The effect of prestressing, such as partial unloading and perfect unloading, on the fatigue crack initiation life was investigated on the notched or precracked specimens of JIS SNCM 439 steel quenched and tempered at 473K.
The crack initiation life and the lower limit mean stress, under which the crack initiation does not occur within 107 cycles at a given stress range, was markedly increased by these two prestress methods, especially by the partial unloading method. The increase in lower limit mean stress by the partial unloading method was large when the range of cyclic stress was middle. The reason why the fatigue crack initiation life was increased by both of the prestress methods could be explained by the decrease of surface tensile stress at the notch root.
