Quantitative study of surface roughness evolution during low-cycle fatigue of 316L stainless steel using Scanning Whitelight Interferometric (SWLI) Microscopy

This paper describes the preliminary study on the evolution of the surface roughness in polycrystalline 316L austenitic stainless steel during low-cycle fatigue using an optical interferometric surface profiling technique. The arithmetic mean surface roughness of four regions with different stress levels was measured at incremental fatigue cycles until the fracture of the specimen. We discovered that there are two different mechanisms contributing to the surface roughness increases in the fatigued specimen. At the initial fatigue stages, the surface roughness increases were dominated by the development of the slip bands. However, the heights of the slip bands stopped growing at certain fatigue cycles. At later fatigue cycles, after the slip band height is saturated, the surface roughness increases are mainly contributed by the out-of-plane grain displacement. Both the slip band development and out-of-plane displacements are correlated to the local stress levels. Local permanent plastic strains were measured from the micrographic images. Analysis on the plastic strains along the loading direction and the transverse direction indicated that the out-of-plane grain displacement is likely to be contributed by the strain differences among individual strains.