Showing papers in "Journal of Composites Technology & Research in 1995"

Compressive Failure of Laminated and Woven Composites

Abstract: The effect of fiber architecture upon the compressive failure mechanisms is investigated for a wide range of fiber composites. Compressive tests are performed on both notched and unnotched specimens made from T800 carbon fiber-924C epoxy laminates, AS4 carbon fiber-PEEK laminates, 2D woven T800 carbon fiber-924 epoxy, and 3D woven AS4 carbon fiber-LY564 epoxy. Additionally, unnotched and notched compressive tests are performed on beechwood and birch plywood. In all cases, the dominant failure mechanism is by fiber microbuckling. An infinite band kinking model is used to estimate the unnotched strength and a large-scale crack bridging analysis is used to predict the notched strength.

Review of the mechanics of embedded optical sensors

Abstract: The published literature on the effect of optical sensors and fibers embedded in host composite materials is reviewed. The literature indicates that improperly embedded optical fibers produce local distortions and resin-rich regions in the host composite that cause structural performance degradation. This is especially true for compressive loading of relatively thin composite laminates. Tensile strength is at most only slightly degraded. The available data are inconclusive for fatigue loading.

Effect of Temperature and Moisture on the Impact Behavior of Graphite/Epoxy Composites: Part I—Impact Energy Absorption

Abstract: Dropped weight impact testing has been used to evaluate the influence of temperature and moisture on the impact resistance of unmodified and modified epoxy/graphite fiber composites. At ambient and low temperatures, moisture was found to have little effect on damage initiation energy or subsequent energy absorption. At elevated temperature, the presence of moisture has a significant effect on damage initiation energy, with the change depending on the particular energy adsorption characteristics of the matrix and the ‘wet’ epoxy phase glass transition temperature. The energy required to initiate damage was found to decrease with temperature, and this is consistent with a reduction in matrix properties at elevated temperatures.

Wrinkling of Composite Sandwich Structures Under Compression

Abstract: The phenomenon of wrinkling as a failure mode of sandwich composite structures under compression is examined using analysis verified by testing. Two cases are considered. In the first case, the facesheets are assumed flat and the wrinkling compression load is determined as a buckling load along with the associated short wavelength. This solution is then compared to finite element predictions and is shown to be in very good agreement. In the second case, the facesheets are assumed to have a wavy shape of known amplitude and wavelength. The various failure modes possible (core tension, compression or shear, adhesive tension or shear, and facesheet bending) are modeled in an effort to predict failure. Test specimens were fabricated and the waviness of the facesheets was measured. The specimens were then tested in compression. Reasonable agreement was found with theoretical predictions.

Pressurized Ring Test for Composite Pressure Vessel Hoop Strength and Stiffness Evaluation

Abstract: With the expanding use of composite pressure vessels, including space, tactical, and civil applications, accurate prediction of performance becomes increasingly important. A critical parameter in performance prediction is the burst strength, as measured by the hoop fiber ultimate strain-to-failure. This paper discusses a newly developed test method that is capable of pressure testing 50.8-cm diameter rings with representative composite pressure vessels' laminate thicknesses and lay-ups. Tests of rings with various laminate thicknesses and lay-ups showed excellent agreement with measured delivered fiber strength and stiffness in full-scale composite pressure vessels. Test data show nearly a one-to-one scaling in strength.

Effect of Temperature and Moisture on the Impact Behavior of Graphite/Epoxy Composites: Part II—Impact Damage

Abstract: In Part I of this paper (this publication), dropped weight impact testing was used to evaluate the influence of temperature and moisture on the impact resistance of various graphite/epoxy composites. In the present paper, subpenetration impact testing and subsequent damage characterization using ultrasonic c-scan and microscopic techniques are used to provide additional insight into the nature of the impact damage and to identify critical parameters affecting impact performance.

Tensile Strength of Unidirectional Composites: The Role of Efficiency and Strength of Fiber-Matrix Interface

Abstract: A micromechanics model has been developed to predict the tensile strength of unidirectional composites. The local stresses are calculated using shear lag analysis, including the effects of interfacial debonding. The tensile strength of the composite is estimated by considering the accumulation of fiber fractures as a function of applied load. A new parameter called ‘efficiency’ of the interface is introduced to account for the effectiveness of load transfer from the matrix to the fiber. A simple scheme is described to estimate this efficiency parameter η using the experimentally measured tensile stiffness in the concentric cylinders model. It is postulated that the interface can be completely characterized by two parameters: interfacial shear strength τi and efficiency of the interface η. Results indicate that the interfacial strength and efficiency can be optimized to maximize the tensile strength of a unidirectional composite. The proposed model was used to predict tensile strength of three different sets of materials that possessed carefully tailored interphase variations. The predicted tensile strengths agree well with the experimental data. The predicted failure modes in these material systems are also consistent with experimental observations.

Thermal Conductivity of Textile Composites with Arbitrary Preform Structures

Abstract: In this paper, a model is constructed to predict the thermal conductivity of textile composite materials under steady state heat transfer conditions. First, the composite under consideration is geometrically characterized to identify relative volume fraction and spatial orientation of each yarn along its path. This is followed by applying finite element analysis (FEA) and virtual work to a “unit cell” of the textile composite. Hybrid hexahedra brick elements with fibers and matrix around each integration point are employed in the finite element formulation by means of micro-level homogenization. Thermal conductivity predictions using this approach are found to agree with experimental results for polymer and ceramic composites. The same approach can be used to solve other field problems including electrical conductivity, electrostatics, and moisture diffusion.

Effects of Stacking Sequences on Mechanically Fastened Joint Strength in Quasi-Isotropic Carbon-Epoxy Laminates

Abstract: The desire to develop adequate strength, particularly at bolt hole, tends to restrict the choice of fiber patterns to those that do not deviate very far from a quasi-isotropic pattern. The weakest parts of a composite structure are the joint parts; therefore, the effective stacking sequences on joint strength while maintaining the isotropic nature have to be cleared. In this study, the effect of stacking sequences on quasi-isotropic T300/2500 joint strength was examined. Two joint geometries that exhibited bearing failure and net-tension failure under pin-bearing loads were chosen. Six types of laminate configurations were examined in quasi-isotropic patterns. According to observation failure mode and acoustic emission (AE) measurement, the evidence of the effect of stacking sequences was made clear. The highest bearing and net-tension strength obtained from the quasi-isotropic laminates tested all had 0° plies on the outer surfaces, 90° plies next to the 0° plies, and ±45° plies interspersed in the middle of the laminate [0°2/90°2(+45°/-45°)2]s.

An Improved Iosipescu Shear Test Fixture

Abstract: A new Iosipescu shear test fixture design that can be used to perform the test procedure outlined in Test Method for Shear Properties of Composite Materials by the V-Notched Beam Method (ASTM D 5379) was designed, fabricated, and tested. The new fixture solves the often reported problems of specimen twisting, variability of results, and differences in measured shear modulus between [0]16, [0/90]4s, and [90]16 laminates. Lateral instability of the Iosipescu specimen is explored as the main agent causing the undesirable effects observed with the previous fixture design.

Characterization of Bolted Joint Behavior: MIL-HDBK-17 Accomplishments at Standardization

Abstract: A testing approach is presented that will provide the user with sufficient data to design bolted joints in composite structures against various failure modes and interactions between them. Test methods for characterizing bolted joint behavior are described. These include double- and single-shear bearing, notched tension and compression, bearing/bypass (tension and compression), shear-out, and fastener pull-through. In addition to test specimens and procedures, test matrices are presented that recommend laminates to be tested, environmental conditions, and replication. The replications represent the minimum data requirements to design bolted joints in composite structures given today's analytic capability. The test methods and testing requirements were established through a working group consensus and approved by the full committee of MIL-HDBK-17.

Time-Dependent Behavior of a Graphite/Thermoplastic Composite and the Effects of Stress and Physical Aging

Abstract: Experimental studies were performed to determine the effects of stress and physical aging on the matrix dominated time dependent properties of IM7/8320 composite. Isothermal tensile creep/aging test techniques developed for polymers were adapted for testing of the composite material. Time dependent transverse and shear compliance's for an orthotropic plate were found from short term creep compliance measurements at constant, sub-T(8) temperatures. These compliance terms were shown to be affected by physical aging. Aging time shift factors and shift rates were found to be a function of temperature and applied stress.

Experimental Study of Three- and Four-Point Shear Test Specimens

Abstract: The measured short beam (three-point) shear strength was found to be strongly influenced by the support span-to-specimen thickness l/t ratio, and to a lesser extent, by the specimen thickness and the diameters of the loading and support cylinders. As the l/t ratio decreased, the shear strength increased. Transverse compressive stresses, induced by the support and loading cylinders, may have progressively inhibited shear failures as the separation of the cylinders decreased with decreasing l/t ratio. Nevertheless, shear failure modes were typically observed. Local compressive damage under the loading cylinder was a minor secondary event, if it occurred at all. Similar results were obtained for the four-point shear test specimen configuration. The four-point shear test method yielded shear strengths similar to those for the short-beam shear test for the unidirectional glass/epoxy composite but lower shear strengths for the unidirectional carbon/epoxy composite.

Testing Composite-to-Metal Tubular Lap Joints

Abstract: Procedures were developed to fabricate, nondestructively evaluate, and mechanically test composite-to-metal tubular joints. The axially loaded tubular lap joint specimen consisted of two metal tubes bonded within each end of a fiberglass composite tube. Joint specimens with both tapered and untapered aluminum adherends and a plain weave E-glass/epoxy composite were tested in tension, compression, and flexure. Other specimens with tapered and untapered steel adherends and a triaxially reinforced E-glass/epoxy composite were tested in tension and compression. Test results include joint strength and failure mode data. A finite element analysis (FEA) of the axially loaded joints explains the effect of adherend geometry and material properties on measured joint strength. The flexural specimen was also analyzed; calculated surface strains are in good agreement with measured values, and joint failure occurs in the region of calculated peak peel stress.

Failure criterion for thick multifastener graphite-epoxy composite joints

Abstract: A method for accurately predicting the strength of multifastener, thick composite joints is discussed. The method is based on the average stress criterion applied around the hole circumference. Basic laminate strength data are obtained from single-fastener and reduced-section notched specimens. Using ABAQUS finite element analyses (FEA), the stress-field distribution around the fastener-loaded hole in both the single-fastener and multifastener joints is determined. The single-fastener test data in conjunction with this FEA and the average stress criterion are then used to predict the multifastener joint strength. Multifastener joint strength of three different laminate lay-ups is predicted to within 1% accuracy. However, the results show that the average stress criterion cannot accurately predict the location of failure initiation. In the current investigation, the maximum strain criterion is used to locate possible sites of failure initiation. When this information is used in conjunction with the average stress criterion, the predicted multifastener joint strength based on single-fastener net-tension data is reasonably accurate.

Effects of Short-Term Environmental Exposure on Axial Strengthening Capacity of Composite Jacketed Concrete

Abstract: Recent studies from the US Department of Transportation (USDOT) estimate that of the 575 000 bridges in the nation, 230 000 (or 40%) are either structurally deficient or functionally obsolete The deterioration has been caused by a variety of factors including corrosion due to marine environments, high chloride content in the air and use of de-icing salts, alkali-silica reactions (ASR), environmental effects on materials, poor initial design, and poor construction and maintenance, or both Cracking and spalling of concrete columns is often seen with corrosion of internal reinforcement steel due to moisture and chloride ingress The loss of cementitious material, as well as the reduction in steel cross-section, leads to drastic reductions in the structural efficiency and load-carrying capacity of columnar supporting elements The high stiffness-to-weight and strength-to-weight ratios of advanced composites, combined with their inherent corrosion resistance, environmental durability, and tailorability make them attractive for use in infrastructure renewal In this paper, we investigate the effect of environmental exposure on the strengthening efficiency of composite-jacketed concrete column stubs The effects of short-term exposure to ambient and 0°F conditions as well as to water and sea water on glass-, carbon-, and aramid-epoxy jackets is investigated

Mixed mode II-mode III fracture of adhesive joints

Abstract: This paper presents approximate closed-form expressions for the local energy release rate in Mode I, Mode II, and Mode III along the idealized straight crack front of a cracked generalized adhesive joint. The development is based on beam theory, the J-integral, and a beam-on-an-elastic-foundation model. Fracture experiments were performed with adhesively bonded split-cantilever-beam (SCB) specimens of different adherend widths and thickness ratios using two different adhesives, and it was seen that the fracture surface had a peculiar stepped arrowhead shape. The analysis and the experiments showed that both the energy release rate and the mode mix vary along the crack front of an SCB specimen, making it unsuitable for the determination of fundamental fracture properties. However, the SCB specimen provided a useful means to test an engineering approach to predict the out-of-plane fracture loads of adhesive joints.

Shear testing of textile composite materials

Abstract: A new in-plane shear-test methodology, developed for laminated composite materials, was extended to textile composites. The methodology incorporates two recently developed advances: a new specimen and a special strain gage. When used together, they produce accurate and consistent shear characterization of laminated and textile composite materials. The “compact shear specimen” incorporates many of the advantages of the similar Iosipescu shear specimen but has a larger test section. A special strain gage called the “shear gage” integrates the shear strain across the entire test section of the specimen to obtain the average shear strain, regardless of the shear-strain distribution in the test section. The methodology was used to test both 2-D braided and 3-D woven textile composites. The entire shear stress-strain response, including the shear modulus and shear strength, was measured for nine textile composite architectures. The variation in shear modulus and shear strength were low and were comparable to the variation of tensile and compressive modulus for the respective materials. Moire interferometry, a full-field optical method, was used to validate the test methodology. Enhanced X-rays were used to document the damage accumulated during failure of the specimens.

Effect of Specimen Tab Configuration on Compression Testing of Composite Materials

Abstract: Standard Test Method for Compressive Properties of Unidirectional or Cross-Ply Fiber-Resin Composites (ASTM D 3410) for compression testing of composite materials recommends that low-carbon steel or a glass/epoxy composite be used as the tab material, and that the tabs be tapered. However, many details remain unspecified. An analytical study of tab configurations was performed in the present study to determine the effect of type of tabbing material and tab-taper angle on the stress distributions within a unidirectional composite specimen in compression. More compliant tab materials and shallower tab-taper angles yielded less stress concentration in the tab-tip region of the specimen. Therefore, specimens with these tab configurations should yield higher measured compressive strengths since premature failure at the tab tip is less likely to occur. Although there are relatively few experimental data available for different tab materials and tab-taper angles, those that are available show the same trends as predicted by the present analytical study.

Properties and analysis of composites reinforced with E-glass weft-knitted fabrics

Abstract: Weft-knitted 1 × 1 rib-fabrics are manufactured from E-glass rovings and are consolidated into flat panels using the resin transfer molding (RTM) process. The panels, with about 50% fiber volume fraction, are tested for their mechanical properties in tension, flexure, compression, and short-beam shear. The property values are reported and the failure mechanisms under different loading conditions are identified and compared. The elastic properties of the knitted composites are analyzed by geometric modeling followed by the finite element analysis (FEA). Good predictions are obtained.

Fastener Hole Reinforcement in Composites Using Cold-Expanded Inserts

Abstract: Cold expansion, a technique widely used to increase the fatigue life of holes in metal structures, has been adapted to composite materials as a means to install metal inserts in fastener holes. Thin-walled cold expanded inserts (grommets) are used to reinforce fastener holes that may be sensitive to damage, for example, from repeated fastener installation and removal, or from lightning strike. Thicker inserts with a nut-anchoring feature are a cost-effective alternative to riveted nut plates currently used in composite-fastened assemblies. A comprehensive test program involving mechanically fastened joints in composites with adhesive bonded and cold expanded grommets showed that, in general, cold expanded grommets perform comparably to, or better than, bonded grommets. In the areas of installation costs and resistance to lightning strike damage, cold expanded grommets significantly out-performed bonded grommets. Work on cold expanded rivetless nut plates in carbon/epoxy has proven the viability of the concept.

Fatigue-life behavior and matrix fatigue crack spacing in unnotched SCS-6/Timetal 21S metal matrix composites

Abstract: Fatigue tests of the SCS-6/Timetal 21S composite system were performed to characterize the fatigue behavior for unnotched conditions. The stress-life behavior of the unnotched (9/90)2s laminates was investigated for stress ratios of R = 0.1 and R = 0.3. The occurrence of matrix cracking was also examined in these specimens. This revealed multiple matrix crack initiation sites throughout the composite, as well as evenly spaced surface cracks along the length of the specimens. No difference in fatigue lives were observed for stress ratios of R = 0.1 and R = 0.3 when compared on a stress range basis. The unnotched SCS-6/Timetal 21S composites had shorter fatigue lives than the SCS-6/Ti-15-3 composites, however the neat Timetal 21S matrix material had a longer fatigue life than the neat Ti-15-3.

Damage Characterization of a Cross-Ply SiC/CAS-II Ceramic Composite Under Fatigue Loading Using a Real-Time Acousto-Ultrasonic NDE Technique

Abstract: The application of a real-time acousto-ultrasonic (AU) nondestructive evaluation (NDE) technique to monitor damage during fatigue without stopping the test is presented in this paper. Real-time AU parameters were used to decipher the true nature of damage mechanisms occurring during cyclic loading of a SiC/CAS-II cross-ply ceramic composite. The results of AU were complemented by other NDE techniques. The S-N curve for a SiC/CAS-II cross-ply ceramic composite at room temperature was generated and a study of the failure process during cyclic loading was performed.

Modes of failure of fibrous composite materials as affected by the orientation-angle of fiber

Abstract: An energy-based failure criterion has been recently developed by the authors. At any stress level, the model deals with the actual composite material as an equivalent linear elastic material. The total strain energy density of the system is considered to be composed of two parts, elastic and plastic. In this paper, the model has been used to identify the modes of failure based on the relative magnitudes of the various energy terms appearing in the failure criterion equation. Accordingly, three types of failure are anticipated: fiber failure, matrix failure (tension or compression), and matrix failure in shear. The critical fiber orientation for a given stress state can be predicted as well. The model has been verified using available experimental data in the literature and compared with other theoretical models as well. Good agreement has been obtained in both cases. The failure envelopes for boron-epoxy Narmco 5505 and glass-epoxy 3MXP 251S are predicted using several stress combinations and fiber orientations ranging from 0 to 90°. Seven failure envelopes are obtained, classified as A, B, C, D, E, F, and G.

Biaxial Bearing/Bypass Testing of Graphite/Epoxy Plates

Abstract: Bolted graphite/epoxy plates were subjected to in-plane biaxial loading. Cruciform shape samples were used. Testing was performed on a biaxial machine developed and built at Concordia University. The bolt hole was constrained using a rigid structure. Four arms of the cruciform samples were independently loaded. Equal and opposite lateral loads were applied along one direction (y). Along the other direction (x), load is applied on one arm (right arm of the cruciform), while the opposite arm (left arm of the cruciform) was held fixed in the grip without applied load. This was possible because the bolt hole at the center of the cruciform sample was held fixed. This opposite arm measured whichever load that was bypassed around the constrained hole (called bypass load). Acoustic emission was used to measure the onset of failure. The applied load in the x direction at the onset of failure is called the joint strength. Subsequent to the onset of failure, the bypass load increased significantly. Also after onset of failure, the bypass load on reloading was larger than the bypass load on previous loading. Results obtained showed that the existence of lateral loads (load along the y direction) had significant effect on the joint strength. Increasing the lateral load decreased the joint strength. These results show that data obtained using uniaxial test are nonconservative.

Residual Stresses in SCS-6/Beta-21S Composites

Abstract: Residual stress profiles were determined using X-ray diffraction (XRD) procedures in SCS-6/Beta-21S composites in unidirectional [0]4, cross-ply [0/90]s, and quasi-isotropic [0/±45/90]s lay-ups by sequentially electropolishing matrix layers off the outer surface. These experimental results were compared with the finite element model (FEM) calculations. Generally, the longitudinal stresses in the unidirectional [0]4 composite was almost twice the transverse stresses, while for the cross-ply [0/90]s, composite, the logitudinal and the transverse stresses were similar in magnitude. The stress profile for the quasi-isotropic [0/±45/90]s lay-up were intermediate between the unidirectional and cross-ply composites. These results show the strong influence of the underlying ply on the residual stresses in the outer ply. A 90° ply has greater influence than a 45° ply on the outer 0° ply of the composite.

Environmental Effects on Mechanical Properties of Thick Composite Structural Elements

Abstract: In this work a consolidated composite structural analysis theory and a suitable treatment of the experimental data obtained from material characterization standard tests has made it possible to the assess the effects of the operating temperature and absorbed moisture on the elastic and ultimate properties of thick composite elements

Electrodeposited Cobalt-Tungsten as a Diffusion Barrier Between Graphite Fibers and Nickel

Abstract: An electrodeposited cobalt alloy coating having a composition of 7.1 ± 0.8 at % tungsten was developed to serve as a diffusion barrier between graphite fibers and nickel. A nickel matrix was electrodeposited around the fibers, and the resulting composite was annealed in vacuum under various time/temperature conditions. The coating was shown to inhibit interdiffusion for up to 24 h at 800°C. This represents an improvement of 200° over the uncoated fibers.

The Effect of High-Temperature Degradation on the Mode I Critical Strain Energy Release Rate of a Graphite/Epoxy Composite

Abstract: The double cantilever beam (DCB) test was used to determine the effects of high-temperature degradation on the Mode I critical strain energy release rate (GIC) of a continuous fiber-reinforced polymer-matrix composite material. The composite material investigated was IM7/8551-7A graphite/rubber-toughened epoxy system. Delamination starter films of two different thicknesses were used: 25.4 µm and 12.7 µm. High-temperature degradation was accomplished by placing the specimens on a wire rack in a forced-air convection oven held at 177°C. A continuous flow of fresh laboratory air existed at all times. GIC decreased with increased exposure, showing a 40% reduction after 8000 h. The specimens with thicker inserts showed a higher GIC. Weight loss of the material occurred throughout the aging cycle, indicating that thermo-oxidation was occurring. GIC was not affected by the wedging open of the precrack during exposure. Crack propagation in the unexposed specimens occurred in the matrix material, whereas for the exposed specimens, it occurred along the fiber-matrix interface. This indicated possible exposure-induced interfacial degradation.

Study of the Residual Stress in Cold-Rolled 7075 Al-SiC Whisker-Reinforced Composites by X-Ray and Neutron Diffraction

Abstract: Many surface treatment techniques are used to improve the mechanical behavior of metal matrix composites (MMC) under dynamic loading. Mill machining, shot-peening, hammering, and cold-rolling are the four techniques commonly used. In this paper, we study the effect of the surface treatment with the cold-rolled technique on the residual stresses in an aluminum alloy 7075 reinforced with 27% vol fraction SiC whiskers. The techniques used to measure the residual stresses are X-ray and neutron diffraction methods. Neutron diffraction is a nondestructive technique that allows one to evaluate the residual stresses in both the Al matrix and the SiC whiskers. The X-ray technique was used to evaluate the residual stresses at the surface of the composite material. The results show us that the cold-rolled treatment induced a steep stress gradient near the surface. The comparison of using the two different measuring techniques will be discussed.