Subcritical Crack Growth Under Single and Multiple Periodic Overloads in Cold-Rolled Steel

Fatigue crack growth retardation, delayed retardation, initial acceleration, and optimum fatigue crack growth life under single, single periodic, and multiple periodic tensile overloads were investigated in cyclic strain softening AISI 1020 cold-rolled steel using compact type specimens. Macroscopic delayed crack growth retardation was observed following both single and multiple periodic overloads and was best observed at high overload ratios and larger crack lengths. Initial crack growth acceleration at the beginning of an overload interval was not quantitatively evident, however initial acceleration could have occurred before the crack extended to the least measurable scale division. Under constant load range testing, multiple periodic overloads of about 10 to 30 gave optimum or maximum fatigue life in most cases, however values of 1 gave maximum life in some cases. The largest increase in fatigue life was 453 percent. Maximum or optimum fatigue life obtained from multiple periodic overloads was always greater than that obtained from a single overload. In general, the results suggested maximum fatigue life can be obtained by applying a few multiple tensile overloads at high overload ratios only when accelerated crack growth begins following low load crack retardation.