Shear zone refraction and deflection in layered granular materials.

Refraction and deflection of shear zones in layered granular materials were studied experimentally and numerically. We show that (i) according to a recent theoretical prediction [T. Unger, Phys.

Extent of force indeterminacy in packings of frictional rigid disks.

Static packings of frictional rigid particles are investigated by means of discrete element simulations. We explore the ensemble of allowed force realizations in the space of contact forces for a given packing structure. We estimate the extent of force indeterminacy with different methods.

Unjamming due to local perturbations in granular packings with and without gravity.

We investigate the unjamming response of disordered packings of frictional hard disks with the help of computer simulations. First, we generate jammed configurations of the disks and then force them to move again by local perturbations. We study the spatial distribution of the stress and displacement response and find long range effects of the perturbation in both cases.

Shear zones in granular media: three-dimensional contact dynamics simulation.

The properties of shear zones forming in slow three-dimensional granular flow are investigated. We simulate a straight version of the split-bottom shear cell. It is shown that the same type of wide shear zones is obtained in the presence as well as in the absence of gravity.

Unjamming of granular packings due to local perturbations: stability and decay of displacements.

We study the mechanical response generated by local deformations in jammed packings of rigid disks. Based on discrete element simulations we determine the critical force of the local perturbation that is needed to break the mechanical equilibrium and examine the generated displacement field. Displacements decay as a power law of the distance from the perturbation point.

Refraction of shear zones in granular materials.

We study strain localization in slow shear flow focusing on layered granular materials. A heretofore unknown effect is presented here. We show that shear zones are refracted at material interfaces in analogy with refraction of light beams in optics.

Force indeterminacy in the jammed state of hard disks.

Granular packings of hard disks are investigated by means of contact dynamics which is an appropriate technique to explore the allowed force realizations in the space of contact forces. Configurations are generated for given friction coefficients, and then an ensemble of equilibrium forces is found for fixed contacts. We study the force fluctuations within this ensemble.

Frictional coupling between sliding and spinning motion.

The tangential motion at the contact of two solid objects is studied. It consists of a sliding and a spinning degree of freedom (no rolling). We show that the friction force and torque are inherently coupled.