Comparative study of uniaxial tension-compression and torsional fatigue behavior of 316LN stainless steel at 550 °C

This study systematically investigates the cyclic deformation behavior, internal stress evolution, and failure mechanisms of 316LN austenitic stainless steel under uniaxial tension-compression and torsional fatigue loadings at 550°C. Results show that the material exhibits similar cyclic stress responses under both loading modes, characterized by initial cyclic hardening followed by progressive softening. The duration of cyclic hardening is primarily governed by strain amplitude and remains nearly identical for the two loading modes, indicating a common underlying mechanism dominated by dislocation multiplication. However, torsional fatigue exhibits a significantly lower cyclic softening rate and longer fatigue life, which is attributed to the through-thickness stress gradient that reduces crack driving force during propagation. Internal stress analysis reveals that cyclic hardening is mainly controlled by the evolution of back stress associated with long-range dislocation interactions, while cyclic softening is dominated by the reduction of friction stress due to dislocation annihilation and rearrangement into low-energy structures. A unified fatigue life prediction model is proposed based on plastic strain energy, which highlights the distinct contributions of tensile and shear deformation to crack propagation.