Fatigue Crack Growth Behavior at Stage 2 and Examination of Oxide Induced Crack Closure

Abstract

The fatigue crack growth behavior at Stage 2 was investigated on two kinds of Cr-Mo steel from the viewpoints of the crack closure and the fracture mechanism. It was confirmed that the crack growth rate at Stage 2a and Stage 2b was correlated with the effective stress intensity factor range, ΔK_eff, regardless of the strength level and the stress ratio. The crack growth rate at Stage 2c, on the other hand, was controlled by the maximum stress intensity factor, K_max, regardless of the stress ratio, as the area percentage of the pseudo-monotonic fracture became more than 70%. Then, it was experimentally clarified that the oxide deposits produced by the fretting of fracture surfaces participated in the crack closure at the very low (near-threshold) stress intensity factor range.