Surface and In‐depth Crack Propagation in High Temperature Low‐cycle Fatigue of Sus 316 Stainless Steel

Abstract

The low-cycle fatigue crack propagation behaviour of surface cracks in SUS316 stainless steel at 700"C, in both the surface direction and the in-depth direction, has been studied with special emphasis on the role of oxidation. The coalescence behaviour of surface cracks is essential for the process of crack propagation in high temperature low cycle fatigue, irrespective of the existence of oxidation effects. For sub-surface cracks the process of crack propagation is divided into two stages characterized by differences in fracture mode. In both stages, the in-depth crack propagation rate in air is higher than that in vacuum. This difference in crack propagation rate is the main reason for the decrease of fatigue life in air compared with that in vacuum. The crack propagation behaviour in the in-depth direction can be estimated from the conversion of the surface crack length into the subsurface depth by the use of an aspect ratio.