Strength and signature of force networks in axially compacted sphere and non-sphere granular media: micromechanical investigations

Abstract

Compaction characteristics of granular materials subjected to axial loading are investigated for both sphere and non-sphere granular assemblies. The computational study is based on the discrete element method (DEM). The compressive stress–strain relation obtained from three-dimensional DEM simulations is compared with that of an idealized two-dimensional plane-strain compression test and physical experiments using a bronze sphere assembly. We observed good agreement between the experimental and three-dimensional DEM simulation results, while two-dimensional simulations significantly underestimate the stiffness of particulate bed, particularly at large strains. This demonstrates that two-dimensional analysis is generally inadequate to model the compaction characteristics of granular systems. We performed a detailed analysis on the force–transmission characteristics of granular materials at microscopic level and present a connection between the directional orientation of force-networks and the invariants of the macroscopic stress tensor: the non-sphere systems were able to build up a strongly anisotropic network of heavily loaded contacts. Several complex phenomena, both geometric and kinematic, that are operative in sphere and non-sphere assemblies due to inter-particle interactions during compression are presented here. It is often assumed that the ratio of invariants of the stress tensor is uniform and constant in uni-axial compression tests. Our results show that the ratio of invariants of the stress tensor is non-uniform and non-constant even when the granular assemblies are subjected to the so-called uni-axial compressive loading, which is in agreement with other recent studies (e.g. Gu et al 2001 Int. J. Plasticity 17 147) performed using the finite element method. The non-homogeneous characteristics that are reported at the particulate scale need to be accounted in considering possible continuum models for the granular systems.

Discrete particle simulation of shock wave propagation in a binary Ni+Al powder mixture

“…15,23 Stress can only be transmitted through interparticle contacts, 23,24 which vary both spatially and temporally. Strictly speaking, such nonequilibrium behavior is not representative of a true shock, but a "catastrophic" shock, as suggested by Graham.…”

Evaluating the aggregate structure in hot-mix asphalt using three-dimensional computer modeling and particle packing simulations

“…Because of this strong dependence on particle arrangement, contact forces will usually be distributed in a complex, non-uniform manner, even when an isotropic and homogeneous assembly of particles is subjected to uniform loading (Antony et al 2005). This complex behavior is revealed in photoelastic studies of two-dimensional discs reported by several investigators (e.g., Drescher and Jong 1972;Behringer et al 1999;Hartley and Behringer 2003).…”

“…The weak force chains are comprised of contacts that carry less than average contact force, and the strong force chains carry a greater than average force. Sliding occurs predominantly within the weak chains, and these contacts primarily contribute to the mean stress (Antony et al 2005).…”

“…Induced anisotropy is the material anisotropy introduced into the sample by the consolidation procedure, and inherent anisotropy stems from the shape of the particles and the way the bedding was formed (Molenda and Horabik 2004). Using DEM simulations, Antony et al (2005) performed a detailed analysis on the force-transmission characteristics of granular materials at microscopic level and presented a connection between the directional orientation of force networks and the invariants of the macroscopic stress tensor: the nonsphere systems were able to build up a strongly anisotropic network of strong force-transmitting contacts.…”

Stress Fluctuations in Triaxial Testing Of Angular Grains

“…Although this method was developed firstly for describing the mechanical behaviour of rock blocks, Cundall and Strack [6] extended the technique to the analysis of granular soils. Over decades, the DEM has been a powerful tool to simulate the mechanics of distinct particles due to its compatibility [7][8][9][10][11][12]. Fortunately, due to the fast processing capabilities of today's computers, model simulations can be performed more effectively.…”