Multi-mechanism damage-coupled constitutive model for ratchetting-fatigue interaction of extruded AZ31 magnesium alloy

Abstract

Magnesium (Mg) alloys have attracted much attention because of their advantage in lightweight design, and a theory of fatigue damage is a key issue for their engineering applications. Therefore, to capture the failure process of extruded AZ31 Mg alloy under complex stress states, a multi-mechanism damage-coupled constitutive model is constructed to reasonably describe the ratchetting-fatigue interaction of extruded AZ31 Mg alloy in the framework of continuum damage mechanics. Based on the multi-mechanism cyclic plastic constitutive model, the so-called pure fatigue damage caused by the plastic deformation resulted from different mechanisms (i.e., dislocation slipping and twinning/detwinning) is innovatively considered. Thus, two distinct evolution rules are formulated to account for two pure fatigue damage parts, respectively, and specifically addressing the coupling of such two damage parts. In addition, in view of significant ratchetting occurred in the extruded AZ31 Mg alloy under the loading conditions with high mean stresses and at elevated temperatures, additional damage caused by ratchetting (denoted as ratchetting damage) is also introduced into the evolution rule of total damage variable. Compared with the experimental results, the multi-mechanism damage-coupled constitutive model proposed in this paper can effectively predict the whole-life ratchetting and fatigue life of extruded AZ31 Mg alloy under different deformation mechanisms and at elevated temperatures.