FAILURE MECHANISMS OF 3D WOVEN COMPOSITES IN TENSION, COMPRESSION, AND BENDING

Observations of failure mechanisms in monotonic loading are reported f or graphite/epoxy composites containing three-dimensional (3D) interlo ck weave reinforcement. The key phenomena are delamination and kink ba nd formation in compression, tow rupture and pullout in tension, and c ombinations of these in bending. The materials exhibit great potential for damage tolerance and notch insensitivity. This is partly due to t he presence of geometrical flaws that are broadly distributed in stren gth and space; and partly to the coarseness of the reinforcing tows, w hich leads to extensive debonding and reduced stress intensification a round sites of failure. Rules of mixture corrected for the effects of tow irregularity suffice to estimate elastic moduli. Rough estimates o f the stress at which the first failure events occur in compression or tension can be made from existing micromechanical models. Ultimate te nsile failure might be modeled by regarding failed tows that are being pulled out of the composite as a cohesive zone. The characteristic le ngth estimated for this zone, which is a direct measure of damage tole rance and notch insensitivity, has very large values of order of magni tude 0.1-0.5 m.