Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relations between those changes and the crack initiation and propagation processes are discussed on the basis of microscopic observations and cellulose acetate replica observations. Strain-controlled fatigue tests were conducted at three constant strain ranges. Surface roughness was measured periodically during the tests, and three roughness parameters were calculated: arithmetic mean roughness Ra, surface profile maximum height Rmax, and maximum valley depth Rv. Until the middle of fatigue life, all three increased linearly with the number of cycles regardless of the strain range, and their rates of increase became smaller with decreasing strain range. Surface observation revealed that small cracks initiated very early in fatigue life, propagated slowly until the middle of fatigue life, and then grew rapidly. Changes in surface roughness are therefore sensitive to fatigue loading even when cracks are very small and crack detection is difficult. The results suggest that surface roughness can probably be used to assess fatigue damage because until the middle of fatigue life it increases linearly with the number of cycles. The definition of each roughness parameter and the changes show that Rmax and Rv are suitable for damage assessment.

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