RESUMEN
We investigate highly polydisperse packings subjected to simple shear by contact dynamics simulations. A major unsolved issue is how granular texture and force chains depend on the size polydispersity and how far they influence the shear strength. The numerical treatment was made possible by ensuring the statistical representation of particle size classes. An unexpected finding is that the internal friction angle is independent of polydispersity. We show that this behavior is related to two mechanisms underlying the stability of force chains: (i) The class of largest particles captures strong force chains, and (ii) these chains are equilibrated by weak forces carried by increasingly smaller particles as the size span broadens. In the presence of adhesion between particles, the Coulomb cohesion increases with size polydispersity as a result of enhanced force anisotropy.
RESUMEN
We present a systematic investigation of the morphology and space-filling properties of polydisperse densely packed granular media in two dimensions. A numerical procedure is introduced to generate collections of circular particles with size distributions of variable shape and span constrained by explicit criteria of statistical representativity. We characterize the domain of statistically accessible distribution parameters for a bounded number of particles. This particle generation procedure is used with two different deposition protocols in order to build large close-packed samples of prescribed polydispersity. We find that the solid fraction is a strongly nonlinear function of the size span, and the highest levels of solid fraction occur for the uniform distribution by volume fractions. As the span is increased, a transition occurs from a regime of topological disorder where the packing properties are governed by particle connectivity to a regime of metric disorder where pore-filling small particles prevail. The polydispersity manifests itself in the first regime through the variability of local coordination numbers. We observe a continuous decrease of the number of particles with four contacts and the growth of two populations of particles with three and five contacts. In the second regime, radial distribution functions show that the material is homogeneous beyond only a few average particle diameters. We also show that the packing orientational order is linked with fabric anisotropy and it declines with size span.
RESUMEN
We investigate the macroscopic mechanical behaviour of wet polydisperse granular media. Capillary bonding between two grains of unequal diameters is described by a realistic force law implemented in a molecular-dynamics algorithm together with a protocol for the distribution of water in the bulk. Axial-compression tests are simulated for granular samples at different levels of water content, and compared to experiments performed in similar conditions. We find good agreement between numerical and experimental data in terms of the rupture strength as a function of water content. Our results show the importance of the distribution of water for the mechanical behaviour.
RESUMEN
We analyze stress transmission in wet granular media in the pendular state by means of three-dimensional molecular-dynamics simulations. We show that the tensile action of capillary bonds induces a self-stressed particle network organized in two percolating "phases" of positive and negative particle pressures. Various statistical descriptors of the microstructure and bond force network are used to characterize this partition. Two basic properties emerge: 1) the highest particle pressure is located in the bulk of each phase; 2) the lowest pressure level occurs at the interface between the two phases, involving also the largest connectivity of the particles via tensile and compressive bonds. When a confining pressure is applied, the number of tensile bonds falls off and the negative phase breaks into aggregates and isolated sites.
RESUMEN
We present a molecular-dynamics study of force patterns, tensile strength, and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Nonuniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as the strongest tensile contacts begin to fail. We find that the tensile strength is well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage or outward from the center upon swelling. We show that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution.