bluuebunny/crossref_metadata_2025 huggingface dataset:
Acoustic Emission Study of Failure Mechanisms in Ceramic Matrix Composite under Longitudinal Tensile Loading
Authors: Luo Jyi-Jiin , Wooh Shi-Chang , Daniel Isaac M.
Published: April 2011

Acoustic emission (AE) measurements were correlated with macroscopic stress-strain behavior and direct microscopic observations of damage mechanisms in a unidirectional ceramic matrix composite under longitudinal tensile loading. Matrix crack initiation and the onset of AE events correspond to the proportional limit of the material. Both the crack density and cumulative AE counts vary linearly up to the point of crack saturation. Thereafter follows a "strain hardening" region corresponding mainly to fiber-matrix debonding and manifested by an increase of low to moderate amplitude AE activity and leveling off of high amplitude AE signals. At higher strains, beyond the "strain hardening" region, there are two characteristic parts in the AE output, one related to further damage (possibly debonding and fiber fracture) and the other clearly related to interfacial frictional sliding. Upon unloading further AE output related to sliding is observed. AE activity of low to intermediate amplitude continues in the last stages of loading and is believed to be associated with fiber fractures.
Delamination Fracture Criteria for Composite Laminates
Authors: Hwu Chyanbin , Kao C. J. , Chang L. E.
Published: April 2011

Due to the singularity nature of delamination, the fundamental concept of fracture mechanics was applied. However, the oscillatory characteristics near the delamination tip prohibited the use of conventional definition assuming the crack embedded into a homogeneous solid. A proper definition for the stress intensity factors and energy release rates of bimaterial interface cracks was introduced in this paper to study the fracture criterion for delamination. To measure the delamination fracture toughness, the test specimen usually used for the unidirectional composites was redesigned to suit for the cases that the delamination lies between two laminae with different fiber orientations. The test results show that the double cantilever beam (DCB) test is near to pure Mode I and the end-notched flexural (ENF) test is near to pure Mode II. Therefore, like the cracks in homogeneous materials, the test methods DCB and ENF are useful for the measurement of delamination fracture toughness GC and G11,. Based upon this conclusion, a series of cracked-lap shear (CLS) and modified end-notched flexural (MENF) tests and finite element simulation are conducted in this paper to establish the mixed-mode delamination fracture criterion. To validate the proposed fracture criterion, two commonly encountered problems are predicted and tested. One is the prediction of the most probable interface for the onset of delamination and its possible minimum load, in which no delamination was embedded prior to loading. The other is the prediction for the test with delamination existing prior to loading. The results show that the prediction by using this proposed criterion is well matched with the experiment.
Delamination Buckling and Growth of Composite Laminated Plates with Transverse Shear Deformation
Authors: Kyoung Woo-Min , Kim Chun-Gon
Published: April 2011

An analytical method was presented to determine the delamination buckling load and the growth of one-dimensional delaminated beam-plates. In this study, the symmetry in delamination with respect to the center of the beam-plate was not assumed. The delamination was arbitrarily located in the longitudinal and thickness directions. The effects of the delamination length, and location on the buckling load and the growth were investigated with consideration of the transverse shear deformation. Results of the analysis showed that the delamination buckling load was dependent on the configuration of the delamination. For the relatively thick delamination, the buckling load of a center located delamination was higher than that of an off-center delamination. The shear deformation effect lowered the buckling loads and increased the energy-release rates. The expreiments on delaminated graphite/epoxy unidirectional specimens were also performed to determine the buckling load and to observe the growth characteristics. Results of the experiments showed good agreement with analytic results and there existed various postbuckling characteristics depending on the configuration of delamination.
Fracture Mechanics Applied to Glass Fibre/Epoxy Matrix Interface Characterization
Authors: Krawczak Patricia , Pabiot José
Published: April 2011

The possibility of the application of mode I and mode II fracture mechanics to the characterization of the glass fibre/epoxy matrix interface has been studied here. The aim is to find out a method much more sensitive and reliable than the commonly used methods such as single filament tests which could allow a separate analysis of cracks initiation and potion phenomena that are involved in the damage of composite structures. One original contribution of this comparative analysis on materials differing only in their fibre sizing is the attempt to use the whole resistance curve of the materials including the propagation phase by assuming that some differences in the amplitude of the well known fibre bridging phenomenon are directly induced by differences in interface quality, which is the only parameter that is not kept constant. Both mode I and mode II tests have revealed a high sensitivity to the fibre/matrix interface quality: According to the property which is considered, variations are noted in a range of 150% to 300% for a modification of the interface by different sizing and in a range of 150% to 700% for an artificial modification of the interface by hydrothermal aging. Nevertheless mode II tests offered rather limited information (only about cracks initiation). On the other hand the interest of mode I fracture mechanics as fibre/matrix interface characterization method has appeared, in so far as this technique makes it possible to separate cracks initiation and then cracks extension phenomena, on which the fibre/matrix interface quality has a specific incidence, that is translated by variations of cracks initiation and propagation energies, of cracks growth rates and of acoustic emissions.
Effect of Treatment Conditions on the Mode I Delamination Fracture Toughness of Sandwich Structures
Authors: Papanicolaou G. C. , Bakos D.
Published: April 2011

The mode I interlaminar fracture behaviour of aluminum-composite sandwich structures has been investigated. Double cantilever beam (DCB) specimens have been used for the determination of G Ic-values. Various different skin treatments were used. The data obtained from these tests have been analysed using several different analytical approaches found in literature and compared with respective G Ic-values derived from a simple analytical method developed by the authors. The original contribution of the present method when compared to all the existing methods is that a variable correction factor for G Ic as well as for G I(prop) describing crack front propagation history is defined. Most important is the fact that the method applies to all types of mechanical behaviour such as linear elastic, nonlinear elastic and inelastic. The results obtained can be used for the optimum design of sandwich structures since suggestions for optimal skin treatment are presented.
A Hybrid Method to Determine Individual Stresses in Orthotropic Composites Using Only Measured Isochromatic Data
Authors: Hawong J. S. , Lin C. H. , Lin S. T. , Rhee J. , Rowlands R. E.
Published: April 2011

Ability to determine the individual stresses photoelastically in orthotropic composites from isochromatic data, but without employing isoclinics or isopachics, is developed and demonstrated. This new capability is achieved by combining isochromatic information, complex stress functions and numerical nonlinear least-squares concepts into a new and effective hybrid method for stress analyzing composite materials. Unlike previous methods of photo-orthotropic-elasticity, the present technique simultaneously smoothes the recorded isochromatic data, provides reliable boundary information, and separates the isochromatics into individual stress components throughout the area of interest. The method is illustrated by evaluating the three individual stresses throughout a region adjacent to a hole in a uniaxially loaded orthotropic composite plate from the measured isochromatics.
The Effect of Processing on the Underwater Explosion Shock Behaviour of GRP Laminates
Authors: Mouritz A. P.
Published: April 2011

This paper compares the impulse (shock) loading response of glass reinforced polymer (GRP) laminates fabricated by the wet hand lay-up and resin transfer moulding (RTM) techniques. The GRP prepared by the hand lay-up technique contained a large number of microstructural defects whereas the GRP fabricated by RTM was almost free of defects. The laminates were tested in a small scale underwater shock facility where they were exposed to increasing levels of shock loading produced by an underwater explosion. Regardless of the fabrication technique, the laminates developed some cracks in the polymer matrix and small delaminations along the polymer/glass fibre interfaces when tested under low shock loading conditions. The damage was caused by the deflection of the laminate under the pressure of the shock wave. Under high shock loading both laminates experienced more severe damage in the form of wide cracks in the polymer, large delaminations, and broken glass fibres, and this resulted in a significant reduction in the tensile and compressive strengths of the GRPs. However, the intensity of the shock wave needed to be significantly higher for the RTM laminate to suffer the same amount of damage as the hand lay-up laminate, proving that RTM produces a more shock resistant GRP laminate.
Calculation of Stresses in Crosslinking Polymers
Authors: Adolf Douglas , Martin James E.
Published: April 2011

A general constitutive model is presented for calculating the evolution of stresses in crosslinking polymers. The analysis is valid for thermo- and chemorheologically simple polymers and for time-dependent strain histories where the total strain at any time is not large, such that linear viscoelasticity applies. We identify the material parameters required and outline experimental and theoretical methods for their determination. This formalism is then used to calculate the development of cure stresses for an epoxy wherein the glass transition temperature exceeds the cure temperature during the reaction. Finally, we discuss practical methods of stress minimization through control of the cure cycle.
Secant Moduli of a Glass Bead-Reinforced Silicone Rubber Specimen
Authors: Chen Chao-Hsun , Cheng Chiang-Ho
Published: April 2011

The influence of volume-fraction of inclusions on the overall stress-strain of a rubber-matrix composite is investigated at the level of dilute concentration. The method developed is based on the approximate mean-field theory developed by Weng and coworkers for plasticity of two-phase composites (Tandon and Weng, 1988; Zhao and Weng, 1989; Qiu and Weng, 1992), using Berveiller-Zaoui's (1979) approximation approach. The constraint due to the matrix phase is characterized by the secant moduli of the matrix, while the interaction of the inclusion is accounted for by the Mori-Tanaka mean-field theory. It is shown that this simple, but approximate theory is capable of predicting the volume fraction dependence of the nonlinear stress-strain relation. An asymptotic solution for this composite system is obtained. By introducing a nonlinear stretch parameter λ m and a stress parameter λ m the stress-stretch master curve of the hardbead-reinforced silicone rubber composite is obtained. The theoretical predictions are in good agreement with the experimental results.
An Effective Crack Growth Model for Residual Strength Evaluation of Composite Laminates with Circular Holes
Authors: Afaghi-Khatibi Akbar , Ye Lin , Mat Yiu-Wing
Published: April 2011

A new effective crack growth model (ECGM) is developed to evaluate residual strength of composite laminates with circular holes. When the local normal stress reaches the tensile strength of the unnotched laminate, damage is assumed to initiate and it propagates with increase of the applied load. The damage is modelled by a fictitious crack with cohesive stress acting on the crack surfaces and the damage growth is simulated by extension of the fictitious crack and reduction of the cohesive stress with crack opening. The apparent fracture energy (G:) is used to define the relationship between the unnotched strength and the critical crack opening. Based on the equilibrium condition, an iterative technique is developed to evaluate the applied load required to produce the damage growth. The residual strength of notched composite laminates is defined by the unstable point of the applied load with damage growth. AS4-carbon/948A1 epoxy [0/90]4s, and [0/±45/90]2s, composite laminates were manufactured and residual strength tests were conducted for composite laminates with various hole diameters and specimen widths. Effect of the 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 predicted from the new model correlates well with experimental data for the different laminate configurations.
A Technique for Locally Increasing Surface Heat Spreading and through-Thickness Thermal Conductivity of Graphite/Epoxy Laminates
Authors: Roberts Jack C. , Luesse Mark H. , Magee Thomas C.
Published: April 2011

The polymer matrix composite through-thickness thermal conductivity is particularly important in applications such as composite spaceborne electronics enclosures where the heat dissipation is primarily dependent on thermal conduction to a heat sink. The spreading of heat at the composite surface and subsequent localized conduction in the through-thickness direction down to high thermal conductivity fiber may be the key to designing a lightweight, thermally efficient enclosure. A finite element model was constructed of a composite with heat applied to a central area. The laminate consisted of a hybrid of high thermal conductivity pitch fiber/epoxy on the outside surfaces interlaminated with low thermal conductivity carbon fabric/epoxy. Three configurations were modeled: (A) a heat source in the middle, (B) Cu plating under the central heat source and (C) Cu plating under the heat source with a centrally located hole that was also Cu plated. The model with Cu on the surface under the heat source had a maximum surface temperature 35% lower than the model with no Cu to spread the heat. The model with a central Cu plated hole had a maximum surface temperature 58% lower than that with no Cu plating on the surface. Therefore, the surface Cu plating with Cu plated hole spreads the heat and increases the through-thickness thermal conductivity. Samples were prepared using the aforementioned hybrid pitch fiber/epoxy interlaminated with carbon fabric/epoxy laminates sandwiched between .015 mm thick Cu foil. In the fabricated samples, all Cu (except that in the immediate vicinity of the heat source) would be removed by etching. A drill size of 2.29 mm diameter and feed rate of 152 cm/min were selected to minimize pitch fiber damage and matrix smearing, which results in increased thermal resistivity. Some of the drilled holes were etched with sulfuric acid and Cu plated and some were just cleaned and Cu plated. SEM photomicrographs of the holes and of the hole edges showed no appreciable increase in Cu adherence of the etched over the unetched samples. The unetched samples appeared to have as intimate contact with the Cu as the etched samples. The etched samples had areas with thicker epoxy layers insulating the pitch fibers from the Cu. The co-cured surface Cu foil with and without a Cu plated hole to spread the heat and increase through-thickness thermal conductivity, respectively, are currently being incorporated into heat transfer test samples.
Thermo-Mechanical Regulation of Residual Stresses in Polymers and Polymer Composites
Authors: Kominar V.
Published: April 2011

Manufacturing processes of polymer and polymer composite products are inevitably linked with buildup of residual stresses which, as a rule, adversely affects the product performance. Mainly used annealing and suitable curing regime may decrease these disapproving effects. Methods of residual stress control in hardened articles using dependence of polymer visco-elastic properties on temperature are considered and some basic features of their efficient application are established. The methods are successfully applied (a) to increase the first crack strain in crossply composite, (b) to enhance "weepage" pressure in glassfiber reinforced polymer pipes and (c) to reduce a central hole influence on ultimate tensile strength in standard polystyrene specimens.
Controlling Thermal Stresses in Composites by Means of Fiber Prestress
Authors: Tuttle M. E. , Koehler R. T. , Keren D.
Published: April 2011

A new method of applying a prestress to the fibers in composite laminates prior to and during cure is described, and several [0 P/90] T laminates produced at varying prestress levels are discussed. Experimental measurements described in this paper confirm that fiber prestressing can reduce or eliminate residual thermal stresses. This is evidenced by a reduction in the thermal warpage of unsymmetric [0 P/90] T laminates. An existing thermal analysis of unsymmetric laminates (developed by Hyer) is modified slightly in this paper to account for fiber prestressing, and is then used to predict out-of-plane deformations of the [0 P/90] T panels. Measurements compare well with theory. Specimens machined from the prestressed panels were also subjected to tensile loadings and inspected for matrix cracks in the 90° ply. It was found that fiber prestressing greatly reduced matrix cracking in [0 P/90] T specimens, but had little effect on [0/90 P] T specimens.
Analysis and Optimum Design of Composite Grid Structures
Authors: Chen Hong-Ji , Tsai Stephen W.
Published: April 2011

An integrated equivalent stiffness model is developed to describe a grid structure with or without laminate skins. The in-plane bending and shear of ribs are included without introducing any extra degree of freedom so that Mindlin's plate theory can still be adopted. As a result, the model can be directly incorporated with the existing FEM technique and take advantage of commercial codes. The proposed model is demonstrated to be efficient and reasonably accurate. If high precision is required, the equivalent stiffness model can be incorporated with the exact FEM modeling to obtain a refined stress analysis. Hygrothermal behavior and local buckling are also taken into account. Using the model as a tool, the performance of composite grid structures can be compared to traditional laminates, sandwich plates and metal grid structures. A method of optimum design is established to design a grid structure with or without laminated skins. Multiple loads and multiple failure mechanisms are considered. Discrete design is also included. Furthermore, the method can be easily incorporated with FEM to perform structural optimization. The optimization code is demonstrated to be a very powerful design tool. Finally, a user-friendly computer code is developed to perform the above tasks.
Progressive Failure Analysis of Laminated Composite Beams
Authors: Kim Youngchan , Davalos Julio F. , Barbero Ever J.
Published: April 2011

A progressive failure model for laminated composite beams is formulated using a beam finite element with layer-wise constant shear (BLCS), which permits accurate computation of stresses on each layer. This is the first study to incorporate the stress-prediction accuracy of a layer-wise element for failure prediction of laminates under bending loads. In the present formulation, based on material degradation factors and existing failure criteria, a linear elastic behavior is assumed, and a damaged layer in an element is substituted by a degraded homogeneous layer. Maximum Stress and Tsai-Wu failure criteria are used to assess failure at the Gauss points. The effect of damage accumulation is accounted for by degrading the stiffness properties of failed element-layers in the equilibrium iterations. After equilibrium is satisfied, the load is increased by a constant percentage of first-play-failure load in a load-controlled failure prediction. A displacement-controlled scheme is also implemented. The predictions of the model correlate well with experimental results for two distinct laminated composite beams: graphite-epoxy and glulam reinforced with GFRP. The study provides guidelines, through parametric studies, for the appropriate selection of material-degradation factors, load increments, and finite element mesh.
Micromechanical Modelling of SCS-6 Fibre Reinforced Ti-6A1-4V under Transverse Tension—Effect of Fibre Coating
Authors: Li Dong Sheng , Wisnom Michael R.
Published: April 2011

Finite element micromechanical modelling of SCS-6 SiC fibre reinforced Ti-6A1-4V has been performed. The fibres have a relatively thick coating consisting of pyrolytic carbon and SiC. The mechanical and thermal properties are estimated by considering both components, among which the properties of pyrolytic carbon are highly anisotropic. The effects of the low stiffness and highly anisotropic coating have been ignored in most other studies on the same material. However, it is shown that the coating has a significant effect on the composite thermal residual stresses, elastic transverse tensile properties and interface debonding. Methods of estimating the interface tensile strength are proposed. The transverse loading-unloading-reloading behaviour of this material is modelled and compared with experimental results. Possible ways of improving the transverse properties of SCS-6/Ti-6-4 unidirectional composite are suggested.
Mode I Interlaminar fracture Toughness and Fracture Mechanism of Angle-Ply Carbon/Nylon Laminates
Authors: Tohgo Keiichiro , Hirako Yutaka , Ishii Hitoshi , Sano Kazunori
Published: April 2011

Interlaminar fracture toughness and fracture mechanism in carbon fiber reinforced nylon laminates have been investigated. Mode I interlaminar fracture toughness tests are carried out on the double cantilever beam (DCB) specimens made of four kinds of angle-ply laminates. These laminates exhibit unstable "stick-slip" crack propagation. In order to obtain the continuous GI resistance curve ( GI- R curve) describing the stick-slip behavior, one modification is made to Kageyama's (1997) modified compliance method. From the continuous GI- R curves, it is found that the stable delamination is characterized by flat region of GIstable≈ 1200 J/m2 for all lay-ups except for 60° // -60° specimen which shows a rising G-R curve to final failure. The interlaminar fracture toughness of these composite laminates is considerably higher than that of carbon/epoxy laminates. Based on the continuous GC-R curves and the observation of the fracture surfaces, a mechanism is proposed in which the unstable stick-slip behavior is explained by the development of the large scale bridging of fiber-bundle or layer and its breakage.
Stress Singularity due to Kink Band Weakening a Unidirectional Composite under Compression
Authors: Chaudhuri Reaz A. , Xie Minsheng , Garala Himatlal J.
Published: April 2011

A Williams type of eigenfunction expansion approach is used to compute the local stress singularity representing a measure of the degree of inherent flaw sensitivity of unidirectional fiber reinforced composites subjected to compression. Previous experimental studies have qualitatively linked the formation of kink bands to the presence of fabrication defects, such as fiber misalignment. These local singular stress regions serve as the primary trigger mechanism for kink band propagation in 0°-plies. The present analysis also explains the previous test results relating to propagation of failure from a notch in a unidirectional composite under compression. Furthermore, the present investigation is the first to quantify, in the context of LEFM (linear elastic fracture mechanics), the sensitivity of these composites to inherent local flaws, such as fiber misalignments, and also shows the inadequacy of the conventional elastic micro-buckling type of analysis to fully explain the experimental results. Although because of the assumed isotropy of the fiber and matrix materials, the present study is primarily suited to glass fiber reinforced composites, the conclusions drawn here are general enough to apply to carbon fiber reinforced composites as well. Numerical results presented include the effects of fiber included wedge angle, and the ratios of fiber-matrix shear moduli and Poisson's ratios on the strengths of the mode I and mode II singularities. Of special practical interest is the present LEFM type of analysis applied to quantitatively investigate the inherent flaw sensitivity of two E-glass/epoxy composites experimentally investigated earlier. Compression fracture type of failure of these composites can be fully explained and quantified by the present two-dimensional LEFM-based method, which is beyond the scope of one-dimensional micro-buckling approach.
Longitudinal and Transverse Moduli and Strengths of Low Angle 3-D Braided Composites
Authors: Kalidindi Surya R. , Abusafieh Abdel
Published: April 2011

This paper presents the results of an experimental study conducted on a class of three-dimensional braided graphite-epoxy composites as well as the results of a modeling study to relate the microstructural variables such as volume fraction of fibers and braid angles to the anisotropic elastic moduli and strengths of these composites. In the experimental program, composite samples with a range of volume fractions (0.2-0.45) and a range of braid angles (0°-30°) were produced and tested in simple compression in both longitudinal and transverse directions. Such data is currently unavailable in literature on a single material system. Extreme care was taken in processing to minimize porosity and non-homogenieties in microstructure, and in proper strain gaging to ensure the reliability and reproducibility of our measurements. In the modeling study, the measured elastic moduli were compared against the currently employed isotrain and isostress models for these materials. At the present time, it is widely reported in literature that the predictions of isostrain model are fairly accurate. This study revealed that the isostrain model predictions can be in significant error and that a weighted average of the isostrain and isostress model predictions yielded the best comparisons against the experiments. For the strength of the composite in the longitudinal and transverse directions, correlations expressing these properties as a function of volume fraction and braid angle were motivated by micromechanics and experimental observations. The predictive capability of these correlations for the selected class of 3-D braided composites was found to be reasonable.
Three-Dimensional Effective Moduli of Symmetric Laminates
Authors: Chen Hong-Ji , Tsai Stephen W.
Published: April 2011

Closed-form expressions of three-dimensional effective moduli of symmetric laminates, which are based on the assumption of constant in-plane strains and constant interlaminar stresses through the thickness, had been proposed by Chou and Carleone (1972) and later re-proposed by Sun and Li (1988). This article modified the effective interlaminar shear moduli to account for the effect of stacking sequence by assuming the parabolic distribution of interlaminar shear stresses through the thickness. In this article, the effective three-dimensional moduli are also re-presented in such forms that it could be seen that several particular forms of the rule of mixture apply for individual stiffness or compliance components. As a result, four stiffness invariants and one compliance invariant are developed. The invariance is helpful in checking the reliability of measurements. The effective moduli predicted by this article are found to be in good agreement with the experimental data measured by ultrasonic methods.
Influence of Void Shape, Void Volume and Matrix Anisotropy on Effective Thermal Conductivity of a Three-Phase Composite
Authors: Krach Ann , Advani Suresh G.
Published: April 2011

By using high conductivity carbon fibers in a graphitizable carbon matrix, very high values for the thermal conductivity of a composite can be obtained in the direction parallel to the fibers. The carbon matrix of this material exhibits anisotropy and is oriented in the direction of the fibers. Thermal conductivity of the matrix material is of the same order as that of the carbon fibers. The processing of such materials invariably introduces voids up to 25% giving rise to a three phase system of fibers, matrix and voids. The void volume, shape and distribution will have a significant impact on the overall or effective thermal conductivity of the composite. Current empirical or simplified models do not show clearly this effect and experimental measurements are non-existent. Hence, we have adopted a numerical approach consisting of a unit cell to explore the influence of void volume and shape on the effective conductivity of a unidirectional sample.

The effective conductivity of a composite consisting of spherical, oval and crack-type voids up to 25% in the direction transverse and parallel to the fiber is calculated and compared with electric circuit analogies and a generalized dispersion formulation for effective thermal conductivity. The results clearly show that the effect of porosity on thermal conductivity cannot be described solely by void volume. The shape and the distribution of the voids affect the effective thermal conductivity which is not captured by any existing models. The finite element models developed should prove useful for the estimation of effective thermal conductivity and hence provide a tool for evaluating analytical continuum models.
On a Technique for Generating Stress-Strain Curves from Flexural Data
Authors: Zhang Yi , Sikarskie David L.
Published: April 2011

A technique is developed in this paper to utilize a four-point-bend test for piecewise construction of the stress-strain curve for the outer fibers (tension and compression) of the sample. The generated flexural stress-strain curves are compared to uniaxial tension and compression stress-strain curves. Initial agreement is excellent while in the latter stages the flexural curves are stiffer and exhibit significantly higher failure stresses. This technique fully allows for the material softening (nonlinear material behavior) due to the onset of damage well below ultimate fracture as well as different stress-strain behavior in tension and compression. The potential importance of this technique in providing corrected estimates of both tensile and compressive elastic moduli and failure stress, cf. ASTM D 790-91, is discussed.
3-D Finite Element Analysis of Curved Beams in Bending
Authors: Wisnom Michael R.
Published: April 2011

The effect of anticlastic curvature in pure bending of curved composite beams is investigated by finite element analysis. A full 3-D model of a slice through the beam is analysed, and also a 2-D model assuming generalised plane strain. It is shown that due to the curvature, there is restraint on anticlastic bending, giving rise to a significant variation of stresses across the width. The commonly assumed state of plane stress is therefore not strictly valid even for relatively narrow specimens. There can be significant discrepancies in fibre direction stresses and strains compared with those calculated on the basis of plane stress. As the width increases, the results at the centre of the beam approach those from the generalised plane strain analysis. This gives the same values of interlaminar tensile stresses as from the plane stress solution, but there are still differences in fibre direction strains. The results show the need for caution in interpreting strain gauge results on curved beams.
The Effect of Surface Tension on Tow Impregnation of Unidirectional Fibrous Preform in Resin Transfer Molding
Authors: Young Wen-Bin
Published: April 2011

Resin transfer molding (RTM) is an efficient technique for fabrication of large polymer composite parts with complicated shapes. The process consists of injecting liquid resin into a mold cavity with preplaced fiber preform. Voids trapped in the composite during the filling stage were believed to be the consequences of the non-uniform micro- and macro-flows in the pore spaces. A two-dimensional flow model is developed to study the impregnation flow in both the micro- and macro-regions during the mold filling process. The capillary force is included in the model to investigate its effect on the micro- and macro-flows. The capillary force is the major cause that results in the leading of the micro-flow for a low filling rate, that cannot be ignored if the analysis of the void formation is to be performed. It is found that mold filling process with capillary number near the value of the critical capillary number is an adequate condition to result in a uniform filling pattern in RTM.
The Influence of Stamping Temperature on the Properties of a Glass Mat Thermoplastic Composite
Authors: Cantwell W. J.
Published: April 2011

The effect of varying the mold temperature on the mechanical properties of a stampable thermoplastic composite has been examined. A series of tensile, flexure and impact tests have shown that a distinct mold temperature dependency exists. This is most evident under impact loading conditions where it has been shown that the energy absorbing capacity of a material stamped at a low temperature is almost twenty-five percent lower than that of a similar composite molded at a much higher temperature. It is concluded that residual stresses resulting from the higher cooling rate may be in part responsible for this significant difference in impact resistance. It is also possible that greater damage occurred during the flow process in the lower temperature mold. A subsequent heat treatment cycle designed to simulate the passage of a component through a paint oven has been shown to improve quite significantly the energy absorbing capability of the material.
Bearing Failure of Bolted Composite Joints. Part I: Experimental Characterization
Authors: Wang Hong-Sheng , Hung Chang-Li , Chang Fu-Kuo
Published: April 2011

An investigation was performed to study the bearing failure of mechanically fastened fiber-reinforced laminated composite joints. Only double-lap metal/composite/metal bolted joints were investigated. The results of this study will be presented in two parts: experimental characterization and analytical prediction. This paper summarizes the experimental work.

The major focus of the experiments was to characterize the bearing failure mechanism and mechanics, and to evaluate the effect of clamping pressure on the bearing response and bearing strength of bolted joints. The bearing damage was characterized either as pure bearing, which had no lateral supports, or as bolt bearing, which contained lateral supports with various degrees of clamping pressure.

A specially designed semi-circular notched specimen was proposed to characterize pure bearing damage. For bolt bearing, a load cell was designed and manufactured which was mounted on the fastener to monitor the bolt clamping pressure as a function of the applied load. T800/3900-2 graphite/epoxy prepregs were selected to fabricate the specimens. All the specimens were x-rayed and sliced at different load levels to examine internal damage.

Based on the experiments, it could be concluded that lateral support is crucial for bolted laminated composite joints. Bearing damage can be catastrophic if there is no lateral support. Shear cracks induced by accumulated compression failure appeared to be the primary failure mode of the bearing damage. Lateral supports could suppress the shear crack propagation and change failure from a catastrophic to a progressive failure mode. Clamping pressure could increase bearing strength.
Strength Envelope of Bolted Composite Joints under Bypass Loads
Authors: Hung Chang-Li , Chang Fu-Kuo
Published: April 2011

An investigation was performed to study the strength and failure of bolted composite joints subjected to bypass loads. The damage accumulation models developed previously by the authors were implemented in a nonlinear finite element code, designated "BJSTAT," for predicting failure and response of bolted composite joints.

Utilizing the code, numerical simulations were conducted to generate the strength envelope for a bolted composite joint under multi-axial bypass loads. The predictions of bolted joints subjected to an axial bypass load were compared with available experimental data. The prediction agreed with the data very well. The envelope can provide information on the strength as well as the failure type of the joint as functions of the bypass loads and, therefore, is very useful in joint design.
Dry Spot Formation and Changes in Liquid Composite Molding: I—Experimental
Authors: Han K. , Lee L. J.
Published: April 2011

Dry spot formation and changes in liquid composite molding (LCM) were investigated by flow visualization experiments. The dry spot size can be reduced by three mechanisms: (1) it can be compressed by the hydrostatic pressure from the surrounding saturated region; (2) it can be reduced by bleeding trapped air into the surrounding saturated region if there is a pressure gradient in the saturated region; and (3) it may gradually disappear by the wicking flow due to the capillary pressure difference between the saturated region and the dry fiber region. Experiments of packing and bleeding, a technique used in industry to eliminate dry spots, were also carried out and the results are explained based on the three types of dry spot changes.
A Comprehensive Model to Predict the Stress Fields in a Single Fibre Composite
Authors: Tripathi Devesh , Chen Fangping , Jones Frank R.
Published: April 2011

Recently, several studies have concluded that the variational method of Nairn is the best among the available analytical models for predicting the stress fields in and around an isolated short fibre embedded in a polymer matrix. However, the variational method is not exact and has several limitations for its application as a data reduction tool for the fragmentation test. The main limitations of the variational model arise because matrix plasticity, debonding and frictional effects which are observed during fragmentation cannot be included. A comprehensive model, known as the plasticity effect model, has been developed which incorporates these aspects into the variational model. It has been argued that the interfacial debonding and shear yielding of the matrix can co-exist in a single fibre composite, thereby, requiring a three zone stress transfer model which can predict the stresses in the bonded and debonded regions along with the shear yielded region of the matrix at and near the fibre-end. Thus, the plasticity effect model is an improvement over the partial debonding model of Piggott which considers stress transfer in the bonded and debonded region; ignoring shear yielding of the matrix at the end of the fibre. The predictions of the plasticity effect model are compared with those from an elastic-plastic finite element model. A glass fibre/epoxy system is used to evaluate the features of the model. The plasticity effect model can be used to analyse the results of laser Raman spectroscopy on single short or continuous-fibre model composites and to develop a data reduction technique to measure fibre-matrix adhesion from the fragmentation test.
Durability of Closed-End Pressurized GRP Pipes under Hygrothermal Conditions. Part I: Monotonic Tests
Authors: Ghorbel I. , Spiteri P.
Published: April 2011

Environmental factors are of extreme importance with respect to the ser-vice life of composite components based on polymeric engineering matrices. Part I deals with the role of matrix ductility and chemical environmental degradation on the composites mechanical properties. Part II is concerned with the lifetime prediction of GRP filament wound pipes under low pressure levels and hygrothermal conditions. There composites are an E glass fiber reinforced polyester resin and an ECR glass fiber reinforced vinylester resin.

The mechanical behavior of the pipes before and after exposure to water for different duration is studied through internal pressurizing tests with closed end procedure. The results indicate that in the hoop direction, fibers carry the complete load while in the axial direction there is a substantial contribution from the matrix. The use of elasticity theory appears as an appropriate method to describe the mechanical behavior of the pressurized pipes under monotonic loading conditions. In the hoop direction the behavior is governed by the fibers one whereas in the axial direction damage must be accounted for in the elastic law.

Chain scission effects on the mechanical properties are investigated. Experimental results show that the mechanical properties, the damage mechanisms and the failure mode of the pipes are changed by water uptake. Relations between the mechanical properties and the exposure duration based on physical considerations are assessed, allowing for the definition of a damage parameter related to hygrothermal degradation. Environmental effects (swelling and chain scission) mainly affect the yielding pressure and the pressure to failure, whereas load condition and initial defects affect the axial modulus. This leads to the definition of two independent damage parameters.
Durability of Closed-End Pressurized GRP Filament Wound Pipes under Hygrothermal Aging Conditions. Part II: Creep Tests
Authors: Ghorbel I.
Published: April 2011

The creep and damage behavior of GRP filament wound pipes are investigated as a function of the resin ductility.

To predict the life time of pipes under low pressure levels and hygrothermal conditions, different methods based on the experimental data and approximations have been used. All these methods involve assumptions that have been discussed and examined carefully.

Under creep tests the response of preconditioned pipes by immersion in water at 60°C follows a nonlinear viscoelastic behavior. A damage creep law is, also, assessed followed by microscopic observations to identify the creep damage mechanisms. Two stages are observed: the first describes the initiation of cracks and the second is related to the crack's propagation. Decohesions between the mechanical part of the pipe and the liner followed by microcracking of the liner lead to the failure. Also, the pipes' life is governed by the critical creep strain to failure of the liner. From microscopic observations and physicochemical analysis it appears that pipes' failure results for low pressure levels from mechanical damage and chemical bond rupture, otherwise cracking and delamination lead to the failure.

Experimental data show that the use of the creep damage law give the more realistic creep lifetime although this criterion is the most pessimistic one. Predictions based on the Eyring-Zhurkov approach and the nonlinear viscoelastic creep law over-estimate the lifetime because assumptions are not appropriate. Moreover it appears using both microscopic observations and extensometric measurements, that the lifetime, the damage kinetics and the failure mechanisms of the pressurized pipes depend strongly on the matrix ductility.
Influence of Thermal Aging on the Damping Properties of an Acrylic-Cored Laminated Steel
Authors: Wang P. C. , Fridrich Richard J.
Published: April 2011

The concern about the thermal stability of polymer materials has led to the studies of the damping properties of polymer-cored laminated steels used for reducing vehicle structure-borne noise and engine noise. While there have been a number of studies concerning the thermal stability of polymer-cored vibration damping laminated steels, very little information is available concerning factors that influence the damping properties of laminated steels after paint bake cycle [Electrophoretic Priming Operation (ELPO) baking oven]. In this paper, results from damping property measurements of an acrylic-cored laminated steel aged for varying times at 180°C (ELPO oven) are presented, and factors contributing to their property degradation are identified.
Solid Particle Erosion Behavior of Metal Matrix Composites
Authors: Miyazaki N. , Funakura S.
Published: April 2011

Phenomenological studies on the erosion of metal matrix composites (MMCs) have not been done enough. So experiments were carried out to clarify the effects of fiber volume fraction, type of reinforcement fibers and impact angle on the solid particle erosion behavior of MMCs. Three kinds of MMCs were used for the erosion experiments. They were the aluminum 6061 alloy reinforced by βSi3N4 whiskers, the aluminum 1070 alloy unidirectionally reinforced by Si-Ti-C-O fibers and the pure aluminum reinforced by SiC plain-woven cloths. In the case where the sizes of reinforcement fibers are much smaller than those of particles impacted, almost all the fibers removed from a MMC are included in a matrix material removed by the impact of particles. In this case, the difference in erosion rate is small between a MMC and the corresponding matrix material. In the case where the reinforcement fibers are included in the form of unidirectionally long fibers or cloths, a lot of fibers are removed from an eroded surface of a MMC. In this case, a MMC has much larger erosion rate than the corresponding matrix material.
Effects of Layer Nesting on Compression-Loaded 2-D Woven Textile Composites
Authors: Breiling Kurtis B. , Adams Daniel O'Hare
Published: April 2011

Layer nesting refers to the interaction between neighboring fabric layers of a textile composite laminate. Idealized layer nesting configurations were investigated in five-harness satin weave carbon/epoxy laminates under static compression loading. A methodology was developed to fabricate three idealized nesting cases: stacked, split-span, and diagonal. All three idealized nesting cases produced reductions in compressive strength and ultimate strain when compared to the conventional randomly-nested laminates. The diagonal nesting geometry produced the largest strength reductions. Finite element results showed consistent strength reduction trends for the idealized nesting cases; however, the magnitudes of compressive strengths were overpredicted. These results suggest that regions of idealized nesting in conventional woven composites, particularly diagonal nesting, may be sites of failure initiation. Additionally, differences in compressive strength associated with idealized nesting configurations may explain the considerable scatter observed in compressive strength for these materials.
Failure Behavior of Plain Weave Fabric Laminates under On-Axis Uniaxial Tensile Loading: I—Analytical Predictions
Authors: Naik N. K. , Ganesh V. K.
Published: April 2011

A two-dimensional woven fabric composite strength model is presented for the prediction of failure strength of two-dimensional orthogonal plain weave fabric laminates under on-axis uniaxial static tensile loading. Different stages of failure such as warp strand transverse failure, fill strand shear/transverse failure, pure matrix block failure and the failure of matrix and fiber in the fill strand in longitudinal tension are considered. Material and geometrical nonlinearities have been considered for predicting the stress-strain behavior. The studies were carried out for three idealized laminate configurations. The possible shift of layers with respect to each other along x-, Y- and -directions were considered for the laminates. A comprehensive experimental program was carried out on different material systems. A very good correlation is observed between the predicted and the experimental results.