Dry granular matter, with infinite tangential friction, is modeled as a connected graph ofgrains linked by purely repulsive contacts. The degrees of freedom of a grain are non-slip rotationon, and disconnection from another. The material stability under shear (jamming) is ensured by oddcircuits of grains in contact that prevent the grains from rolling on each other. A dense granularmaterial with high stiffness-to-load ratio has two possible states: fragile solid, blocked by odd circuits,and dry fluid or bearing, in the absence of odd circuits, that flows under shear by creation and glide ofa pair of dislocations as in plasticity of continuous media. In this paper, we introduce the notions of blob, a region of the material containing only even circuits, and of critical contact that closes an oddcircuit. The granular material is then represented, at low energies and critical applied shear, as a chainof blobs connected by critical contacts. The transition between dry fluid and fragile solid occurs byintermittency.
Directional shear jamming of frictionless ellipses
