Abstract
It has been clarified in the previous investigation that the intensity of initial dislocation locking makes an important contribution to the fatigue limit at room temperature, through an increase in kw in the relationship, σw0=Cw+kwd-1/2. In this paper, the nonpropagating crack length lnpc, the grain size d and the ratio r=lnpc/d were observed. The results were analyzed by Weibull's distribution function and thereafter the role of cyclic strain aging on the fatigue limit at room temperature was discussed. The following results were obtained.
(1) Most of microcracks remained within a grain (one grain type). A small number of cracks (about 24%), however, grew over several grains, beyond grain boundaries and then arrested (several grains type). When the nonpropagating cracks were separated into these two types, the shape parameter of lnpc was nearly equal to that of the grain size. When they were treated as one group, the distribution of lnpc was expressed well by using the Weibull distribution of mixed type.
For a small number of cracks, the crack tips nearly reached grain boundaries, that is, r=1, but the crack tip of most cracks (about 84%) stayed within a grain.
(2) The cyclic strain aging may contribute to crack arrest in the course of crack growth until a crack reaches grain boundaries. The effect of crack arrest results in an increase in other characteristic value Cw.
