bluuebunny/crossref_metadata_2025 huggingface dataset:
A Self-Aligned Test Fixture for Interlaminar Tensile Testing of Two-Dimensional Carbon-Carbon Composites
Authors: Roy Ajit K.
Published: April 2011

The interlaminar tensile testing of two-dimensional carbon-carbon laminates requires special attention on aligning the specimen with the test fixture. A selfaligned test fixture is designed to efficiently carry out such tests. The self-alignment of the two pull rods of the test fixture is achieved by holding the threaded pull rods by means of a nut prior to gripping the pull rods with the loading frame. It is demonstrated that the fixture can easily be used to measure interlaminar tensile stiffness and strength of thin carbon-carbon specimens. The interlaminar tensile stiffness and strength of three carbon-carbon laminates ([0/90]2s, [0/90/ ±45]s, and [0/90/ ±45]3s) measured by the present test fixture are reported.
Strain Rate and Coupling Agent Effects in Discontinuous Glass Fiber Reinforced Polypropylene Matrix
Authors: Agbossou A. , Mele P. , Alberola N.
Published: April 2011

Mechanical tensile behaviour studies are reported for discontinuous glassfibers reinforced polypropylene matrix. Samples were prepared by varying fiber/matrix coupling agents. The influence of strain rate on mechanical properties was analyzed in low and moderate (10-sec-1 to 10 sec-1) strain rate ranges. It was found that (1) in the moderate strain rate range, optical extensometry is invaluable in determining the actual specimen strain, (2) a drastic transition of mechanical tensile behaviour occurs with increasing strain rates, and (3) fiber/matrix coupling agents affect particularly stress-strain behaviour in the moderate strain rate range. A strong interfacial bond seems to improve the ability of the fibers to take up load in regions of matrix cracking; this leads to higher tensile strength.
The Yeh-Stratton Criterion for Composite Materials
Authors: Yeh Hsien-Yang , Kim Chang H.
Published: April 2011

A new criterion for composite failure is proposed based on the Yeh Stratton (Y-S) criterion which is proven to work well for isotropic materials. Since the Y-S criterion requires three yield strengths from simple tension, compression, and torsion tests, the Y-S criterion can be applicable for ductile materials as well as brittle materials with different strengths in tension and compression. Like many other theories of anisotropic materials which were developed from those of isotropic materials, the Y-S criterion is now generalized in this paper to examine the failure of composite materials. One unique feature of the Y-S criterion is in its ability to change its format depending upon what types of stresses are applied and what types of material strengths are considered. Thus, it is shown that the entire closed failure surface by the Y-S criterion is composed of piecewise surfaces. When a material is in compression as an example, the compressive strength should be the dominant critical strength, not the tensile strength. Accordingly, the failure function must be adjusted so that it can accommodate the compressive strength. It is shown that the failure surface of the generalized Y-S criterion will consist of piecewise surfaces depending on types of stresses and strengths of composite materials. The validity of the generalized Y-S criterion is examined with experimental results of off-axis uniaxial strength tests of unidirectional fiber composites, and cross-ply and angle-ply laminated composites. Comparisons were made with other well-known anistropic theories.
Failure of Braided Carbon/Epoxy Composites under Biaxial Compression
Authors: Smith L. V. , Swanson S. R.
Published: April 2011

Although textile composites have been studied for some time, experimental stiffness and failure data have been limited. This study presents the experimental results from compressive tests involving triaxially braided fibrous composite cylindrical specimens. The specimens consisted of four different braid architectures, and were loaded both uniaxially and biaxially. Predicting the failure of textile composites has received relatively little attention in the literature in comparison to the work that has been done on predicting stiffness. A maximum fiber-direction strain criterion was applied to the four architectures considered in this study, and was found to correlate failure of the braided specimens as well as it has in the past for laminated materials. The failure strains of the braided specimen's axial yarns were observed to be quite sensitive to small undulations resulting from the braiding process, and fell between -0.5% and-1.4%. The failure strains of the braid yarns, with relatively large undulation, were less sensitive to undulation changes within the architecture or even between the architectures, and fell between-0.3% and -0.5%.
Loading Rate and Temperature Dependence on Flexural Fatigue Behavior of a Satin Woven CFRP Laminate
Authors: Miyano Yasushi , McMurray Michael K. , Enyama Juichi , Nakada Masayuki
Published: April 2011

Generally, the mechanical behavior of polymer resins depends remarkably on time and temperature, which is called viscoelastic behavior. Thus, it can be presumed that the mechanical behavior of a CFRP using polymer resins is also time and temperature dependent. In this paper, the loading rate and temperature dependence on flexural fatigue behavior of satin woven CFRP laminates consisting of a matrix resin with a high glass transition temperature is investigated. The results show that not only flexural static strength but also flexural fatigue strength of the CFRP laminate depends on time and temperature even at temperatures that are much lower than the glass transition temperature. The reciprocation law of time and temperature was found to be applicable for both the flexural static and fatigue strengths. The fatigue and static fracture modes were found to be similar for the wide ranges of time and temperatures tested. The slope of the S-N curves also remains the same despite the large temperature range and two frequencies tested. From the above results a prediction method for flexural fatigue strength at an arbitrary temperature and frequency is proposed.
Micromechanics of Crenulated Fibers
Authors: Carapella E. E. , Hyer M. W. , Griffin O. H. , Maahs H. G.
Published: April 2011

The stresses at the interface of fully bonded and fully disbonded crenulated carbon fibers in a carbon matrix are presented. Interface stresses for two loading cases, transverse tensile loading and a negative temperature change, are discussed. A crenulated fiber is one which has a wavy or scalloped outer radius, the amplitude and frequency of the waviness resulting from the manufacturing process. A square-packed array of fibers is assumed and the results are obtained using the finite-element method. For comparison to the crenulated fiber, the stresses for similarly loaded bonded and disbanded circular fibers are also presented. The level of disbond in each case is measured by the size of the radial gap between the fiber and the matrix. When fully disbanded, the fiber and matrix are assumed to interact only through frictionless elastic contact. Results from the fully disbonded case show that mechanical interference occurs between the fiber and the matrix. In general, the results of the study show that the stress levels present with the crenulated fiber are significantly higher than for the circular fiber. In particular, large stress concentations arise in the matrix tangential stress component at high crenulation amplitudes and high crenulation frequencies. Overall, the work described provides new insight into the complex interactions between fiber and matrix, specifically interactions that reflect on the reality of disbond, and innovations regarding fiber architecture.
Fiber Clustering: A Comparison of Two Resin Flow Models
Authors: Mignolet Marc P.
Published: April 2011

The present paper provides a comparative analysis of the potential of two different resin flow models, the Ideal Fibre-Reinforced fluid and the Stokes' flow model, to account for fiber clustering. It is shown that both of these models represent well the resin flow during different periods of the curing process. On that basis, it is shown that fiber clustering occurs early in the curing process when viscosity is still low. Then, as time increases, the resin becomes more viscous and prevents any further relative motions of the fibers.
Fracture Scaling Parameters of Inhomogeneous Microstructure in Composite Structures
Authors: Cairns Douglas S. , Ilcewicz Larry B. , Walker Tom , Minguet Pierre J.
Published: April 2011

The influence of inhomogeneous microstructure on fracture properties is presented in this study. Previous tests have shown an improvement in the notched tensile strength of laminates manufactured with fiber placement versus hand layup for the same materials and laminate configuration. Fiber placement results in structures with a more inhomogeneous microstructure than in conventional hand layup manufacturing. Equivalent continuum modeling was employed to study this phenomenon. A specialty, couple stress (Cosserat Theory) finite element was developed to explore the influence of inhomogeneities in regions of high stress gradients such as notches. The results indicate that the addition of the couple stresses mitigates singularities at notch tips. In particular, it was shown that geometry alone is not sufficient for proper scaling if inhomogeneities exist. A two-parameter failure criterion, based on critical strain energy release rates and an inhomogeneity parameter, is proposed. Good correlations were found between predictions and experimental results.
Experimental and Finite Element Analytical Guidelines for Fabricating Continuous Fiber (SCS-6) Metal Matrix (Ti-6A1-4V) Composites via the Foil/Fiber/Foil Technique
Authors: Nicolaou P. D. , Piehler H. R. , Saigal S.
Published: April 2011

The consolidation behavior of Ti-6A1-4V matrix SCS-6 silicon carbide fiber metal matrix composites fabricated using the foil/fiber/foil technique was studied by both experimental physical modeling and finite element analysis. The experiments were carried out at 8750C (T/Tm = 0.597 for the matrix material) for varying levels of uniaxial compressive stress and times to characterize the stages of consolidation observed prior to reaching full density. Three stages of consolidation were identified: (1) instantaneous plasticity, (2) time-dependent foil deformation prior to establishing contact between neighboring foils, and (3) time dependent consolidation to close the pores at each side of the fiber that remain after neighboring foils have contacted between the fibers. Finite element analysis using the commercial code ABAQUS was employed to model the process and to examine the role of fiber spacing and applied stress level on the time required to densify the composite. The simulation predictions for the times necessary to achieve various densities were in good agreement with the experimental observations. The FEA results were used to establish analytical relations that describe the entire densification process. These equations are quite general and can be used to characterize any foil/fiber/foil composite fabrication process since they include as variables the matrix flow properties, processing stress and temperature, foil thickness and fiber diameter. The results predicted a slowing in densification at the latter stages of pore closure. This slowing was attributed to the low stresses in the matrix near a pore. Also, as the fiber spacing decreases, the time to densify increases because the load per fiber is now smaller.
Forming of Woven Fabric Composites
Authors: Laroche D. , Vu-Khanh T.
Published: April 2011

Forming woven fabric composites into complex parts consisting of double curvature shells leads to complex redistribution and reorientation of fibers in the composite. The thermo-mechanical performance of the molded part is highly dependent on this rearrangement. Consequently, prediction of the properties of double curvature parts requires good understanding of the structural rearrangement in each layer of the composite laminate. This paper presents the results of a study on the prediction of the complex rearrangement that occurs during molding of complex components made of woven fabric prepregs. The deformation of the composite laminate to conform to a given geometry has been modeled using the pin-jointed net idealization and the volume conservation assumption. Experimental measurements of the fiber orientation and the ply thickness variation have also been carried out on molded parts with double curvature shape made of different fabric structures. The numerical prediction has been found to be in good agreement with experimental measurements.
A Tension-Mode Fracture Model for Bolted Joints in Laminated Composites
Authors: Schulz Kurt C. , Packman Paul F. , Eisenmann James R.
Published: April 2011

A model capable of predicting bolted joint failure in generally orthotropic laminated composite plates which fail in the tension mode under bearing loading conditions has been developed. Ply-by-ply laminated analysis is not required. To facilitate practical application of the model, the plate is analyzed as a bulk orthotropic plate whose material properties are determined through the application of classical lamination theory. The analysis is based on fracture mechanics concepts applied to a pseudo-flaw which is related to the physical cracking of the laminate at the joint. The maximum circumferential stress concept is extended to orthotropic materials where both the fracture toughness and stress intensity vary with orientation. Experimental tests were conducted on single-bolt joints for two laminate layups; excellent correlation between experimental data and analytical prediction was found.
Theoretical Model of Laser Grooving for Composite Materials
Authors: Sheng P. , Chryssolouris G.
Published: April 2011

The influence of directionally-dependent thermal properties on laser machining effectiveness is analyzed theoretically and experimentally. A laser grooving model was developed which incorporated the effects of directionally-dependent thermal conductivity. Laser grooving experiments on graphite/vinylester composites showed differences in groove depth and profile depending on the laser beam direction relative to fiber orientation. The groove depth predictions from the composite grooving model showed an improved correlation with experimental data compared to predictions from the model with homogeneous material properties. The predictions of groove profiles and centerlines from the composites model are also presented for laser beam impingement parallel and perpendicular to fiber orientation.
Nonlinear Analysis of the Poisson's Ratio of Negative Poisson's Ratio Foams
Authors: Choi J. B. , Lakes R. S.
Published: April 2011

This article contains an analytic study of Poisson's ratio of re-entrant foam materials with negative Poisson's ratio. These materials get fatter when stretched and thinner when compressed. The Poisson effect is so fundamentally important to the properties of a material that a large change in the value of the ratio will have significant effects on the material's mechanical performance. Isotropic foam structures with negative Poisson's ratio have been fabricated through a permanent volumetric transformation. The cells were converted from the convex polyhedral shape of conventional foam cells to a concave or "reentrant" shape. Mechanical behavior of a re-entrant open cell foam material will differ from that of a conventional foam in ways not addressed by existing theoretical treatment. Poisson's ratio as a function of strain is obtained by modeling the three-dimensional unit cell as an idealized polyhedron unit cell. Poisson's ratio is predicted to approach the isotropic limit of −1 with increasing permanent volumetric compression ratio of idealized cells, in comparison with experimental values as small as −0.8.
Fracture Mechanics and Mechanisms of Impact-Induced Delamination in Laminated Composites
Authors: Wang H. , Vu-Khanh T.
Published: April 2011

The fracture mechanism of impact-induced delamination is studied in carbon fiber/PEEK (polyetheretherketone) cross-ply laminates under drop weight impact. The study is based on the energy theory of fracture mechanics and the concept of crack arrest toughness. The damaged laminate is modeled by a finite element method which simulates delaminations and transverse cracks. The numerical results are combined with test data to study the delamination behavior. It is found that the delamination occurs in a deflection-controlled condition and is a process of Mode II dominated unstable crack growth and subsequent arrest. The fracture behavior can be described by strain energy release rate and the delamination size is governed by the delamination arrest toughness of the composite.
Vibration Analysis of Delaminated Composite Laminates in Prebuckled States Based on a New Constrained Model
Authors: Chen Hsin-Piao , Tracy John J. , Nonato Ramon
Published: April 2011

An analytical model of free vibration of a delaminated composite laminate in prebuckled states has been developed. The formulation is based on a new constrained model which includes both effects of the compressive force and bending-extension coupling. These two effects on the natural frequency of delaminated plates have not been studied by such a model before. It is found that the compressive force, laminate lay-up, delamination length, and delamination locations in the thickness-wise and spanwise directions are significant factors to determine the vibration characteristics. Experiments have been conducted to validate this analytical model. Good agreements between the analytical results and test data have been obtained.
Micromechanics and Creep Behavior of Fiber-Reinforced Polyether-Ether-Ketone Composites
Authors: Chen Chao-Hsun , Chang Yi-Haw , Cheng Chiang-Ho
Published: April 2011

A micromechanics formulation of composite properties in terms of the constitutent properties using Eshelby's inclusion principle and Mori-Tanaka's mean field method. A linear four-element Burger mechanical model has been employed for application to viscoelastic polymers. The influence of volume fractions of inclusions on the overall creep strain of a polyether-ether-ketone (PEEK)-matrix composite is investigated. The creep rate of the PEEK-matrix which depends linearly on the creep strain and the primary creep, resulting from the viscous flow of creep deformation, is also considered in addition to the usual steady state, or secondary creep. In order to examine the applicability of the model to pure PEEK resin and carbon/PEEK, a series of experiments on PEEK 150P and unidirectional carbon fiber-reinforced PEEK composites has been carried out, which included constant rate of tensile tests and constant load tensile creep tests. It is found that the pure PEEK resin and its composite satisfies the four element Burger model, and that the model agrees with the experimental results extremely well. Also, it is demonstrated that this simple, albeit micromechanic theory is capable of predicting the volume fraction dependence of the time dependent creep, with a characteristic consistent with the known elastic behavior.
An Experimental Investigation on Infiltration Limit and the Mechanical Properties of Al2O3 Short Fiber Reinforced Metal Matrix Composites Fabricated by Squeeze Casting
Authors: Kang C. G. , Ku G. S.
Published: April 2011

Melt infiltration of fibrous preforms is an attractive fabrication route for metal matrix composites. To understand infiltration limits in squeeze casting processes, experimental data are presented for injection of aluminum alloy-based melts into preforms of volume fraction 10%-23% which were composed of δ-alumina fiber with a silica binder. Base alloys used in this study were A16061, A17075, A12024, A1514.0, A356.0 and pure A1. The infiltration distance and deformation of fiber preforms were observed with variation to manufacturing parameters such as preform temperature, die temperature, applied pressure and molten metal temperature. Tensile strength and microstructures were also examined to investigate the effects of processing parameters on infiltration states.
Boundary Effects in Woven Composites
Authors: Whitcomb John , Kondagunta Gopal , Woo Kyeongsik
Published: April 2011

Two dimensional finite elements were used to study boundary effects in plain weave composite specimens subjected to extension, shear, and flexure laods. Effective extension, shear, and flexural moduli were found to be quite sensitive to specimen size. For extension and flexure loads stress distributions were affected by a free surface, but the free surface boundary effect did not appear to propagate very far into the interior. For shear load the boundary effect appeared to propagate much further into the interior.
Fatigue Behavior of a Plain-Woven Glass Fabric Laminate under Tension/Torsion Biaxial Loading
Authors: Fujii Toru , Lin Fan
Published: April 2011

Fatigue strength and stress-strain response of a plain-woven glass fabric laminate subjected to pulsating tension/pulsating torsion biaxial loading were investigated. Biaxial loads were proportionally applied. S-N curves at different biaxial stress ratios were obtained. For the S-N diagram, the normalized fatigue strength defined by the ratio between the cyclic biaxial stress and the static strength, was used. All data roughly locate on a slightly wide band on the normalized S-N diagram, although the tendency that the slope of the S-N curve becomes low with an increase of the shear stress component is distinguishable. Modulus decays both in tension and shear with respect to loading cycles under biaxial loading are almost the same as those under uniaxial tension and pure torsion loadings, respectively. The direction of matrix cracks is dependent on the principal stress direction under biaxial loading although it does not coincide with the direction of the principal plane.
Analytical and Experimental Results of the Coefficient of Thermal Expansion of High-Modulus Graphite-Epoxy Materials
Authors: Romeo Giuuo , Frulla Giacomo
Published: April 2011

The coefficient of thermal expansion (CrE) of an arbitrary laminate can be determined by the Classical Laminate Theory. However, the results obtained have shown that the CFE is very sensitive to some orthotropic elastic constants and to the laminate layup. In particular, the non-Hookean behaviour of an unidirectional lamina in the fibres direction should have to be taken into account to exactly predict the CTE.

In order to verify the theoretical analysis, a new test facility has been designed to carefully measure the CTE in specimens made of advanced composite materials having a quasi zero value of CTE. The facility is basically composed of two thermostats and contained in a vacuum chamber (10−2 Pa). Four specimens are enclosed in the first thermostat and the operating temperature range is limited to −20°C up to 80°C. Four displacement transducers are enclosed in the second one at a temperature of 20°C and kept rigorously constant within 0.1°C. The specimens are mounted on Ultra Low Expansion (ULE) frames having a CTE of 1 × 10−8/°C. It has been possible to miniize the measurement error in the CTE, by a careful choice of displacement sensors and the high control of their thermal stability.

The following CTEs have been obtained along the fibre direction, normal to the fibre direction and for a quasi-isotropic laminate, respectively, for two High-Modulus (HM) graphite/epoxy materials.

GY70/M18: α1 = −1.04 × 10−6/°C; α2 = 33.9 × 10−6/°C; α x = -0.095 × 10−6/C

M55J/M18: α1 = −1.02 × 10−6/°C; α2 = 35.2 × 10−6/°C; α x = −0.162 × 10−6/°C

The analytical results have shown as a variation of ±1° only in the lamina orientation can change the CTE of the quasi-isotropic laminate up to ±50.5% of the theoretical value; as well as, a variation of ±5% in the physical and mechanical properties can change the CTE up to ±48%.
Compression Loading of Laminates: In Situ Failure of 0° Unidirectional Plies
Authors: Whitney James M. , Crasto Allan S. , Kim Ran Y.
Published: April 2011

Consideration is given to failure of filament-dominated laminates subjected to compression loading. Of particular interest is the interaction between longitudinal compression loading and transverse loading (tension and compression). Data obtained in conjunction with a mini-sandwich specimen are used to generate failure stresses. Classical lamination theory, including thermal residual stresses due to cure is utilized for determining ply stresses. Residual stresses are based on initial elastic properties, while mechanical stresses are based on secant modulus at failure (compression stress-strain curve for both unidirectional and laminated composites is nonlinear at high strain levels). Experimental results reveal little interaction between longitudinal compression and transverse stresses as long as matrix-dominated modes such as transverse cracking and delamination are suppressed.
Elastic Constants of Thick Orthotropic Composite Plates
Authors: Ayorinde E. O.
Published: April 2011

This paper gives a method of obtaining the three-dimensional elastic constants of a completely free orthotropic plate from experimental plate vibration data, using an optimized three-mode Rayleigh formulation that incorporates through-the-thickness shear and rotatory inertia. The author believes this is the first reported attempt employing this formulation for obtaining the frequencies of thick plates with the incorporation of these two phenomena. The primary purpose of this paper is to develop the basic theory and method for this class of plates. The author had earlier utilized the classical lamination theory (which does not include these two effects) with both three and six optimized modes. The forward problem of computing plate vibration frequencies from given elastic constants by the Rayleigh method is solved for three thickness ratios (.075, .1, .125) for an isotropic (aluminum) and an orthotropic (graphite/epoxy) material. The inverse method for extracting the elastic constants from experimental vibration data is then applied to two thick plates fabricated from these materials. The results suggest that the proposed method is potentially useful for rapid, inexpensive extraction of in situ elastic constants of composite plates. The major original contribution of this paper is the novel approach for formulating and solving both the forward and inverse problems for thick plates.
Analytical and Experimental Studies of Crushing Behavior in Composite Laminates
Authors: Bolukbasi Akif O. , Laananen David H.
Published: April 2011

The objective of this research effort has been to gain a better understanding of energy absorption behavior in composite laminates. Crush tests of nine flat plate specimens with three different lay-ups of T650-35/F584 graphite/epoxy fabric were performed to evaluate the mechanisms that contribute to the energy absorption process. A nonlinear finite element approach was used to model the sustained crushing of the flat plate, and a progressive failure model was implemented as part of the finite element analysis to enable modeling of the fundamental mechanisms involved in the crushing behavior. The progressive failure model was based on linear elastic fracture mechanics for prediction of crack growth and a set of failure criteria for predicting fiber/matrix failures that occurred as a result of large deformations. Friction between the specimen and the crushing surface was included in the model. Predicted values of sustained crushing stress agreed with experimental results for the flat plate specimens within nine percent.
Expert Cure System for the Carbon Fiber Epoxy Composite Materials
Authors: Choi Jin Ho , Lee Dai Gil
Published: April 2011

In this paper, the expert cure system for thermosetting resin matrix composites, which controls the temperature and pressure of the autoclave according to the several rules, was developed to manufacture better composite products in shorter curing time. The rules were based on the on-line measured quantities such as the dielectric properties, temperature of the composites and the pressure of the autoclave.

The curing time and the mechanical properties of the composite materials manufactured with the expert cure system were compared to those of the specimens manufactured with the conventional cure cycles
Influence of Stacking Sequence on the Transverse Matrix Cracking in Continuous Fiber Crossply Laminates
Authors: Xu Luo-Yu
Published: April 2011

The influence of stacking sequence to the transverse matrix crack was studied experimentally and analytically. Five crossply laminates of GR/BMI and GL/BMI composites were tested. Stacking sequence was found to have a significant effect on the matrix crack growth in composite laminates, it is behaved by the thickness effect to the cracked 90° plies. If the total volumes of the 00 and 90° plies are identical but the stacking sequences are designed to be different, the matrix crack growth processes are quite different. The 90° layer with smaller thickness has the higher cracking loading, matrix crack growth rate and final saturation crack density. Although stacking sequence affects the damage material state of the matrix crack, a final damage mechanics state may exist in the matrix crack growth for the laminates with the same total volumes of the 0° and 90° plies but with different stacking sequences, this damage state is characterized by the final residual laminate stiffness and seems to be independent of the laminate stacking sequence. This phenomenon can be accounted by a modified shear-lag model. Further study shows that the stiffness degradation of the cracked 90° plies has a low boundary which is independent of the laminate layup and material.
Biaxial Flexural Testing of a Carbon Fiber Reinforced Epoxy Composite
Authors: Tsangarakis N. , Taleghani B. K.
Published: April 2011

The response of a 3501-6 resin composite to biaxial flexural loading was assessed using disk specimens. The composite was cross-ply reinforced with continuous carbon fibers with a volume content of 60%. Under monotonic flexural loading the formation of delaminations limited the useful range of the composite's deformation to a biaxial tensile strain of 0.0067. Under cyclic flexural loading and for a fatigue life of 106 cycles, the useful strain range was limited to 0.003. The latter strain is only 19% of the composite's uniaxial tensile strain to failure. Fatigue failure mechanisms included delaminations, matrix cracking, fiber and possibly bundle debonds. Some delaminations were identified with ultrasonic C-scanning
Notched Strength of Thermoplastic Woven Fabric Composites
Authors: Marissen R. , Brouwer H. R. , Linsen J.
Published: April 2011

Woven fabric composites with a thermoplastic matrix form a class of promising materials for industrial use, due to their balance of properties. They combine excellent mechanical performance with ease of processing.

Failure behavior of notched woven fabric composites shows a number of peculiarities, the most notorious being the dependence of notched tensile strength on notch radius below a certain hole size. This dependence, which is observed for all fiber reinforced composites, has been modelled by other authors with the aid of semi-empirical continuum mechanics models. If at all based on physical principles, the modelled fracture process rarely coincides with experimentally observed failure phenomena.

This study investigates failure of notched woven fabric (0-90)4, E-glass laminates with a PETP matrix.

A model is proposed which is, unlike other previous modelling efforts, related to experimentally observed fracture phenomena. It introduces the ineffective length of a critical element in the notch root, obtained by a simple shear lag-type analysis. The notched strength is shown to depend on the ratio of notch radius to the critical element ineffective length. The model is able to predict changes in notched strength with composite parameters like fiber volume fraction, fiber bundle diameter and interfacial or matrix shear strength, and takes into account the composite microstructure.
Effect of Loading Method on Compression Testing of Composite Materials
Authors: Xie Ming , Adams Donald F.
Published: April 2011

Compression testing methods for composite materials usually employ either shear loading or end loading to apply the compressive load to the specimen. The effect of both loading methods on compression testing of unidirectional composite materials was studied. A combined loading scheme, i.e., the compression specimen was subjected to a combined end and shear loading, was also investigated. The stress distributions within a compression specimen subjected to each of these three different loadings were studied using a three-dimensional finite element analysis. It was found that the shear loading method yielded the most severe stress concentration at the tab tip of the specimen, while the end loading yielded a high shear stress concentration at the loading end of the specimen. The combined loading method was found to be a good compromise to reduce the stress concentration at both the tab tip and the loading end of the specimen, when compared to the shear loading and the end loading methods, respectively. The effect of loading method on a thick-section compression specimen was also analyzed. The end loading method was found to be superior to the shear loading method. The analytical results were also correlated with available experimental data. End crushing of the end-loaded specimen is often experimentally observed, due to the high stress concentration at the end of the specimen as predicted in the present study. However, if the end crushing problem can be overcome, available experimental results show that the end loading method generally yields slightly higher compressive strengths for unidirectional composites than the shear loading method. This agrees with the present analytical results, the end loading method yielding less stress concentration at the tab tip.
Thermal Conductivity of Uniaxial Coated Cylindrically Orthotropic Fiber-Reinforced Composite with Thermal Barriers
Authors: Lu Y. , Donaldson K. Y. , Hasselman D. P. H. , Thomas J. R.
Published: April 2011

The theories of Hasselman et al. and Benveniste et al. were extended to derive expressions for the thermal conductivity of continuous matrix composites containing dilute volume fractions of uniaxial coated cylindrically orthotropic fibers with interfacial thermal barriers at the fiber-coating and coating-matrix interfaces. The transverse thermal conductivity of this type of composite was found to depend on ten variables: the thermal conductivities of the matrix and coating, the radial and tangential components of the fiber thermal conductivity, the volume fractions of fibers and coating, the absolute dimensions of the fiber and coating, and the interracial thermal conductances at both interfaces.
Thermochemical and Mechanical Aspects of Composite Tape Laying
Authors: Sarrazin Hugo , Springer George S.
Published: April 2011

The overall objective of this investigation was to study the manufacture of organic matrix composites by the tape laying process. Specifically, the effects of the processing conditions on the quality of composite parts were investigated. Models were developed which relate the applied heating, applied pressure, tape laying speed, and mold material to the thermochemical and mechanical response of the material. On the basis of these models, computer codes were written to generate numerical results. With these computer codes, parametric studies were performed which indicated the influence of the various processing parameters on the temperature, degree of cure (for thermosetting matrix composites), crystallinity (for thermoplastic matrix composites), viscosity, and stresses inside the composite. From the numerical results, the processing conditions required for manufacturing good quality parts were identified.
Creep Phenomenon of Simple Composite Structures
Authors: Lee One-Chul , Ueng Charles E. S.
Published: April 2011

Fiber composites which are suitable for low cost engineering applications exhibit remarkable viscoelastic behavior. Therefore creep effect becomes an important consideration in the design of such structures subjected to a sustained load. In order to extrapolate the limited range of creep data, the capability of predicting the creep behavior for a long period is desirable. A "law of mixture" model is proposed for the creep behavior of a unidirectional composite. In the 3-bar truss problem, the stress in each bar and the vertical displacement at the joint are obtained while in the bending problem, the stress expression and the deflection at the mid span of a simple beam are determined based on the Timoshenko beam theory. The results are compared with the case where the members are made of isotropic materials, with and without creep. Finally, the trends of results are examined as certain parameters are varied.
Behavior of Stitched Laminates under In-Plane Tensile and Transverse Impact Loading
Authors: Wu Enboa , Wang Jyue
Published: April 2011

Experimental results for in-plane tensile and transverse impact responses of stitched composite laminates were presented in this paper. (02/902). E-glass/epoxy laminates of 2.8 mm nominal thickness fabricated by resin transfer molding were used as the specimens. The through-the-thickness reinforcement was provided by untwisted Kevlar-29 ravings of 1,000 and 3,000 denier. In the in-plane tensile test, although the damage mechanism of the stitched laminates was much affected by the loading directions, the stiffness was not significantly affected by the addition of the stitch threads. In the impact test using a hemispherically tipped impactor, stitching was found to significantly reduce the delamination crack area, and the 3,000 denier threads provided a better resistance to the propagation of these cracks. It was also found that, although the stitch step, stitch spacing, as well as the impact location relative to the stitch threads affected the size and the shape of the delamination area during impact, the penetrating density of the stitch threads could be better used as a criterion to evaluate this delamination resistance capability, and an optimal penetrating density could always be obtained. Consequently, the impact-induced delamination damage for composite laminates can always be alleviated by the stitch reinforcement.
A Closed-Form Solution of the Transverse Effective Thermal Conductivity of Woven Fabric Composites
Authors: Ning Qiong-Gong , Chou Tsu-Wei
Published: April 2011

This paper examines the transverse effective thermal conductivity of plain weave fabric composites. A three-dimensional thermal resistance network model is proposed. Based upon this model, a closed-form representation of the transverse thermal conductivity has been achieved. The inputs to the analytical solution are constituent material properties, fiber volume fraction, and fabric architectural parameters. No empirical parameters are adopted. The predictability of this model is substantiated by the good agreement between the present results and the published analytical and experimental work. Parametric studies have been conducted to determine the sensitivity of composite effective transverse thermal conductivities to fiber thermal anisotropy, degree of fiber undulation, and fabric weave style.
A Numerical Study of the Compression and Shear Failure of Woven Carbon-Carbon Laminates
Authors: Anand Kamal , Gupta Vijay , Y-He Ming
Published: April 2011

A finite element analysis was performed to determine the influence of microstructural inhomogeneities on the compressive and shear strength of 2-D carbon-carbon laminates. In particular, the effects of amplitude and distribution of the fiber bundle crimps, and lengths and distribution of the interply and transverse bundle cracks are determined. The results of the numerical study are successfully compared with experimental observations made in an earlier study under compression and shear loading. Under a warp-aligned compression load, the local material parameter which controls the overall instability in the form of multiple interply delaminations is the critical energy release rate for growth of pre-existing interply cracks, except for when the maximum length of these cracks is less than 0.6 mm when the failure is through bundle kink-induced shear faulting. For all pre-existing crack lengths, the same parameter was responsible for initiating a planar shear fault following the crimp boundaries under an interlaminar shear loading. Accordingly, the numerical cell-model considered here conforms to the above mechanism by allowing the participation of the energy release rate as the failure parameter.
Processing and Properties of IM7/PETI Composites
Authors: Hou T. H. , Jensen B. J. , Hergenrother P. M.
Published: April 2011

A phenylethynyl terminated imide (PETI) oligomer from the reaction of 3,3',4,4'-biphenyltetracarboxylic dianhydride, an 85:15 molar ratio of 3,4'-oxydianiline and 1,3-bis(3-aminophenoxy)benzene and 4-phenylethynylphthalic anhydride as the end-capper at a theoretical number average molecular weight (Mn) of - 5,000 g/mol was evaluated as a composite resin matrix. Unidirectional prepreg was made by coating an N-methylpyrrolidinone solution of the amide acid oligomer onto unsized IM7. The thermal and Theological properties and the solvent/volatile depletion rates of the amide acid/NMP system were determined. This information was used to successfully design a molding cycle for composite fabrication. Composites molded under 1.38 MPa at 371°C consistently yielded good consolidation as measured by C-scan and optical photomicrography. Composite mechanical properties at various temperatures included short beam shear strength, flexural strength and modulus, longitudinal compression strength and modulus (IITRI), longitudinal tensile strength and modulus and open hole compression (OHC) strength. These composites exhibited excellent resin dominant properties. The OHC strength values, in particular, suggested that PETI composites have excellent damage tolerance.
Experimental and Numerical Study of Composite T-Joints
Authors: Theotokoglou Efstathios E. , Moan Torgeir
Published: April 2011

The present paper deals with experimental and nonlinear finite element analysis of sandwich T-joints which are formed by two panels connected by lap joints. Such joints are for instance commonly used in high-speed marine vehicles, and the joints considered herein are typical for such structures. This study is concerned with T-joints subjected to tension (pull-out) force and the behaviour up to ultimate failure. The behaviour of such sandwich joints is very complex, since it involves several types of materials such as GRP, glue filler as well as foam material, and a geometry with potential high stress concentrations. Also, nonlinear material and geometrical effects may influence the ultimate behaviour. In this study experimental specimens have been produced and tested to failure. A nonlinear finite element method is adopted throughout the T-joint, which is taking into account the possible high-stress concentration positions, the nonlinear properties of both the core and the glue materials and the nonlinear displacement deformation of the T-joint. In this way we succeeded in making a significant contribution to furthering the understanding of the response and failure of sandwich T-joints. The effect of some design parameters of such joints affecting joint strength and flexibility is also investigated.
Effective Crack Growth and Residual Strength of Composite Laminates with a Sharp Notch
Authors: Afaghi-Khati Akbar , Ye Lin , Mai Yiu-Wing
Published: April 2011

The main objective of this study was to evaluate the residual strength of composite laminates with a sharp notch using an effective crack growth model (ECGM). Damage is assumed to initiate when the local normal stress ahead of the notch tip reaches the tensile strength of the unnotched laminate. The damage was modelled by a fictitious crack with cohesive stress acting on the cracks surfaces and the damage growth was simulated by extension of the fictitious crack and reduction of the cohesive stress with crack opening. The apparent fracture energy (G?) was used to define the relationship between the unnotched strength and the critical crack opening. Based on the global equilibrium condition, an iterative technique was developed to evaluate the applied load required to produce the damage growth. The residual strength of notched composite laminates was defined by the applied load instability with damage growth. The effect of damage increment on the convergence of the residual strength was investigated and the stress redistribution with damage growth in the composite laminates was discussed. The residual strength simulated from the new model correlated well with experimental data in the published literature for the different laminate configurations.
Analysis of Local Delaminations Caused by Angle Ply Matrix Cracks
Authors: Salpekar Satish A. , O'Brien T. Kevin , Shivakumar K. N.
Published: April 2011

Two different families of graphite/epoxy laminates with similar layups but different stacking sequences, (0θ/-θ) s and (-θ/θ/0) s, laminates, were analyzed using three-dimensional finite element analysis for 0 = 15 and 30 degrees. Delaminations were modeled in the -θ/θ interface, bounded by a matrix crack and the stress free edge. The total strain energy release rate, G, along the delamination front was computed using three different techniques: the virtual crack closure technique (VCCI), the equivalent domain integral (EDI) technique, and a global energy balance technique. The opening fracture mode component of the strain energy release rate, G1, along the delamination front was also computed for various delamination lengths using VCCT. Although the finite element model did not have an orthogonal mesh, VCCT still yielded accurate results which were in agreement with the global energy balance and yielded similar G distributions across the delamination front as the EDI technique. For both layups analyzed, the matrix crack length influenced the magnitude of G for delamination. Furthermore, the opening mode, G., was greatest near the matrix crack and decreased near the free edge. The laminate stacking sequences with a matrix crack in the surface angle ply had a greater GI value than the laminate stacking sequences with an angle ply matrix crack in the interior of the specimen thickness. This is consistent with test results in the literature that show delamination occurs earlier in the fatigue life of laminates with matrix cracks in the surface plies than in the interior plies.
Analysis of Geometric Imperfections Affecting the Fibers in Unidirectional Composites
Authors: Paluch Bernard
Published: April 2011

The compressive strength of unidirectional composites is partly limited by the presence of geometric imperfections, mainly including undulation and general misalignment of the fibers. By examining a number of regularly spaced cut sections under an optical microscope, the fiber paths within the material can be reconstructed using image processing techniques to determine the coordinates of the fiber centers in each cut. For each fiber, these coordinates are projected in main planes in inertia to determine the overall misalignment angle and smooth the undulation using a sinusoidal waveform. Four carbon fiber composites were examined in this way. The complete analysis of the parameters relative to misalignment and undulation shows the magnitude of the imperfections affecting each composite. Main component analysis is performed to identify the interdependence between several of these factors. In addition, correlating the positions of the different fibers show that they interact only at very short distances. These results call into question the approximation generally applied up to now, which was to consider unidirectional composites as a series of mutual parallel fibers undulating in phase. This method therefore opens the way to more realistic modeling of geometric imperfections as is required both for numerical and finite element analyses of the compression behavior of such materials.