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 quas i-static biaxial loading at small to moderate strains. Deformations within individual voids were computed from the relative motions of surrounding par ticles. Evolution of the local fabric was measured in terms of void-based p arameters, including effective void ratio, void cell valence, and shape-elo ngation of the voids, all of which increased monotonically during loading. A direct correlation was measured between local void shape and dilation, wh ich accounts for the transition from compressive to dilatant behavior. Defo rmation was very nonuniform at the microscale of individual voids. The pred ominant deformation structures were thin obliquely trending bands of void c ells within which slip deformation was most intense. These "microbands" app eared spontaneously throughout the test, even at the start of loading. The microbands ranged in thickness between one and four particle diameters. Unl ike shear bands, the microbands were neither static nor persistent: they wo uld emerge, move, and disappear. Their orientation angle increased as defor mation proceeded. Dilation was slightly larger within the microbands than i n the surrounding material. Void shapes within the microbands were somewhat elongated, with an elongation direction that was related to the orientatio ns of the microbands. Energy dissipation was concentrated within the microb ands, even at small strains. In a small cycle of loading and unloading, loc al fluctuations in the elastic and plastic slips occurred in opposite direc tions. No spatial relation was found between the deformation microbands and chains of the most heavily loaded particles. Particle rotations were struc tured, with the most rapid rotations occurring within and near microbands. The rotations tended to relieve sliding between most particles, but transfe rred the sliding to a few contacts at which frictional slipping was most in tense. (C) 1999 Elsevier Science Ltd. All rights reserved.