Investigating the relation between elastic and relaxation properties of dry, frictional granular media during shear deformation

Using discrete element simulations based on molecular dynamics, we investigate the mechanical behavior of two-dimensional sheared, dry, frictional granular media in the "dense" and "critical" regimes. We find that this behavior is partitioned between transient stages and a final stationary stage. While the latter is macroscopically consistent with the predictions of the viscous μ(I) rheology, both the macroscopic behavior during the transient stages and the overall microscopic behavior suggest a more complex picture. Indeed, the simulated granular medium exhibits a finite elastic stiffness throughout its entire shear deformation history, although topological rearrangements of the grains at the microscale translate into a partial degradation of this stiffness, which can be interpreted as a form of elastic damage. When letting the system relax under constant volume at different stages of shear deformation, the relaxation of stresses follows a compressed exponential, also highlighting the role of elastic interactions in the medium, with residual stresses that depend on the level of elastic damage. The relations we establish between elastic and relaxation properties point to a complex rheology, characterized by a damage-dependent transition between a viscoelastoplastic and a viscous behavior.