Influence Factors of the Formation of ODA in Ultralong Fatigue Life Regime

摘要

Recent studies have provided a warning that fatigue failure can occur in the life regime exceeding N = 10~7 and at stresses lower than the conventional fatigue limit. In the present study, the mechanism of fatigue failure in extremely high cycle regime was studied on bearing steels, SAE 52100. Specimens having a longer fatigue life had a particular morphology beside the inclusion at fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area, ODA. The ODA looks a rough area when observed by SEM and AFM. In order to investigate the growth of ODA with cycles, the fatigue tests of multi-step loadings were carried out and the formation of ODA was compared with those of the specimens tested under a constant amplitude loading. The condition for the critical size of ODA for the start of conventional fatigue crack growth mechanism was analysed by fracture mechanics. The critical stress intensity factor range _(ODA) can be correlated with the threshold stress intensity factor'range ΔK_(th) for small cracks expressed by the (area) parameter model.