Shearing of a confined granular layer: tangential stress and dilatancy

Friction of a slider on a granular layer: nonmonotonic thickness dependence and effect of boundary conditions

We investigate the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. The observed friction has minima for a small number of layers before it increases and saturates to a value that depends on the roughness of the sliding surface. We use an index-matched interstitial liquid to probe the internal motion of the grains with fluorescence imaging in a regime where the liquid has no significant effect on the measured friction. The shear profiles obtained as a function of depth show a decrease in slip near the sliding surface as the layer thickness is increased. We propose that the friction depends on the degree of grain confinement relative to the sliding surfaces.

Characterization of the material response in granular ratcheting

The existence of a very special ratcheting regime has recently been reported in a granular packing subjected to cyclic loading. In this state, the system accumulates a small permanent deformation after each cycle. The value of this permanent strain accumulation becomes independent of the number of cycles after a short transient regime. We show in this paper that a characterization of the material response in this peculiar state is possible in terms of three simple macroscopic variables. The definition of these variables is such that they can be easily measured both in the experiments and in the simulations. A thorough investigation of the micro- and macromechanical factors affecting these variables has been carried out by means of molecular-dynamics simulations of a polydisperse disk packing, as a simple model system for granular material. Biaxial test boundary conditions with periodically varying load were implemented. The effect on the plastic response of the confining pressure, the deviatoric stress, and the number of cycles has been investigated. The stiffness of the contacts and friction has been shown to play an important role in the overall response of the system. Especially illustrative is the influence of the peculiar hysteretical behavior in the stress-strain space on the accumulation of permanent strain and the energy dissipation.