Fracture Mechanics Correlation of Boron/Aluminum Coupons Containing Stress Risers

The mechanical behavior of boron/aluminum near stress risers has been studied and reported. This effort was directed toward defining the tensile behavior of both unidirectional and (0/ ±45) boron/aluminum using linear elastic fracture mechanics (LEFM). The material used was 5.6-mil boron in 6061 aluminum, consolidated using conventional diffusion bonding techniques.

Mechanical properties are reported for both unidirectional and (0/ ±45) boron/aluminum, which serve as control data for the fracture mechanics predictions. Three different flawed specimen types were studied. Tension coupons containing circular centerholes, double edge notches, and center notches as well as control specimens were tested. The circular hole and double edge notched specimens were tested with various flaw sizes. In each case the series of specimens remained geometrically similar to eliminate variations in finite size correction factors. The center notched specimens were used to obtain conditions more in line with conventional plane-strain fracture testing.

The fracture data from these tests were reduced using two techniques. They both used conventional LEFM methods, but the existence of a characteristic flaw was assumed in one case and not the other. Both the data and the physical behavior of the specimens support the characteristic flaw hypothesis. Cracks were observed growing slowly in the (0/ ±45) laminates, until a critical crack length was reached at which time catastropic failure occurred.