M. Kumosa

Trigger mechanisms in energy-absorbing glass cloth/epoxy tubes

Abstract A study has been made of chamfer-based trigger mechanisms for use with energy-absorbing glass cloth/epoxy composite tubes. Tubes with external chamfers ranging from 10 to 90° were crushed to different extents and the resulting microstructures were examined microscopically. The mechanical properties of the material were determined and used in analysing the initial stages of the crushing process by finite element modelling. It was found that, for this material, the crushing process was usually initiated by local bending of the chamfered parts of the tubes and by internal cracking. These events gave rise to the generation of wedges of crushed material which were pushed to the inside of the tube wall. The stresses generated at the root of the wedge gave rise to lateral cracks, which caused small rings of material to be sheared off. This sequence of events dominated the initial stages of the crushing process. At a later stage, the mode of crushing changed to one of frond-wedge-frond geometry.

Corrosion of E-glass fibers in acidic environments

This study is concerned with the corrosion behavior of E-glass fibers exposed to oxalic, hydrochloric, nitric and sulfuric acids. The results reveal that acid corrosion of E-glass fibers is primarily attributed to calcium and aluminum ion depletion. The leachability of these metallic ions is not only determined by the hydrogen ion concentration but also strongly affected by the anions in the acids. The formation of insoluble salts or complex ions between the anions in oxalic and sulfuric acids and calcium and aluminum ions leached out from the glass accelerates fiber corrosion considerably. Furthermore, acid corrosion may generate axial or spiral cracks on the fiber surface, depending on acid type and concentration. In this study, an ion-depletion-depth model is proposed to explain the mechanisms of fiber surface cracking; the axial crack formation is most likely associated with a larger ion-depletion depth than the spiral crack formation.

Determination of shear strength of unidirectional composite materials with the Iosipescu and 10° off-axis shear tests

Abstract The purpose of this research was to determine the shear strength of a unidirectional carbon-fibre/epoxy composite by means of the 10° off-axis and 0° Iosipescu specimens subjected to shear. Detailed non-linear finite-element computations of these two tests were conducted, taking into account the actual non-linear material behavior of the composite. The tests were compared in terms of stresses and strains at failure. It was found that the shear strength of the composite can be very accurately determined by using the two independent testing techniques only if fully non-linear finite element computations of the tests are performed. The stresses and strains at failure in the 10° off-axis specimen closely match the stresses and strains at the onset of intralaminar damage near the roots of the notches in Iosipescu specimens. Owing to the difficulties associated with the measurement of the shear strength of the composite using the Iosipescu test, and in particular, with the interpretation of the experimental data, this test was found to be almost impractical for the determination of shear strength. The test can only be used if fully non-linear finite element computations of uncracked and axially cracked Iosipescu specimens are conducted in conjunction with the continuous monitoring of intralaminar damage near the roots of the notches during testing. In addition, the shear strength results obtained from the Iosipescu specimen should be independently verified by using another method, such as the 10° off-axis test.

Analysis of the Iosipescu shear test as applied to unidirectional carbon-fibre reinforced composites

Abstract Intralaminar shear properties of unidirectional carbon-fibre reinforced epoxy and PEEK composites were investigated using the Iosipescu shear test. The apparent shear strength and shear moduli were measured using specimens with two different fibre orientations. Finite element analysis was applied to determine the stress distribution within the Iosipescu specimen. Using numerical correction factors to account for the non-uniform shear stress distribution in the gauge-section of Iosipescu specimens, the actual shear moduli were established. The Iosipescu shear test also provided a reasonable estimate of the shear strength.

Theoretical and experimental evaluation of the Iosipescu shear test

Abstract Use of the Iosipescu shear test for measurement of shear properties of unidirectional laminae has been studied both analytically and experimentally. The intralaminar shear strength and shear stiffness of glass-reinforced polyester material have been measured using specimens with two different fibre orientations. Acoustic emission has been monitored and a fractographic study carried out. A finite element analysis has been conducted to evaluate the stress distribution within the specimen, assuming isotropic and orthotropic elastic properties of the material. There is a complicated stress distribution in the specimen, particularly in the vicinity of each notch root, depending on the elastic properties of the composite. The shear stress region in the specimen gauge section is almost uniform, though small normal compressive stresses exist. The experimental results have shown that the measured shear modulus does not depend on reinforcement orientation. However, it has been observed that different failure modes occur in each case. This results in a change in apparent shear strength of the composite with fibre orientation. Some explanations of these differences have been found in a detailed analysis of the local stresses at the roots of the notches. It is considered that the presence of tensile stresses in this area is primarily responsible for the apparent reduction in the shear strength.

Elastic-plastic and failure properties of a unidirectional carbon/PMR-15 composite at room and elevated temperatures

Abstract A series of off-axis tensile tests at room and elevated temperatures have been conducted up to 316°C (600°F) to determine the elastic and plastic properties of a unidirectional carbon/PMR15 composite as a function of temperature. The transverse tensile and shear strengths of the composite as a function of temperature have also been determined. The effect of the specimen preparation process (type of machining) on the strength properties of the composite has also been evaluated. It has been shown that elastic (with the exception of Poisson ratios ν 12 and ν 21 ), plastic, and strength properties of the composite are significantly affected by elevated temperatures. It has also been demonstrated that the quality of machining can noticeably influence the normal and shear strength data at room and elevated temperatures. Even if the quality of machining is very high, failure of the specimens can occur either in the gage or grip sections. At room temperature, all specimens failed in the grip areas influencing the transverse tensile and shear strength measurements. However, the type of specimen failure does not noticeably affect the strength data at elevated temperatures. The transverse tensile and shear strength properties of the composite at room temperature could only be estimated by extrapolating the normal and shear strength vs temperature curves to room temperature.

Micro- and mesomechanics of 8-harness satin woven fabric composites: I — evaluation of elastic behavior

Abstract In part I of this two-part paper, simplified two-dimensional micromechanics and mesomechanics models have been introduced to predict the elastic behavior of 8-harness satin (8HS) woven fabric composites. The woven warp and fill tows were independently treated as unidirectional composites and composite cylinder assemblage (CCA) theory was adopted to predict tow elastic properties from constituent fiber and matrix properties. Since evaluation of woven lamina stiffness requires an accurate description of tow geometry, a method was also developed to describe arbitrary tow geometries by mathematically fitting cubic splines and/or polynomials to micrographs of composite cross-sections. Finally, classical lamination theory was introduced to determine the overall elastic behavior of an n-layered composite laminate, assuming the woven lamina was a modified, two-layer laminate. The simplified mechanics model was evaluated using results from numerical strain energy and equivalent force approaches and results from a series of experimental Iosipescu shear tests and off-axis tensile tests on T650-35(3k), 8HS woven graphite-PMR 15 composites. Issues regarding exclusion of a matrix layer in the simplified, 2-layer laminate analysis were addressed in the strain energy analysis of an idealized 3-D, representative volume element (RVE). The mechanics model was found adequate in estimating the lower bounds of 8HS woven fabric, composite elastic properties. The model also provided a reasonable estimation of symmetric cross-ply composite properties.

An evaluation of the critical conditions for the initiation of stress corrosion cracking in unidirectional E-glass/polymer composites

The purpose of this work is to establish the critical surface conditions leading to the initiation of stress-corrosion cracks on the as-supplied surfaces of three unidirectional E-glass/polymer composites with modified polyester, epoxy and vinyl ester resins subjected to a nitric acid solution without mechanical loads. The composite materials considered in this study are commonly used in composite high-voltage insulators on overhead transmission lines with the line voltages ranging from 69 to 735 kV. The initiation of stress-corrosion cracks in exposed glass fibers on the composite surfaces was observed in the absence of externally applied mechanical loads. However, the crack initiation rates are strongly dependent on the amount of exposed fibers. After the initial stage of crack initiation, no further stress-corrosion damage is observed in the composites. The E-glass/vinyl ester system appears to be more resistant to the initiation of stress-corrosion cracking in comparison with the other two composite systems investigated. This system exhibits the lowest number of stress-corrosion cracks and the lowest total surface area of exposed fibers on the composite surfaces. The E-glass/epoxy composite shows the lowest resistance to stress corrosion with the largest areas of exposed fibers. The effect of exposed fibers on the stress-corrosion process in unidirectional E-glass/polymer composites used in high voltage insulators has not been previously reported. It is clear that in order to reduce the rates of failure of composite high-voltage insulators by stress-corrosion cracking (brittle fracture), the presence of exposed fibers on their rod surfaces should be minimized.

An analysis of residual thermal stresses in a unidirectional graphite/PMR-15 composite based on X-ray diffraction measurements

The purpose of this research is to determine residual thermal stresses in a unidirectional graphite-fiberT/PMR-15 polyimide composite by using crystalline inclusions. X-ray diffraction (XRD) measurements have been made to determine residual stresses in embedded aluminum and silver inclusions placed between the first and second plies of six-ply unidirectional graphite/PMR-15 composite specimens. In the modeling part of this research, residual thermal stresses in unidirectional graphite/polyimide composite plates and in the embedded aluminum and silver inclusions with interlaminar and intralaminar particle distributions have been modeled by using elastic and visco-elastic laminate theories and the Eshelby method. The numerically determined residual stresses in the particles have been subsequently compared to the residual stresses determined from the XRD analysis. It has been shown in this research that the residual stresses in the unidirectional graphite/polyimide composite can be obtained with reasonable accuracy by using the X-ray diffraction technique in conjunction with the application of the visco-elastic Eshelby method of multiple inclusions. The modeling has also shown that the distribution of the aluminum and silver particles and their geometries have a strong effect on the XRD data and the thermal stress analysis based on the concept of embedded crystalline inclusions.

Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.