Abstract
The effect of induction-hardened depth on torsion fatigue strength of several types of notched specimens were investigated. The results are as follow 1) Induction-hardened specimens show the best torsion fatigue strength when the effective case depth/radius ratio is about 0.5. 2) In the case of shallow case depth, the fatigue cracks initiate at the hardened/non-hardened boundary. On the other hand, the fracture of deeply hardened specimens originates at the surface layer.
