bluuebunny/crossref_metadata_2025 huggingface dataset:
A Numerical Investigation of Three Dimensional Effects in Cracked Orthotropic Plates
Authors: Prabhu Santosh , Lambros John
Published: April 2011

In this study, an investigation of the three dimensional nature of stress fields in the near tip region of a cracked orthotropic plate was conducted. The finite element method was used to investigate the relative extent of regions of three dimensional to two dimensional (plane stress or plane strain) deformation in the cracked plate. The properties used in the orthotropic material system simulated those of a unidirectional graphite/epoxy composite with a crack oriented either parallel or normal to the fiber direction. A three point bend loading geometry was simulated. In analogy to isotropic materials, it was observed that a plane stress K-dominant region does not arise arbitrarily close to the crack tip because of the existence of a three dimensional zone. However, it was seen that the shape and the size of this three dimensional deformation region in the cracked composite plate is substantially different from that of an isotropic plate and depends intimately on material properties. For a crack parallel to the fiber direction the three dimensional zone extends to 0.46 h ( h = specimen thickness) ahead of the crack tip but only to 0.27 h at 30°. Fibers perpendicular to the crack produce a highly elongated three dimensional zone in the direction of the fibers (up to 0.78 h). This zone is also sensitive to the variations in Poisson's ratios of the orthotropic solid.
Die Inlet Contour Impact on the Pressure Rise in a Pultrusion Die
Authors: Raper K. S. , Roux J. A. , McCarty T. A. , Vaughan J. G.
Published: April 2011

The quality of a pultruded composite depends on many process control parameters and variables. The pressure rise in the pultrusion die inlet is one such variable which can impact the quality of a pultruded product. An appreciable die inlet pressure rise can suppress void formation and enhance fiber "wet out." In this study, a model based on Darcy's law for flow in a porous medium and which employs the finite volume solution method is developed to predict the pressure and velocity fields as a function of various die inlet contours. Investigating the behavior of the pressure rise in the pultrusion die inlet as a function of the shape of the die inlet contour can be a very useful and effective tool for analyzing and designing pultrusion die inlets. Various die inlet contours such as the wedge, circular tapered die inlet, and some hypothetical die inlet contours were investigated to determine the impact on die inlet pressure rise. The original contributions of this paper are results which are useful for designing pultrusion dies and for producing an optimum pressure rise which will enhance the quality of the pultruded product.
Ultrasonic and Interlaminar Properties of Highly Porous Composites
Authors: Mouritz A. P.
Published: April 2011

The effect of increasing void content (up to 30%) on the ultrasound attenuation coefficient, interlaminar shear strength and Mode I interlaminar fracture toughness properties of a glass fibre reinforced polymer (GFRP) composite were investigated. Also studied was the effect of increasing the thickness (up to 150 mm) of highly porous GFRP composites on ultrasound attenuation. A pulse-echo ultrasonics technique was used to measure the amount of attenuation through the GFRP, and this increased linearly with thickness up to 150 mm. Attenuation also increased rapidly with void content, and these measurements were used to further test the accuracy of models proposed by Martin and by Hale and Ashton for calculating the attenuation coefficient of porous composites. Poor agreement was found between the measured and calculated attenuation coefficients, and the causes for the discrepancies are discussed. As expected, the interlaminar shear strength and interlaminar fracture toughness decreased rapidly with increasing void content.
Strength Prediction of Plain Woven Fabrics
Authors: Kollegal Manohar G. , Sridharan Srinivasan
Published: April 2011

The mechanical behavior of plain woven fabric composites under inplane loading is investigated using 3-D finite element analysis (FEA) in conjunction with a micro-mechanical model. The analysis incorporates both geometric and material nonlinearities. The use of finite elements facilitates accurate description of the geometry, i.e., the lenticular shape of the cross-section and the curvilinear profile of the yarns. Sandwiched between the yarns is a thin layer of matrix whose nonlinear shear response and local tensile failure mimic the anticipated slip and separation of the yarns. Intra-yarn damage is treated using a simple micro-mechanical model installed at the integration stations of the FEA. The results of the analysis make it possible to spell out the dominant modes of failure under inplane tension, compression and shear respectively. Excellent agreement is found to exist between the results of the present analysis and currently available experimental results.
Dynamic Testing of Composite Laminates with a Tensile Split Hopkinson Bar
Authors: Eskandari H. , Nemes J. A.
Published: April 2011

A tensile version of the split Hopkinson bar, suitable for dynamic testing of laminated composite materials, is introduced. A pulse duration of 200-250 microseconds and peak strain rates of up to 350 s5l are obtained. Tests performed on a quasi-isotropic lay-up of graphite-epoxy show good repeatability. To determine the significance of rate-dependency, static tests were also performed using an identical specimen configuration as in the dynamic tests. Comparison of the Hopkinson bar tests to those performed at a quasi-static rate on a hydraulic test machine shows the rate-dependency of this lay-up of graphite-epoxy. Tensile strength and fracture strain are found to be higher for dynamic testing.
Inelastic Response of a Woven Carbon/Copper Composite— Part II: Micromechanics Model
Authors: Bednarcyk Brett A. , Pindera Marek-Jerzy
Published: April 2011

Part II of this paper presents a detailed development of an efficient micromechanics model for woven metal matrix composites (MMCs). The approach was to employ Aboudi's (1995) three-dimensional generalized method of cells (GMC-3D) as a global model, and to embed Aboudi's (1987) original method of cells within GMC-3D as a local model. Inelastic deformation is modeled on the local level, in the original method of cells. For this reason the field quantities must be passed between the local and global models continuously, as opposed to using the local model once simply to determine local effective properties. This process is computationally intensive, and GMC-3D, in its original form, is not adequately efficient to analyze refined composite geometries. Hence, GMC-3D was reformulated using mixed concentration equations rather than traditional strain concentration equations, improving the model's computational efficiency considerably. The model is now capable of readily analyzing the thermomechanical behavior of elaborate and varied woven and braided composites. Experimental test results for a particular woven MMC, 8-harness satin carbon/copper, were presented in Part I of this paper. The predictions of the model described herein will be correlated with these experimental results in Part III.
Inelastic Response of a Woven Carbon/Copper Composite-Part III: Model-Experiment Correlation
Authors: Bednarcyk Brett A. , Pindera Marek-Jerzy
Published: April 2011

This paper compares the predictions of a new micromechanics model for woven metal matrix composites with experimental results for 8-harness (8H) satin carbon/copper (C/Cu). Experimental results for this advanced woven metal matrix composite were presented in Part I of this paper (Bednarcyk and Pindera, 1999), while Part II discussed the development of the micromechanics model (Bednarcyk and Pindera, 2000). The experiments included monotonic, cyclic, and combined tension, compression, and Iosipescu shear tests. The micromechanics model is based on an embedded approach in which Aboudi's (1987) two-dimensional method of cells (MOC) is embedded within a reformulated version of Aboudi's (1995) three-dimensional generalized methods of cells (GMC-3D). In order to obtain accurate and realistic predictions, the model accounts for the effects of porosity, residual stresses, imperfect fiber/matrix bonding, and grip constraints. These effects, and the selection of the parameters, which characterize these effects, are discussed. By judiciously selecting the parameters, good qualitative and reasonable quantitative correlation is obtained between the model predictions and experiment. In particular, the unexpected trend based on matrix alloy type observed in the experimental tensile data has been correctly predicted.
Concise Derivation of Formulas for 3D Sublaminate Homogenization
Authors: Whitcomb John , Noh Jae
Published: April 2011

Typically, lamina by lamina modeling of thick composites is excessively expensive. Although formulas for sublaminate homogenization appear in the literature, there are discrepancies and the derivations are either not given or are complex. Herein, a simple, concise derivation of homogenization formulas is presented. The formulas are easily specialized for special classes of laminates. The formulas are expressed in an efficient form, which makes them suitable for use in numerical simulations. Three-dimensional finite element analysis was used to demonstrate that the formulas are exact.
Buckling Analysis of Elliptically Delaminated Composite Laminates with Consideration of Partial Closure of Delamination
Authors: Sekine H. , Hu N. , Kouchakzadeh M. A.
Published: April 2011

By taking account of partial closure of delamination, the buckling analysis of elliptically delaminated composite laminates under in-plane compressive loads is made using a finite element method. In order to deal with the contact problem for the partial closure of delamination, a penalty function method is proposed. In the method, fictitious springs are inserted iteratively at all of the overlapped points. Stiffness of each spring is calculated by considering the overlap and stiffness of the structure at that point. Numerical calculations were carried out to clarify the effects of delamination size, shape, position through thickness and ply angle of delaminated layer on the buckling load and buckling mode. The results are shown in graphs.
Effects of Moisture Absorption on the Dynamic Interlaminar Fracture Toughness of Carbon/Epoxy Composites
Authors: Todo Mitsugu , Nakamura Takashige , Takahashi Kiyoshi
Published: April 2011

Effects of moisture absorption on the dynamic mode II interlaminar fracture behavior of two different kinds of carbon/epoxy composites at a low impact rate of 0.9 m/s were studied. ENF specimens were preconditioned under three different environmental conditions, that is, dry, 80°C-90% RH and 80°C-wet. Mode II interlaminar fracture toughnesses, G IIC, of the composites were then evaluated under quasi-static and dynamic loading. The results showed that moisture absorption had an influence on the GIIC values of the composites. The experimental results also exhibited that the impact loading condition tended to decrease the GIIc values. Scanning electron micrography was also performed to characterize the micromechanisms of mode II interlaminar fracture. Effects of moisture absorption on the mode II interlaminar fracture behavior were then discussed on the basis of the results of the interlaminar fracture testing and microscopic studies.
Prediction of the Tensile Load Capability of Co-Cured Steel-Composite Tubular Single Lap Joints Considering Thermal Degradation
Authors: Lee Dai Gil , Cho Durk Hyun
Published: April 2011

Co-cured joints composed of composite and steel structures are realized by the excess resin extracted during cure of composite prepreg and the coupling pressure due to residual thermal stresses generated by the difference of coefficients of thermal expansion between the two materials. Although the excess resin from the composite prepreg is less dependent on the environmental temperature compared to conventional epoxy adhesives in which rubber is added to strengthen toughness, the coupling pressure is much dependent on the environmental temperature.

In this study, the tensile load capability of co-cured joints composed of carbon fiber epoxy composite and steel adherends was experimentally investigated with respect to the stacking sequence of the composite and the environmental temperature. Also, the stress distributions in the composite were calculated by finite element method, from which the failure index of the composite adherend was calculated considering thermal degradation of the composite.

The co-cured steel-composite tubular single lap joint under tensile loads might be applied to hybrid structures such as golf clubs and automotive composite propeller shafts which have a conventional adhesively-bonded joint by epoxy adhesives with fillers between a carbon/epoxy shaft and a metal shaft.
Derivation of Boundary Conditions for Micromechanics Analyses of Plain and Satin Weave Composites
Authors: Whitcomb John D. , Chapman Clinton D. , Tang Xiaodong
Published: April 2011

Efficient 3D analysis of periodic structures depends on identifying the smallest region to be modeled and the appropriate boundary conditions. This paper describes systematic procedures for deriving the boundary conditions for general periodic structures. These procedures are then used to derive the boundary conditions for plain and satin weave composites.
The Stress Corrosion Experiments on an E-Glass/Epoxy Unidirectional Composite
Authors: Ely T. , Kumosa M.
Published: April 2011

A series of stress corrosion experiments were performed on a pultruded E-glass/epoxy composite system commonly used in high voltage composite insulators. The tests were performed in nitric acid solutions (pH 1.2) using constant K1 specimens specifically designed for stress corrosion testing of unidirectional fiber-polymer matrix composites. The effect that the magnitude of the applied loads had on the stress corrosion fracture process was investigated. The process was monitored using acoustic emission (AE) techniques. It was found that when the specimens were subjected to static loads (ranging from 71.2 to 124.6 N) in the presence of nitric acid planar cracks formed and propagated perpendicular to the fiber direction without generating a significant degree of fiber debonding or pullout. In addition, for various stress intensity factors at the crack tip both the normal and log-normal distribution functions were used to model the peak amplitude data generated by stress corrosion induced fiber fracture. Both the AE data and mechanical compliance results clearly indicated that the specimens used in this project provided consistent constant K1 conditions over the entire range of loads considered. The results from the tests performed in this project clearly demonstrate that the constant K1 stress corrosion experiments are simple and informative evaluation methods for monitoring stress corrosion cracking in unidirectional composites.
Studies on Impact Fracture Properties of Wood-Polymer Composites
Authors: Razi P. S. , Raman A.
Published: April 2011

Composite plates containing pine wood chips and high density polyethylene (HDPE) matrix were prepared and studied for their impact fracture behavior using an instrumented drop weight tester. Maximum load sustained, deflection to maximum load, energy to maximum load and energy for total penetration of the test specimens were determined for the above-mentioned wood-polymer composites. Effects of pine chip concentration in composites and its particle size on impact damage resistance were analyzed. Large size wood chips in the 50 to 60 vol.% concentration range increased the energy to fracture and damage resistance considerably. This is attributed to the internal friction forces encountered between such large sized chips restricting the deformation and contributing to uneven perforation. On the contrary, specimens with small sized wood chips in similar composition range perforated evenly consuming less energy than needed to perforate even the polymer alone.
Predicting Shrinkage in Polyester Reinforced by Glass Fabrics
Authors: Vu-Khanh T. , Do-Thanh V.
Published: April 2011

Polyester is one of the most common resins used in contact lay-up method because of its low cost, room-temperature curing, wide availability, ease of handing, etc. However, the main disadvantage of this resin is the large volumetric shrinkage after curing (up to about 0.5%). This represents a major problem because it can cause unexpected defects in the molded composite parts such as warpage, distortion, rippled surface, etc. The effect of resin shrinkage on composite deformations is very complex because of the anisotropic properties induced by the fibers, especially in woven fabric composites with interlacing yarns. Moreover, in many applications, when the part geometry has a double curvature, the forming process usually results in significant in-plane shear deformation of the interlaced yams. The angle between the fill and the warp threads is no longer orthogonal because the fabric must follow the shape of the mold. In this work, an approach to measure the shrinkage coefficients of the interlaced yarns of fabric structure has been developed. The recently proposed sub-plies model has been used to predict deformations due to resin shrinkage in woven fabric composite. Resin shrinkage can lead to an expansion in the laminates with specific angles between undulated yams, due probably to a straightening effect on the fibers. Expansions due to matrix shrinkage were verified on several woven laminates. Prediction of deformations due to matrix shrinkage by the sub-plies model is in good agreement with experimental measurements.
A Numerical and Experimental Study of Woven Composite Pin-Joints
Authors: Pierron F. , Cerisier F. , Grediac M.
Published: April 2011

A numerical and experimental study was carried out to determine the stiffness and the bearing strength of bolted woven composite joints. The main objective was to investigate the possibility of predicting the properties of the joint from the properties of the material measured with standard tests. A refined finite element model was developed in which the nonlinearities due to both the material and the contact angle between the pin and the hole were taken into account. Particular attention was paid to account for the influence of the clearance which has been shown to be very significant. In conclusion, good agreement between experimental results and numerical predictions has been obtained.
Progressive Fatigue Damage Modeling of Composite Materials, Part I: Modeling
Authors: Shokrieh Mahmood M. , Lessard Larry B.
Published: April 2011

In this research a modeling technique for simulating the fatigue behaviour of laminated composite materials, with or without stress concentrations, called progressive fatigue damage modeling, is established. The model is capable of simulating the residual stiffness, residual strength and fatigue life of composite laminates with arbitrary geometry and stacking sequence under complicated fatigue loading conditions. The model is an integration of three major components: stress analysis, failure analysis, and material property degradation rules. A three-dimensional, nonlinear, finite element technique is developed for the stress analysis. By using a large number of elements near the edge of the stress concentration and at layer interfaces, the edge effect has been accounted for. Each element is considered to be an orthotropic material under multiaxial state of stress. Using the three-dimensional state of stress within each element, different failure modes of a unidirectional ply under multiaxial states of stress are detected by a set of fatigue failure criteria. An analytical technique, called the generalized residual material property degradation technique, is established to degrade the material properties of elements. This analytical technique is not restricted to the application of failure criteria to limited applied stress ratios. Based on the model, a computer code is developed that simulates cycle-by-cycle behaviour of composite laminates under fatigue loading.
Impact Behavior of Aramid Fiber/Glass Fiber Hybrid Composite: Evaluation of Impact Behavior Using Delamination Area
Authors: Park Rohchoon , Jang Jyongsik
Published: April 2011

One-layer, two-layer, and three-layer aramid fiber/glass fiber hybrid composites were fabricated to investigate the impact behavior of thin composites using the analysis of delamination area. The effect of stacking sequence and surface treatment on the delamination area of hybrid composites was examined by using penetrant injection and de-plying technique. The impact energy and delamination area of the laminates depended on impact characteristics of fiber at impacted surface. When aramid layer was at impacted surface, the laminate exhibited the higher impact energy and delamination area. The surface-treated composites exhibited the reduced delamination area due to the restriction in deformation of aramid fiber. The impact behavior of three-layer hybrid composites was affected by the delamination area at each interface. The deformation of neighboring aramid layer increased the deformation at adjacent interface.
Simple, Conservative Criteria for Buckling and Delamination Propagation in the Presence of Stitching
Authors: Cox B. N.
Published: April 2011

The problem of delamination cracks in laminates that are bridged by stitches is revisited. Analytical solutions for the critical strain for buckling of a bridged, delaminated layer are extended to consider propagation of the delamination crack under in-plane compression in the limit that the crack is long. Numerical estimates for common laminate parameters show that delaminations of interest will not be far from the long crack limit. Therefore, the analytical results should be conservative but not wasteful design guides.
Testing and Analysis of Composite Skin/Stringer Debonding under Multi-Axial Loading
Authors: Krueger Ronald , Cvitkovich Michael K. , O'Brien T. Kevin , Minguet Pierre J.
Published: April 2011

A consistent step-wise approach is presented to investigate the damage mechanism in composite bonded skin/stringer constructions under uniaxial and biaxial (in-plane/out-of-plane) loading conditions. The approach uses experiments to detect the failure mechanism, computational stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for delamination growth. In a first step, tests were performed on specimens, which consisted of a tapered composite flange, representing a stringer or frame, bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-point bending, and combined tension/bending to evaluate the debonding mechanisms between the skin and the bonded stringer. For combined tension/bending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. For all three load cases, observed failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from skin.

In a second step, a two dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, computed principal stresses exceeded the transverse strength of the material in those areas of the flange where the matrix cracks had developed during the tests. In a third step, delaminations of various lengths were simulated in two locations where delaminations were observed during the tests. The analyses showed that at the loads corresponding to matrix ply crack initiation computed strain energy release rates exceeded the values obtained from a mixed mode failure criterion in one location. Hence, unstable delamination propagation is likely to occur as observed in the experiments.
Surface Oxidation and Its Effect on the Observed Elastic Modulus of SiC Whisker/A1203 Composites
Authors: Chiu Chin-Chen , Case Eldon D. , Chiu Chingchen S.
Published: April 2011

This article presents an analysis of the 27 wt.% SiC whisker/A1203 c01 posites exposed to air for periods up to 180 hours at 1100 C to 1450 C. Experimental results indicate that SiC whisker reacts with oxygen and alumina to produce a mullite-rich surface layer on tested composite specimens. The surface oxidation reaction degrades the observed elastic moduli of specimens. A theoretical analysis based on the Bernoulli-Euler beam equation shows that the surface layer has lower elastic modulus than the unreacted core.
Analysis of Modulus and Strength of Dry and Wet Thermoset and Thermoplastic Composites Loaded in Transverse Tension
Authors: Pomies Frëdëric , Carlsson Leif A.
Published: April 2011

Micromechanical finite element analysis of modulus and strength of transverse tension loaded continuous fiber composites is presented and related to experimental data for dry and wet glass/epoxy, carbon/epoxy, glass/polyphenylsulfide (PPS) and carbon/bismaleimide (BMI)-epoxy composites. Transverse tensile strength predictions employed the combined mechanical and residual stress states at the fiber/matrix interface and in the matrix in conjunction with debonding and matrix failure criteria. Due to lack of experimental data on fiber/matrix interfacial strengths, an ideal bond was assumed by assuming that the interfacial tensile and shear strengths were the same as those for the matrix. The Cooper-Kelly transverse strength model was also examined. Predictions of the transverse Young's modulus using finite elements were overall in good agreement with experimental values at dry and wet conditions. The transverse stress levels corresponding to debond initiation and matrix failure generally decreased with increased amounts of absorbed moisture which is consistent with experimental observations, but due to the ideal fiber/matrix bond assumption, the strength predictions tended to be unconservative. The Cooper-Kelly Model with a weak fiber/matrix interface yielded highly conservative strength predictions.
Buckling of Elliptically Delaminated Composite Plates
Authors: Yeh Meng-Kao , Tan Chung-Ming
Published: April 2011

The buckling of laminated plates with elliptic delamination under compressive loading was studied experimentally and analytically. In the experiment, a tensiletest machine was used to determine the load-displacement behavior of the delaminated plates under uniaxial loading and the buckling strength was extracted therefrom. In analysis, a nonlinear finite element program based on the updated Lagrangian formulation was developed to analyze the response of the laminated plates. The formulation includes large displacements and large rotations. The plates were divided into finite elements and the degenerated shell elements were used. The Newton-Raphson method was used to solve the resulting equation for the nonlinear system and a displacement-controlled scheme was used in the solution procedure near the buckling load. This process was repeated until a desired accuracy was achieved. The buckling behavior of mixed and global types of the delaminated composite plates were examined. Four parameters, including the size of the delaminated region, the orientation of the fiber direction, the position of the delaminated region in the thickness direction and the orientation of the major axis of the elliptic region with the loading axis, were varied to assess their influence on the buckling behavior of the plates. Good agreement was obtained between the analytical and experimental results.
Buckling of Multiply Delaminated Beams
Authors: Adan Matti , Sheinman Izhak , Altus Eli
Published: April 2011

An analytical model is developed for buckling of multiply delaminated composite beams or long plates under cylindrical bending, and their interactive effects are studied. The prebuckling and buckling equations, formulated for one-dimensional cases with an arbitrary configuration of through-the-width cracks, are solved in a closed-form manner. The effects of crack length and location are studied, showing different trends of behavior for global and local mode shapes, including a decrease in the buckling load when cracked zones overlap. In some cases, where inadmissible interlaminar penetration in the first mode is involved, higher eigenvalues and the corresponding eigenmodes are needed to define the buckling state. In other cases, where all eigen modes exhibit penetration, the present model can be used only as indicated for the behavior, and a nonlinear analysis with contact constraints is called for. Considering the scanty work done to-date in the field of multiply delaminated beams, the proposed model-even though restricted to a contactless situation-can also serve as the basis for more general nonlinear analysis. A parametric study of several beam configurations is presented through examples.
Residual Stresses and Delamination Problems Induced by Cocuring of Damped Composite Laminates
Authors: He Shulin , Rao Mohan D.
Published: April 2011

A study to understand the delamination of laminated composite beams with a viscoelastic damping layer due to residual stresses resulting from cocuring is presented in this paper. The effects of residual stress concentration near the free edges of the laminate on the delamination is discussed. Numerical study is conducted to show the effects of laminate and viscoelastic material properties and various structural parameters such as the width of the composite beam, the thickness of the 0° ply angle layers, and the thickness of the compliance layers, on the failure of the composite beam.
Non-Fickian Moisture Diffusion in Polymeric Composites
Authors: Cai L. -W. , Weitsman Y.
Published: April 2011

This article concerns the effects of non-Fickian water diffusion in fiber-reinforced polymeric composites. The departure from classical diffusion is attributed to the time-dependent response of the polymer, akin to viscoelastic mechanical response.

The purpose of the present work is to propose a methodology to reduce non-Fickian moisture weight gain data in a manner which enables the evaluation of the diffusion coefficient and moisture profiles across the thickness of composite laminates. The evaluation of those moisture profiles is essential to the computation of residual stresses within composite laminates.

It is demonstrated that in some circumstances the non-Fickian moisture profiles result in residual hygro-thermal stresses which differ by about 25% from predictions based upon classical diffusion.
An Analytical Method for the Evaluation of Interlaminar Stresses Due to Material Discontinuities
Authors: Bhat Narendra V. , Lagace Paul A.
Published: April 2011

Interlaminar stresses arise in regions with in-plane stress gradients. These in-plane gradients could arise due to loading conditions, geometric or material characteristics. One of the material characteristics which could cause such gradients is a sudden transition in material properties occurring at regions of implants within a structure, making them potential delamination sites. Such implants are being used in the design of adaptive structures. The proposed method efficiently evaluates interlaminar stresses at material discontinuities. The laminate is divided into two regions with different layups which are joined together. The stresses in each region are expressed in terms of eigenfunctions which satisfy equilibrium. The principle of minimum complementary energy is used to obtain the differential equations of the problem and thus solve for the eigenfunctions in each region. The analytical form of the stresses enables easy application of averaging type criteria to predict delamination initiation.
Ultrasonic Evaluation of Thin, Fiber-Reinforced Laminates
Authors: Veidt Martin , Sachse Wolfgang
Published: April 2011

This paper reports on the characterization of one-or two-ply, unidirectional and [0,90,0]t, cross-ply graphite/epoxy laminates whose thicknesses were approximately 145 μm, 275 μm and 400 μm, respectively. The measurements are based on the application of the ultrasonic point-source/pointreceiver technique (PS/PR) using either a focused laser beam operating as a dipole source or a small aperture, piezoceramic shear transducer serving as a monopole source. Detection of the signals is with a piezoceramic sensor that directly responds to the lateral or shear motions of the specimen surface. The detected time signals represent the detailed spatial and temporal characteristics of the elastic wave field. The experimental group velocity data is analyzed using a simple plane-wave, plane-stress model that describes the propagation of quasi-longitudinal and quasi-transverse membrane waves in the plane of the plate. We demonstrate in this paper the strength of the PS/PR measurement technique when coupled with appropriate signal processing to recover the stiffness tensor of thin, fiberreinforced laminates. Additionally, preliminary experimental results on the detection of flaws in thin specimens from in-situ measurements are included.
Solid Particle Erosion of Thermoplastic Resins Reinforced by Short Fibers
Authors: Miyazaki N. , Hamao T.
Published: April 2011

Experiments were carried out to study the effects of matrix materials, reinforcement fibers, impact angle and particle velocity on the solid particle erosion behavior of thermoplastic resins reinforced by short fibers. All of the previous research on the erosion of fiber-reinforced plastics deals with steady state erosion, and no work has been done on the initial process of erosion. So special attention was focussed on an incubation period of erosion. Two types of fiber-reinforced plastics were used for erosion tests. One was a new type of thermoplastic polyimide (New-TPI) resin reinforced by short glass or carbon fibers. The other was polyetheretherketone (PEEK) reinforced by short glass or carbon fibers. The fiber-reinforced plastics with different fiber contents were utilized as test materials to examine the effect of fiber content. Initial and eroded surfaces of test specimens were observed with a scanned electron microscope and an optical microscope. The results of the experiments show that the mass of an incubation period is strongly dependent on initial surface roughness. The erosion rate at a steady state depends on the matrix material and on the volume content of fibers, regardless of fiber type.
Thermomechanical Modeling of Irradiated Polymer Composites
Authors: Chaturvedi Shive K. , Chao Chen-Ping
Published: April 2011

Thermoelastic models are presented to evaluate constitutive response of fiber-reinforced polymer composites subjected to irradiation and temperature exposure. These models are based upon thermodynamic considerations involving Helmholtz and Gibbs free energy functions, and micro-mechanical models involving Eshelby's modified approach. The proposed union between the energy functions and micro-mechanics provides a unifying framework that can deal with the potential property changes in fibers, matrix and at the fiber-matrix interface. The results for a graphite/epoxy (T300/934) composite show a good agreement with the available experimental data. The parametric study for all the principal elastic constants involving matrix stiffening as well as matrix softening exhibit a number of interesting results.
Fatigue Damage and Fracture Mechanism of Notched Woven Laminates
Authors: Xiao Jiayu , Bathias Claude
Published: April 2011

Three E-glass/epoxy notched woven laminates (two orthotropic and one quasi-isotropic) were studied under tensile fatigue loading with a stress ratio of 0.1 and a frequency of 20 Hz (sine wave form). At a fixed ratio of maximum fatigue stress to ultimate tensile strength (UTS) (0.5), the apparent secant stiffness changes with the cycle number were measured and the fatigue damage mechanism was investigated. The S-N curve and notched fatigue strength (at 1,000,000 cycles) were determined. All results obtained were compared with those for unnotched specimens.

The results show that the normalized apparent secant stiffness change can be divided into three stages. During the first and second stage, damage may be presented in the form of microcracks within the interlacement structure of the woven plies. But they have not been observed and verified with traditional experimental methods such as radiography and microscopy. The last stage corresponds to initiation and evolution of damage observed directly on the specimen surface with a video-camera system. The fatigue life at a same ratio of maximum fatigue stress to UTS for the three laminates is very slightly influenced by the notch. The quasi-isotropic laminate has a lower fatigue strength and an earlier first visible damage than the two orthotropic laminates. However, for the two orthotropic laminates, the fatigue strength and the appearance of the first visible damage are very slighly influenced by the stacking sequences, but, the normalized secant stiffness curve appears to be different from one to another. The notched specimens have the same ratio of the fatigue strength to UTS as the unnotched ones, almost equal to 0.4. The ratio of the notched fatigue strength to the unnotched fatigue strength is amost equal to the static strength reduction factor (SRF) which is equal to the ratio of the static notched strength to the static unnotched strength.
Microstructural Design of Advanced Multi-Step Three-Dimensional Braided Preforms
Authors: Kostar Timothy D. , Chou Tsu-Wei
Published: April 2011

The "multi-step" braiding process is introduced as an extension to the four-step braiding process. This process greatly enlarges the possibilities of braid micro-structures. An assumed yam packing arrangement within the braided preform and geometric considerations were used to relate yam orientation to non-dimensional pitch length. Further, study of the fundamental principles of track and column braiding yielded a relationship between track/column shifting sequence and yam orientation, pitch length, and yarn diameter. Coupled via. computer software, these relationships provide a link between design parameters such as preform size, yarn diameter, and microstructure to operating parameters such as pitch length and shifting sequence. Additionally, the volume of yam in a given pitch length divided by that pitch length has been predicted. This supplies a simple method of calculating fiber volume fraction. Experimentally measured yam volume fraction and yarn orientation have shown close agreement with theoretical predictions.
Fracture Behavior of Continuous Alumina Fiber Reinforced Epoxy
Authors: Gu Y. H.
Published: April 2011

This work investigates fracture behavior of a continuous alumina fiber (FP) reinforced epoxy under compression at two different temperatures. Effects of fiber orientation, stacking sequence, and temperature on failure mechanisms of the composite material are analyzed through a scanning electron microscope (SEM). Fiber orientation is found to affect the failure mechanisms of the FP/epoxy. While the failure of the 00 laminae is mainly caused by fiber breakage, that of the 90° laminae by sheared epoxy matrix. The [02/902]s laminates not only have the combined failure modes of the 00 and 900 laminae but also show fiber micro-buckling in the 0° plies and ply delamination at the interfaces of the 0° and 900 plies. Failure mechanisms are found to be different between the symmetric and antisymmetric crossply laminates. In addition, the composites fractured at 77 K display more fiber-matrix debonding, smoother fractured surfaces of fiber and matrix, and cleaner ply delamination at the interfaces of the 00 and 900 plies than the composites fractured at 295 K.
Global/Local Finite Element Analysis for Textile Composites
Authors: Woo Kyeongsik , Whitcomb John
Published: April 2011

Conventional finite element analysis of textile composite structures is impractical because of the large, complex microstructure. Global/local methodology combined with special macro elements is proposed herein as a practical alternative. Initial tests showed dramatic reductions in the computational effort with only a relatively small loss in accuracy for stresses away from the global/local boundary.
Low Velocity Impact of Optical Fiber Embedded Laminated Graphite/Epoxy Panels. Part I: Macro-Scale
Authors: Sirkis J. S. , Chang C. C. , Smith B. T.
Published: April 2011

This paper uses nondestructive evaluation techniques to investigate the development of low velocity impact induced delamination in composite laminates with a single optical fiber embedded at the laminate mid-plane. Six different types of optical fibers ranging in diameter from 80 um to 600 Itm are embedded in [902/04 /OF(0)/04/902], [906 /OF(0)/906], [(+45/-45)3/OF(0)/(-45/+45)31 AS4/3501-6 plates. With the exception of the 600 Am diameter fiber, it is found that optical fiber sensors embedded at the mid-plane of these laminates did not influence the size or distribution of the delamination damage. Micro-scale damage will be presented in Part II.
Optimization of Fiber Coatings for Transverse Performance: An Experimental Study
Authors: Case Scott W. , Carman Greg P.
Published: April 2011

In this manuscript we use a concentric cylinder model to determine the stresses in a composite material subjected to transverse loading. After selecting a failure criterion, it is possible for us to determine a fiber coating which optimizes the strain to failure of the composite material. Experimental results are presented for a model composite system and the results are compared to the analytical predictions. The model is shown to give reasonable agreement with the experimental data.
Analytical Characterization of the Anisotropy and Local Heterogeneity of Short Fiber Composites: Fiber Fraction as a Variable
Authors: Pan Ning
Published: April 2011

A statistical analysis to characterize the system anisotropy and the local heterogeneity of short fiber composites is presented in this report. Firstly, using a fiber orientation probability density function (pdf), the statistical mean values and variations of the fiber cut ends and the parameters of the matrix material distribution on an arbitrary cross section of a composite are derived. The fiber area fraction Af, which is shown to be a function of the fiber orientation and the direction of the cross section, is used to calculate the composite elastic properties such as the tensile modulus associated with the cross section so that the direction-dependence or the anisotropic nature of the composite properties is predicted.

However, to study other important system properties such as the strength, the fiber-matrix interaction, failure process and impact resistance, neither the overall fiber volume fraction Vf nor the fiber area fraction Af at cross sections is proved to be adequate. Local fiber volume fraction Vaf is hence defined in this work to specify the local heterogeneity of the composite, i.e., the non-uniformity of the distribution of the fibers and the matrix material at local level. This local fiber volume fraction can be inferred from the statistical features such as the radius r and the free length I of the spaces occupied by the matrix material. The allowable local minimum and maximum fiber volume fractions, two concepts from the existing theories, as well as the allowable variances of the radius r and the free length 1 of the matrix material can be used to predict in composites the weakest locations and the weakest cross sections in anisotropic cases.

The theory has been applied to two special cases with hypothetic fiber orientation pdf functions, and the effects of the important variables involved are investigated and illustrated.
Normalized Impact Response and Damage in a Thin Composite Laminate Supported by a Rigid Substrate
Authors: Christoforou A. P. , Yigit A. S.
Published: April 2011

The local impact response and damage of thin composite laminates are investigated. The impact model uses a static contact law which incorporates damage effects due to local contact stresses. A major difference between this contact law and the others used previously is that the damage is accounted for during the loading rather than during the unloading phase which is expected to yield a more reasonable prediction of the maximum impact force. Normalization of the governing differential equations and initial conditions yields a single governing parameter, which is a dimensionless group of all the pertinent impact parameters (i.e., impact velocity, impactor mass, material and geometric properties). The introduction of this parameter not only simplifies the problem but also provides the necessary scaling rules for model testing.
Effect of Stitching on the Mechanical and Impact Properties of Woven Laminate Composite
Authors: Kang Tae Jin , Lee Sung Ho
Published: April 2011

Mechanical and impact-resistant properties of stitched woven laminate composite have been studied. The S-2 Glass/Polyester, Kevlar/PVB-Phenol and Spectra/Vinylester composites were stitched at different stitch densities to improve its mechanical and impact properties of the composite. Both chain and plain stitches were used for reinforcing the laminate in the Z-axis of the composite. The mechanical properties of woven laminate composite were improved at the optimum stitch density compared to those of conventional unstitched one. Energy absorption capability of stitched composite was also higher than that of unstitched composite, and the stitched composite showed a significant increase in energy absorption capability under repeated impact. The damaged areas caused by the repeated impact or by the penetration of projectile were far smaller in the stitched composite than in the unstitched laminate one. The stitched composite showed about 10% improvement in ballistic efficiency over the conventional woven laminate composite at the optimum stitch design.
A Self-Correcting, Thermal-Curvature-Stable Bending Element
Authors: Wetherhold Robert C. , Wang Jlanzhong
Published: April 2011

Under a through-thickness temperature variation, thin structural elements such as beams, plates, and shells tend to bend. This bending deformation can distort the elements of a structure and cause thermal stresses. In this paper, the requirements for a laminate which is "self-correcting" are given. These thermal-curvature-stable (TCS) laminates exhibit zero bending curvature under steady-state heat transfer conditions. The TCS effect is independent of the actual temperatures, within the context of linear theory. This effect is demonstrated for an aluminum/composite laminate, where the solution appears as the ratio of composite lamina to aluminum lamina thicknesses. The example laminated beams demonstrate a reduced net in-plane thermal expansion when compared to mono-lithic aluminum beams. The efficiency of the iCS laminate is shown to be more sensitive to lamina thickness than to lamina properties.

An additional design parameter for TCS laminates is the fiber volume fraction (Vf), which affects the composite modulus, thermal expansion, and transverse thermal conductivity. It is shown that at higher Vf, the TCS effect is relatively insensitive to Vf. At lower Vf, the in-plane thermal expansion can be further reduced at the expense of increased composite layer thickness.
A Micromechanics Model for Compressive Failure of Unidirectional Fibre-Reinforced Plastics
Authors: Haberle J. G. , Matthews F. L.
Published: April 2011

In this paper a method is presented for predicting the compressive strength of unidirectional fibre-reinforced plastics. Initial fibre misalignment and nonlinear shear response are included in the model which is based on a combination of other published methods. Predictions are shown to compare well with test data from an improved test method for carbon fibre-reinforced materials; one with an epoxy matrix and one with a PEEK matrix. Acceptable predictions are shown to depend on an appropriate assessment for fibre misalignment. The influence on compressive strength of moisture uptake by the composite was seen to be correctly predicted by the model.