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The stress field in the short-beam shear test specimen is very complex. Failure of the short beam (and every other shear specimen used to date) occurs due to a combination of multiple failure mechanisms, since a state of pure shear can not be achieved. However, test results for brittle unidirectional composites reported in the literature indicate that development of transverse (interlaminar) shear cracks cause specimen failure for a wide range of span-to-depth ratios. In this study, stress analyses have been performed for graphite and glass fiber-reinforced beams for various span-to-depth ratios. The results are correlated with test data reported in the literature, and it is shown that the failure loads for small span-to-depth ratios can be predicted based on the maximum shear stress at failure that may sometimes differ significantly from the apparent interlaminar shear strength obtained from beam theory. In addition, high transverse compressive stresses at critical locations, which provide frictional resistance to sliding, may increase the failure load resulting in a higher apparent shear strength. For larger span-to-depth ratios, a subcritical yield phenomenon due to compressive flexural stress may increase the maximum shear stresses and yield lower apparent shear strengths.
short-beam shear test, unidirectional composites, transverse shear failure, compressive stresses, data correlation, graphite/epoxy composites, glass/epoxy composite materials, testing, design
Staff scientist, Materials Sciences Corporation, Fort Washingtin, PA