STRESS CONCENTRATION EFFECT OF NONMETALLIC INCLUSIONS AND ITS EFFECT ON FATIGUE CRACK INITIATION IN RAIL STEEL

Fatigue crack initiation has been examined under reverse bend loading conditions to determine the effect of sulfide stringer inclusions in fully pearlitic rail steels. All the specimens were machined either from the head of rails or from the web or tread of wheels. Fatigue initiation occurs at sulfide inclusions located near the surface. The initiation data show that the endurance limit in the rail steels is a function of the inclusion content when the sulfide stringer inclusions are aligned parallel to the specimen axis. The endurance limit is greatly reduced, and becomes independent of the inclusion content in the rail material, when the stringer inclusions are aligned perpendicular to the specimen axis. The endurance limit for the wheel material, which has a random sulfide orientation, is independent of inclusion content for the narrow range in inclusion content considered. A correlation between the endurance limit and the finite failure is proposed for these steels. The difference in the endurance limit and the failure between the rail and wheel steels can be explained in terms of an effective stress concentrator arising from the variation in inclusion morphology and orientation.