Previous studies of fatigue under two-step loading have shown that the value of cumulative cycle ratio is usually smaller than unity when the first stress level is higher than the second, but in some cases it becomes larger than unity. This phenomenon may be related to the existence of work hardened layer in the specimen.
In the present study, rotating bending fatigue tests were performed by using the annealed or the pre-strained 0.21%C steel specimens under two-step loading. The behavior of fatigue cracks was observed successively by the replication method and the effect of the work hardened layer on the cumulative fatigue damage was examined. The main results obtaind are as follows.
(1) The Miner's rule holds good in the crack initiation process under double repeated stress in two stress levels.
(2) On the annealed specimens:
(a) When σ1<σ2, the value, of the cumulative cycle ratio based on fracture, n1/Nf1+n2/Nf2, is nearly equal to unity in general.
(b) When σ1>σ2, the value of n1/Nf1+n2/Nf2 is nearly equal to unity excepting the case where the cycle ratio of the first stress level, n1/Nf1, is small. In that case, the value of n1/Nf1+n2/Nf2 is often larger than unity. This may be due to the work hardening under the first stress level.
(3) On the pre-strained specimens:
(a) When σ1<σ2, the value of n1/Nf1+n2/Nf2 is larger than unity.
(b) When σ1>σ2, the value of n1/Nf1+n2/Nf2 is smaller than unity including the case where n1/Nf1 is small. This may be due to the work softening under the first stress level.
(4) The results of the pre-strained specimens agreed with the previous results showing that n1/Nf1+n2/Nf2>1 when σ1<σ2 and n1/Nf1+n2/Nf2<1 when σ1>σ2. Therefore, the validity of the Miner's rule depends upon the existence of work hardened layer.
