抄録
Stepwise S-N curves have been obtained at 500°C under rotating bending fatigue tests for the notched specimens of a low alloy steel SCM 435. The behavior of crack propagation under such fatigue conditions was examined, and the reason why such stepwise S-N curves appear and the effect of oxide formed at crack surface on the crack propagation were discusseed. The conclusions obtained were as follows:
(1) On the fracture surface failed at the longer life side in the stepwise S-N curve, a certain region from the root of notch, designated as A region, was covered with thick oxide layer.
(2) In the A region, a crack propagated with a very slow rate and the propagation rate scarcely depended on the crack length.
(3) By examining the change in compliance under rotating bending conditions, it was clarified that the crack opening level was increased by the oxide formed on the crack surface. It is considered that with increasing the crack opening level the effective stress intensity factor range decreases and then the crack propagation rate decreases.
(4) The stepwise change seems to occur at the boundary where the crack propagation rate slows down by the oxide formation.
