Changes in surface roughness during low cycle fatigue process of austenitic stainless steel

Changes in surface roughness during low cycle fatigue loading were investigated on austenitic stainless steel, SUS316NG, commonly used in the piping systems of nuclear power plants. The fatigue damage process, including crack initiation and propagation, was observed using cellulose acetate replicas, and the relationship between the process and changes in surface roughness was discussed. Strain-controlled fatigue tests were conducted on mirror polished specimens at constant strain range conditions Δε = 8, 4, and 1%. During the cyclic loadings, the surface roughness was measured at cycles determined with respect to the usage factor UF_pre. As a result, the surface roughness was found to increase roughly linearly until approximately UF_pre = 0.4 regardless of the strain range conditions. The rate at which surface roughness increased with UF_pre became smaller with decreasing applied strain range. In the damage process, small cracks were observed to initiate very early in fatigue life. The cracks propagated slowly until about the middle of fatigue life, however it grew rapidly after around UF_pre = 0.6. These results showed that the change in surface roughness is sensitive to fatigue loading even when cracks are very small and crack detection is difficult. The obtained results suggest that surface roughness can probably be used to assess fatigue damage until the middle of fatigue life because of its linear increase with respect to the number of cycles. (author)