Transient rheology and the direct velocity effect in simulated sheared granular materials*

Here we use short-range molecular dynamics simulations to examine the transient rheology of confined sheared granular materials in response to perturbations in sliding velocity within the quasi-static shearing regime (inertial number 10^-8 to 10^-1). The granular systems are composed of spherical and disk-shaped grains that interact with each other via the Hertzian and Hookean contact laws in normal direction and a constant grain-grain friction coefficient in tangential direction. We find that sheared layers show an immediate transient effect (“direct velocity effect”) following velocity perturbations, the size of which depends on the ratio of fluctuating kinetic energy (δE_k) to the stored elastic potential energy (U_E) of the layer. The Hookean simulations in low-sliding velocity show low δE_k values and no appreciable direct velocity effect. Our observations can be explained by a modified version of a thermally-activated creep model for the direct velocity effect seen in crystalline materials, that instead uses energetic terms measured and estimated for the granular layer. Using these findings, we can begin to explain a possible source for a similar transient friction effect seen in fault rocks and other geological shear zones, referred to as the rate- and state-dependent friction.