308 高速度工具鋼SKH51の超長寿命疲労挙動と破壊形態(OS1-1 高強度材の超高サイクル疲労(1))(OS1 高強度材・表面改質材の疲労と破壊)  [in Japanese] 308 Gigacycle Fatigue Behavior and Fracture Morphology of High Speed Steel, JIS SKH51  [in Japanese]

Gigacycle fatigue characteristics of high speel steel, JIS SKH51 was investigated under cantilever-type rotary bending fatigue tests in an open environment at room temperature. As a result of fatigue tests, S-N curve was clearly classified into two types of fracture modes by the different crack origins. One fracture mode occurred at the surface inclusion in the region of short fatigue life and high stress amplitude level. The other was at the subsurface inclusion in the region of long fatigue life and low stress amplitude level. The transition of crack initiation site from surface to subsurface appeared at about the stress amplitude of 1050MPa and 2x10^4 cycles, which was not affected by the tempering temperature of the specimen. Fatigue fracture mechanisms were discussed through the detailed observation of subsurface crack initiation site.