Experimental Study of Three- and Four-Point Shear Test Specimens

The measured short beam (three-point) shear strength was found to be strongly influenced by the support span-to-specimen thickness l/t ratio, and to a lesser extent, by the specimen thickness and the diameters of the loading and support cylinders. As the l/t ratio decreased, the shear strength increased. Transverse compressive stresses, induced by the support and loading cylinders, may have progressively inhibited shear failures as the separation of the cylinders decreased with decreasing l/t ratio. Nevertheless, shear failure modes were typically observed. Local compressive damage under the loading cylinder was a minor secondary event, if it occurred at all. Similar results were obtained for the four-point shear test specimen configuration. The four-point shear test method yielded shear strengths similar to those for the short-beam shear test for the unidirectional glass/epoxy composite but lower shear strengths for the unidirectional carbon/epoxy composite.