Multiaxial creep-fatigue behavior and damage mechanisms of 316L stainless steel at 550 °C: effects of strain ratio, non-proportionality and holding type

Abstract

This study investigates the multiaxial creep-fatigue behavior of 316 L stainless steel at 550 °C affected by strain ratio, non-proportionality, and holding type. Experimental tests, including multiaxial low cycle fatigue (MLCF) and creep-fatigue interaction (MCFI), were conducted under a uniform von Mises equivalent strain amplitude of 0.4 %. Macro-mechanical responses and microstructural evolution were analyzed via internal stress decomposition and advanced micro-characterization techniques. In which a novel method was proposed to extract equivalent internal stresses and inelastic strain energy densities under multiaxial non-proportional fatigue conditions. Results reveal that non-proportional loading induces significant hardening, peaking at a strain ratio of $\sqrt{3}$, which is dominated by back stress. Holding period causes lower stress amplitude and reduces fatigue life, with axial holding having a stronger life reduction than shear holding for non-proportional multiaxial fatigue. Microstructure analysis reveals that non-proportional loading promotes dislocation cross-slip, forming equiaxed dislocation cells and increasing geometrically necessary dislocation density. Moreover, creep-fatigue interaction facilitates dislocation climb and elongated dislocation cells to accommodate additional inelastic deformation. Furthermore, the extracted equivalent inelastic strain energy density and the maximum normal stress complement each other as damage parameters for fatigue life, suggesting their combined use for damage evaluation.