The mechanism of fatigue failure in the ultra-high cycle regime was studied using a bearing steel, JIS SUJ2 (SAE 52100). In the case of tension-compression fatigue testing, the S-N curve for high strength steels consisted of a single straight line in contrast to the dual S-N curve observed in rotating bending. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behavior has been discussed by Murakami et al. In order to investigate further the influence of hydrogen trapped by various types of inclusions, four materials of different chemical compositions were prepared. Those specimens having a longer fatigue life had a particular morphology designated as ODA (Optically Dark Area) surrounding the inclusion at the fracture origin. In order to investigate the growth of ODA with cycling, fatigue tests involving multi-step loading were carried out, and the appearance of the ODA was
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