The Effect of Geometry on the Mode of Failure of Composites in Short-Beam Shear Test

Abstract

The interlaminar shear strength of composite materials is often determined by the short-beam shear test method. This type of test is satisfactory only so long as a pure shear failure occurs. Nonshear or mixed-mode failures, hence invalid data, can result if the interlaminar shear strength is too high with respect to the ultimate flexure strength, or if span-to-depth ratio or width-to-depth ratio is improperly chosen. Since the flexural stress is dependent on span length, various span-to-depth ratios of interest were investigated using beam theory to determine the interaction of shear and flexure stresses present in the short-beam specimen. These interactions are shown as a function of the direct stress in the short beam. Examination of the shear-flexure interaction was made by employing a failure criterion based on distortion energy. Failure was found limited to either shear mode at the center of the beam or flexural mode at the outer fiber. The combination of shear and flexure at intermediate locations is not of sufficient magnitude to cause failure. Since the probability of nonshear failure increases as interlaminar shear strength is improved without accompanying increases in flexural strength a means of choosing proper length-to-depth ratios knowing the approximate flexural-to-shear strength ratio is presented. The possibility of a flexural failure is minimized by incorporating a flexural stress to flexural strength ratio which will not be exceeded at the chosen length-to-depth ratio. The effect of the width-to-depth ratio on the shear stresses in the specimen is developed by a theory of elasticity solution. The formula currently being used to calculate the shear stresses is based on beam theory and assumes that the shear stresses are constant along the width of the specimen. The exact solution for an ortho-tropic beam shows a significant effect of the width-to-depth ratio on the magnitude and distribution of shear stresses in the short beam. The inter-laminar shear stress is highest at the edges of the specimen and lowest at the middle. Calibration curves for graphite-polymer, boron-polymer, and boron-aluminum have been developed to correct the shear stresses as given by the conventional beam theory formula for various values of width-to-depth ratios. Experimental data has been obtained on a graphite-polymer system for fiber orientations of both zero and ±45 deg. Conventional beam theory calculations show the interlaminar shear strength of the ±45-deg cross ply to be reduced significantly over that of the unidirectional specimens. Application of the theory of elasticity correction factors to these data indicate that the actual interlaminar shear strength for both orientations are approximately equal.