Effect of pre-strain on low cycle fatigue strength of austenitic stainless steel SUS316NG

Low cycle fatigue tests were performed for austenitic stainless steel, SUS316NG. The effect of pre-strain history on the crack growth was investigated in detail using cellulose acetate replicas. In strain range, Δe = 2.5 % and 8 % constant tests, cracks initiated from slip bands near grain boundary, while intergranular crack initiations were prevailing when Δe = 16 % constant test. The cracks grew with several coalescences until the final fracture. Pre-strains were applied at Δe = 8 or 16 % followed by fatigue loading at Δe = 2.5 %.These tests revealed that contribution of cyclic strain hardening and softening were remarkable depending on the pre-strain history. Crack growth characteristics were well evaluated by J integral range, ΔJ, which is an elasto-plastic fracture mechanics parameter. Crack growth characteristic under Δe-constant tests as well as the pre-strained tests coincided with that measured by fracture mechanics experiment. Fatigue crack growth rates were predicted by elasto-plastic fracture mechanics approach in conjunction with cyclic stress-strain response.