Tensile Strengths of Notched Composites

Abstract

The tensile strengths of several different aligned fiber-epoxy resin com posites containing cracks perpendicular to the fibers have been measured as a function of crack size. The fibers were surface treated and surface untreated, high and low modulus, carbon; and S-glass. Thus, the effect of varied fiber- matrix bond strengths were observed. Surface treated carbon fiber com posites were notch-sensitive but the surface untreated carbon fiber compos ites and the glass fiber composites were notch-insensitive. This effect is due to differences in the shear strengths of the materials. The tensile strengths of the notch-sensitive materials are discussed in terms of various failure criteria, and it is shown that as the relative sharpness of cracks is increased, strengths tend towards the prediction of linear elastic fracture mechanics. The fracture surface energies of all these materials were measured by a work of fracture technique. The works of fracture of the carbon fiber composites are best explained by a model in which shear stress at the interface is maintained during pull-out. The works of fracture of the glass fiber composite are best explained by a debonding model.