Grain size dependent shear instability induced by geometry softening for NC/UFG materials under dynamic loading

Adiabatic shear localization of viscoplastic materials has been an area of great interest in the past few decades. Many numerical and theoretical investigations have been performed, yet few have taken into account the influence of microstructure (grain size, e.g.) of the material. For instance, experimental evidence has suggested enhanced shear instability for some nanostructrued (NS) metals compared to their coarse-grained counterparts[1]. Recently, Joshi and Ramesh proposed a rotation diffusion mechanism for the quasi-static shear localization behavior of nanostructured materials. Since shear band formation is generally enhanced under dynamic loading where diffusive processes are no longer essential, the adiabatic shear localization behavior at high strain rates could be different. In this work, a geometry softening mechanism is presented to study the adiabatic shear instability of nanostructured metals under dynamic loading.