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This study was made to determine the effect of test variables on the strength of unidirectional composites. Specimens of glass fiber reinforced epoxy of varying geometry were tested in tension and flexure at several strain rates. Both experimental and theoretical results show that specimen geometry strongly influences strength measurements for off-axis specimens having a small length-to-width ratio. This is a result of nonsymmetrical anisotropy in the specimens which causes twisting in flexure tests and shear coupling in tension tests. For slender specimens, having a free length-to-width ratio greater than six, both strength and stiffness measurements are almost unaffected by the boundary conditions of the test. A normalization procedure was used successfully to relate the strength of an off-axis composite to its transverse strength. While changes in loading mode, strain rate, and specimen geometry may produce significant changes in absolute strength values, normalized strengths proved to be essentially invariant.
composite materials, fiber, composites, mechanical properties, glass fibers, epoxy laminates, evaluation, tests
Visiting associate professorPersonal member ASTM, Washington UniversityIsrael Institute of Technology, St. Louis, Mo.
Lavengood, R. E.
Materials engineer, Monsanto Co., St. Louis, Mo.