This work evaluates the effects of static and cyclic loading conditions on the stress corrosion process in unidirectional glass/modified polyester composite in the presence of a nitric acid solution. The composites were investigated under static, cyclic, and static with overload conditions using a rectangular specimen with centrally positioned grooves. The stress corrosion process was monitored using acoustic emission (AE). It was found that the time-to-failure of the composite specimens were only slightly affected by loading conditions. However, the large scatter in the time-to-failure of the specimens did not allow for the proper evaluation of the effect of the cyclic loads on the stress corrosion fracture process. It seemed that the time-to-failure were more influenced by the specimen variations than by the type of loading. By using an AE digital set-up based on four transducers, not only can the fiber fracture by stress corrosion be very accurately monitored, but also the location of AE sources can be precisely determined.
Anisotropic behavior of damping in fiber-reinforced composites is well established. An attempt is made to measure all the six loss factors for glass fiber-reinforced epoxy (three in normal and three in shear) experimentally using free decay method. Different types of specimen such as beam, tubular and cuboidal in shape made from glass fiber-reinforced epoxy are tested under different loading conditions. The six loss factors (eta(11), eta(22), eta(33), eta(12), eta(13) and eta(23)) so determined are compared with the analytical results.
An analysis based on plate finite elements, nonlinear spring elements and the virtual crack closure technique has been implemented to study the effect of stitching on strain energy release rates for debond configurations. The stitches were modeled as discrete nonlinear spring elements with a compliance determined by experiment. The axial and shear behavior of the stitches was considered, however, the two compliances and failure loads were assumed to be independent. Both a double cantilever beam (mode I) and a mixed mode skin-stiffener debond configuration were studied. In the double cantilever beam configuration, G(I) began to decrease once the debond had grown beyond the first row of stitches and was reduced to zero for long debonds. In the mixed-mode skin-stiffener configuration, G(I) showed a similar behavior as in the double cantilever beam configurations; however, G(II) remained nonzero over the range of debond lengths considered.
The wrinkling, or skin buckling, behavior of sandwich panels with skins consisting of equally spaced strips was investigated. These so-called zebra skin sandwiches proved to be superior to full skin sandwiches in terms of weight and wrinkling strength. Two and three-dimensional finite element analyses were performed and the full skin sandwich results were compared to various results in the open literature. An approximate analytical estimate, using Euler-Bernoulli kinematics in the skin and a simple exponentially decaying field in the core, was derived and compared with finite element results.
This study was carried out with the aim of developing a rational and effective design procedure for the development of cost-effective composite masts that can be manufactured by automated manufacturing procedure. This was in response to the lack of availability of systematic design and analytical procedure for such structural components, and the relatively high cost of the currently available composite masts in market. To achieve this objective, a systematic evaluation of the loads on the mast was performed, followed by a relatively accurate design procedure that considered structural design of an actual mast for a commonly used YD-40 boat. It will be shown that the proposed mast will be less expensive than the commercially available all carbon-epoxy masts.
This paper describes the development and results of a tension-tension fatigue life test program. The fatigue life of a quasi-isotropic graphite/epoxy laminate was experimentally measured at three temperatures: - 195degreesC, 22degreesC, and 121 degreesC. To perform testing at these temperatures a supplemental gripping fixture and a cold cell were designed and manufactured. Issues addressed during the test program include failure of the gripping system, grip-induced failure of the test laminate, and temperature stability. The test results showed that the test laminate's nominal fatigue life improved when tested at - 195degreesC, and decreased when tested at 121degreesC.
The dynamic response of a localized, heat-damaged, fiberglass-reinforced epoxy cantilever beam is obtained as a function of damaged length and damage severity. A heat-damaged area causes a reduction in the local stiffness of the beam and introduces a complex damping in the damaged zone. These variations in the local mechanical properties could result in changing vibration characteristics of the beam. The variations in the system characteristic could then be used to assess the structural integrity of the composite beam. A cantilever beam made of a glass fiber-resin composite material and damaged by a hot tip contact element and a laser beam is evaluated for its dynamic response using numerical methods. The laser beam caused local melting of many fibers in the damaged area and thus reducing the stiffness of the beam in the damaged area. The beam is analyzed by modeling it as a lumped system and a continuous system and by a finite element. The goal of the research is to find out whether the changes in the frequency response of the damaged beam can be used as a diagnostic tool for estimating the severity of the damage. The results show that the size and location of damage are equally as important as the local stiffness and damping of the damaged region in terms of their effects on the beam resonant frequencies. The results indicate that the resonance frequencies may not be suitable parameters for estimating the residual tensile strength of the composite. A 50% reduction in the local bending rigidity produced relatively little change in the system first resonance frequency. In contrast, it significantly reduced the residual tensile strength of the composite.
The use of advanced composite materials for cryogenic fuel tanks reduces the mass of the structure but the leakage of fuel can become a problem. Since the amount of leakage will increase with crack opening volume, it is essential that we understand the factors that affect the crack opening. In a previous study by the authors, the degradation behavior of lamina properties due to cracking was studied extensively. The study examined the effects of a variety of parameters such as adjacent ply orientation, material properties of adjacent plies, initial properties of the cracked ply, and cracks in adjacent plies on the effective properties of the cracked ply. It was shown that the rate of degradation is not very sensitive to these parameters. In the current paper, it will be shown that the crack opening volume can be directly related to changes in effective moduli. A simple calculation of the crack opening volume for a laminate based on the degradation of the E22 of a cracked lamina is presented. The effect of various parameters on the crack opening volume shows a similar pattern as the E22 degradation of a cracked ply. This suggests that the opening volume is expected to also be quite insensitive to the laminate design parameters listed above.
The influence of specimen configuration and size on the transverse tensile strength of two glass/epoxy materials; and, one carbon/epoxy material, loaded in three and four-point bending was evaluated: Transverse tensile strength: was typically lower for longer span lengths due to the classical weakest link effect. However, strength was less sensitive to volume changes achieved by increasing specimen width. The Weibull scaling law typically over-predicted changes in transverse tensile strengths in three-point bend tests and under-predicted changes in transverse tensile strengths in four-point bend tests. Furthermore, the Weibull slope varied with specimen configuration, volume, and sample size. Hence, this scaling law was not adequate for predicting transverse tensile strength of heterogeneous, fiber-reinforced, polymer matrix composites.
Finite element (FE) analyses were performed on 3-point and 4-point bending test configurations of glass-epoxy and carbon-epoxy unidirectional tape beams tested at ninety degrees to the fiber direction to identify deviations from beam theory predictions. Both linear and geometric non-linear analyses were performed using the ABAQUS® finite element code. The 3-point and 4-point bending specimens were first modeled with two-dimensional elements. Three-dimensional finite element models were then performed for selected 4-point bending configurations to study the stress distribution across the width of the specimens. For 3-point bend test configurations, both the linear and geometric non-linear 2D plane-strain and plane-stress analyses yielded similar results. The maximum tensile stresses under the center load nose calculated from the FE analysis were slightly lower than stresses predicted by beam theory. The difference (maximum of 4%) was greatest for the shortest span analyzed. For 4-point bend test configurations, both the plane-stress and plane-strain 2D linear analysis results agreed closely with beam theory except right below the load points. However, 2D geometric non-linear analyses deviated slightly from beam theory throughout the inner span as well as below the load points. Plane-stress results deviated from beam theory more than plane-strain results. The maximum tensile stresses between the inner span load points were slightly greater than the beam theory result. This difference was greatest (maximum of 4%) for configurations with the shortest spans between inner and outer load points. A contact analysis was also performed in order to investigate the influence of modeling the roller versus modeling the support as a simple boundary condition at one nodal point. The discrepancy between the FE and beam theory results became smaller (max. 2–3%) when the rollers were modeled in conjunction with contact analysis. Hence, the beam theory yields a reasonably accurate value for the maximum tensile stress in bending compared to 2D FE analysis. The FE results are primarily for guidance in the choice of beam thickness, width, and configuration. For the 3-point bend configuration, longer spans are preferred to minimize the error in beam theory data reduction. Similarly, for the 4-point bend configurations, a longer span between the inner and outer load noses, at least equal to the span between the inner load noses, results in less error compared to beam theory. In addition, these FE results indicate that the span between the inner load noses should not be too long to avoid obtaining a non-uniform maximum stress between the inner load noses. Finally, the 3D analysis indicates that specimens should be sufficiently wide to achieve a fully constrained state of plane-strain at the center of the specimen width.
The mixed-mode bending (MMB) test for delamination toughness was first introduced in 1988. This simple test is a combination of the standard Mode I (opening) test and a Mode II (sliding) test. This MMB test has become widely used in the United States and around the world for mixed-mode toughness measurements. Because of the widespread use of this test method, it is being considered for standardization by ASTM Committee D30. This paper discusses several improvements to the original test method. The improvements to the MMB test procedure include an improved method for calculating toughness from the measured test quantities, a more accurate way of setting the mixed-mode ratio to be tested, and the inclusion of a new alignment criterion for improved consistency in measured values.
This paper investigates the tensile behavior of plain regular braided fabric reinforced composites subjected to uniaxial load. An experimental program is performed to characterize the stiffnesses and strengths of a number of braid composites. Two different material systems, i.e., carbon/epoxy and glass/epoxy, were investigated in this study, each with three different braiding angles. A theoretical approach, based on a bridging micromechanics model, is employed to predict the tensile properties of the braid composites only using monolithic fiber and matrix properties and the fabric geometric information as input parameters. These parameters are easily obtainable before or after composite fabrication, and determination of them is described in the paper. Unit cell geometry of the braided fabric in the composite was represented by either elliptic or sinusoidal cross section combined with the same undulation function, and a comparative study has been performed. After the unit cell of the braid composite has been divided into slices and the bridging model has been applied, an assemblage based on iso-stress or iso-strain assumption was adopted to obtain the overall properties of the composite. Although both the assumptions give reasonable predictions for the stiffness of glass/epoxy braid composites, significant differences exist between the predictions from the iso-stress approach and those from the iso-strain approach for the strength of the glass/epoxy composites and for the stiffness and strength of the carbon/epoxy composites. The iso-strain approach combined with the elliptic geometric description exhibits the best accuracy, and the predicted stiffnesses and strengths for the two material systems thus obtained are all within 13% discrepancy with the experimental data.
Failure and stiffness properties of a woven 8 harness satin (8HS) graphite/PMR-15 composite have been investigated at room temperature and at 315degreesC by performing the +/-45degrees tensile and Iosipescu tests. Acoustic emission has been monitored during testing. The critical loads for the initiation of damage in the composite have been determined. In particular, the specimen width effect has been investigated in the case of the +/-45degrees specimens by testing the specimens with their width ranging from 12.7-50.8 mm. The results from the high temperature tests have been compared with the room temperature data presented in Refs. 1 and 2. Similar to the room temperature +/-45degrees tests, the shear stresses at the onset of intralaminar damage in the specimens and the shear stresses at the maximum loads at 315 C are significantly affected by the specimen width effect. The trends in the damage initiation stresses and the maximum stresses as a function of specimen width at 315degreesC have been found to be very similar to the room temperature data with the stresses increasing almost linearly with the specimen width. It has also been shown in this project that the shear stresses at the onset of intralaminar damage and at the maximum load at 315 C depend very strongly on the specimen type. The shear stresses determined at the onset of damage and maximum loads from the Iosipescu tests at 315degreesC are noticeably higher than the stresses from the +/-45degrees tests. The +/-45degrees tensile test significantly underestimates the room and elevated temperature shear strength properties of the 8HS graphite/PMR-15 composite in comparison with the Iosipescu shear test.
Throughout the United States including New York, many reinforced concrete bridges on county and state highway systems have deteriorated to the certain degree that structural strengthening is necessary to extend their service life. Fiber reinforced polymer (FRP) composite systems appeared to be one of the options to address the issues of cost-effective load-rating improvement. Recently, an FRP deck has been installed on a state highway, located in New York State, as an experimental project. This paper describes multi-step linear static analyses that were conducted using the finite element method to study the possible failure mechanisms of the deck-superstructure system. Finite element model was verified using the load tests of the bridge deck. Furthermore, the thermal behavior of the FRP deck was investigated and presented in this paper. Analytical results reveal several potential failure mechanisms for the FRP deck and truss bridge system.
Alloying elements were added to the copper matrix to produce unidirectional carbon-fiber-reinforced copper-matrix composites with different interfacial bonding strengths (IBS). The thermal expansion coefficients of these composites were determined to investigate the influence of the IBS on the thermal expansion behavior at a low temperature range. The results showed that the thermal expansion coefficient (CTE) at low temperatures (elastic region) of the composites was controlled by the IBS. Furthermore, the IBS of the composites was observed to control the thermal stresses in the matrix and, therefore, to affect the temperature of the onset of matrix yielding. However, the relationship between IBS and expansion behavior at a high temperature range (beyond elastic region) for the present carbon-fiber copper composite system cannot be determined because of the influence of matrix strength. Further work is underway to develop a model so as to correlate the IBS with the CTE of the composites under elastic stage, and a suitable composite system is needed to correlate the IBS to the CTE at high temperatures.
The purpose of this paper is to describe and explain some highlights in the history of adhesive bonding of fibrous composite structures. Successes are described, along with opportunities to do better in future. The importance of making simple details is stressed, in the context of making high-strength composite parts free from wrinkles in the fibers. A case is made that secondary bonding is frequently less expensive than co-curing of complex "single-step" parts, despite the consistent projections that the opposite should be true. Some explanations are presented to explain how this misconception arises, based mainly on the issue that the estimates used for comparison are traditionally based on the costs of making only the first articles, whereas what should have been compared was the cost of making possibly very differently designed lower-cost parts and structures for series production. Some common misunderstandings about variable-thickness adhesives layers are put to rest by explaining how intense load transfer occurs only in small areas, where the adhesive layer is thinnest, with the rest of the bond area inevitably lightly stressed. The problem created by variable thickness bonds is not that of weak bonds. The thicker, and often porous, bond lines usually shed most of the load they were intended to transmit to nearby thin and stiffer areas. Indeed, the loss of load caused by softening can exceed the loss of strength caused by the porosity. No, the problem caused by variable thickness adhesive layers is that it increases the local stresses in the skin whenever the glue layer is thinnest. Traditional problems with release-agent-coated peel plies that prevent adhesion of the glue to the substrate are recounted. This issue is now fairly well known. What is far less understood is the adverse influence of pre-bond moisture that is unable to escape during the bond cycle. This prevents adhesion perhaps even more effectively than a layer of silicone. The importance of bonding only dry parts is explained. This is far easier to do at the time of original manufacture and less easy for in-service repairs and repainting. The paper concludes with a discussion of the important factors to be considered during design and analysis of bonded composite joints.
The aerospace industry lacks a validated, practical analysis method for the strength, durability, and damage tolerance evaluation of composite bonded joints. This; paper presents the results of a combined strength and fracture analysis approach applied to typical bonded joint configurations found in rotorcraft composite structures. The analysis uses detailed 2-D non-linear finite element models of the local bondline. Strength-of-materials failure criteria are used to predict critical damage initiation loads and locations. A fracture mechanics approach is used to predict damage growth and failure under static and cyclic loads based on test data for static fracture toughness (G(Ic), G(IIc)) and crack growth rate (da/dN). Results are presented from the application of the analysis approach to two joint configurations: 1) a skin-stiffener T-joint and; 2) a bonded repair lap joint. The results show that the proposed approach can be used to predict critical failure modes, damage initiation loads and locations, crack and/or delamination stability, static strength, residual strength, and fatigue life. Discussion is also included on how this approach can be applied in damage tolerance evaluations of composite bonded joints.
This paper presents results from an analytical and experimental study of the effect of temperature and geometrical variations on the Mode II interlaminar fracture toughness in glass-cloth/epoxy laminates. The end-notched flexure (ENF) test geometry was used for Mode II experiments, which were performed at room temperature (R.T.), liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). The fracture surfaces were also examined by scanning electron microscopy to verify the fracture mechanisms. A finite element model was further used to perform the delamination crack analysis. Critical load levels, and the geometric and material properties of the test specimens were input data for the analysis, which evaluated the Mode II energy release rate at onset of delamination crack propagation. The results of the finite element analysis are used to supplement the experimental data.
The mechanical response of a woven eight-harness satin graphite/polyimide composite has been investigated by performing +/-45degrees tensile and Iosipescu shear tests at room temperature. Nonlinear finite element simulations of the tests have been conducted to determine internal stress distributions in the +/-45degrees tensile and Iosipescu fabric specimens as a function of load. In the experimental part of this study, a series of tensile and Iosipescu shear tests have been performed. Acoustic emission techniques have been employed to monitor damage initiation and progression in the composite. The Finite element computations have shown that the internal stress distributions in the Iosipescu and tensile fabric specimens are significantly different. In the gage sections of Iosipescu specimens, the state of stress is essentially pure shear, whereas the tensile tests generate biaxial stress conditions. It has been shown in this research that the shear strength of the composite determined from the maximum loads obtained from the Iosipescu shear tests is significantly higher than the shear strength obtained from the +/-45degrees tensile tests. Moreover, the initiation of intralaminar damage in the tensile specimens occurs at much lower loads than in the Iosipescu specimens. It appear,; that the +/-45degrees tensile test significantly underestimates the shear strength of the composite evaluated from the onset of intralaminar damage and the maximum loads.
