Abstract: According to fracture modes, the S-N curve obtained from cantilever-type rotary bending fatigue tests using hourglass shaped specimens of high carbon-chromium bearing steel is clearly classified into two groups of different crack origin. One is called the S-N curve of surface fracture mode. It is governed by a crystal slip on the specimen surface, which occurs in the region of short fatigue life and high stress amplitude level. The other is called one of internal fracture mode. It is governed by nonmetallic inclusion at subsurface, which occurs in the region of long fatigue life and low stress amplitude. Because fatigue fractures of internal fracture mode occurs in a wide range of stress amplitude not only below the Fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit, the shape of the S-N curve for the material used in this study is different from a step-wise one reported in previous literature and is characterized as duplex S-N curves. Through the detailed observation of fatigue crack origin and calculation of crack initial size parameter, mechanisms for internal fracture mode are discussed and fatigue life prediction procedure is indicated based on crack initial size parameter.

Key words: Bearing steel, Fisheye, Internal crack, Nonmetallic inclusion, S-N curve, Super-long life fatigue, Surface crack