ANALYSIS OF 5-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 presen ted, using orthotropic plane elasticity to investigate the interlamina r shear failure of unidirectional composites. The approach uses Fourie r series expansions of the applied concentrated loads, together with t he Saint Venant's semi-inverse method in which the stresses are obtain ed by a non-dimensional stress function. The computations of the stres ses and displacements are conducted by a GRAY XM-P/25 supercomputer. T he comparison of stress analyses between the five-point and three-poin t bending configuration is presented for a T300/5208 graphite/epoxy un idirectional composite with a span-to-height ratio of four. It has bee n found that five-point loading provides the advantage of a specimen h aving 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 domina ted stress distributions, the five-point bending fixture has several a dvantages against other test methods, such as simple fabrication of th e specimen, economical testing jig, and easy stopping of test for stud y of phenomenon, and it can be used to take photographs of a failure. An optimal design in terms of the loading configuration, especially th e 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.