A numerical study of microstructure effect on adiabatic shear instability: Application to nanostructured/ultrafine grained materials

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 (e.g., texture, grain size) of the material. For instance, experimental evidence has suggested enhanced shear instability for some nanostructured metals compared to their coarse-grained counterparts. Recently, Joshi and Ramesh proposed a rotational 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 can be different. In this work, a geometry softening mechanism is presented to study the adiabatic shear instability of viscoplastic materials. We further use the analysis to study the adiabatic shear instability in nanostructured metals under dynamic loading. This mechanism is based on grain rotation and is directly related to the grain size of the material. The mechanism was implemented into the governing equations of adiabatic shear localization and a one-dimensional numerical model was employed by using the characteristic line method. The effects of strain rate hardening, strain hardening and initial microstructure condition have been studied numerically. Examples are given for some specific metals and the results are compared with those of experiments.