Analysis of five-point bending for determination of the interlaminar shear strength of unidirectional composite materials

Analyses of a beam under five-point and three-point bending are presented, using orthotropic plane elasticity to investigate the interlaminar shear failure of unidirectional composites. The approach uses Fourier series expansions of the applied concentrated loads, together with the Saint Venant's semi-inverse method in which the stresses are obtained by a non-dimensional stress function. The computations of the stresses and displacements are conducted by a CRAY XM-P/25 supercomputer. The comparison of stress analyses between the fivepoint and three-point bending configuration is presented for a T300/5208 graphite/epoxy unidirectional composite with a span-to-height ratio of four. It has been found that five-point loading provides the advantage of a specimen having both high shear stresses and almost zero bending stresses over a substantial region of the specimen. This provides the possibility of using the five-point test configuration to measure interlaminar shear strength of unidirectional composites. In addition to the shear dominated stress distributions, the five-point bending fixture has several advantages against other test methods, such as simple fabrication of the specimen, economical testing jig, and easy stopping of test for study of phenomenon, and it can be used to take photographs of a failure. An optimal design in terms of the loading configuration, especially the load line distance of the upper loading applicators, is recommended for the most favorable stress states for interlaminar shear failure of a given composite beam.