bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-Dimensional, Nonlinear Viscoelastic Analysis of Laminated Composites
Authors: Kennedy Timothy C. , Wang Min
Published: April 2011

A three-dimensional finite element analysis treating the nonlinear viscoelastic response of laminated composites is presented. The individual plies in the laminate are modeled using 20-node isoparametric solid elements. The transient creep compliance in the viscoelastic model is represented as an exponential series plus a steady-flow term, which allows for a simplification of the numerical procedure for handling hereditary effects. Time-dependent stress and strain distributions are calculated for unnotched and notched laminates subjected to tensile loading or bending.
Matrix Cracking Effect on Delamination Growth in Composite Laminates Induced by a Spherical Indenter
Authors: Liu Sheng , Chang Fu-Kuo
Published: April 2011

An investigation was performed to study the mechanics of delamination growth in fiber-reinforced laminated composites resulting from transverse concentrated loads. The effect of matrix cracking on delamination propagation was the primary focus of the study. For simplicity's sake, only cross-ply laminates were investigated. An analytical model was proposed for determining the response of the laminates containing a matrix crack-induced delamination and subjected to a concentrated load through a spherical indenter. Two types of matrix cracks were considered in the model: a surfacebending crack and an internal shear crack. The model consists of three parts: a stress analysis, a contact analysis, and a failure analysis. A nonlinear three-dimensional finite element model based on an updated Lagrange formulation was developed for calculating stresses and strains in composites. An augmented Lagrangian method was utilized to simulate the interfacial contact condition of the embedded delamination and the indentation between the composite and the indenter. Fracture mechanics was applied to predict the delamination growth in three dimensions.

In order to verify the model, results of the calculations from the analysis were compared with the existing solutions and test data available in the literature. Experiments were also conducted to further substantiate the model and the predictions from the analysis. Overall, the predictions agreed well with the existing solutions and the test data.
Numerical Analysis for Design of Composite Specimens for Through-the-Thickness Tensile Measurements
Authors: Shan Hui-Zu , Parvizi-Majidi Azar , Chou Tsu-Wei
Published: April 2011

The research reported in this paper was motivated by the need for measuring through-the-thickness tensile properties of composite panels. Such measurements are especially necessary for three-dimensional textile structural composites, in which the rein-forcement effect in the thickness direction is significant. A multi-section specimen design which includes gauge section, transition section and grip section is proposed. The two-dimensional boundary element technique has been adopted for analyzing the characteristics of the stress distribution in the specimen. A general singular boundary element with arbitrary order of the stress singularity has been formulated. Verification studies show good accuracy of the analytical method. The geometric and material factors affecting the stress distribution and stress singularity have been identified. Finally, recommendations have been made for multi-section specimen design in achieving high uniformity of stress distribution within the gauge section and low intensity of the stress singularity in the interface between the gauge and transition sections.
On Sandwich Beams with Laminate Facings and Honeycomb Cores Subjected to Hygrothermal and Mechanical Loads: Part III-Timoshenko Beam Theory
Authors: Mukhopadhyay Achintya K. , Sierakowski Robert L. , Yu Yi-Yuan
Published: April 2011

Following Timoshenko beam theory [4], the present problem of a sandwich beam with laminate facings is formulated in terms of a single angular parameter 4 that was first used in the Timoshenko beam equations, subsequently in the Mindlin equations for plates [4] and previously in equations for sandwich beams and plates [5,6]. The governing equations are obtained in terms of this new variable. Using energy principles and the Rayleigh-Ritz method, a new and general expression is obtained for the kinematic parameter X. It is shown that the earlier results obtained in References [1] and [2] are a particular case of the present formulation. In addition, the classical results are recovered from the present general results.
Role of Interphase on the Elastic Behavior of Composite Materials: Theoretical and Experimental Analysis
Authors: Lagache M. , Agbossou A. , Pastor J. , Muller D.
Published: April 2011

The study and design of composite structures raises numerous difficulties due to the high level of heterogeneity and to the small size of the fillers. One of the most usual solutions consists in replacing the composite material by a homogeneous medium which is considered as "macroscopically" equivalent. This kind of approach, named homogeneization, aims to provide every mechanical characteristic of the composite according to the mechanical and geometrical features of its constituents. The aim of the present paper is twofold: (1) to review the most usual homogeneization approaches for linear elastic behavior in order to obtain their own range of validity in comparison with experimental data, and (2) to determine experimentally and numerically the effect of a third phase, named interphase, on the linear elastic behavior of fiber composites.
Failure of Spectra® Polyethylene Fiber-Reinforced Composites under Ballistic Impact Loading
Authors: Lee B. L. , Song J. W. , Ward J. E.
Published: April 2011

Failure modes of Spectra polyethylene fiber-reinforced composites were examined under ballistic impact loading. Both woven fabric-reinforced laminates and angle-plied unidirectional fiber-reinforced laminates were found to exhibit sequential delamination, cut-out of a plug induced by through-the-thickness shear, and combined modes of shear and tensile failure offibers as obserxed in the cases of glass and graphite fiber composites. At low areal density, both laminates demonstrated similar ballistic limits. However, as areal density increased, differences in ballistic limit became more apparent, with angle-plied composite laminates showing higher values. When subjected to the repeated impact of a constant striking velocity below the ballistic limit, a progressive growth of local delamination was observed until gross failure of composites occurred. The use of lower striking velocity of the projectile led to the increase in cumulative numbers of impacts for full penetration defining an impact fatigue lifetime profile. The results of impact testing indicated that Spectra fiber-reinforced composites with vinyl ester resin matrix have a higher ballistic limit and longer impact fatigue life at a given striking velocity than the polyurethane matrix composites. Less effective absorption of impact energy by flexible polyurethane matrix composites was attributed to much more restrained pattern of delamination growth. Correlated with the results of dynamic mechanical analysis, these trends indicated that the stiffness of resin matrices plays an important role in controlling the ballistic impact resistance of Spectra fiber composites. Damping factor and resultant stress wave attenuation characteristics of materials seem to be less important parameters.
Quantitative Acoustic Emission for Fracture Behavior of Center-Hole GFRP Laminates
Authors: Caprino G. , Teti R.
Published: April 2011

Residual strength prediction through acoustic emission (AE) detection and analysis is attempted for tensile loading of GFR composite laminates containing center holes. By utilizing a previously developed two-parameter fracture mechanics model, the mechanical behavior of circular center hole composite specimens was interpreted. The AE response of the specimens was shown to be controlled by stress intensity factor K, rather than stress. In order to establish a quantitative basis for AE analysis aimed at predicting the fracture behavior of center hole composite laminates, a description of the experimental AE response through a mathematical model was considered necessary. An analytical expression of exponential form was assumed and experimentally verified. The excellent agreement between theory and experiment quantitatively confirmed the validity of the model. Through AE detection and analysis, it is therefore possible to assess hole criticality in composite laminates by direct prediction of their residual strength under tensile loading.
Three-Dimensional Finite Element Analysis of Free Edge Stresses and Delamination of Composite Laminates
Authors: Yang Henry T. Y. , He C. C.
Published: April 2011

A 24-node, 64-degree-of-freedom, solid hexahedronal finite element is used to analyze the interlaminar free edge stresses and delamination of composite laminated plates. The difficulty due to the requirement of large storage space which is frequently encountered in the modeling of contoured delamination problems is resolved by using the preconditioned conjugate gradient method. One of the advantages of this method is that there is no need to store the fill-in elements in the stiffness matrix decomposition process, which leads to a significant reduction in storage space required, thereby making the numerical analysis of the complicated contoured delamination problem feasible as well as effective. To evaluate the accurateness of the present formulation, numerical algorithm and computer program, free edge stress analyses are performed for a [0/901 cross-ply and a [+ 45], angle-ply laminated plate. Both plates are subjected to an axial tensile strain loading. The present three-dimensional results are found to be in good agreement with those obtained using an alternative quasi-three-dimensional methods. To further demonstrate the applicability of the present development, the strain energy release rate of a [+25/90], graphite-epoxy rectangular laminated plate subjected to tensile strain is studied. Two initial delaminations are assumed to be located in the midspan of the two opposite free edges and also at the middle of the thickness. The delamination contour is assumed to be a straight line segment joined by two circular arcs. The strain energy release rates along the delamination front are calculated using the modified crack closure method. As a step beyond the solution using quasi-3D assumption, the present results show the non-uniform distribution of mode I strain energy release rate G1 along the boundary of the delamination front which fluctuates to a maximum value when approaching both ends at the free edge of the laminate.
Fatigue Failure Model for Fibre-Reinforced Materials under General Loading Conditions
Authors: Fawaz Z. , Ellyin F.
Published: April 2011

A new model is presented which predicts the fatigue failure of fibrereinforced materials (FRM's) under multiaxial stresses, varying minimum to maximum cyclic stress ratios and different fibre orientations with respect to the loading directions. The concept of multidirectional elementary blocks is introduced. The model predicts the fatigue failure of these and unidirectional elementary blocks, based on the establishment of a reference stress versus life line and the determination of two simple functions. This methodolgy reduces considerably the number of parameters to be determined experimentally. Correlation with published fatigue failure results is presented. It is shown that the model succeeds in accurately predicting fatigue failure of different unidirectional and bidirectional FRM's subjected to uniaxial and biaxial stresses and different minimum to maximum cyclic stress ratios.
Difficulties in the Theories for Predicting Transverse Thermal Conductivity of Continuous Fiber Composites
Authors: Wetherhold Robert C. , Wang Jianzhong
Published: April 2011

A large number of theories of varying degrees of sophistication are available for predicting the transverse thermal conductivity of continuous fiber composites. These theories assume a knowledge of fiber and matrix properties and volume fractions, interphase properties (if required), and fiber architecture. The fiber transverse conductivity is normally difficult to measure directly due to the small fiber diameter. The usual suggested method is to measure matrix and composite conductivity, and infer ("back out") the fiber conductivity. This paper reviews several pathological conditions that can occur for composites where the fiber conductivity is relatively large compared to the matrix conductivity, with particular application to graphite fibers. These conditions include great sensitivity to measured composite conductivity, to the model used, and to the measured fiber volume fraction, as well as failure to predict fiber values. Some suggestions for improving the results are discussed.
Low Velocity Impact of Optical Fiber Embedded Laminated Graphite/Epoxy Panels. Part II: Micro-Scale
Authors: Sirkis J. S. , Chang C. C.
Published: April 2011

Optical Microscopy and Scanning Electron Microscopy are used to investigate the micro-crack distribution in the vicinity of optical fibers embedded in laminated graphite/epoxy plates subjected to low velocity impact. The test matrix consists of six different types of optical fibers embedded in [902/04/OF(0)/04/902], [906/OF(0)/906], [(45/-45)3/OF(0)/(-45/45)3] AS4/3501-6 plates. The results show that the optical fiber diameter and coating influence the micro-crack distribution in the [902/04/OF(0)/04/902], [906/OF(0)/906], [(45/-45)3/OF(0)/(-45/45)3] plates, and that the host damage is insensitive to the presence of the optical fiber for [906/OF(0)/906] laminates. The results further indicate that optical fibers less than 100 μm in diameter do not alter the indigenous micro-crack distribution for these material systems and this optical fiber placement.
Mode II Free-Edge Delamination in Linainated Composites under Uniaxial Compression
Authors: Kim D. M. , Hong C. S.
Published: April 2011

The influence of interlaminar shear stress on mode II free-edge delamination at the free-edge in composite laminate is studied experimentally and analytically. Delamination tests are conducted under uniaxial compressive load to eliminate transverse cracking. Test laminates are designed to have compressive interlaminar normal stress and various ratios of interlaminar shear stresses at the interfaces of interest. The laminates treated in this paper are [θ4/-θ4/904] s, [(±θ2)2/904] s, and [(±θ)4/904] s, where θ is 30 and 45 degrees. The experimental results are discussed with the analyzed results obtained by the finite element method. The fracture surface of delamination is further examined for the determination of the predominant mode of delamination using a scanning electron microscope. It is found that some laminates can be delaminated only by the effect of mode II interlaminar shear stress even though it is zero at the free-edge.
Numerical and Experimental Failure Analysis of Composite Laminates with Bolted Joints under Bending Loads
Authors: Chen Wen-Hwa , Lee Shyh-Shiaw
Published: April 2011

A three-dimensional contact mechanics analysis has been successfully made on the composite laminates with bolted joints under bending loads. The AS4/3501-6 laminates of [08/908] s and [908/08] s lay-up with two types of countersunk bolted joints, MS-24UNF-3A (ψ = 80°) and NAS-24UNJF-3A (ψ = 100°), subjected to bending loads are analyzed, respectively. The effects of friction, stacking sequence and countersunk bolt head on the contact tractions around the bolted joint are studied systematically. The progressive failure analysis based on the maximum stress theory, Ye delamination criterion and the complete ply failure approach is performed for predicting the maximum bending load that the laminate can sustain before it catastrophically fails. Toward this end, experimental tests were also conducted to measure the maximum bending load for the laminates with the countersunk bolted joints under bending loads. From the results, it is shown that the joint configuration of [908/08] s laminate and NAS-24UNJF-3A (ψ = 100°) countersunk bolt has more higher bending strength while the joint configuration of [08/908] s laminate and NAS-24UNJF-3A (ψ = 100°) countersunk bolt has more higher bending stiffness. Good agreement between the computed and experimental results shows the applicability of the technique developed. The present work should be of help for the design of composite laminates with the countersunk bolted joints.
Ultrasonic Detection of Damage in CFRPs
Authors: Kaczmarek H.
Published: April 2011

A good knowledge on damage processes in CFRPs can be given by developing non-destructive testing. In order to improve ultrasonic capabilities to detect matrix cracking development, a water tank has been mounted directly on a servo-controlled machine and an automatic scanning by points has been investigated. More accurate testing procedures have been implemented including mechanical improvements, amplitude measurement accuracy, resetting points under loading, comparative evaluation of the attenuation between undamaged and damaged state. A comparison between X-ray and ultrasonic results has been made. Another aim was to control hidden damage due to impact and especially to measure the size and shape of delaminations at each interply location using an improved pulse echo method. Hence, conventional equipment has been modified in order to explore the material by thickness segments. A new procedure has been added to a classical equipment providing in a single scan multiple C-scan images of a coupon with a very limited amount of data. The main purpose of this paper is to describe the methods used and to discuss the original procedures that have been performed. The new capabilities of an improved ultrasonic set-up are going to be examined mainly in relation to crack-density measurement, matrix cracking development under tension and ply by ply delamination sizing and location.

With these features, an ultrasonic set-up can find its application in many aeronautical fields: improving composites, moisture influence on damage, detecting of damage resulting from impact, structures lifetime.
Experimental Verification of a Progressive Damage Model for IM7/5260 Laminates Subjected to Tension-Tension Fatigue
Authors: Coats Timothy W. , Harris Charles E.
Published: April 2011

The durability and damage tolerance of laminated composites are critical design considerations for airframe composite structures. Therefore, the ability to model damage initiation and growth and predict the life of laminated composites is necessary to achieve structurally efficient and economical designs. The purpose of this research is to experimentally verify the application of a continuum damage model to predict progressive damage development in a toughened material system. Damage due to monotonic and tension-tension fatigue was documented for IM7/5260 graphite/bismaleimide laminates. Crack density and delamination surface area were used to calculate matrix cracking and delamination internal state variables to predict stiffness loss in unnotched laminates. A damage dependent finite element code predicted the stiffness loss for notched laminates with good agreement to experimental data. It was concluded that the continuum damage model can adequately predict matrix damage progression in notched and unnotched laminates as a function of loading history and laminate stacking sequence.
On the Effect of Temperature on the Creep Behavior of Neat and Carbon Fiber Reinforced PEEK and Epoxy Resin
Authors: Katouzian M. , Bruller O. S. , Horoschenkoff A.
Published: April 2011

Schapery's nonlinear formulation for viscoelastic materials has been successfully used by many investigators at room temperature [1-3]. Little work has been presented in the literature where the above approach is applied to viscoelastic materials at elevated temperature. In the present work, Schapery's constitutive equation is used to study the nonlinear viscoelastic creep response of neat and carbon fiber-reinforced Polyether-etherketone (PEEK) and epoxy resin at different temperatures. As reinforced materials, the laminates [904] s and [±454] s were investigated. Series of 10-hour isothermal tensile creep tests were conducted on each laminate at four temperatures (up to 140°C for the epoxy system and up to 120°C for the PEEK system) and different stress levels. For comparison reasons the same type of experiments was conducted on the respective neat polymers. Schapery's approach was used to characterize the nonlinear viscoelastic response of the above materials. The stress and temperature dependence of the nonlinearity factors was evaluated using a numerical procedure based on least squares techniques. The results show that the linear viscoelastic limit is shifted to lower values with increasing temperature. This was observed for both neat polymers as well as for the [±454] s laminates investigated. On the other hand, for the [904] s laminates the influence of the temperature on the linear viscoelastic limit seems to be relatively restricted. Moreover, for all resins and laminates studied it is shown that the influence of temperature on the nonlinearity of the instantaneous material response is significantly lower than that on the transient nonlinearity. For the investigated temperature range it can therefore be assumed that the instantaneous creep response is linear and independent of temperature over a stress range relevant in practical applications. On the other hand, the influence of temperature on the transient creep response was found to be nonlinear. The transient creep response of the composite materials subjected to intralaminar shear stress showed higher temperature sensitivity than that under normal stress.
Micromechanics for Ceramic Matrix Composites via Fiber Substructuring
Authors: Mital S. K. , Murthy P. L. N. , Chamis C. C.
Published: April 2011

A generic unit cell model which includes a unique fiber substructuring concept is proposed for the development of micromechanics equations for continuous fiber reinforced ceramic composites. The unit cell consists of three constituents: fiber, matrix and an interphase. In the present approach, the unit cell is further subdivided into several slices and the equations of micromechanics are derived for each slice. These are subsequently integrated to obtain ply level properties. A stand-alone computer code containing the micromechanics model as a module is currently being developed specifically for the analysis of ceramic matrix composites. Towards this development, equivalent ply property results for a SiC (silicon carbide fiber) /Ti-15-3 (titanium matrix) composite with a 0.5 fiber volume ratio are presented and compared with those obtained from customary micromechanics models to illustrate the concept. Also, comparisons with limited experimental data for the ceramic matrix composite, SiC/RBSN (Reaction Bonded Silicon Nitride) with a 0.3 fiber volume ratio are given to validate the concepts.
Mechanical Behavior of Mullite Fiber Reinforced Aluminum Alloy Composites
Authors: Canumalla Sridhar , Dynan S. A. , Green D. J. , Bhagat R. B. , Pangborn R. N.
Published: April 2011

Discontinuously reinforced aluminum alloys are viewed as candidate materials for elevated temperature applications because of their attractive high temperature strength properties and wear resistance. The elevated temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attention in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fibers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Young's modulus of the composite exceeds that of the unreinforced alloy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The primary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the degradation in the matrix stiffness. Reinforcing the alloy with mullite fibers results in only a moderate improvement in strength at room temperature but the elongation to failure is reduced considerably. Increasing the amount of shot, although not appreciably degrading strength, further reduces the ductility. Shot is found to play an important role in the damage evolution by fracturing early in the loading process, and thus, the composite integrity when subjected to slow stable crack growth, as in fatigue, for example, could be adversely affected.
Failure of Braided Composite Cylinders under Biaxal Tension
Authors: Smith L. V. , Swanson S. R.
Published: April 2011

Braided preforms of advanced fiber composites are being considered for structural applications with potential cost and through-the-thickness length advantages relative to conventional laminates. However, the in-plane strength properties of braided preforms are relatively unknown. The present study reports on biaxial tension tests on AS4 braided reforms that were resin transfer molded with 1895 resin, in four different sets of braiding parameters. The fiber direction figure strain was observed to be independent of the biaxial stress state in the failure tests. Accordingly a maxim fiber direction strain lure criterion was used to correlate the results of this study. Failure properties of these materials appear to be sensitive to the material processes from which they ar produced, and scatter of the current results was observed to be relatively large. Examinaton of the specimen surfces as well as the braiding parameters has provided clues regarding the inconsistent material performance. The specimens were observed to be sensitive to parameters which influence yarn straightness. Braider coverage and preform bulk are believed to be primary among these parameters. Scatter of the experimental data make comparison difficult, but in general the data are in good agreement with the failure envelopes.
A Comparison of Finite Element and Test Results on the Dynamic Characteristics of Butted and Overlapped Ply Laminated Cylinders
Authors: Roberts Jack C. , Schaefer Edward D.
Published: April 2011

Numerical and experimental tests were performed on three thin-wall carbon fabric/epoxy cylinders with end flanges, and their vibration characteristics were compared. Two different laminates with [(O°)]-2 ply and t(0)]3-3 ply configurations were used. All cylinders were fabricated using four gore sections per ply. One of the two 3-ply configurations was fabricated with the gore sections butted together, and the other with the sections overlapped approximately 6 mm. There was very little difference in the natural frequencies from tests with butted or overlapped ply joints. However, there was a 1-20% difference in natural frequencies between the finite element analysis (FEA) and test results for the 3-ply cylinders over the frequency range 88 to 577 Hz. The 2-ply configuration had overlapped ply joints and a 0.50 mm thick PTFE/copper substrate adhesively bonded to the outside. In this configuration there was approximately a 7% difference in natural frequencies between the FEA and test results. The effect of the added mass of the substrate in the test samples had a greater influence on the modal analysis than the assumption of a continuum at the overlapped ply joints. The mode shapes for each of the cylinders were comparable for all configurations tested. Damping increased by 12-38% over most of the frequency range for the 3-ply overlapped compared to the butted ply joint configuration. Damping in the 2-ply overlapped joint cylinder with PTFE/copper substrate showed a 48-93% (depending on frequency) increase over that of the 3-ply overlapped joint cylinder. An increase in damping for both 3-ply configurations and the 2-ply configuration was attributed to increased matrix shearing as a result of changes in mode shape, increased warping with mode shape, and the resultant increase in internal strain.
Predicting Scaling Effect on the Notched Strength of Prepreg and Fiber Tow-Placed Laminated Composites
Authors: Shahid Iqbal , Sun Hsien-Tang , Chang Fu-Kuo
Published: April 2011

The effect of scaling on the notched strength and response of composite plates was investigated. Unidirectional prepreg and tow-placed laminated composites, both of which were made of the same fibers and resin system, were considered. In order to predict the scaling effect, the local damage accumulative inside the notched composites was considered. A finite element analysis based on an accumulative damage model previously proposed was developed for analyzing the response and tensile failure of composite laminates containing a slit or a circular hole of various sizes. The model consists of a constitutive equation which relates the damaged material to an accumulated damage state; and damage accumulation criteria which predict the state of damage as a function of the applied stresses. The free-edge induced damage was not considered in the model.

The effect of scaling on the notched strength of laminated composites was evaluated by comparing the results of the calculations from the analysis with the test data generated at Stanford University and the Boeing Company. The predictions agreed with the test data very well for the notch sizes ranging from as small as 0.25 inch up to 2.5 inches.

It was found that the larger the notch size, the more accumulated damage was predicted. Hence, the scaling effect on the notched composite strength is strongly related to the extent of damage accumulated inside the composites. The model also shows that the fiber interaction zone affects significantly the residual strength of notched composites. The higher fiber interaction zone tends to reduce fiber tensile strength, but to increase tensile notched strength. With the same material system, although the fiber tensile strength of unidirectional tow-placed composites was lower than prepreg composites, tow-placed composites exhibited higher notched strength than did the prepreg composites.
The Continuous Cuing Process for Thermoset Polymer Composites. Part 1: Modeling and Demonstration
Authors: Kim Cheol , Teng Hong , Tucker Charles L. , White Scott R.
Published: April 2011

This paper presents a new manufacturing method, continuous curing, that is specifically engineered for extremely thick parts made from thermosetting polymer matrix compounds. In this process, matrix curing takes place continuously as the precursor material is being laid down. The energy for curing is obtained from the exothermic curing reaction through controlled heat release; once started, the process is self-sustaining. Thermal spiking, an unavoidable feature of autoclave curing, can be completely eliminated in continuous curing by manipulating the process parameters. In addition, manufacturing times are reduced by an order of magnitude or more.

A simple analytical model is presented first to show the inherent features of continuous curing. Next, a detailed thermo-chemical process model is developed and numerical results are obtained for a glass/polyester composite. The process is demonstrated by curing a 100 mm thick graphite/epoxy lamina. Finally, thermal spiking behavior and manufacturing times are compared for conventional autoclave curing and continuous curing.
Effects of Hydrostatic Pressure on the Compressive Properties of Laminated, 0° Unidirectional, Graphite Fiber/Epoxy Matrix Thick-Composite
Authors: Rhee K. Y. , Pae K. D.
Published: April 2011

Compression tests were performed on laminated, 0° continuous, unidirectional, graphite fiber/epoxy matrix thick-composites at various superposed hydrostatic pressures to 4 kbar (400 MPa) at room temperature. Cylindrical samples were made by laminating continuous filament prepreg tapes (3M, SP-319) with fibers oriented along the direction of the cylinder axis. The samples were cured at 135°C for 4 hours in a platen press and post-cured at 135°C for 4 hours in a vacuum oven. The samples contained 60% fiber by volume. The tests were conducted in a high pressure apparatus which was capable of containing pressures to 7 kbar (700 MPa) and of operating at temperatures between −100 and 300°C.

The test results show that the compressive stress-strain curves undergo significant changes with increasing pressure. The stress-strain curves show perfectly linear elastic behavior at lower pressure range but exhibit curvature toward the end of the curves at higher pressure range. The compressive modulus obtained from the stress-strain curves increases bilinearly with increasing pressure with a discontinuity located at ∼2 kbar. This continuity has been associated with the low temperature secondary glass transition which is being shifted to room temperature from sub-zero temperature by the applied pressure. The modulus increases from 23.84 GPa at atmospheric pressure to 26.60 GPa at 4 kbar. The increase of the modulus at high pressure is due to finite compressive deformation of matrix by high pressure. Both fracture strength and strain increase linearly with pressure. The fracture strength increases from 376 MPa at atmospheric pressure to 607 MPa at 4 kbar. One of the important findings of our experimental study is that the compressive properties of the thick-composite are far less than the tensile properties of prepreg tapes.

It was apparent that fracture of samples occurred by end-crushing due to a combination of fiber buckling, delamination of plies in the transverse direction, and kinking at all pressure levels. High pressure makes it more difficult for the fracture of the samples to be initiated; thereby the fracture strength increases with increasing pressure.
Energy Release Rates for the ENF Specimen Using a Beam on an Elastic Foundation
Authors: Corleto Carlos R. , Hogan Harry A.
Published: April 2011

A new modified beam theory analysis is presented for the ENF specimen, which is used to evaluate the mode II delamination fracture toughness of fiber reinforced composite materials. The analysis combines the solution of a beam on a generalized elastic foundation to incorporate the effect of crack tip deformation, and a Timoshenko Beam Theory solution of the ENF to incorporate transverse shear on the predicted energy release rates. A distinctive feature of this approach is that crack tip deformation and shear deformations are treated separately and explicitly in accounting for deviations from simple beam theory. Two unknown parameters are introduced, however, that must be determined by comparison with finite element solutions. The resulting solution nevertheless demonstrates considerable accuracy over a wide range of material properties (e.g., axial to shear modulus ratios) and crack lengths. In addition, the current analysis compares very favorably with other analyses of the ENF that incorporate the effect of crack tip deformations on the energy release rate.
Constant and Variable Amplitude Fatigue Behavior and Modeling of an SRIM Polymer Matrix Composite
Authors: Kallmeyer A. R. , Stephens R. I.
Published: April 2011

Strain-controlled fatigue behavior of smooth specimens of an SRIM polymer matrix composite under constant and variable amplitude loading was investigated, including the effects of mean stresses/strains. Significant degradation of the macroscopic stiffness was observed during cyclic loading, and SEM examination of the failed specimens revealed the degradation was due to a variety of damage mechanisms, including matrix cracking, fiber/matrix debonding, fiber fracture, and fiber buckling. Fatigue life predictions made using common strain-based models overestimated the experimental results for both constant amplitude and variable amplitude loading. An improved strain-based model for making life predictions using an effective strain amplitude was proposed which was in much closer agreement with experimental results.
A New Compression Test Method for Thick Composites
Authors: Hsiao H. M. , Daniel I. M. , Wooh S. C.
Published: April 2011

A test method was developed for determination of compressive properties of thick composite materials. A new fixture was designed and built based on the concept of transmitting the initial load through the tabs by shear loading and thereafter engaging the ends to apply the additional load to failure by end loading. This fixture was used to measure the longitudinal compressive properties of thick composite materials. It is shown that longitudinal compressive strengths of 1660 MPa (240 ksi) are obtainable experimentally for thick composites. Several factors were found to affect the results of these compression tests. They include alignment, specimen geometry, stress concentrations, specimen end-crushing, and timing of end-loading engagement.
Composite Materials Which Exhibit High Stiffness and High Viscoelastic Damping
Authors: Brodt M. , Lakes R. S.
Published: April 2011

Composite micro-structures are studied, which give rise to high stiffness combined with high viscoelastic loss. We demonstrate that such properties are most easily achieved if the stiff phase is as stiff as possible. Incorporation of a small amount of damping in the stiff phase has little effect on the composite damping. Experimental results are presented for laminates consisting of cadmium and tungsten and of InSn alloy and tungsten. The combination of stiffness and loss (the product E tan δ) exceeds that of well-known materials.
Failure in Carbon/Polyimide Laminates under Biaxial Compression
Authors: Grape Johan A. , Gupta Vijay
Published: April 2011

The failure mechanisms of a woven 2-D carbon/polyimide (C/P) laminate have been determined under in-plane uniaxial and biaxial compressive loads, and the associated failure envelopes that account for the effect of loading directions were obtained. The overall failure was primarily in the form of axial interply delaminations aligned with the principal axis of loading, with only a few isolated events of kinking in the bundles. While the biaxial strength varied significantly with the ratio of in-plane stresses, R, there was no variation in the local failure mechanisms. Accordingly, it was found that the composite fails upon achieving a maximum strain along the primary axis of loading.
The Nonlinear Viscoelastic-Viscoplastic Behavior of IM7/5260 Composites Subjected to Cyclic Loading
Authors: Tuttle Mark E. , Pasricha Arun , Emery Ashley F.
Published: April 2011

A constitutive model consisting of a combination of the Schapery nonlinear viscoelastic heredity integral and a nonlinear viscoplastic functional employed by Zapas and Crissman is described. Material constants associated with the constitutive models are measured for graphite-bismaleimide (IM7/5260) composites at elevated temperatures and stress levels. These results are then combined with classical lamination theory, so as to predict the response of a multi-angle laminate to cyclic thermomechanical loadings. Predictions are favorably compared with measurements obtained during a 50-hr test involving ten 5-hr loading cycles.
Improved Mechanical Properties in Hydrodynamically Aligned, Short-Fiber Composite Materials
Authors: Guell David C. , Graham Alan L.
Published: April 2011

A hydrodynamic method has been employed to align short, reinforcing fibers in polymer matrix composite materials. Samples of composite materials were prepared and tested two at a time (one with randomly oriented fibers and the other with aligned fibers) to isolate and directly measure the effect on mechanical properties of aligning the fibers. Data were collected on the ultimate tensile strength, modulus of elasticity, and ultimate tensile strain of the composite samples prepared. The results show that the aligned fibers were approximately twice as effective as randomly oriented fibers at stiffening and strengthening the composite in the alignment direction.
Response of SiC/Ti under Combined Loading Part III: Microstructural Evaluation
Authors: Lissenden Cliff J. , Lerch Bradley A. , Herakovich Carl T.
Published: April 2011

A post-failure microstructural evaluation of unidirectional [04] and angle-ply [+45], silicon carbide/titanium (SiC/Ti) tubular specimens that were tested under multiaxial loading is presented. This evaluation indicates that damage is concentrated near the fiber/matrix interface. Radial interface cracks and debonding as well as plastic slip in the matrix were observed in the [0.] tubes. In addition to the presence of interfacial debonding in the as-fabricated condition, the [+45], tube was observed to contain much more damage and plastic deformation after mechanical loading than the [04] tubes. Transverse fiber cracks and isolated-areas of matrix cracking were also observed in the [I45], tubes. The observed permanent deformation mechanisms are consistent with measured strains. A micromechanical model has been used to predict the evolution of damage and matrix plasticity for the loadings applied in the experimental program. The theoretical predictions are consistent with observed permanent deformation mechanisms.
A Study of Fatigue Damage and Fatigue Life of Composite Laminates
Authors: Wu Wen-Fang , Lee L. J. , Choi S. T.
Published: April 2011

The fatigue damage and fatigue life of a batch of [ 45 °h. graphite/epoxy composite laminates are investigated in the present study. Residual stiffness is selected as a parameter to describe the degradation behavior of the composite laminates. In order to predict the fatigue life and residual stiffness of such laminates, a simple model which contains a residual stiffness degradation equation and the randomization of its parameters is proposed. A Markov chain model is also employed. Both models can be used to predict the probability distribution of the degraded stiffness at a specified loading cycle. The probability distribution of loading cycles for the residual stiffness to degrade to a certain amount can be predicted as well. Experimental data verify the applicability of both models to the studied composite laminates. According to the data, a simple relation between the ultimate strength and fatigue life of the laminates is also found. The relation provides us with quick estimation of the fatigue life of a component based on its ultimate strength which can usually be obtained very easily.
Effect of Fatigue Damage in Woven Composites on Thermo-Mechanical Properties and Residual Compressive Strength
Authors: Mitrovic Milan , Carman Greg P.
Published: April 2011

The long term mechanical and thermal fatigue of a Celion G30-500/PMR-15 woven composite system is investigated to study the interrelationship between thermo-mechanical properties, namely the thermal expansion coefficent (TEC) and the compressive strength. Qualitative trends in the experimental data generated by either thermal or mechanical loadings are shown to be similar to properties predicted by a shear lag based model. Residual compressive strength measurements (IITRI fixture) conducted on specimens subjected to tension-tension fatigue cycling indicate that this material property is sensitive to cracks and delaminations which form during mechanical cycling. Measured compressive strength degradation is as large as 49% for this material undergoing mechanical fatigue cycling with TEC degradation as large as 61%. Experimental results show that a correlation exists between TEC measurements and compressive strength. This correlation suggests that TEC measurements may be used as a damage evaluation technique.
An Enhanced Cell Model with Nonlinear Elasticity
Authors: Aidun John B. , Addessio F. L.
Published: April 2011

Nonlinear elasticity effects are added to an existing homogenization technique for unidirectional fiber reinforced composites. This extension of the homogenization technique increases the accuracy of numerical simulations for high strain-rate loadings. It is particularly important for any dynamic loading in which shock waves might be produced, including crash safety, armor, and munitions applications. Examples illustrate that elastic nonlinearity can make substantial contributions at strains of only a few percent. These contributions are greatest during post-yield inelastic deformation. The micromechanics-based homogenization technique is shown to facilitate use of an efficient approximate treatment of elastic nonlinearity in composites with isotropic matrix materials.
An Experimental Investigation of Stiffness Reduction and Cracks Geometry in a Unidirectional Brittle Matrix Composite
Authors: Baste Stephane , Bouazzaoui Rachid El
Published: April 2011

The nonlinear mechanical behavior of a unidirectional ceramic matrix composite is investigated by means of an ultrasonic method. This paper presents measurements of all the stiffness tensor coefficients during each step of a tensile test, and consequently the variation of all the components of the anisotropic damage tensor. A description in terms of compliance makes possible comparison between mechanical and ultrasonic measurements of the composite deterioration and the re-construction of the stress-strain curve from ultrasonic data. Good agreement between both experimental and predicted compliances of an anisotropic medium with distributed slit cracks is found. From this comparison, the crack density parameter is estimated and the matrix cracks geometry is characterized.
Failure Analysis of Bolted Composite Joints Exhibiting In-Plane Failure Modes
Authors: Hollmann Kam
Published: April 2011

The Damage Zone Model (DZM) is applied for simultaneous in-plane, net-tension, and shear-out failure analysis of T300/914C graphite/epoxy [0/90/±45,]-type laminates containing a single, perfect-fit, finger-tight, bolt-loaded hole. DZM is a general in-plane failure model for composites with arbitrarily shaped cutouts or in other ways experiencing stress gradients. The DZM parameters (laminate strength and fracture energy properties), two for each failure mode, are determined from the strength of standard notched and unnotched coupon specimens. Neither DZM, nor its parameters, are in any way modified or calibrated for bolted-joint application. For the given joint geometry, DZM correctly predicts a dominant shear-out failure for all laminates. The joint strength predictions are well within ± 10% of the experimental results.
Thermal Model for Three-Dimensional Irregular Shaped Pultruded Fiberglass Composites
Authors: Chachad Y. R. , Roux J. A. , Vaughan J. G. , Arafat E. S.
Published: April 2011

Computer simulation of the manufacturing of pultruded composite materials has been limited strictly to one or two-dimensional modeling of simple flat or circular shaped composites. This research presents the modeling of unsteady-state temperature and degree of cure distributions for the manufacturing of fiberglass-epoxy composite materials with irregular cartesian geometries in three-dimensions. The model is capable of predicting temperature and degree of cure distributions for composites with cartesian shapes in three dimensions, and temperature profiles in pultrusion dies without the aid of predetermined temperature values used as die wall boundary conditions. One of the benefits of this model is in designing the heating section of pultruder machines.

Using a differential scanning calorimeter (DSC) the chemical kinetic parameters for Shell EPON 9420 epoxy resin were obtained. A finite difference control volume technique was utilized in the development of the numerical model for solving the governing energy and species equation used in modeling the entire heating section of the pultruder. The combinations of pull speed, fiber volume, and die temperature profiles can be modeled very economically in manufacturing composites for very specific needs. Since this research is not limited in terms of predetermined temperature values, it can be easily tailored to predict a multitude of temperature profiles suited for a pultrusion process. This research is also important because it provides realistic modeling of irregular cross-sectional geometries.