Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N_D/N_CAL vs overload ratio and crack length (when U = U_min) vs overload ratio fit a straight line, from which the power laws are developed.