The effect of compressive underloads and tensile overloads on fatigue damage accumulation in SAE 1045 steel

Block histories containing either compressive underloads or tensile overloads followed by smaller fully reversed cycles were applied to smooth specimens to determine the load interaction effect of large stress cycles on the fatigue behaviour of subsequent smaller cycles. Interactive damage was obtained by subtracting the damage due to the overload and the steady state damage due to the small cycles from unity. As in previous tests on an aluminium alloy, the interactive damage per cycle decays as a power law function of the number of cycles following an underload or overload. The results indicate that small cycles, including those below the constant-amplitude fatigue limit, can contribute significantly to damage accumulation, and therefore small cycles should not be ignored when predicting fatigue lives for variable-amplitude histories.