Showing papers in "Journal of Composites Technology & Research in 1990"
TL;DR: In this paper, the effects of several physical properties defining the performance of the constituent fiber, matrix, and interface are examined relative to their effect on composite's behavior, and it is shown that the initial transverse modulus for the composite will be the same regardless of whether there is a well-bonded or an unbonded interface.
Abstract: The mechanics of transversely loaded high-temperature composites with a thermally induced residual stress field and a vanishingly weak fiber-matrix interface strength was investigated using two analytical models. In particular, the effects of several physical properties defining the performance of the constituent fiber, matrix, and interface are examined relative to their effect on composite's behavior. Both models demonstrate that, if there is a thermally induced residual stress field in the composite, the initial transverse modulus for the composite will be the same regardless of whether there is a well-bonded or an unbonded interface. 10 refs.
93 citations
TL;DR: In this article, an integrated micromechanics methodology for the prediction of damping capacity in fiber-reinforced polymer matrix unidirectional composites has been developed.
Abstract: An integrated micromechanics methodology for the prediction of damping capacity in fiber-reinforced polymer matrix unidirectional composites has been developed. Explicit micromechanics equations based on hysteretic damping are presented relating the on-axis damping capacities to the fiber and matrix properties and fiber volume ratio. The damping capacities of unidirectional composites subjected to off-axis loading are synthesized from on-axis damping values. Predicted values correlate satisfactorily with experimental measurements. The hygro-thermal effect on the damping performance of unidirectional composites caused by temperature and moisture variations is also modeled. The damping contributions from interfacial friction between broken fibers and matrix are incorporated. Finally, the temperature rise in continuously vibrating composite plies is estimated. Application examples illustrate the significance of various parameters on the damping performance of unidirectional and off-axis fiber reinforced composites.
83 citations
TL;DR: In this paper, a failure theory which combines classical lamination theory with free-edge interlaminar stress was employed to predict the strength of laminates with free edges, which was capable of predicting failure loads for [± θ]2s and [±θ/90]s laminate as well as delamination on-set loads for π/4 lamination.
Abstract: A failure theory which combines classical lamination theory with free-edge interlaminar stress was employed to predict the strength of laminates with free edges. Laminates studied were [±θ]2s angle-ply laminates, [±θ/90]s laminates, and fiber-dominated π/4 laminates. The present theory was capable of predicting failure loads for [±θ]2s and [±θ/90]s laminates as well as delamination on-set loads for π/4 laminates.
40 citations
TL;DR: In this paper, the room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured.
Abstract: The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained approx 30 volume fraction of aligned 142-micron diameter SiC fiber in a porous RBSN matrix. Three types of laminate studied were unidirectional: (1) (0) sub 8, (2) (10) sub 8, and (3) (45) sub 8, and (90) sub 8; cross plied laminates (0 sub 2/90 sub 2); and angle plied laminates: (+45 sub 2/-45 sub 2). Each laminate contained eight fiber plies. Results of the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane properties. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Splitting parallel to the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. In-plane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for (0 sub 2/90 sub 2)sub s and (+45/-45 sub 2) sub s laminates showed that their matrix failure strains were similar to that for (0) sub 8 laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle-ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.
38 citations
TL;DR: In this paper, a prepreg layer is applied on an identical layer of the same material that has already been cured and postcured, and the resulting warpage, consisting of elliptical equal deflection contours, is related to the chemical shrinkage.
Abstract: Residual stresses in composites (microscopic and macroscopic) result from shrinkage of the matrix during cure. It is usual to distinguish between the chemical and thermal components of shrinkage. The former depends on the polymerization reaction, whereas the latter is caused by the difference between the thermal expansion coefficients of the reinforcement and the matrix. A method was developed for determination of the chemical cure shrinkage. A prepreg layer is applied on an identical layer of the same material that has already been cured and postcured. The assembled laminate is then cured, and the resulting warpage is recorded by means of the shadow moire method. The resulting warpage, consisting of elliptical equal deflection contours, is related to the chemical shrinkage.
37 citations
TL;DR: In this paper, a broad range of geometric configurations, as far as the location of the delamination through the thickness, was considered, and both the initiation resistance and growth resistance were quantified for each configuration.
Abstract: Compression tests on delaminated kevlar/epoxy specimens were conducted in order to determine the buckling and post-buckling behavior of the system and observe the characteristics of the deformation including growth of the delamination. A broad range of geometric configurations, as far as the location of the delamination through the thickness, was considered. Both the initiation resistance, defined as the applied displacement per specimen length and the growth resistance, defined as the applied displacement per unit delamination growth during the postbuckling stage were quantified for each configuration. For the particular case studied, it was found that the growth resistance is infinite (that is, no growth) for delamination thickness/total thickness ratio H/T = 1/15, becoming 0.52 for H/T = 2/15 and dropping to a value of only 0.07 for H/T = 4/15. The initiation resistance is also lowered as the delamination is located further away from the specimen surface and for H/T = 4/15 growth initiation occurred before peak load. The experimental program investigates also the development of the deformation regarding the postbuckled shape, the load-displacement curve and the corresponding growth of the delamination. Furthermore, a comparison with analytical solutions for the postbuckling behavior at large applied displacements is performed.
29 citations
TL;DR: In this article, the Iosipescu and the asymmetric four-point bending (AFPB) methods have been proposed for the measurement of shear stress-strain behavior and shear strength of fiber reinforced composite materials.
Abstract: In recent years two notched beam flexure tests, the Iosipescu and the asymmetric four-point bending (AFPB) methods, have been proposed for the measurement of shear stress-strain behavior and shear strength of fiber reinforced composite materials. The methods have made significant inroads into the testing community for continuous, woven fabric and discontinuous fiber composites, and a test standard is being developed in ASTM Committee D-30. This paper describes the test methods and summarizes the results of a round-robin test program to define the within- and interlaboratory precision and bias.
28 citations
TL;DR: In this article, a new approach to the prediction of tensile strength after impact is presented based on the premises that the stress concentration associated with the constraint of the undamaged material around the impacted region dominates the remaining strength calculation and that the physical parameter that enters the model should be the size and shape of the damage region.
Abstract: A new approach to the prediction of tensile strength after impact is presented. The method is based on the premises that the stress concentration associated with the constraint of the undamaged material around the impacted region dominates the remaining strength calculation and that the physical parameter that enters the model should be the size and shape of the damage region. The damaged area resulting from impact was used to evaluate the reduction in the compliance properties of the inclusion. The predictions were made by using a closed-form solution for an infinite plate containing an elliptical inclusion and by invoking an average stress criterion applied at the ply level. The laminates studied were [0/±45/90]2s, [0/±450/0]2s, and [(0/90)4]s.
26 citations
TL;DR: In this paper, damage development under monotonic loading was studied in five cross-ply graphite/epoxy laminates, and three characteristic ranges of varying stiffness were found: linear elastic, decreasing stiffness and transverse matrix cracking in the 90° layer up to the characteristic damage state (CDS).
Abstract: Damage development under monotonic loading was studied in five cross-ply graphite/epoxy laminates. The laminates exhibited three characteristic ranges of varying stiffness. The first range is linear elastic and is characterized by the absence of any measurable damage. The second range is one of decreasing stiffness and corresponds to transverse matrix cracking in the 90° layer up to the characteristic damage state (CDS). The third range is a nearly linear one of stabilized or even slightly increasing stiffness. Ultimate failure was governed by the ultimate tensile strain in the 0° plies with small variations attributed to residual stresses, statistical scatter and local strain concentration.
25 citations
TL;DR: In this article, the residual strength of T300/934 graphite epoxy laminates, in tension and in compression, after the samples were exposed to tension-compression fatigue loading (R = -1), was measured.
Abstract: Results are presented on the measurements of the residual strengths of T300/934 graphite epoxy laminates, in tension and in compression, after the samples were exposed to tension-compression fatigue loading (R = -1). Four laminate ocnfigurations were tested: unidirectional, cross-ply, angle-ply, and quasi-isotropic. It was found that the fatigue behavior of laminates was dependent on the quasi-static strengths and the specific structure of the laminate. No direct correlation was found between remaining residual strengths and the percentage of average fatigue life. However, a correlation scheme was developed for the individual specimen under test, based on a cumulative damage model and a stiffness change of the material.
22 citations
TL;DR: In this article, a review of compression test methods for evaluating the ultimate compression strength of composite materials has been presented for the purpose of guidance in the future development of a standardized test methodology.
Abstract: Compression test methods for evaluating the ultimate compression strength of composite materials have been reviewed for the purpose of guidance in the future development of a standardized test methodology. Test considerations, including approximate specimen and test fixture configurations and loading configurations, have been examined to assess the state of the art for compression tests.
TL;DR: In this article, the behavior and prediction of unidirectional off-axis, angle ply, and [0/ ± θ]s laminates are discussed and a method for determining the characteristic dimension for the average stress criterion is presented.
Abstract: The notched tensile strength of composite laminates was examined. The behavior and prediction of unidirectional off-axis, angle ply, and [0/ ± θ]s laminates is discussed. It was found that good predictions for off-axis laminates can be obtained since the same failure mode occurs in such laminates. Similar results were also obtained for the [0/ ± θ]s laminates. However, for angle-ply laminates, accurate predictions were not achieved because of the diversity of failure modes among the configurations tested. For fiber dominated laminates, a method for determining the characteristic dimension for the average stress criterion is presented. Moreover, the notched strength of quasi-isotropic composite laminates loaded at an angle φ to the material x axis was investigated. Good correlation between experimental and predicted values was obtained.
TL;DR: Six different bolted joint analysis programs will be examined, and some numerical comparisons of the results will be made to help users determine which program will best suit their needs.
Abstract: Several different composite bolted joint analysis programs are now available to composite structures designers. All of the programs vary widely in the amount of input required, output received, and required user knowledge of composite materials. Six different bolted joint analysis programs will be examined. The various programs are (1) A4EJ, (2) BJSFM, (3) SASCJ, (4) SAMCJ, (5) SCAN, and (6) JOINT. Other analysis programs exist, but they apply many constraints on the problem definition and depend heavily on empirical test results. The relative merits of each program as well as their disadvantages will be discussed. The most appropriate applications of each of the programs will be presented, and some numerical comparisons of the results will be made. Each of the programs has a useful place in the design and analysis arena. An examination of the programs will be made to help users determine which program will best suit their needs.
TL;DR: In this article, the joint parameters considered were the edge margin in terms of the (e/d) ratio, the hole parameter, and the washer geometry and the lateral pitch.
Abstract: Bolted joints in SMC-R18 sheet molding compound composites were studied with the goal of determining joint proportions that provide long fatigue lives. The joint parameters considered were the edge margin in terms of the (e/d) ratio (where e is the edge margin and d is the hole parameter), the lateral pitch in terms of the (w/d) ratio (where w is the specimen width) and the washer geometry. It was observed that (e/d) should be at least three, and the lateral pitch should be at least six to eight to assure a fatigue failure originating at the edge of the washer. Smaller margins lead to bearing failures almost immediately or to splitting failures at relatively short fatigue lives. It was also observed that washer geometry (larger diameter) can lead to significantly longer fatigue lives.
TL;DR: In this article, computer generated plots of pullout load Pm versus embedded length l are used to identify important factors influencing the experimental results obtained in the single filament pull-out test.
Abstract: In this paper, computer generated plots of pull-out load Pm versus embedded length l are used to identify important factors influencing the experimental results obtained in the single filament pull-out test. The computer results suggest that random variation in the initial crack size is an important source of scatter in the experimental data. Furthermore, variation in fiber surface energy was computed to play a much smaller role than crack size variation in producing the observed data scatter. The results also suggest that the initial crack size corresponding to observed pull-out loads is surprisingly large. Finally, residual stress detrimental to the interfacial bond was identified as the factor causing an apparent displacement of the data points to the right, as observed experimentally.
TL;DR: In this paper, the authors address the need to create new test methods pertinent to automotive structural application and standardize the existing test methods, and present a list of selected tests covering static short-term properties as well as long-term durability properties.
Abstract: This paper addresses the need to create new test methods pertinent to automotive structural application and standardize the existing test methods. Ten cases in point are briefly discussed. A list of selected tests has been proposed covering static short-term properties as well as long-term durability properties.
TL;DR: In this article, a fatigue envelope is presented to indicate the specific failure mode for any stress ratio and number of loading cycles of both tensile-tensile and compressive-compressive fatigue.
Abstract: Laminated composite materials tend to fail differently under tensile or compressive load. Under tension, the material accumulates cracks and fiber fractures, while under compression, the material delaminates and buckles. Tensile-compressive fatigue may cause either of these failure modes depending on the specific damage occurring in the laminate. This damage depends on the stress ratio of the fatigue loading. Analysis of the fatigue behavior of the composite laminate under tension-tension, compression-compression, and tension-compression had led to the development of a fatigue envelope presentation of the failure behavior. This envelope indicates the specific failure mode for any stress ratio and number of loading cycles. The construction of the fatigue envelope is based on the applied stress-cycles to failure (S-N) curves of both tensile-tensile and compressive-compressive fatigue. Test results are presented to verify the theoretical analysis.
TL;DR: In this article, the tensile properties of unidirectional silicon carbide fiber-reinforced reaction-bonded silicon nitride (SiC/RBSN) composite specimens were carried out in air at 25, 1300, and 1500 C, using a new testing technique and a specially designed gripping system that minimizes bending moment and assures that failure always occurred in the gage section.
Abstract: Measurements of tensile properties of unidirectional silicon carbide fiber-reinforced reaction-bonded silicon nitride (SiC/RBSN) composite specimens were carried out in air at 25, 1300, and 1500 C, using a new testing technique and a specially designed gripping system that minimizes bending moment and assures that failure always occurred in the gage section. The material was found to display metallike stress-strain behavior at all temperatures tested, and a noncatastrophic failure beyond the matrix fracture. The tensile properties were found to be temperature dependent, with the values of the ultimate tensile strength decreasing with temperature, from 543 MPa at 25 C to 169 at 1500 C.
TL;DR: The feasibility of mixing hardwood aspen pulp and surface-treated mica as the hybrid reinforcing filler for polystyrene composites has been investigated by evaluating the mechanical properties of the resulting composite materials.
Abstract: The feasibility of mixing hardwood aspen pulp [chemithermomechanical pulp (CTMP)] and surface-treated mica as the hybrid reinforcing filler for polystyrene composites has been investigated by evaluating the mechanical properties of the resulting composite materials. Compatibility of wood fiber and polystyrene, as well as mica, was improved by surface treatments of CTMP, for example, coating with polystyrene + isocyanate or with silane and grafting with polymer. Mechanical properties deteriorated when nontreated CTMP was used in addition to treated mica. However, properties improved when surface-treated CTMP was used as a hybrid filler. Experimental results indicate good compatibility between treated CTMP/mica and polystyrene composites.
TL;DR: In this article, the effects of fatigue damage on the tensile failure strength and the opening mode fracture toughness of an SMC-R50 composite were investigated, which had been conditioned to various preselected fatigue damage stages, were measured for their residual properties.
Abstract: The present study is part of a series of research work on microscopic damage evolution and associated macroscopic property degradation of random short-fiber composites under cyclic fatigue loading. In this paper, the effects of fatigue damage on the tensile failure strength and the opening mode fracture toughness of an SMC-R50 composite were investigated. Test specimens, which had been conditioned to various preselected fatigue damage stages, were measured for their residual properties. The results indicated that both the failure strength and fracture toughness were degraded during the cyclic fatigue loading. The reduction in strength was appreciable, but was less than that observed in the matrix-controlled elastic properties. The reduction of the residual fracture toughness, as a result of the local high-stress field, was affected even less appreciably.
TL;DR: In this paper, a study on measuring residual stress on metal matrix composites (MMC) components by the incremental hole-drilling and X-ray diffraction methods is presented.
Abstract: Silicon carbide (SiC) whiskers and particule-reinforced aluminum alloys show promise as metal matrix composites (MMC) stiff and in high-strength lightweight applications. When a MMC is cooled down to room temperature from the fabrication or annealing temperature, residual stresses are induced in the composite as a result of the mismatch of the thermal expansion coefficients between the metal matrix and fibers. The magnitude of the residual stresses has a very important effect on the yield stress and fatigue strength of the MMC. Many techniques are available for residual stress measurement on mechanical parts. X-ray diffraction, bending deflection, and hole drilling are three techniques that are commonly used. This paper discusses a study on measuring residual stress on MMC components by the incremental hole-drilling and X-ray diffraction methods. The modified hole-drilling method is used for measuring the macroscopic residual stress gradient in depth, and the X-ray method is used for measuring surface residual stresses. New developments have shown that the strain measured on the surface during incremental drilling can be related to the residual stress gradient. Finite element software has been developed for the calculation of calibration coefficients for the hole-drilling method. Some special aspects of this technique, drilling system, and stress gradient will be presented. Different aluminum matrices (2024, 2124, and 6061) and SiC fiber proportions have been tested. The effects of heat treatment, machining, and shot-peening treatment on the residual stress distribution of these materials will be analyzed and compared. Comparison between measurement with the X-ray diffraction method and the incremental hole-drilling method will also be discussed. This study shows that it is possible to optimize the residual stress distribution of MMC materials with adequate posttreatment.
TL;DR: In this article, a practical example of applying two-to-three-dimensional finite element analysis to laminated composites is presented, where cross-ply graphite/epoxy laminates with central circular holes ranging from 1 to 6 in. (2.54 to 15.2 cm) in diameter, subjected to in-plane compression were analyzed.
Abstract: A practical example of applying two- to three-dimensional (2- to 3-D) global/local finite element analysis to laminated composites is presented. Cross-ply graphite/epoxy laminates of 0.1-in. (0.254-cm) thickness with central circular holes ranging from 1 to 6 in. (2.54 to 15.2 cm) in diameter, subjected to in-plane compression were analyzed. Guidelines for full three-dimensional finite element analysis and two- to three-dimensional global/local analysis of interlaminar stresses at straight free edges of laminated composites are included. The larger holes were found to reduce substantially the interlaminar stresses at the straight free-edge in proximity to the hole. Three-dimensional stress results were obtained for thin laminates which require prohibitive computer resources for full three-dimensional analyses of comparative accuracy.
TL;DR: In this article, the results of ultrasonic testing, in particular pulse-echo scans and full waveform scans, performed on composite test specimens are presented, in order to find correlations between these defects and their displayed images.
Abstract: This paper presents the results of ultrasonic testing, in particular pulse-echo scans and full waveform scans, performed on composite test specimens. Defects, such as backing paper, excess resin, Teflon®, sand, acetone, and Frekote® spray are implanted artificially in samples manufactured from carbon fiber prepregs. Cracks and delaminations are generated by impact and bending tests. The specimens are scanned in order to find correlations between these defects and their displayed images. Finally, the cut specimens are analyzed under the microscope.
TL;DR: In this paper, a nonlinear finite element program, designated as PDTUBE, was developed to assess damage in the laminates and predicting residual stiffness and strength of composite cylinders subjected to transverse crushing loads.
Abstract: An investigation was performed to study the load-carrying capability of fiber-reinforced composite shells subjected to quasi-static transverse crushing loads. Both experimental and analytical work have been performed. Experiments on T300/934 and T300/976 graphite/epoxy materials were first conducted on composite cylinders subjected to crushing loads. An analytical model was also developed which was based on a progressive damage analysis. The model consists of a stress analysis and a failure analysis. Deformations, stresses, and strains in the composite cylinders were analyzed based on the large deformation theory. Damage accumulation and the residual stiffness and strength of the tubes were evaluated by a combination of appropriate failure criteria with property degradation models. Based on the model, a nonlinear finite element program, designated as “PDTUBE,” was developed. The code is capable of assessing damage in the laminates and predicting residual stiffness and strength of composite cylinders subjected to transverse crushing loads. An excellent agreement was found between the computer simulations and the test results. A parametric study was also conducted to determine the effects of material properties, ply orientation, and geometries on the response of cylindrical composite shells.
TL;DR: The degradation of graphite fibers in intimate contact with nickel at high temperatures is due to the interdiffusion of carbon and nickel as mentioned in this paper, and there are two mechanisms by which the damage occurs: diffusion of carbon into the nickel and recrystallization of the fibers.
Abstract: The degradation of the mechanical properties of graphite fibers in intimate contact with nickel at high temperatures is due to the interdiffusion of carbon and nickel. There are two mechanisms by which the damage occurs: diffusion of carbon into the nickel and recrystallization of the fibers. There has been much disagreement in the literature and seemingly conflicting data. These apparent conflicts in data have lead to two different schools of thought, each favoring one mechanism over the other. On careful analysis of the entire body of the literature, however, a unified picture emerges. Below a critical temperature Tg the fiber is weakened by diffusion into the nickel matrix. Above Tg, the fiber recrystallizes at an appreciable rate, catalyzed by the diffusion of nickel into the fiber. The value of Tg varies with the type of fiber, and it is significantly higher for high modulus fibers than for high tensile strength fibers.