Local Rheology for Vibrated Granular Shear Flows

The rheology of granular flows under vibration is essential for understanding landslides, earthquakes and even river beds. Both endogenous and exogeneous sources of seismic waves can trigger catastrophic weakening as part of failure processes. Despite the ubiquity of the phenomenon, detailed study is still required to map out the effect of vibration on shear and separate the local and global effects that interact in complex, natural geometries. We computationally study the frictional properties of sheared granular media subjected to harmonic vibration applied at the boundary. Based on a dimensional analysis and DEM simulations, we show that system-wide weakening requires that the absolute amplitude of the displacement is sufficiently large relative to the grain size in addition to sufficiently high acceleration. We also examine frictional weakening from a local perspective, correlating local microstructure and granular temperature with the local stress tensor. We observe power law scaling functions that relate friction, shear rate, and granular temperature. Our results represent a step toward formulating a constitutive law for shear vibrated granular materials that could be used to solve for flows in complex geometries.