The respective influence of microstructural and thermal softening on adiabatic shear localization

It has recently been shown that dynamic shear failure of crystalline solids can be initiated by local microstructural changes (dynamic recrystallization, DRX), instead of the commonly assumed thermal softening mechanism. We systematically investigate the respective contribution of thermal and microstructural softening to the initiation of dynamic shear localization, by means of a fully coupled numerical model incorporating the two softening mechanisms in an adjustable manner. Our results indicate that, for those materials that exhibit early DRX, (e.g. Ti6Al4V), the role of thermal softening is negligible, whereas for materials with late (e.g. pure Ti) or no DRX, thermal softening effects become dominant. The strength of the thermomechanical coupling term (thermal softening) is found to determine the local temperatures, with the strongest effect being achieved in the absence of coupling, together with the formation of thermal "hot spots". Thermal softening is found to regulate the evolution of the local temperature, in the sense that the softened material both stores and dissipates smaller increments of strain energy. The results of this study allow for a general classification of the material proneness to dynamic shear localization as a function of its thermo-physical characteristics.