random Glass Fibre Reinforced Plastics
Authors: Caprino G. , D'Amore A. , Facciolo F.
Published: April 2011

Static and fatigue tests were carried out in four-point bending on three random Glass Fibre Reinforced Plastics (GFRP), different in resin type (thermosetting and thermoplastic), resin content, and fibre form. The experimental results were evaluated in the light of a recent two-parameter model, allowing for a statistical analysis of the fatigue data. It is shown that, despite the different nature of the materials considered, the theoretical model accurately predicts the fatigue response, permitting the calculation of the scatter in fatigue life from the scatter in static strength. A physical interpretation is given for the two parameters appearing in the model, correlated with the material sensitivity to stress amplitude and number of cycles, respectively. From the values of the two parameters, evaluated for the GFRPs tested, a common trend is observed: the higher the material sensitivity to stress amplitude, the lower its sensitivity to number of cycles. This implies that comparing different materials on the basis of their fatigue response at relatively low cycles can be misleading, because a better behaviour at high stress levels can result in shorter life-times at low stress levels.
Stress Rupture of Unidirectional High Performance Thermoplastic Composites in End-Loaded Bending at Elevated Temperatures. Part I: Experimental Characterization of the Failure Mode
Authors: Mahieux Céline A. , Russell Blair E. , Reifsnider Kenneth L.
Published: April 2011

Macro- and micro-observations of unidirectional thermoplastic composites under end-loaded quasi-static bending have been made. The presence of stable microbuckles has been identified on the compressive side of the specimen. The formation of the microbuckles has been videotaped. Frames of this film are examined and the failure process is identified. The stress rupture of these specimens at elevated temperatures in bending seems to be driven by the formation and propagation of microbuckles. Videotaping from a different angle also gives information on the change of shape of the bent specimen in the oven as a function of time. These unique data form the basis for an understanding of the stress rupture of unidirectional thermoplastic composites in bending that was not previously available.
Elastic Response of Porous Matrix Plain Weave Fabric Composites: Part I—Modeling
Authors: Kuhn J. L. , Charalambides P. G.
Published: April 2011

In this first of a two-part series of papers, improved two-dimensional modified lamination theory and three dimensional finite element models are developed for the elastic analysis of plain weave fabric composites. New more accurate surface functions are proposed to model the geometry of the woven unit-cell. These geometry functions are implemented in both the analytical and finite element models which allows for direct model comparison. These models take into account the effects of the complex microstructure exhibited by both Chemical Vapor Infiltrated (CVI) woven ceramic matrix composites and traditional woven polymer matrix composites. The spatially dependent directional orthotropy of the fiber bundles is incorporated in both the analytical and numerical 3-D finite element models. In addition to the interbundle matrix porosity. the models also account for the presence of porosity within the bundles, as well as the presence of thin fiber and bundle coatings. The models are developed in a general manner such that they can be used to assess the effective elastic response of both stiff-ceramic and soft-polymer matrix woven composites. Case studies of pure remote tension and shear are formulated, and the theoretical platform required for the analytical and numerical evaluation of the in-plane effective elastic properties of stiff-ceramic and soft-polymer matrix woven composites is presented. Extensive parameter studies, discussion of the results and overall conclusions are reported in the accompanied manuscript with the title extension, Part II-Results. This work extends and refines previous micromechanical models aimed at predicting the macromechanical response of woven composite systems. It also establishes for the first time a hierarchical modeling framework, which is needed in the modeling of the effects of complex microstructures often exhibited by soft-polymer and stiff-ceramic porous matrix woven composites.
Elastic Response of Porous Matrix Plain Weave Fabric Composites: Part II—Results
Authors: Kuhn J. L. , Charalambides P. G.
Published: April 2011

In this second of a two-part series of papers, the analytical and three-dimensional finite element models developed in Part I are implemented to predict the effective elastic response of both Stiff-porous ceramic and Soft-dense polymer matrix plain weave fabric composites. Analytical results obtained using a Modified Lamination Theory (MLT) model and which are based on four methods of unit-cell property averaging are presented. Effective elastic in-plane properties predicted via the finite element method for a single woven ply and for a laminate comprised of symmetrically arranged woven plies are also reported. Comparisons between the analytical estimates and the numerical predictions suggest that the Parallel (P-MLT) property averaging scheme yields, in an overall sense, good unit-cell effective property estimates. Thus, the P-MLT model was employed to conduct extensive parameter studies aiming at assessing the effects of the woven geometry and the overall microstructure on the effective elastic properties of Soft- and Stiff-matrix woven composites. Soft-matrix systems were shown to exhibit higher sensitivity to the unit-cell woven morphology and fiber/matrix elastic mismatch when compared to their Stiff-matrix counterparts. On the other hand, the effective elastic properties of Stiff-matrix systems were shown to be substantially reduced with increasing interbundle matrix porosity, and were also shown to be rather sensitive to the elastic properties of the thin fiber and bundle coatings. Surface contour data obtained from 3-D finite element analysis provide strong evidence of local micro-bending and stress concentrations within the unit cell. In both Soft- and Stiff-matrix systems, the out-of-plane normal and shear stresses along the bundle/matrix interface surfaces were shown to be at least an order of magnitude smaller than the predicted normal bundle stress along the fiber direction. This work presents detailed analytical and numerical parameter studies, and explores for the first time the relations between the microstructure and macromechanical woven composite response.
Interlaminar Fracture Toughness of GFRP Influenced by Fiber Surface Treatment
Authors: Rikards R. , Korjakin A. , Buchholz F. G. , Wang H. , Bledzki A. K. , Wacker G.
Published: April 2011

The interlaminar fracture behavior of unidirectionally glass fiber reinforced composites with fiber surface treatments have been investigated in Mode I, Mode II and for a fixed mixed Mode 1/11 ratio. With respect to the loading conditions at the crack tip the double cantilever beam (DCB), the end notched flexure (ENF) and the mixed mode flexure (MMF) specimens were used in order to obtain Mode I, Mode II and mixed Mode I/II critical energy release rates. The data obtained from these tests have been analyzed by using analytical and finite element approaches. The present investigation is focused on the influence of the glass fiber surface treatment on the interlaminar fracture toughness. Glass fibers with two different fiber surface treatments have been investigated. For the DCB test the data reduction from the experiments have been carried out by using six different approaches: the area method. the Berry method, the modified beam analysis, the compliance method and the geometric linear and non-linear finite element analysis. The geometric non-linear finite element analysis of the DCB test was performed firstly. For the ENF test contact and friction along the crack surfaces have been taken into account.
Co-Cured Extension-Twist Coupled Damped Composite Strut
Authors: Napolitano K. L. , Kosmatka J. B.
Published: April 2011

A co-cured damped composite strut that uses the extension-shear coupling mechanism of off-angle composite material to enhance the damping performance of a viscoelastic damping material is presented. The damping material was placed between two graphite shells. Composite material replaced the damping material at each strut end. The inner shell contains plies oriented at a positive ply angle in the first half of the strut length and plies oriented at a negative ply angle in the second half. Due to the extension-shear coupling, the shell center section rotates when the strut undergoes extension deformation. The induced torque of each shell cancel each other so that there is a negligible net twist at each strut end. The outer shell plies are oriented in the opposite manner so that the outer shell center rotates in the opposite direction. The relative rotation at the strut center between the two shells places the damping material into shear, providing damping. Finite element analysis was used to determine optimal damping material shear modulus and ply orientation to maximize strut imaginary stiffness. Modal tests were performed on undamped and damped struts. Damping increased by factors of 6.2, 12.8, and 29.3 for first bending, extension, and torsion modes, respectively, over the undamped strut. Corresponding strut bending, extension, and torsion stiffness decreased by 50, 14, and 50%, respectively.
Influence of Ply Orientation on Delamination in Composite Laminates
Authors: Tao Jianxin , Sun C. T.
Published: April 2011

An experimental investigation in interlaminar fracture behavior and toughness of multi-directional composite laminates was conducted. ENF-like specimens were used for three-point-bending tests first to determine the crack-jumping behavior in various laminates. It was found that a delamination crack could propagate only along the 0/0 interface. Based on the result of this investigation, an experimental method to determine Mode II dominated interlaminar toughness was proposed and used to obtain interlaminar toughness of 0/0 interfaces in AS4/3501-6 composite material with 0 = 30°, 45°, 60°, and 90°. It was found that the critical strain energy release rate decreases as the off-axis angle 0 increases.