Fatigue properties of 316 stainless steel and its failure due to internal cracks in low-cycle and extremely low-cycle fatigue regimes

It is important to know residual fatigue strength of materials in the extremely low-cycle fatigue (ELCF) regime in order to assess structural integrity after large cyclic strain caused by an earthquake. In the ELCF regime, it was pointed out that not only surface cracking but also additional bulk damage reduces the fatigue life. In this study, the fatigue strength of stainless steel in the low-cycle and ELCF regimes and the effects of bulk damage on fatigue strength were investigated. Type 316 stainless steel was subjected to axial cyclic strain up to strain amplitude of 6%. Also, the change in fatigue life was investigated when the surface layer of the specimens was removed during the fatigue tests. It was shown that not only the surface cracking but also the bulk damage affected the fatigue life. It was also shown that the bulk damage consisted of two factors: internal cracking and local damage. Internal cracks were observed on the fractured surface when the strain amplitude was more than 1%, and in some cases the specimen was fractured by these internal cracks. Local damage reduced the incubation period before crack initiation and fatigue life decreased as the strain amplitude increased. In all cases, the experiment conducted under the strain amplitude εa = 0.5%, neither the internal cracks nor the local damage affected the fatigue life.