Initiation and Propagation Behavior of High Cycle Fatigue Cracks on HSP-treated Type 316L steel(Fatigue 2)

Abstract

In order to investigate fatigue crack behavior of the hard-shot peening (HSP) treated Type 316L austenitic stainless steel, the rotating bending fatigue test were carried out. As for a series of studies for rotating bending fatigue properties in the HSP-treated type 316L, the mechanism of fatigue strength improvement was investigated and estimation method of HSP-treated austenitic stainless steel was suggested. In this study, observation of a longitudinal section with the optical microscope, a fractographical observation with the SEM, surface crack observation with the replication technique and non-destructive inspection with the SH-ultrasonic wave were performed for discussing the crack behaviors in detail. As the results of these investigations, the fatigue crack behavior at the early fatigue propagation stage was clarified. (1) Although the fatigue crack initiated at the surface in early stage of fatigue, the crack does not propagate during the fatigue process until just before the final fracture. (2) The fatigue crack propagates toward inside of the radial direction at first, and followed gradually changing to axial direction at a depth of about lOOμm from surface. (Generation of axial-direction crack) (3) Secondary fatigue crack initiates from the axial-direction crack and propagates toward inside of the radial direction.(4) After the fatigue crack propagated radically for a certain depth, the surface fatigue crack propagated quickly at the final fracture.