Shear-weakening of the transitional regime for granular flow

Abstract

This paper experimentally investigates the rheology of dense granular flow through itssolid-like to fluid-like transition. Between the well-established flow regimes – quasi-static and grain-inertial – the physical description of the transition remains elusive. Our experiment uses a top-rotating torsional shear cell capable of ± 1 μm accuracy in height and 5 decades (10−3 − 100 rad s−1) in rotation rate. The data on beach sand shows that shear and normal stresses exhibit an inverse rate-dependence under a controlledvolume environment in the transitional regime, while in the limiting regimes the results are in agreement with previous work. Theshear-weakening stresses illustrate a previouslyunknown ‘dip’ with increasingshear rate. Under a controlled-pressure environment, however, the shear-compacting volume-fraction ‘peaks’ with increasing shear-rate. We combine these results from both configurations to infer a constitutive law based on a rate-invariant granular fluid compressibility. The formulation provides an equation-of-state for dynamic granular systems, with state variables of pressure, strain rate and free-volume-fraction. Fitting parameters from independent constant-volume and constant-pressure data shows good agreement in validating our model. Moreover, the degree of grain jaggedness is essential to the rate-dependence within the transitional regime. The results on the solid–fluid transitionmay elucidate the evolution of granular flow anisotropies.