Structured deformation in granular materials

Microscale deformations are investigated in numerical DEM experiments of a large two dimensional assembly of disks. The assembly was subjected to quasi-static biaxial loading at small to moderate strains. Deformations within individual voids were computed from the relative motions of surrounding particles. Evolution of the local fabric was measured in terms of void-based parameters, including effective void ratio, void cell valence, and shape-elongation of the voids, all of which increased monotonically during loading. A direct correlation was measured between local void shape and dilation, which accounts for the transition from compressive to dilatant behavior. Deformation was very nonuniform at the microscale of individual voids. The predominant deformation structures were thin obliquely trending bands of void cells within which slip deformation was most intense. These “microbands” appeared spontaneously throughout the test, even at the start of loading. The microbands ranged in thickness between one and four particle diameters. Unlike shear bands, the microbands were neither static nor persistent: they would emerge, move, and disappear. Their orientation angle increased as deformation proceeded. Dilation was slightly larger within the microbands than in the surrounding material. Void shapes within the microbands were somewhat elongated, with an elongation direction that was related to the orientations of the microbands. Energy dissipation was concentrated within the microbands, even at small strains. In a small cycle of loading and unloading, local fluctuations in the elastic and plastic slips occurred in opposite directions. No spatial relation was found between the deformation microbands and chains of the most heavily loaded particles. Particle rotations were structured, with the most rapid rotations occurring within and near microbands. The rotations tended to relieve sliding between most particles, but transferred the sliding to a few contacts at which frictional slipping was most intense.