Abstract
Rotating bending fatigue tests were carried out on notched specimens of an age-hardened Al-alloy, an annealed 0.17% steel and a heat-treated 0.54%C steel. Microscopic observations on the surfaces of notch roots were made successively by using the plastic replica method with special attention being given to the starting points of fatigue cracking. The relationship between the crack initiation process and the notch sensitivity in fatigue was investigated.
The main results obtained are as follows;
(1) Initiation process of one grain size crack was different from one metal to another. In an Al-alloy, the crack initiated at a much smaller region than the grain size and then propagated toward the grain boundary. This process corresponds to the so-called stage I crack propagation process. In the two steels the region to become a crack in the future was damaged as a whole, with almost no increase in the size on the specimen surface, and gradually turned into a crack. This process in each steel is essentially different from the later crack propagation process.
(2) An Al-alloy and a heat-treated steel were more notch sensitive in fatigue than an annealed steel. This is ascribed to the difference in thickness of the surface layer affecting the crack initiation among the metals used.
