Shear-weakening of the transitional regime for granular flow

This paper experimentally investigates the rheology of dense granular flow through its solid-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 controlled volume environment in the transitional regime, while in the limiting regimes the results are in agreement with previous work. The shear-weakening stresses illustrate a previously unknown ‘dip’ with increasing shear rate. Under a controlledpressure 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 ratedependence within the transitional regime. The results on the solid–fluid transition may elucidate the evolution of granular flow anisotropies.