Synopsis The effect of induction-hardened depth on torsional fatigue of hole notched specimens was investigated. The results are as follows. 1 Induction-hardened specimens show the best torsional fatigue strength when the effective case depth / radius ratio is about 0.5. 2 In the case of shallow case depth, the cracks initiate at the hardened / non-hardened boundary. On the other hand, the fracture of deeply hardened specimen originates at the surface layer. 3 The cracks initiate at the hole

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... face in the normal direction of principal stress, because the principal shear stress is smaller than principal stress at hole surface.