How do interparticle contact friction, packing density and degree of polydispersity affect force propagation in particulate assemblies?

Interparticle contact friction, packing density and polydispersity are known to be major contributors to the macroscopic strength of particulate assemblies, that is, their bulk resistance to deformation. For example, when a solid object penetrates a particulate material, the penetration resistance (i.e. the force opposing the object) increases concomitantly with an increase in the degree of polydispersity, packing density and interparticle friction. To establish the underlying mechanisms by which these properties govern the macroscopic response, we characterize quantitatively force propagation at length scales beyond that of the interparticle contact region. Using data derived from discrete element simulations of a two-dimensional granular assembly subject to indentation by a rigid flat punch, we examine the properties of force chains and the force chain network as they evolve during the course of the deformation. Findings indicate that increasing interparticle friction, packing density and degree of polydispersity promotes the formation of straighter chains and a greater degree of branching in the force chain network. Although the force chain length appears to be independent of friction and polydispersity, on average, denser systems tend to favour shorter chains. Thus, straighter and shorter force chains, combined with a greater degree of branching in the force chain network, result in a macroscopically stronger granular material.