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An experimental investigation was conducted to study the behavior under biaxial tensile loading of graphite/epoxy plates with cracks and to determine the influence of crack length on failure. The specimens were 40 by 40-cm (16 by 16 in.) [0/±45/90]s laminates with cracks oriented at 30 deg to one side of the specimen. Four crack lengths, 2.54, 1.91, 1.27, and 0.64 cm (1.00, 0.75, 0.50, and 0.25 in.), were investigated. Deformations and strains were measured using strain gages, birefringent coatings, and moiré grids. The specimens were loaded in biaxial tension in a specially designed loading frame. Results were presented in terms of the ratio of the normal to the crack stress and the unnotched tensile strength of the laminate. This ratio was found to be approximately 21 percent below that for uniaxially loaded plates with transverse cracks, indicating an appreciable contribution to failure of the shear stress. This percentage of strength ratio reduction remained nearly the same for all crack sizes. Experimental results were compared satisfactorily with those predicted on the basis of a tensor polynomial failure criterion for the lamina and a progressive degradation model.
test methods, biaxial testing of composites, fracture of composites, graphite/epoxy, composite laminates with cracks, failure modes, birefringent coatings, moiré grids, effect of crack length, failure prediction, crack-opening displacement, crack-shearing displacement, composite materials
Science advisor, Materials Technology Division, IIT Research Institute, Chicago, Ill.