In high-strength steel, fatigue fracture initiating from internal inclusions occurs in the very high-cycle fatigue region; however, the fracture behavior under cyclic shear stress has not been elucidated yet. In this study, ultrasonic torsional fatigue tests and ultrasonic axial fatigue tests were performed on the same bearing steel to compare the fracture behaviors. The effect of load type on very-high-cycle fatigue characteristics was also examined. Both the torsional and axial fatigue tests resulted in fracture originating from inclusions, and an optically dark area (ODA) was observed in the vicinity of each fracture origin; however, no difference in load type was recognized in the relationship between the ⊿K value, obtained from the inclusion size and ODA size, and the number of cycles. Nevertheless, there are differences in the types of inclusions that cause fracture. It has been found that, in the case of the torsional fatigue test, inclusions elongated in the rolling direction tend to become the originating points of fractures.
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