抄録
In order to clarify the fatigue process of comparatively hard materials under a combined stress state, electron and optical microscopic observations were made successively on torsional fatigue damage on the surface of quenched and tempered 0.54%C steel by using the two stage plastic replica method which makes it possible to observe the fatigue process in reversed sequence, with special attention being given to the starting points of fatigue cracking. The process was then compared to that under rotating bending fatigue.
The following conclusions were reached:
(1) The process of crack initiation is such that the region to become a crack in the future is damaged as a whole without increasing the size at the specimen surface and is gradually turned into a crack.
This process of crack initiation is essentially different from that of crack propagation, and the difference between these two processes was also observed in annealed steel. The process of crack initiation under torsional fatigue or rotating bending fatigue of the material used is, however, slightly different from that of annealed steel; the process in the material used is such that minute portions disrupted by the repetition of slip appear and scatter discretely over the region to become a crack in the future, and then they are joined together to become a crack.
(2) The torsional fatigue crack initiated in the direction of maximum shearing stress propagates slightly in the same direction and then follows the direction perpendicular to that of maximum tensile stress. In this instance, the feature of the deformation of the material in the vicinity of the crack tip is similar to that of rotating bending fatigue at the beginning of the tensile mode of crack propagation.
