Crack initiation in B-AL composites

The toughness of notched, unidirectional B--Al composites under 3-point bending has been examined as a function of filament volume fraction, filament diameter, and matrix yield strength. The results illustrate that the work-of-fracture, γ/sub f/, criterion for composite toughness has only limited utility since γ/sub f/ was found to be a strong function of notch length. The linear, elastic fracture toughness, K/sub c/, was observed to be independent of both notch length and filament diameter. However, the K/sub c/ values increased significantly as the yield strength of the matrix increased. The critical energy release rate, L/sub c/, was shown to be independent of crack length, filament diameter and matrix yield strength. It has been proposed that L/sub c/ is a reflection of the strain energy stored in the filaments at crack initiation and is thus a measure of the parameter which controls composite crack initiation--strain in the filaments at the notch root. (U.S.)