Abstract
In order to investigate high cycle long life fatigue characteristics, cantilever-type rotary bending fatigue tests were conducted in an open environment at room temperature using hourglass shaped specimens of high carbonchromium bearing steel. The S-N curve obtained was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed 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. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. An examination of the initial stress intensity factor range of fracture origin, ΔKini, showed that the ΔKini of the slip governed fracture mode was larger than the threshold stress intensity factor range of the surface crack growth, ΔKth. On the other hand, cracks originate and propagate from inclusion at subsurface with smaller ΔKini than the slip-originating crack and the ΔKth.
