bluuebunny/crossref_metadata_2025 huggingface dataset:
Life Prediction Methodology for Composite Structures. Part I—Constant Amplitude and Two-Stress Level Fatigue
Authors: Schaff Jeffery R. , Davidson Barry D.
Published: April 2011

A strength-based wearout model is presented for predicting the residual strength and life of composite structures subjected to constant amplitude or two-stress level fatigue loadings. It is assumed that the structure undergoes proportional loading, that its residual strength is a monotonically decreasing function of the number of loading cycles, and that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary load history may be represented by two parameter Weibull functions. The model also incorporates a "cycle mix factor" to account for the degradation of strength and life that may be associated with frequent changes in the stress amplitude of the loading. Experimental results from two-stress level, uniaxially loaded laminates are used to evaluate the model's predictive capability for sequencing effects, as in a low-high or high-low test, and for cycle mix effects, as in a low-high-low-high repeating test. Good correlation between theory and experiment is obtained for all loadings and laminates studied.
Damage Detection by Piezoelectric Patches in a Free Vibration Method
Authors: Jian X. H. , Tzou H. S. , Lissenden C. J. , Penn L. S.
Published: April 2011

An experimental program was undertaken to evaluate the feasibility of using piezoelectric patches for damage detection in composite materials. The fact that damage development can cause shifts in the natural frequencies of a structural component suggested an impulse-frequency response approach, in which free vibration was initiated by a single external mechanical pulse and was sensed by piezoelectric patches. Patches in both surface-bonded and embedded configurations were tried. The investigation was conducted on unidirectional glass fiber/epoxy laminated plates containing controlled levels of damage, present as interply delaminations. The data obtained from the piezoelectric patches showed the expected decrease in frequency of the natural vibrational modes with increase in damage. The attractiveness of this method lies in its convenience and its potential for use in the field.
Three Dimensional Effective Properties of Composite Materials for Finite Element Applications
Authors: Alexander Amos , Tzeng Jerome T.
Published: April 2011

A model has been developed to compute the effective properties for an arbitrarily shaped element with multiple anisotropic material regions. The analysis utilizes a finite element technique to resolve the complexity of three-dimensional layer geometry, anisotropy, ply orientations, and multi-material regions within an element. Accordingly, the model accounts for complex geometries requiring changes in mesh density and/or arbitrarily shaped elements that cannot be readily aligned with the layers of the laminate. Discontinuity due to ply drop-off or layer terminations within an element is also considered. The computed elastic constants are accurate, especially for the transverse shear properties. The analysis is particularly suitable for finite element applications of near-net shaped thick-section structures. A preprocessor was developed incorporating the effective property model to allow for generation of finite element representations for computer codes such as DYNA3D and ABAQUS.
Prediction of Flexural Fatigue Strength of CRFP Composites under Arbitrary Frequency, Stress Ratio and Temperature
Authors: Miyano Yasushi , Nakada Masayuki , McMurray Michael K. , Muki Rokuro
Published: April 2011

This paper deals with fatigue strength of a class of CFRP laminates satisfying certain hypotheses and proposes a prediction method of the fatigue strength at an arbitrary combination of frequency, stress ratio and temperature. Three-point bending tests were conducted for satin-woven CFRP laminates T400/3601 under static, creep and fatigue loadings. The experimental data support the prediction method and the hypotheses.
Stresses and Deformations Induced during Manufacturing. Part II: A Study of the Spring-in Phenomenon
Authors: Jain Lalit K. , Lutton Brian G. , Mai Yiu-Wing , Paton Rowan
Published: April 2011

It has been observed that residual stresses always build up during the fabrication of composite components. Such stresses can change the shape of a component when it is removed from the mould. This paper examines the spring-in induced during fabrication of composite angle components by such residual stresses. The sample composite components were fabricated from unidirectional carbon/epoxy pre-preg tape and plain-weave carbon/epoxy pre-preg fabric, as well as from dry carbon fabric and resin using the resin transfer moulding (RTM) technique. The effects of lay-up, orientations, part thickness, tool angle, tool material, etc. were investigated. The results show that (1) the tool material has an insignificant effect on the degree of spring-in, (2) spring-in is insensitive to the lay-up sequence for symmetric lay-ups, (3) spring-in does not depend on ply orientation angles in the case of fabric laminates, (4) spring-in remains constant for almost all radius/thickness ratios and (5) spring-in decreases with an increase in the tool angle. Comparison with the theoretical model developed by Jain and Mai [1] is provided and good agreement is found.
Moisture and Temperature Effects of the Compressive Failure of CFRP Unidirectional Laminates
Authors: Soutis C. , Turkmen D.
Published: April 2011

Problems in measuring the compressive strength properties of continuous fibre composites are still encountered at present day even though ISO and ASTM standards exist, and the strength prediction remains an unresolved topic. The objective of the present work is to evaluate the compressive response of the T800/924C carbon fibre-epoxy composite system (currently available for aerospace structural applications) in hot-wet environments using a modified Celanese test rig. The weight gains, maximum moisture contents and through-thickness diffusion coefficients of unidirectional laminates immersed in boiling water (accelerated ageing) are reported. Data are also presented on the effects of moisture and temperature on the compressive strength and failure mode of these laminates. It is observed that failure of specimens tested in hot-wet conditions always occurs as a result of out-of-plane fibre microbuckling. This is attributed to the reduction in matrix strength arising from elevated temperatures and environmental conditioning. In addition, two recent microbuckling models are employed to predict the compressive stress-strain response and failure load of the composite; agreement between theory and experiment is acceptable.
The Effective Elastic Constants of Solids Containing Spherical Exclusions
Authors: O'Rourke John P. , Ingber Marc S. , Weiser Martin W.
Published: April 2011

This paper considers the effective elastic properties of solids containing random arrangements of spherical exclusions. The fully three-dimensional response of an elastic material containing voids undergoing a uniaxial tensile test is simulated using the boundary element method (BEM). The effective Young's modulus and the effective Poisson's ratio are calculated as a function of the void volume fraction by comparing numerical solutions of matrix materials with and without voids. Because these effective properties depend on the microstructure, scatter about a mean value is observed for an ensemble of random configurations of voids at a given volume fraction. The effect of the number of voids on the scatter is studied. To validate the numerical results, comparisons are made with analytic solutions and experimental results. Since the authors could fine no experimental results for the effective Poisson's ratio, experiments were conducted on porous rubber samples to confirm the numerical results.
Evaluating Weibull Parameters for Transverse Cracking in Cross-Ply Laminates
Authors: Li Dong Sheng , Wisnom Michael R.
Published: April 2011

The conventional method of statistical analysis of multiple transverse cracking in cross-ply laminates is improved by correcting the strains in the cracked plies using finite element modelling. The correction takes into account the non-uniform stress distribution associated with each crack and the stress interactions between adjacent cracks. These effects are normally ignored, although they become important as the crack density increases. The strain distribution in the cracked 90° plies is determined first. An equivalent constant strain is calculated for a given crack density to give equal probability of failure to the actual variable strain distribution. This is then used to evaluate the Weibull parameters based on the true non-uniform strain distribution in the 90° plies. The cases studied include a cross-ply [0/90]. laminate, a hybrid GFRP/8-ply CFRP and a hybrid GFRP/16-ply CFRP. The resultant Weibull plots show much greater linear regions and the evaluated Weibull parameters are not sensitive to the segment length provided that it is short enough to avoid multiple cracking. The normal problem of choosing the appropriate segment length is therefore overcome, and more reliable Weibull parameters can be obtained.
Buckling of Beam-Plates Having Multiple Delaminations
Authors: Wang James Ting-Shun , Pu Hua-Nan , Lin Chien-Chang
Published: April 2011

An analysis procedure for determining the buckling load of beam-plates having multiple single delaminations is presented. Free and constrained models based on the usual beam-column theory are used for perfect and separated parts of delaminated segments. Successive corrections of buckled delaminated segments are made by inserting constrained models in the portions where overlapping of the upper and lower parts occurred in the segments of assumed free mode deformations. It appears to be the first time such analysis leading to buckling load with physically realistic mode of deformation has been considered. Recurrence equations relating integration constants of adjacent segments are established which lead to a 2 by 2 buckling determinant and allow direct determination of buckling mode along the entire beam-plate. Some numerical examples are presented for illustrative purposes. The procedure established appears to be simple, systematic and effective which should be useful for engineering applications.
Optimal Design of Process Parameters for Resin Transfer Molding
Authors: Yu Hon-Wen , Young Wen-Bin
Published: April 2011

Resin transfer molding is a widely used technique to fabricate large composite parts in the industry. The process involves injecting reactive resins into a closed mold with preplaced fiber mats. While in applications, proper selection of the process parameters is the key point to yield successful molding results. In this study, the effects of process parameters (the mold heating rate, mold temperature, resin filling and curing temperatures) on the thermal responses of the composites were studied first using a simulation model to ensure their relevance to the process. Genetic algorithms were used to search for the parameters that could reduce the cycle time and enhance property uniformity of the part. The RTM process simulation model was integrated with genetic algorithms to create an efficient design tool for the RTM process. Genetic algorithms efficiently determine the design parameters for a better molding cycle.
A Micromechanical Model for Textile Composite Plates
Authors: Marrey Ramesh V. , Sankar Bhavani V.
Published: April 2011

A novel finite element based micromechanical method is developed for computing the plate stiffness coefficients (A, B, D matrices) and coefficients of thermal expansion (α's and β's) of a textile composite modeled as a homogeneous plate. Periodic boundary conditions for the plate model, which are different from those for the continuum model, have been derived. The micromechanics methods for computing the coefficients of thermal expansion are readily extended to compute the thermal residual stresses due to curing. The methods are first verified by applying to several examples for which solutions are known, and then applied to the case of woven composites. The plate stiffness coefficients computed from direct micromechanics are compared with those derived from the homogenized elastic constants in conjunction with the classical plate theory. It is found that the plate stiffness coefficients of textile composites, especially the B and D matrices, cannot be predicted from the homogenized elastic constants and the plate thickness.
An Analysis of Axial Crushing of Composite Tubes
Authors: Gupta N. K. , Velmurugan R. , Gupta S. K.
Published: April 2011

Composite tubes of different D/t and H/D ratios and made of glass woven mats with epoxy resin, were tested under axial compression. The crushing mode and the energy absorbing capacity of these tubes were studied. Some of the tubes were filled with foam before testing, and the effect of the infill foam on the failure mode and the energy absorbing capacity of the tubes is discussed. The variation of the tube crush zone length(length crushed in single crush cycle) with variation in its D/t ratio is studied. An analysis has been carried out to find the average crush stress and the length crushed in each cycle of the crushing process when a tube is squashed between two platens. The results thus obtained have been compared with the experimental results.
Delamination Failure of a Woven Glass Fiber Composite
Authors: Ebeling T. , Hiltner A. , Baer E. , Fraser I. M. , Orton M. L.
Published: April 2011

Delamination fracture of a woven glass fiber composite was studied with the double cantilever beam (DCB) test. The mechanism of stop-start crack propagation was determined by direct observation of the crack front, complemented by fractographic analysis. The results were compared with an earlier study of interlaminar fracture that considered the effect of yarn undulation without the effect of weave. The constraint imposed by the interwoven fill yarns forced the fracture from an intrayarn path to an interply path which approximately doubled the interlaminar fracture toughness. Increasing the matrix toughness by increasing the amount of soft segment in the urethane methacrylate resin translated directly to increased composite toughness. The resin-rich regions, especially the interstitial areas where four yarns intersected, strongly influenced the mechanism of stop-start crack propagation. In the most brittle matrix, these regions fractured ahead of the crack front and thus served as internal flaws or weak points that facilitated crack propagation. With a tougher matrix, the interstitial areas were points of increased toughness and functioned as pinning points that temporarily arrested crack growth.
Investigation of the Effect of Part Length on Process-Induced Fiber-Wrinkled Regions in Composite Laminates
Authors: Kugler Danielle , Joyce Peter J. , Moon Tess J.
Published: April 2011

A detailed survey was conducted of the localized fiber waviness which develops in unidirectional thermoplastic laminates (T300/P1700) in order to determine how part length affects the distribution of fiber waviness. Eleven laminates of varying length were manufactured using identical processing histories. Each plate exhibited discrete wrinkle regions concentrated in the lengthwise center of the part. The amplitude, wavelength, and distribution of these regions were characterized for each plate processed, while penetration depths were studied for sample plates of each length. The results indicate that the waviness severity increases slightly with increasing part length. Furthermore, the major component is in-plane and the waviness frequently penetrates several plys below the surface.
Formation and Characterization of Carbon/Polycarbonate Towpregs and Composites
Authors: Nunes J. P. , Bernardo C. A. , Pouzada A. S. , Edie D. D.
Published: April 2011

A powder coating technique was used to produce carbon fiber towpregs with low fiber content. Unidirectional composites obtained from this material were subjected to mechanical testing. The tensile strength of the carbon fibers was determined from experimental data, using the Weibull and log-normal distribution functions. The properties of the composites are comparable with those of a commercial Sheet Molding Compound (SMC), and could be predicted using the Classical Lamination Theory. It is shown that the tensile strength of the composites is more sensitive than the modulus to processing derived defects.
Directional Dependence of Fracture Toughness in Hot-Pressed SiC-Whisker Reinforced Alumina at Room and Elevated Temperatures
Authors: Wereszczak Andrew A. , Ferber Mattison K. , Parvizi-Majidi Azar
Published: April 2011

The directional dependence of fracture toughness was investigated in commercially available hot-pressed SiC-whisker reinforced alumina composites at room and elevated temperatures. Whisker orientation was transversely isotropic with respect to the pressing axis in these composites. Composite bend bars were chevron-notched and loaded in four-point bending to induce stable crack propagation in two different crack systems: defined here as a combination of crack plane and direction with reference to the pressing axis. The average fracture toughness, KIc, was found to be approximately 30% less for a crack system with its crack plane parallel and crack direction perpendicular to the pressing axis compared to a crack system with both its crack plane and direction parallel to the pressing axis through 1200°C. The crack systems with the lower fracture toughness exhibited a characteristic high concentration of whiskers aligned parallel to the crack direction within the crack plane; this appeared to he associated with decreased or unpromoted crack deflection. This fracture mechanism evidently dominated the fracture process through 1200°C because measured fracture toughnesses were independent of temperature; crack-wake toughening mechanisms, whose efficiency is a function of the residual thermal stresses, are expected to be temperature-dependent.
A Lamination Theory Incorporating the Effect of Interlaminar Deformation
Authors: Bai Q. Sh. , Murakami S. , Kanagawa Y.
Published: April 2011

The effect of interlaminar deformation due to interlaminar shear stresses and delamination on the in-plane behavior of FRP laminates was investigated. The interlaminar shear stresses considered were induced by in-plane external load and stiffness mismatch. In order to develop a simple lamination theory which could take account of the effect of interlaminar shear deformation, a simplified laminate model and linear shear slip theory were employed. The lamination theory was applied to a series of two-layer angleply laminate tubes and the results show that interlaminar shear deformation affects significantly the mechanical behavior of the angle-ply laminate tubes with imperfect interfacial bonding.
Cure Dependent Lamina Stiffness Matrices of Resin Transfer Molded Composite Parts with Woven Fiber Mats
Authors: Golestanian Hossein , El-Gizawy A. Sherif
Published: April 2011

Models are presented for determination of cure dependent stiffness matrices for composites manufactured with woven fiber mats. Five-harness carbon and eight-harness glass mats with EPON 826 resin composites are considered. The models presented here take into account important material/process parameters with emphasis on: (1) the effects of cure-dependent resin mechanical properties, (2) fiber undulation due to the weave of the fiber fill and warp bundles, and (3) resin interaction with the fiber bundles at a microscopic scale. In the present study these effects are considered in the determination of mechanical properties of a Resin Transfer Molded (RTM) part
The Effect of Temperature Gradient on the Free-Edge Interlaminar Stresses in Multi-Layered Structures
Authors: Yin Wan-Lee
Published: April 2011

For a multilayered anisotropic laminate, the intense and localized interlaminar stresses in the vicinity of a free edge may be significantly affected by non-uniformity of the temperature load. An efficient analysis method, based on polynomial expansions of the stress functions with respect to the thickness coordinate and the principle of complementary energy, is further developed in the present work to evaluate localized free-edge stresses under non-uniform temperature loads. The variational method yields closed-form solutions of the interlaminar stresses in terms of eigenfunctions. Interlaminar stresses are obtained for symmetric, four-layer, cross-ply and angle ply laminates subjected to a uniform temperature load and to linearly varying non-uniform temperature loads. It is shown that a temperature gradient may contribute significantly to the free-edge interlaminar stresses even if the local value of the temperature increment vanishes at the intersection of the interface and the free edge.
Multiple Delaminations and their Severity in Nonlinear Circular Plates Subjected to Concentrated Loading
Authors: Suemasu Hiroshi , Majima Osamu
Published: April 2011

Instability problem of multiple penny-shape interlaminar delaminations in circular nonlinear axisymmetric plates subjected to a transverse loading is theoretically studied. It is an idealized problem of damage accumulation in composite laminates due to low velocity impact loading. All the delaminations having a same size are located at a same interval. Nonlinear behaviors of the plates are approximately solved through Rayleigh-Ritz method considering only two mode functions, that is, global and local mode functions which are based upon a linear exact solution. Energy release rate due to a simultaneous propagation of all the delaminations is obtained approximately and the results are compared with those obtained via a finite element analysis. The present solutions agree excellently with the finite element results except when the nonlinear effect is extremely large. The energy release rate significantly decreases with the propagation of the delaminations due to the nonlinear effects, particularly when the number of the delaminations is large. The load must be increased to keep the delaminations to propagate. The conclusions from the results obtained based on the linear assumption that the more delaminations exist, the easier delaminations propagate, may not be true when the nonlinear effect is considered.
Tow Impregnation Model and Void Formation Mechanisms during RTM
Authors: Binetruy C. , Hilaire B. , Pabiot J.
Published: April 2011

A simple analytical model has been developed for the impregnation of fiber bundles where the macroscopic flow is parallel to the fiber axis. Based on the study of the contribution of the axial and transverse flow mechanisms inside tows, this model shows that the main tow impregnation process is transverse to the fiber axis. A criterion has been established to indicate when the axial flow can be neglected to simplify the tow impregnation model. This case represents the majority of situations in the RTM of woven fabrics and the model predicts that the flow front in tows has a pointed meniscus shape whose length depends on the effective permeability of the large pores formed between the tows, transverse permeability of tows and thickness. A new boundary condition at the unsaturated tow surface is proposed. It conveys the interactions between the flows occurring inside and outside axial tows during a constant pressure driven impregnation. Two air entrapment mechanisms have been observed. Although the source of these void formation processes is the local difference in fiber arrangement, the dependence of the amount of formed bubbles on the impregnation front length has been clearly identified. In addition, a few means for voids mobilization or bubbles dividing have been investigated.
Compression Behavior of FRP Sandwich Specimens with Interface Debonds
Authors: Hansen Ulrich
Published: April 2011

Local regions of bond failure between face sheet and core are often observed when sandwich structures are exposed to low energy impacts. Similarly, impact loads frequently result in delaminations within the face sheets. In the absence of any delaminations in the sandwich structure the buckling strain of the debonded region is in this work shown to be the key parameter in assessing the structural integrity of a sandwich component in compression. The objective of this study was to predict the buckling of the debonded region and to investigate the parameters controlling this instability. A large number of analyses of the delamination buckling problem, which in many ways may appear similar to the debond problem, have been carried out in the past. However, the debond-buckling problem has not been addressed before and as shown in this work there are significant differences. The Finite Element method was found able to predict all the experimentally observed phenomena accurately. These calculations were expensive, and a simple one-dimensional closed form solution is proposed for qualitative parametric investigations. Effects of material properties and initial imperfections were examined. Analyses of the buckling of a delamination suggest that the effect of the core can be ignored. In contrast, the results presented in this paper show that the core cannot be ignored in the case of buckling of a debond nor can it be simulated in terms of simple boundary conditions.
Impact Study of Sandwich Composite Structures with Delamination
Authors: Kwon Y. W. , Wojcik G. W.
Published: April 2011

Finite element analyses were conducted to analyze the dynamic response of an unbalanced sandwich beam, a balanced sandwich beam, and a balanced sandwich plate subject to a low energy impact. In particular, strain versus time history, failure location and mode, and the influence of an existing delamination crack on the failure were investigated. It was found that, in the presence of a small delamination crack, the failure load of a sandwich composite structure increased compared to that of the non-delaminated structure. For a small crack, failure under impact loading was core shearing, that occurred away from the crack tip. However, as the delamination crack length increased, the core shearing failure occurred at the crack tip under a reduced impact load. Thus, the failure location shifted moving away from the crack tip to the crack tip as the delamination crack size increased from a small size to a large size. The failure load for a large crack structure was significantly smaller than that of the non-delaminated structure. Core shearing failure was followed by delmaination at the core/faceplate interfaces. When the initial delamination crack size became longer than half of the beam length, the crack further propagated through the interface. Furthermore, a delamination crack did not result in a substantial increase in the maximum deflection compared to that of the non-delaminated specimen. Thus, detection of delamination cracks was difficult from the deflection measurement under a low energy impact. Finally, the effect of crack surface friction on the dynamic behaviour and failure of the sandwich structures was small.
Effects of Thermo-Mechanical Properties of Composites on Viscosity, Temperature and Degree of Cure in Thick Thermosetting Composite Laminates during Curing Process
Authors: Yi Sung , Hilton Harry H.
Published: April 2011

In the present study, the effects of thermo-mechanical properties of composites on viscosity, temperature and degree of cure in thick laminated composites have been investigated. Thermal properties such as the density, specific heat and thermal conductivity are taken to be both temperature and degree of cure dependent. The internal heat generation term is expressed as a function of the cure rate. A non-linear transient heat transfer finite element model is employed to simulate the curing process. Hercules AS4/3501-6 graphite/epoxy composites are considered. The results show that temperature and degree of cure calculations are significantly influenced by the values of thermal conductivities when thickness of laminates increase. The present study indicates that it is necessary to appropriately measure thermal properties in order to accurately predict temperature, viscosity, and degree of cure in thick composites. The effects of fiber volume fraction ratios and bagging materials are also studied. Increasing fiber volume fraction ratios decreases the maximum temperature at the center of laminates. It is also seen that the difference of viscosity values between inside and outside of the laminate increases remarkably as the thickness increases.
Spin Burst Test of Carbon-Carbon Composite Disk
Authors: Kogo Yasuo , Hatta Hiroshi , Kawada Hiroyuki , Shigemura Takashi , Ohnabe Hisaichi , Mizutani Tomoaki , Tomioka Fumiki
Published: April 2011

In order to establish design criteria for a turbine disk made of Carbon-Carbon (C/C) composites, spin burst tests were performed on quasi-isotropically laminated C/C composite disks. Un-notched and notched flat disks were prepared to evaluate the effect of stress concentration on fracture behavior.

Strain measurements during rotation tests revealed that deformation of the C/C composite disks with or without notches was in agreement with analytical or finite element calculations in the elastic region, which suggests that the test fixtures of the spin tester have a negligible effect on the deformation of C/C composite disks.

In the spin burst tests, the un-notched C/C composite disks burst at an average rotation speed of 25000 r.p.m. with a peripheral velocity of 393 m/s. A comparison between the mean hoop stress averaged over the radius and the tensile strength of smooth specimen suggests that the burst of the un-notched C/C composite disks obey the mean stress criterion. On the other hand, the fracture toughness criterion was shown to be applied for the notched specimen. The difference in the fracture criterion between the un-notched and the notched C/C composite disks is believed to correspond to that between the double edge notched specimens and specimens with a hole in the static tensile tests, (which was explained by the difference in the magnitude of the damaged zone where the stress concentrations would be relaxed).
Integrated Product and Process Design for Resin Transfer Molded Parts
Authors: Spoerre Julie , Zhang Chuck , Wang Ben , Parnas Richard
Published: April 2011

Resin transfer molding (RTM) is an attractive composite processing method due to its potential for providing consistently superior parts at low costs. The RTM process involves a large number of variables related to the product and process performance. In this study, an integrated product and process design (IPPD) approach is developed and implemented to optimize RTM product and process design variables for good part quality and process performance. Genetic algorithms (GA), in conjunction with the cascade correlation neural network architecture (CCA-NN), are utilized to establish a model that predicts and optimizes performance and quality of RTM parts. The integrated product and process design approach is implemented and illustrated with experimental data.
Progressive Delamination Using Interface Elements
Authors: Mi Y. , Crisfield M. A. , Davies G. A. O. , Hellweg H. B.
Published: April 2011

The paper describes a method for modelling progressive mixed-mode delamination in fibre composites. The procedure, which is incorporated within the non-linear finite element method, is based on the use of interface elements in conjunction with softening relationships between the stresses and the relative displacements. Fracture mechanics is indirectly introduced by relating the areas under the stress/displacement curves to the critical fracture energies.
Failure of ±55 Degree Filament Wound Glass/Epoxy Composite Tubes under Biaxial Compression
Authors: Kaddour A. S. , Soden P. D. , Hinton M. J.
Published: April 2011

New experimental results are presented which describe the crushing behaviour of ±55° filament wound E-glass/epoxy tubes subjected to biaxial compression at various ratios of circumferential to axial stress. The tubes were tested to fracture under combined external pressure and axial compression using a recently developed high pressure testing rig. In some cases it was necessary to use tubes with radius to thickness ratio less than 5, in order to avoid buckling under external pressure. These new results have been combined with previous results from tests involving tensile loads to form a comprehensive biaxial failure stress envelope. The experimental results showed that for ±55° tubes under biaxial loading the biaxial compressive strength is substantially greater than the uniaxial strength, reaching a maximum at a hoop to axial stress ratio of -2/-1. The maximum compressive strength under -2/-1 loading was up to three times greater than the uniaxial compressive strength. The experimental envelope is compared with theoretical predictions which utilise simple failure theories. Good correlation was observed between the experimental results and the prediction of simple 2-D laminate model which incorporated a progressive maximum stress ply failure criterion.
Interlaminar Fracture of Commingled GF/PET Composite Laminates
Authors: Svensson Niklas , Shishoo Roshan , Gilchrist Michael
Published: April 2011

The Mode I, Mode II and mixed mode (Mode I:II ratios of 4:1, 1:1 and 1:4) fracture behavior of novel textile glass fiber reinforced polyethylene terephthalate (GF/PET) laminates has been investigated. The laminates were manufactured by compression molding two different fabrics produced by weaving and warp knitting commingled GF/PET yarns. The initiation fracture toughness of the woven laminates in pure Mode I and Mode II were slightly higher than those of the warp knitted laminates. For the mixed modes, the difference between the fracture toughness of the two materials was smaller. An extensive scanning electron microscopy (SEM) investigation of the fracture was conducted to identify any characteristic failure mechanisms. The main fractographic features of the Mode I dominated failures were a brittle matrix failure and large amounts of fiber pull-out. As the Mode II loading component increased, the amount of fiber pull-out was reduced and the matrix had a more sheared appearance. A relatively large amount of cusps were found in pure Mode II and mixed mode I:II = 1:4; such features are seldom seen in thermoplastic matrix composites. A general mixed mode failure criterion, which accounts for the appearance of the fracture surface, was evaluated and was seen to give a good fit to the experimental fracture toughness values.
The Effect of Time and Temperature on the Flexural Strength of a Silica Particle Filled Epoxy Resin
Authors: McMurray Michael K. , Amagi Shigeo
Published: April 2011

Composite materials that use an epoxy as a matrix resins have superior mechanical properties but these materials exhibit time and temperature behavior when used for long periods and under high temperatures. This time and temperature behavior has not been fully explained. The purpose of this paper is to further describe this time and temperature behavior, therefore increasing the reliability of these types of composite materials. The time and temperature dependence of flexural strength was examined by flexural static and fatigue testing. Initially the neat resin's viscoelastic mechanical behavior was determined experimentally to be later compared to the composites. Static tests were carried out at various constant deflection rates and temperatures below the glass transition temperature of the epoxy resin. It was shown that the time-temperature superposition principle is applicable for the flexural static strength. Comparison of the neat resin and the composite results show that the neat resin is a major influence on the time and temperature behavior of the composite materials static strength behavior. Fatigue tests were carried out at several stress ratios and various temperatures below the glass transition temperature of the matrix resin. It was shown that the time-temperature superposition principle is applicable for the initial flexural fatigue strength by using the time-temperature behavior of the matrix resin. As temperature and stress ratio increased, the creep influence increased in both the flexural strength and the modulus.
Quantitative Analysis of the Resin Transfer Molding Process
Authors: Velten Kai
Published: April 2011

We develop a general method based on implicit integral equations that allows the derivation of analytical expressions for wet length, filling time, pressure distribution and flow rate in the mold filling step of the resin transfer molding (RTM) process. Our method is based on Darcy's law, avoids unnecessary assumptions on the process parameters and considers pressure, inflow and general mixed boundary conditions. Temperature and curing effects as well as non-Newtonian flow conditions are dicussed. Previously derived analytical solutions of the RTM flow equations appear as special cases of our general equations. Our equations provide a framework for the toolbox-like construction of new analytical expressions. The general mathematical formulation allows for sensitivity and optimization studies; examples are given. A simple numerical solution procedure for the implicit analytical equations is suggested.
Tensile Strength of Fiber-Reinforced Composites: I. Model and Effects of Local Fiber Geometry
Authors: Curtin W. A. , Takeda N.
Published: April 2011

Predictions of the ultimate tensile strength of 3-dimensional fiber-reinforced composites as a function of the fiber statistical strength distribution and fiber geometry (square vs. hexagonal packing) are presented for materials in which the load transfer from broken to unbroken fibers is very localized. The predictions are obtained using a previously-developed simulation model adapted here for hexagonal fiber arrays. The model includes (1) the Hedgepeth and Van Dyke load transfer model to determine in-plane load transfer and (2) fiber slip in the longitudinal direction via a shear-lag model. Results show that, although the load transfer does depend on fiber geometry, the average composite tensile strength and the statistical distribution of strengths do not depend strongly on the fiber geometry. The size scaling of strength is then also shown to be nearly-independent of local fiber geometry. These results are physically reasonable since the critical clusters of fiber damage causing failure are observed to be larger than 15-20 fibers, so that the detailed local geometry at smaller length scales is not crucial to failure. Hence, analytic models developed previously for square fiber arrangements can be used with reasonable accuracy independent of fiber arrangements. Applications of the model to polymer matrix composites are discussed in a companion paper (Part II).
Experimental Design of Fire Retardant Formulations: SMC Low Profile Formulations
Authors: Hernangil A. , Rodriguez M. , Leon L. M. , Ballestero J. , Alonso J. R.
Published: April 2011

The problems originated by fire are important enough to carry out studies related to ignition, flame spread, combustibility, and smoke suppression for the purpose of obtaining materials with better fire behaviour.

An improvement in fire properties can be achieved using fire retardants and smoke suppressants. Fire retardants can act by means of chemical reactions where the combustion presents radicals with high energy in the gas phase. They can also react physically by several methods: developing a layer which protects the material from ignition on the surface of the substrate, giving off non flammable gases such as H2O, CO2, SO2, HCl, etc., in order to dissolve the flammable gases and withdrawing the oxygen from the fire area in the gas phase, improving certain reactions such as endothermic ones or surface dehydrations. On the other hand, smoke suppressants can also act physically and chemically (by means of Red-Ox reactions) in the condensed and gas phases.

The present work focuses on the study of SMC-LP(Sheet Moulding Compound Low Profile) materials which are widely used in sectors such as: automotive, transport and building. In SMC low profile process, the polyester resins are mixed with thermoplastic additives. In this way, the formulation obtained is called low profile due to the contraction inside the mould tends to zero. On the other hand, using the proper process it is possible to obtain fibre reinforced plastics formulations with different fire retardant and smoke suppressants.

The main objective of the present work has been an exhaustive analysis of the possible effects and interactions in fire and mechanical properties due to the different fillers added.

This study has been carried out by means of an experimental design with statistical analysis of the results. The scope is focused on a further optimisation of these sorts of materials, in order to obtain a wide range of suitable formulations.
Delamination Fracture of Multidirectional Carbon-Fiber/Epoxy Composites under Mode I, Mode II and Mixed-Mode I/II Loading
Authors: Choi N. S. , Kinloch A. J. , Williams J. G.
Published: April 2011

The purpose of the present study was to characterize the delamination fracture of continuous carbon fiber/epoxy multidirectional-laminates under Mode I, Mode II and Mixed-Mode I/II loading conditions. The present study considers the variation of the interlaminar failure energy, G C, with the extent of crack jumping, and ensuing fiber bridging, which arises during the growth of the delamination in the multidirectional-laminates under the various modes of loading. The main type of laminate which was studied was a multidirectional fiber composite prepared from 24 ply lay-ups of (-45°/0°/+45°)2S (+45°/0°/-45°)2S. The initial delamination was located at the +45°/-45° mid-plane of the specimen. It has been found that when the values of the interlaminar fracture energy, G c, are ascertained as a function of the length of the propagating crack, a, then very complex relationships are observed. This was the case for all the different modes of loading, and these observations reflected the complex failure paths which occurred as the delamination propagated through the multidirectional fiber composites. It was, however, possible to define clearly the onset of crack initiation. These results also revealed that the values for the interlaminar fracture energy, G C-(initiation), at crack initiation for the (-45°/O°/+45°)2S (+45°/O°/-45°)2S multidirectional laminates were always significantly greater than that for the corresponding unidirectional (i.e., 0°/0°) laminates.
A Semi-Analytical Method for the Calculation of the Elastic Micro-Fields in Plain Weave Fabric Composites Subjected to In-Plane Loading
Authors: Kuhn J. L. , Haan S. I. , Charalambides P. G.
Published: April 2011

In this paper, a computationally efficient semi-analytical method is developed for the calculation of the elastic micro-fields dominating plain weave composites under remote in-plane loading. The framework needed to obtain analytical displacement estimates is derived using the Rayleigh-Ritz method for a composite plate with spatially varying laminate stiffnesses. The method is developed with the aid of a symmetric woven unit-cell geometry. Three separate boundary value problems, corresponding to remotely applied biaxial and uniaxial displacement, and simple shear boundary conditions are formulated. Unit-cell geometry and material characteristics for both polymer and ceramic matrix composites are incorporated in the formulation of the above boundary value problems.

Extensive solution convergence studies are presented. As expected, convergence is shown to depend on the number of terms used in the Ritz series solution and the number of integration stations used to integrate the spatially laminate stiffnesses within the woven unit-cell geometry. Convergence results for the optimal selection of both of the above parameters are presented. The implications of the method's potential to yield accurate elastic microfields on the evolution of stress induced microdamage in woven systems is discussed.
A Variational Approach to Analyzing Laminates with Cracks and Viscoplasticity—The Model and its Validation
Authors: Gao Zhanjun , Xiang Xiaoyun
Published: April 2011

For cross-ply laminates, matrix cracks and viscoplasticity of the 90 degree ply are two major forms of damage which affects the performance of the composite laminates, especially at high temperatures and under complicated loading patterns. To predict the durability of such kinds of materials, one of the tasks is to analyze stress distribution in each ply of the laminates taking into the interaction between the cracks and viscoplastic deformation.

A unified viscoplasticity model is utilized together with the principle of minimum complementary energy to establish a general framework for cross-ply laminates containing cracks and undergoing general viscoplastic deformation. The problem is reduced to an ordinary differential equation for a stress function with an inhomogeneous "force" term related to the viscoplastic strain accumulation. An algorithm to solve the equation for the stress function is presented. The results are compared with those from the finite element method to validate the proposed approach. It is found that although the variational approach, like most of the analytical solutions, does not represent the near crack tip stress singularity field, it does provide fairly accurate through-thickness average stress within the 0 and 90 degree plies. In reality, local inelastic deformation occurs near the crack tip so that the stress is finite. If the local inelastic deformation near the crack tip is taken into consideration, the results from the variational approach match more closely with the finite element solution. Furthermore, the variational approach yields rather accurate in-plane shear stress along the interface between the 90 and 0 degree plies.
The Effective Elastic Properties in Anisotropic Composites with Ellipsoidal Inclusions
Authors: Demakos C. B. , Theocaris P. S.
Published: April 2011

Parallel series of ellipsoidal inclusions, aligned along their major axes, whose centers are arranged in arrays inside a matrix, result generally in an orthotropic composite. When these arrays of inclusions constitute stacks placed at equal spacings through the thickness of the composite, the material is transformed into a transversely isotropic material.

Considering this simplified problem, the effective elastic properties, i.e., the longitudinal and transverse elastic and shear moduli, as well as the respective Poisson's ratios, are evaluated in this paper. The solution of the problem was made in terms of the aspect ratio of the inclusions, their volume fraction and, finally, the succession of the constituent phases inside the composite.

Employing Eshelby's theory for the stress field around a single ellipsoidal inhomogeneity, which is surrounded by the effective anisotropic composite medium, and Mori-Tanaka's concept for considering the mutual interaction of the neighboring inclusions inside the matrix, we evaluated the effective properties of composites containing ellipsoidal inclusions parallel dispersed inside the matrix.

The whole analysis provides, in a closed-form solution, interesting results concerning the anisotropy, as well as the stiffness of anisotropic composites, reinforced with ellipsoidal inclusions. These findings reveal that the anisotropy is merely increased in composites consisting of soft matrices and reinforced by hard needle-shaped inclusions, whereas the stiffness is affected by both the shape of inclusions, as well as by their mutual concentrations. Furthermore, introduction of different constituent phases inside the composites indicated that intense variations of stiffness occur, especially for composites consisting of hard matrices containing soft inclusions.

The results of the paper concerning effective elastic properties of anisotropic composites with ellipsoidal inclusions can be applied in case of unidirectional discontinuous fiber composites, where the fibers are simulated by ellipsoidal inclusions. Also, the evaluation of the elastic properties in hard composites with enough soft inclusions provide information for the mechanical as well as the fracture behavior of hard composites weakened by internal through cracks.