Fatigue Damage Assessment of SUS316NG based on Surface Roughness Measurement and the Applicable Scope
Nao FUJIMURA, Takashi NAKAMURA, Hiroyuki OGUMA, Tomohiro NAKAJIMA and Tomoki SHINKO
Abstract:Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relationship between those changes and the fatigue damage processes, including evolution of surface relief due to active slip systems, were discussed on the basis of microscopic observations. Strain-controlled fatigue tests were conducted at three constant strain rangesƒ¢ƒÃ= 8, 4, and 1%. During the tests, surface roughness was measured periodically at cycles determined with respect to the usage factor UF, and then the surface was observed directly to clarify the surface morphology. As a result, until the middle of fatigue life, the arithmetic mean roughness Ra increased linearly with the number of cycles regardless of the strain range conditions. Clear changes in surface roughness were obtained even at a strain range as small as 1%. The results suggest that the surface roughness measurement can probably be used to assess fatigue damage because it increases linearly with the number of cycles until the middle of fatigue life. The rate at which surface roughness increased with UF became smaller with decreasing applied strain range. In the damage process, the surface became uneven because of active slip systems and the asperity became larger when the number of cycles increased. Surface observations revealed that the number of slip bands decreased and the development of the asperity became more difficult with decreasing applied strain range. The change tendencies of surface roughness were similar to those of the density of slip bands at each strain range. This shows that the rate at which surface roughness increased with UF is affected by the number of slip bands at each strain range condition. Key Words:Low cycle fatigue, Damage assessment, Surface roughness, Fatigue damage process, Austenitic stainless steel