For the purpose of studying how the torsional fatigue strength of steel shafts changes with the distance between a step and a key groove end, the completely reversed torsional fatigue tests were performed on the low carbon steel specimens having steps with a comparatively sharp fillet and a key groove, and the fatigue crack initiation and propagation were discussed macroscopically from the fatigue data obtained and the location of fracture.
The conclusions obtained are as follows:
(1) For the specimens with a key groove, the reversed torsional fatigue strength of the specimen with a groove end was about 10% lower than that of the specimen with a key groove extending over the reduced section of the specimen.
(2) On the fatigue crack initiation in the specimen with a groove end, as long as the circular arc of the key end was located in the portion of fillet, there existed an interaction between two notches of a filleted step and a key groove (that is, the “double notch effect”). On the other hand, the distance between a step and a groove end did not directly influence the fatigue strength.
(3) Fracture mostly occurred at a key end in the parallel portion (the reduced section) of the specimen.
(4) The three-dimensional stress concentration appeared at the groove end not only from the shape change in the transverse cross-section but also from that along the longitudinal section of the specimen.
