Spatial correlation and temporal evolution of plastic heterogeneity in sheared granular materials

Granular materials will yield and undergo serrated plastic flow when subjected to mechanical loading. The microscopic dynamics of serrated plastic flow in granular materials is of great importance for earthquake physics, condensed physics, and material science. It has been found the stress avalanches of granular materials is expected to be related to the spatial concentration and temporal evolution of the microscopic plastic events. Although the research on it is abundant, the mechanism of the initialization and localization of the plastic events within timescale much less than the structural relaxation time is still poorly understood. Limited by the temporal resolution of experimental techniques available, we turn to seek help from the computer simulation, i.e., the combined finite and discrete element method (FDEM). We use FDEM to model the simple shearing of an assembly of irregularly shaped particles and track the particle scale dynamics after the sample reaches steady state. Then we illustrate the initialization, annihilation, and merging of the zones of intense plastic activity over a wide range of timescales (i.e., strain scales). At small strain scales, nonaffine motions originate at particles experiencing large stress fluctuations.The cooperative dynamics and long-ranged dynamic heterogeneity is responsible for the animation and merging of plastic zones at the timescale close to the structural relaxation time, and the plasticity is manifested as structural rearrangement and shear transformation at large enough strain scales. The spatiotemporal variability of plastic activity is confirmed by the transient nature of the high mobile clusters.