The Effect of Mean Stress and Mean Strain on Fatigue Damage Following Overloads

The influence of mean stress and mean strain on fatigue damage following overloads of yield stress magnitude was investigated. Strain controlled loading spectra each consisting of a periodic fully-reversed overload cycle followed by a number of constant amplitude small cycles were applied to normalized SAE 1045 steel and 2025-T351 aluminum axial specimens. The small cycle strain range was kept constant for all tests but their position within the overload hysteresis loop was varied to observe the effect of mean stress on fatigue damage. The crack opening stress and strain were calculated as a function of the number of cycles after the overload and the effective stress range was defined as the difference between the maximum stress and the greater of either the minimum stress or the crack opening stress. Fatigue life estimates were made using a linear cumulative damage summation based on the effective stress-life fatigue curves for these materials. The experimental results indicate that the mean stress of cycles following an overload has a small but consistent effect on fatigue damage that is not explained by the effective stress range or crack closure concepts alone. Nevertheless, the calculated fatigue lives were within a factor of 2.4 of the test results.