FATIGUE STRENGTH OF SHAFT WITH A KEY GROOVE UNDER COMBINED BENDING AND TORSION.

Stepped specimens with or without key ways were used and fatigue tests were carried out under combined stress of reversed torsion and reversed plan bending. It was found that fatigue strength increased in the order of bending, torsion and combined bending and torsion. This tendency was more remarkable as the fatigue life became longer. Fracture on the surface of shafts with key ways was located in most cases at the end of the key way in the parallel portion of the specimen regardless of testing methods. The location of crack initiation and the direction of crack propagation in the shafts with key ways subjected to fatigue tests under combined stress of bending and torsion showed an intermediate behavior between those of bending and torsion. Cracks initiated in the plane of the principal stress, especially in the early stage of fatigue process.