Quantitative Characterization of the Changes in Surface Topography of Austenitic Stainless Steel under Low Cycle Fatigue Loading

Low-cycle fatigue tests of SUS316NG austenitic stainless steel were conducted for several strain ranges (0.76, 1.26, 2.1, 4.1, and 8.1%) in which the specimen’s surface topography was regularly measured using a laser scanning microscope. The surface topographies obtained were analyzed by frequency analysis to separate the surface relief due to persistent slip bands (PSBs) from that due to crystal grain deformation. The PSBs-induced surface relief evolution and the grain-deformation-induced one were quantitatively evaluated by using arithmetic mean roughness Ra and arithmetic mean waviness Wa, respectively. The ΔRa and ΔWa, the increments in Ra and Wa from the initial values, increased with the usage factor (UF) for each strain range. Moreover, the rates of increase in ΔRa and ΔWa with respect to UF increased with the strain range. ΔWa/ΔRa was larger for higher strain ranges. This tendency of ΔWa/ΔRa indicates that the surface relief due to grain deformation develops more prominently than that due to PSBs for larger strain ranges. The results for Ra and Wa agree with the results of surface topography observation. On the basis of these results, a method was developed for estimating the UF of fatigued material and the applied strain range by using only the measured Ra and Wa.