Rotating bending fatigue tests have been carried out on smooth specimens of an annealed aluminum-killed steel, an annealed rimmed steel and a low temperature quenched rimmed steel. Successive observations on the surfaces of the specimens and fractographic observations have been made by SEM with special attention being given to the starting points of fatigue cracking, in order to investigate the effect of strain aging on fatigue crack initiation process.
The main results obtained are as follows;
(1) A low temperature quenched rimmed steel, which has the highest strain aging ability, had a higher resistance to initiation and propagation of fatigue cracks and had an about 50% higher fatigue limit comparing with other two steels. However, no qualitative difference was recognized among the steels in the process of initiation of a crack when the crack size was about the size of a grain. This process is such that the region, which is to become a crack in the future, is damaged as a whole with little increase in size at the specimen surface and gradually turns into a crack.
Moreover, this process is essentially different from the later crack propagation process. Fractography of the initial stage of fatigue cracking also seems to support the above statement.
(2) Non-propagating microcracks were observed on the unbroken specimen surface subjected to more than 107 cycles at the respective fatigue limit. Although some of them remained within the grain, the largest one propagated into neighboring grains and then stopped propagating.
