Showing papers in "Journal of Composite Materials in 1986"
TL;DR: In this article, an end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading and a simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented.
Abstract: The end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading. A simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented. Results indicate that interlaminar shear effects may be significant for tough resin systems requiring large thickness-to-length geometries. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.
436 citations
TL;DR: In this paper, a new concept called fatigue modulus, defined as a slope of applied stress and resultant strain at a specific cycle is introduced, which is used to predict the fatigue life of glass fiber reinforced epoxy composite materials better than S-N curve or Basquin's relation.
Abstract: Fatigue behavior of glass fiber reinforced epoxy composite materials has been studied analytically. A new concept called "fatigue modulus," which is defined as a slope of applied stress and resultant strain at a specific cycle is introduced. Fatigue modulus degradation is studied using an assumption that the fatigue modulus degrada tion rate follows a power function of fatigue cycle. Theoretical equation for predicting fatigue life is formulated using the fatigue modulus and its degradation rate. This rela tion is simplified by strain failure criterion for the practical application. It is proved that the final formula predicts the fatigue life of a glass fiber epoxy composite material better than S-N curve or Basquin's relation. An attempt is made to find the relation ship between fatigue modulus and elastic modulus by the geometric relation from stress-strain curve under the cyclic loading.
319 citations
TL;DR: In this article, the elastic properties of three-dimensional textile structural composites were analyzed based on the classical laminated plate theory, and the effective in-plane elastic properties were derived.
Abstract: This paper describes a methodology for predicting the elastic properties of three- dimensional textile structural composites. A "Fiber Inclination Model" is established, which treats the unit cell of a composite as an assemblage of inclined unidirectional laminae. The analysis is based upon the classical laminated plate theory. The effective in-plane elastic properties are derived; comparisons between theoretical predictions and experimental data are also presented.
309 citations
TL;DR: In this paper, three different cumulative damage models were defined using several physical variables such as fatigue modulus and resultant strain, and the proposed models were derived as functions of nor malized applied stress level, r, and number of fatigue cycle, n.
Abstract: Cumulative damage during fatigue is studied analytically. Extensive reviews are per formed on the published damage models. Three different cumulative damage models are defined using several physical variables such as fatigue modulus and resultant strain. Proposed model I is defined using fatigue modulus, while models II and III are defined using resultant strains. Proposed models are derived as functions of nor malized applied stress level, r, and number of fatigue cycle, n. It is verified that the proposed cumulative damage model III has better agreement with the two stress level fatigue experimental data than other models.
257 citations
TL;DR: In this article, a static crushing test was conducted on graphite and Kevlar reinforced epoxy tubes to examine the influence of specimen geometry on the energy absorption capability of composite materials.
Abstract: Static crushing tests were conducted on graphite and Kevlar reinforced epoxy tubes to examine the influence of specimen geometry on the energy absorption capability of composite materials. Tube inside diameter to wall thickness (D/t) ratio was determined to significantly affect the energy absorption capability of composite materials. As D/t ratio decreases, the energy absorption capability increases nonlinearly. The energy absorption capability of K/E tubes was found to be geometrically scalable but energy absorption of Gr/E tubes was not geometrically scalable.
166 citations
TL;DR: In this article, a damage zone analysis based on more fundamental concepts is used to predict fracture of laminates with circular holes of various radii, and oval and rectangular holes of different sizes.
Abstract: The methods currently used for predicting static fracture of notched composite laminates in tension, such as the point and average stress criteria and the inherent flaw criterion, are of semi-empirical nature and have limited applicability with respect to size and shape of the notch. In this paper, a damage zone analysis, based on more fundamental concepts, is used to predict fracture of laminates with circular holes of various radii, and oval and rectangular holes of various sizes. The damage is represented by a linear cohesive zone. Based on the two fundamental parameters unnotched tensile strength (σ0) and apparent fracture energy (G*c), this model excellently predicts the strength of notched laminates for a number of specimens tested.
144 citations
TL;DR: In this article, the influence of fiber and matrix maximum strain at failure on the energy absorption capability of graphite reinforced composite material was examined and the results indicated that to achieve maximum energy absorption from a composite material a matrix material that has a higher strain at fault than the fiber reinforcement should be used.
Abstract: Static crushing tests were conducted on graphite composite tubes to examine the influence of fiber and matrix maximum strain at failure on the energy absorption capability of graphite reinforced composite material. Fiber and matrix maximum strain at failure were determined to significantly effect energy absorption. The higher strain at failure composite material system, AS-4/5245, exhibited superior energy absorption capability compared to AS-4/934, T300/5245 or T300/934 composite material. Results of this investigation suggest that to achieve maximum energy absorption from a composite material a matrix material that has a higher strain at failure than the fiber reinforcement should be used.
132 citations
TL;DR: In this article, an effort has been made to develop bamboo-fibre reinforced plastic (BFRP) composites using a simple casting technique and these composites have been tested for tensile strength, impact strength and Young's modulus of elasticity.
Abstract: Many researchers in the past have developed composites with natural fibres such as of sisal, henequen, jute and palm; but the work on the bamboo-fibre reinforced com posites is not available in the literature published so far.In the present work, an effort has been made to develop bamboo-fibre reinforced plastic (BFRP) composites using a simple casting technique. These composites have been tested for tensile strength, impact strength and Young's modulus of elasticity. It has been observed that these composites possess high strength and ductility and are useful for many structural applications. The ultimate tensile strength of some BFRP composites is more or less equal to the ultimate tensile strength of mild steel, while their density is approximately one eighth of the density of mild steel. The mechanical behaviour of these composites is similar to other commonly used composites such as Glass fibre reinforced plastics.
128 citations
TL;DR: In this article, an analytical procedure is presented for predicting the loss in integrity of composite structures subjected to simultaneous intense heating and applied mechanical loads, which considers the effects of surface ablation, re-irradiation losses, and temperature dependent thermophysical properties.
Abstract: An analytical procedure is presented for predicting the loss in integrity of composite structures subjected to simultaneous intense heating and applied mechanical loads An in tegral part of the method is a nonlinear, two-dimensional, finite difference thermal analysis which considers the effects of surface ablation, re-irradiation losses, and temperature-dependent thermophysical properties Another important feature of the structural survivability model is a flat-plate finite element code, based on the Mindlin theory, which is coupled to a maximum stress failure criterion Predictions from the analysis methodology are compared with experimental results obtained on 24, 48, and 96 ply graphite epoxy tension specimens which were spot-irradiated at various intensity levels
122 citations
TL;DR: In this paper, the finite element analysis of laminated anisotropic composite thin shells including imperfections is presented, and the derivations of the nonlinear geomagnetic equations are derived.
Abstract: Formulations and computational procedures are presented for the finite element analysis of laminated anisotropic composite thin shells including imperfections. The derivations of the nonlinear geom...
96 citations
TL;DR: In this paper, a variational model for a laminated plate consisting of an arbitrary number of fiber-reinforced composite material layers has been developed, using the variational principles.
Abstract: Governing equations of motion for a laminated plate consisting of an arbitrary number of fiber-reinforced composite material layers have been developed, using the variational principles. Each layer was considered to be of an orthotropic material with its directional elastic properties depending on the fiber orientation. The extension, bending, inplane shear, and transverse shear deformations in each separate layer were considered. The analytical results were verified with the literature-reported data. A study of the optimum fiber orientations in a criss-cross laminated plate has shown that different fiber orientations lead to the maximum frequency and the maximum damping. For a cross-ply laminated plate, the maximum flexural frequency ratio has been obtained for large aspect ratio plates and with large values of elastic modular ratio E11/E22. The maximum loss factor for a cross-ply plate was obtained with square plate and with small values of E11/E22. 15 references.
TL;DR: In this paper, a generalized Hele-Shaw model is used to predict flow and defor mation during mold filling, which is appropriate for sheet molding with fibers much longer than part thickness and a cold material molded in a hot mold.
Abstract: A method is presented for predicting the distribution of orientation of rigid short fibers in thin compression molded parts. The method extends Folgar and Tucker's model for fiber orientation in a concentrated suspension to the case of spatially non-uniform flows and orientation states. A generalized Hele-Shaw model is used to predict flow and defor mation during mold filling. Other assumptions are appropriate for sheet molding com pound : fibers much longer than the part thickness and a cold material molded in a hot mold. The predictions compare favorably to experiments on sheet molding compound and on a model suspension of nylon monofilaments in silicone oil.
TL;DR: In this article, a method called the Damage Zone Model (DZM) is used for predicting strength of composites with through-the-thickness cracks, based on the two fundamental parameters unnotched tensile strength (σ0) and apparent fracture energy (G*c).
Abstract: In this paper a method, called the Damage Zone Model (DZM), is used for predicting strength of composites with through-the-thickness cracks. The DZM is based on the two fundamental parameters unnotched tensile strength (σ0) and apparent fracture energy (G*c). The damage zone, developed at a notch in the composite, is modelled as a crack with cohesive forces acting on the crack surfaces. Redistribution of stresses and change in stiffness is accounted for in the model. For comparison, strengths are also calculated by semi-empirical methods such as the inherent flaw and the point stress criteria.Experimental results for three point bend (TPB), single edge notch (SEN) and compact tension (CT) quasi-isotropic carbon/epoxy specimens are presented. Some results for specimens made from randomly oriented short glass fiber/polyester specimens are also discussed. The damage zone model is shown to accurately predict fracture load, load- deformation behaviour and damage zone sizes in these types of laminates.
TL;DR: In this paper, prominent anisotropy in mechanical prop erties is observed in unidirectional CFRP, which has higher fracture strength and stiffness along the carbon fiber strengthening component.
Abstract: As a common feature of composite materials, prominent anisotropy in mechanical prop erties is observed in unidirectional CFRP, which has higher fracture strength and stiffness along the carbon fiber strengthening component. Since it is well known that a resin matrix exhibits characteristic time and temperature dependence on mechanical behavior, that is, viscoelastic behavior, the CFRP is expected to exhibit similar behavior.
TL;DR: In this paper, an analysis is presented for calculating the stresses and strains in bends made of fiber reinforced composites and for estimating the strength of bends, using the Tsai-Hill criterion for inplane failure and the Chang-Springer criterion for out-plane failure.
Abstract: An analysis is presented for calculating the stresses and strains in bends made of fiber reinforced composites and for estimating the strengths of bends. The stress field is calculated by a finite element method. The strength is predicted using the Tsai-Hill criterion for inplane failure and the Chang-Springer criterion for out of plane failure. Results are presented which illustrate the effects of geometry and ply orientation on the strength and on the mode of failure.
TL;DR: In this paper, an analysis was performed to evaluate the effect of the assumed pin load distribution on the calculated strength and predicted failure mode of pin loaded holes in laminated composites.
Abstract: An analysis was performed to evaluate the effect of the assumed pin load distribution on the calculated strength and predicted failure mode of pin loaded holes in laminated composites. The calculat...
TL;DR: An elasto-plastic finite element analysis of pin loaded joints in laminated composites has been investigated and comparisons made with both existing 2D linear elastic plane stress analytical solutions and experimental results for a graphite/epoxy laminate as discussed by the authors.
Abstract: An elasto-plastic finite element analysis of pin loaded joints in laminated composites has been investigated and comparisons made with both existing 2-D linear elastic plane stress analytical solutions and experimental results for a graphite/epoxy laminate. The finite element analysis included nonlinear material behavior after initial failure by assuming an elastic-perfectly plastic bimodular material model. Laminated plate theory was used to obtain lamina stresses and the Hill yield criterion applied in each layer to create a ply-by-ply failure analysis. The effect of including friction forces along the fastener hole interface on the stress distribution around the hole was also studied. Based upon the results, failure criteria are proposed for each of the basic failure modes, bearing, shearout and net-tension. Failure maps for each ply were developed to characterize the damage progression and identify critical failure strength and mode. For the [0i/±45j/90k] family of laminates the elasto-plastic finite ...
TL;DR: In this paper, the results of biaxial tension tests on AS4/3501-6 carbon/epoxy are presented for a quasi-isotropic [90, ± 45,0]s laminate.
Abstract: The results of biaxial tension tests on AS4/3501-6 carbon/epoxy are presented for a quasi-isotropic [90, ±45,0]s laminate. These tests were performed using a tubular spec imen subjected to internal pressure and axial tension. The specimen design appears to minimize stress concentrations in the gage section. The measured stress-strain response shows a small but definite reduction in stiffness associated with progressive matrix failure. The failure stresses and strains are consistent with a maximum fiber strain failure criterion, and are most accurately modeled with a progressive failure model that incor porates ply stiffness changes.
TL;DR: In this article, the authors used the Real-Time Ultrasonic C-San (RUSCAN) experimental facility and the Composite Durability Structural Analysis (CODSTRAN) computer code to predict the fracture progression.
Abstract: Refined models and procedures are described for determining progressive composite fracture in graphite/epoxy angleplied laminates. Unique Lewis Research Center capabilities are utilized including the Real-Time Ultrasonic C-San (RUSCAN) experimental facility and the Composite Durability Structural Analysis (CODSTRAN) computer code. CODSTRAN is used to predict the fracture progression based on composite mechanics, finite element stress analysis, and fracture criteria modules. The RUSCAN facility, CODSTRAN computer code, and scanning electron microscope are used to determine durability and identify failure mechanisms in graphite/epoxy coomposites. Results indicate that RUSCAN/CODSTRAN is an effective method of Studying progressive fracture of composites.
TL;DR: In this paper, the stability of delamination cracks in angle-ply laminate is investigated for uniform displacement and load loading, where a quasi-three-dimensional finite element method is used to obtain strain energy release rate.
Abstract: Delamination, a frequently observed failure mode in composite laminate, is a key issue in structural design and integrity consideration. In this paper, the stability of delamination crack is investigated for angle-ply laminates subjected to uniform displacement and ther mal loading. Due to the complexities of the problem, a quasi three-dimensional finite ele ment method is used to obtain strain energy release rate. The effect of parameters, such as temperature drop, loading type, stacking sequence, laminate width and laminate thick ness will be considered. Also, the effect of delamination crack on the tensile strength of angle-ply laminate for small fiber orientation is evaluated by using a rule of mixtures and energy release rate concept in classical fracture mechanics. Finally, failure mode and ten sile strength are predicted and compared with known experimental data to assess the pre diction accuracy of the delamination growth model.
TL;DR: In this article, the split Hopkinson bar was used to determine the stress, strain and strain rate response of woven graphite-epoxy laminates during high shear strain rate deformation.
Abstract: The split Hopkinson bar was used to determine the stress, strain and strain rate response of woven graphite-epoxy laminates during high shear strain rate deformation. The specimens were oriented and tested in both interlaminar shear and in transverse shear at strain rates ranging from 6000 to 18000 sec-1. The results show that the strain rate effect in interlaminar shear is small. The fracture surface is characterized primarily by interfacial failure between the fibers and the matrix. In transverse shear, however, the strain rate effect is significant due to the greater role played by the resin matrix in the deformation process. In both cases, at the high strain rates imposed in this investigation, sparks were produced by the specimens during deformation. These are believed to be caused by the combustion of fragments resulting from the high rate deformation.
TL;DR: In this article, the width tapered double cantilever beam test and double torsion test were used to evaluate the fracture toughness of composite composites and the results indicated that the two failure modes have similar failure mechanisms.
Abstract: Delamination and transverse cracking failure modes of composites have been char acterized by using the width tapered double cantilever beam test and the double torsion test, respectively. The fracture toughness GIC values obtained from the two tests are found to be quite comparable. This indicates that the two failure modes have similar failure mechanisms. The phenomenon of fibers bridging between crack surfaces observed in the tests is also discussed for its relevance to the laminate fracture.
TL;DR: In this paper, the effect of anisotropy and biaxial, far field, loading on the direction of crack growth in unidirectional off-axis composite materials is investigated.
Abstract: The purpose of this study is to gain a better understanding of the parameters affecting crack growth direction in unidirectional composite materials. To achieve this, the effect of anisotropy and biaxial, far field, loading on the direction of crack growth in unidirectional off-axis composite materials is investigated. Specific emphasis is placed on defining the crack-tip-stress field and finding a consistent criterion for predicting the direction of crack growth. An anisotropic crack-tip-stress analysis was implemented using three criteria (the normal stress ratio theory, the tensor polynomial failure criterion, and the strain energy density theory) to predict the direction of crack extension in unidirectional off-axis graphite-epoxy. The theoretically predicted crack extension directions were then compared with experimental results. It was determined that only the normal stress-ratio criterion correctly predicts the direction of crack extension.
TL;DR: In this article, the authors used T300/934 graphite-epoxy that was subjected to 1.0 MeV electron radiation at a rate of 5.0 x 10 to 7th rads/hour, simulating a worst-case exposure equivalent to 30 years in space.
Abstract: This investigation of composite material properties utilized T300/934 graphite-epoxy that was subjected to 1.0 MeV electron radiation for a total dose of 1.0 x 10 to the 10th rads at a rate of 5.0 x 10 to the 7th rads/hour, simulating a worst-case exposure equivalent to 30 years in space. Mechanical testing was performed on 4-ply unidirectional laminates over the temperature range of -250 F (116 K) to +250 F (394 K). In-plane elastic tensile and shear properties as well as strength were obtained. The results show that electron radiation degrades the epoxy matrix and produces products that volatilize at the temperatures considered. These degradation products plasticize the epoxy at elevated temperatures and embrittle it at low temperatures, thereby altering the mechanical properties of the composite.
TL;DR: In this paper, a compact finite element formulation based on singularity transformation is presented and is used to analyze the stress singularity at the boundary layer of an interface between adjacent layers in a laminated composite.
Abstract: A compact finite element formulation based on singularity transformation is presented and is used to analyze the stress singularity at the boundary-layer of an interface between adjacent layers in a laminated composite. The composite is subjected to uniform axial ex tension in the plane of the layers. For illustration, results from few elements are shown to demonstrate the accuracy and efficiency of the present approach. Excellent agreement between the present results and the analytical solutions is obtained.
TL;DR: In this article, acoustic emissions were monitored during single filament tension tests of Kevlar 49 fiber bundles, and the results of these later tests were carried out in such a way that the individual filament breaks could be independently verified.
Abstract: Acoustic Emission (AE) was monitored during single filament tension tests of Kevlar 49 fiber. AE was also monitored during dry and lubricated bundle tests of the same material. These later tests were carried out in such a way that the individual filament breaks could be independently verified. Statistical studies were made of the AE event characterization parameters for the source mechanism of filament failure. Studies of effect of cumulative damage, test fixturing, first vs. second AE sensor hit, and friction between fibers were made. Characterization by AE events made up of multiple filament fractures was examined relative to events due to single filament fracture in a bundle. These results are preliminary studies prior to using AE to study fundamental variables which control damage progression in fiber composites upon repeated loading.
TL;DR: In this article, an analysis for the preliminary design of boxbeams made of fiber-reinforced composites is presented, which provides deflections, bending and torsional stiffnesses, stresses, and the conditions for buckling and first-ply failure.
Abstract: This paper presents an analysis for the preliminary design of boxbeams made of fiber-reinforced composites. The analysis provides deflections, bending and torsional stiffnesses, stresses, and the conditions for buckling and first-ply failure. Use of the analysis is illustrated through the example of a single cell, three bay, cantilevered box beam. This example demonstrates a design procedure for selecting the configuration which results in the required strength and stiffness at a minimum weight.
TL;DR: In this article, the task of designing symmetric laminated beams with optimal stiffness and damping is considered by incorporating the damping model of Adams et al. into an optimization algorithm.
Abstract: The task of designing symmetric laminated beams with optimal stiffness and damping is considered by incorporating the damping model of Adams, et al. into an optimization algorithm. The design variables are the fiber orientation in each ply, the stacking se quence, and also the thickness of each ply in the laminate. A recursive quadratic program ming algorithm is employed for the optimization process and numerical results are presented for several test problems including the constraint condition of invariable flexural rigidity.
TL;DR: In this paper, the authors developed an efficient boundary element method (BEM) for use in the analysis of composite structures, principally loaded holes in mechanically fastened composites, and verified the validity of the method by computing the stress concentration factor in a double lap joint for different hole sizes and various boundary conditions.
Abstract: The purpose of this paper is to develop an efficient boundary element method (BEM) for use in the analysis of composite structures, principally loaded holes in mechanically fastened composites. The idea is to modify the BEM by an analytical scheme to evaluate boundary stresses and deformations accurately. The validity of the method is verified by computing the stress concentration factor in a double lap joint for different hole sizes and various boundary conditions.
TL;DR: In this article, the effects of cure and sizing on the bond strength between graphite fiber bundles and epoxy resins were evaluated, and the results indicated the influence of the cure temperature on the fracture strain and on the stiffness of the pure resins.
Abstract: Experiments were performed to evaluate the effects of cure and sizing on the bond strength between graphite fiber bundles and epoxy resins. Fiberite 976 and 934 resins and Thornel T300 (3K) fibers were used in the tests. The fibers were either uncoated (no siz ing) or coated with different types of sizing. Pure resin specimens and resin specimens containing a single coated or uncoated fiber bundle were prepared by curing in a mold at different temperatures. The specimens were subjected to three-point bending, and the load versus deflection curves were recorded. Photomicrographs of fiber bundles em bedded in the resin were also taken. The data show the influence of the cure temperature (and the corresponding degree of cure) on the fracture strain and on the stiffness of the pure resins. The data also indicate the effects of cure temperature and sizing on the fiber bundle-resin bond strength (as manifested by the specimen's fracture strain), and the effect of sizing on the geometry of the bundle and on the ...