Triological Aspects and Fluidity of Dense Granular Flow

A key difficulty in describing the fluid-like behavior of granular materials is the transitional regime between the fully dynamic regime, i.e. the grain-inertial regime, to a quasi-static one. In the present investigation, we aim to establish a constitutive relation between stress and strain rate for a granular layer over a range of deformation rates within the transition/mixed regime. Our objective is to understand the relationship between granular flow and tribological-related issues, such as friction and stress behaviors, in terms of dependences on packing density, shear rate, and grain size. For the experimental setup, we utilize a Tribo-Rheometer to infer shear stress directly from the applied torque through steps of constant velocities. The configuration is a torsional-plane-shear geometry between two plates and the granular layer is confined laterally by a self-lubricating Teflon cylindrical sleeve. To analyze granular motion, we consider the powder layer as a compressible, frictional, athermal, and elasto-plastic material with solid, fluid, and gas-like properties. The granular flow regimes, defined by the dominant mode of interaction, are then analyzed as a function of different dimensionless parameters to illustrate the process that is controlling the tribological behavior. We find that shear stress and normal stress both decrease with increasing shear rate within the transition regime between the two limiting regimes. Also, the granular flow up to the grain-inertial regime shows a Coulomb-like friction behavior with no correlation to shear rate. However, the granular sample displays a quadratic rate-dependence of friction coefficient leading into and within the grain-inertial regime. A decrease in grain-size results in a dramatic increase in friction at the boundary.