Abstract
In the previous paper, the authors had discussed the effect of a high tensile load prior to stress cycling on fatigue strength. In the present study, in order to make clear the effect of a high tensile load during stress cycling on rotating bending fatigue strength, the following fatigue tests were carried out on smooth specimens of 0.33% carbon steel: a high tensile load was applied (i) during stress cycling in the ordinary fatigue test and (ii) at changing of stress amplitude in the two steps fatigue test.
The conclusions obtained are summarized as follows:
(1) The high tensile load σH=39.5kg/mm2, that is the lower yield stress, reduces the fatigue life. Especially, its effect is more pronounced at the lower stress level. This is due to non-uniformity of work hardening, as discussed in the previous paper.
(2) The high tensile load σH=60.0kg/mm2 increases the fatigue life at the higher stress level, but decreases at the lower stress level.
(3) There are two reasons to explain the effect of a high tensile load on fatigue life. One is the work hardening caused by the high tensile load. The other is the growth of cracks by the high tensile load to macro cracks from slip lines or micro cracks created during stress cycling. Therefore, the fatigue life is determined by the competitive process of both factors. It seems that the effect of residual stress caused by the high tensile load is very little.
