Rotation bending fatigue tests were performed. The higher the stress level, or the shorter the fatigue life at the same stress level, the larger is the number of main fracture origin. At the stress level close to the fatigue strength at 10
7 cycles, fracture originates mostly at a point in the specimen endured above certain number of cycles. The relation between the fatigue life and the number of main fracture origin is approximately estimable on the P-N
f diagram. The transition site lies at a position on the fracture surface nearer to the specimen surface, as the stress level becomes higher or the fatigue life shortens at the same stress level. At the stress level close to the fatigue strength at 10
7 cycles, the transition site lies at nearly a constant depth in the specimen endured above certain number of cycles. The relation between the fatigue life and the depth to the transition site is also approximately estimable on the P-N
f diagram.
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