The mechanism of fatigue failure in extremely high cycle regime was studied on bearing steels, SAE 52100. In case of tension-compression fatigue test, the S-N curve for high strength steels consists one straight line in contrast to the dual S-N curve observed in rotating bending. Specimens having a longer fatigue life had a particular morphology named ODA(Optically Dark Area) beside the inclusion at fracture origin. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed by the fracture mechanics. The critical stress intensity factor range DELTA K_(ODA) for the critical size of ODA can be correlated with the threshold stress intensity factor range DELTA K_(th) expressed by the area~(1/2) parameter model for small cracks. Fatigue design method for ultralong fatigue life regime was proposed by using the growth behavior of ODA, the statistics of extremes of inclusions, and the area~(1/2) parameter model.
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