Computer simulation of model cohesive powders: Plastic consolidation, structural changes, and elasticity under isotropic loads

The quasistatic behavior of a simple two-dimensional model of a cohesive powder under isotropic loads is investigated by discrete element simulations. We ignore contact plasticity and focus on the effect of geometry and collective rearrangements on the material behavior. The loose packing states, as assembled and characterized in a previous numerical study [Gilabert, Roux, and Castellanos, Phys. Rev. E 75, 011303 (2007)], are observed, under growing confining pressure P, to undergo important structural changes, while solid fraction Phi irreversibly increases (typically, from 0.4-0.5 to 0.75-0.8). The system state goes through three stages, with different forms of the plastic consolidation curve, i.e., Phi as a function of the growing reduced pressure P* = Pa/F-0, defined with adhesion force F-0 and grain diameter a. In the low-confinement regime (I), the system undergoes negligible plastic compaction, and its structure is influenced by the assembling process. In regime II the material state is independent of initial conditions, and the void ratio varies linearly with ln P [i.e., Delta(1/Phi) = lambda Delta(ln P*)], as described in the engineering literature. Plasticity index lambda is reduced in the presence of a small rolling resistance (RR). In the last stage of compaction (III), Phi approaches an asymptotic, maximum solid fraction Phi(max), as a power law Phi(max)-Phi proportional to (P*)(-alpha), with alpha similar or equal to 1, and properties of cohesionless granular packs are gradually retrieved. Under consolidation, while the range xi of fractal density correlations decreases, force patterns reorganize from self-balanced clusters to force chains, with correlative evolutions of force distributions, and elastic moduli increase by a large amount. Plastic deformation events correspond to very small changes in the network topology, while the denser regions tend to move like rigid bodies. Elastic properties are dominated by the bending of thin junctions in loose systems. For growing RR those tend to form particle chains, the folding of which, rather than tensile ruptures, controls plastic compaction.