Low cycle fatigue behavior of a high nitrogen austenitic stainless steel under uniaxial and non-proportional loadings based on the partition of hysteresis loops

Total strain controlled low cycle fatigue tests under uniaxial loading and non-proportional loading have been conducted on a high nitrogen austenitic stainless steel (AISI 316LN). Based on the partition of hysteresis loops under both loading modes, the evolution of internal stress, effective stress and elastic modulus is presented. Firstly, the cyclic stress response and cyclic stress-strain curve of 316LN under both loading modes were determined mostly by the internal stress component. Therefore, the non-proportional hardening cannot be simply modeled by using the change of yield stress as in several commonly used plasticity models. Moreover, 316LN exhibited greater cyclic softening in the effective stress and internal stress compared with 316L. The behavior is related to the increasingly heavy destruction of the short range order in 316LN under cyclic loading. Finally, the greatly enhanced multiple-slip and cross slip due to the continuous rotation of principal stress orientation increase the effective stress and internal stress of 316LN under non-proportional loading.