Basic Mechanical Properties of Wet Granular Materials: A DEM Study

Abstract

International audienceNumerical simulations, by the discrete element method (DEM), of a model granular assembly, made of spherical balls, are used to investigate the influence of a small amount of an interstitial wetting liquid, forming capillary bridges between adjacent particles, on two basic aspects of granular material rheology: (1) the plastic response in isotropic compression, and (2) the critical state under monotonic shear strain, and its generalization to steady, inertial flow. Tensile strength F0=πΓa, in contacts between beads of diameter aa joined by a small meniscus of a liquid with surface tension Γ, introduces a new force scale and a new dimensionless control parameter, P∗=a2P/F0, for grains of diameter a under confining stress P. Under low P*, as cohesion dominates, capillary cohesion may stabilize very loose structures. Upon increasing pressure P in isotropic compression, such structures gradually collapse. The resulting irreversible compaction is well described by the classical linear relation between logP* and void ratio in some range, until a dense structure forms that retains its stability without cohesion as confinement dominates for large P*. In steady shear flow, with uniform velocity gradient γ˙=∂v1/∂x2γ˙=∂v1/∂x2 under normal stress P=σ22, the apparent internal friction coefficient, which is defined as μ∗=σ12/σ22, depends on P* and inertial number (reduced shear rate) I=γ˙√m/aP, and so does solid fraction Φ. The material exhibits, as P* decreases, a strongly enhanced resistance to shear (larger μ*). In the quasistatic limit, for I→0, it is roughly predicted by a simple effective pressure assumption by which the capillary forces are deemed equivalent to an isotropic pressure increase applied to the dry material as long as P*≥1, while the yield criterion approximately assumes the Mohr-Coulomb form. At lower P*, such models tend to break down as liquid bonding, causing connected clusters to survive over significant strain intervals, strongly influences the microstructure. Systematic shear banding is observed at very small P

Additive rheology of complex granular flows

“…The model of capillary bridge. We assume that the liquid inside the agglomerate is in the 'pendular' state with a uniform distribution of capillary bridges between particles 38,[53][54][55][56][57][58][59] . This distribution may be a consequence of mixing the liquid with the particles, drainage of a saturated packing, or capillary condensation from a vapor.…”

Investigation into the isotropic compression of wet granular soils using discrete element method

“…For P * << 1, initial loose structures are stabilized by adhesion and the relevant force scale is F 0 . For large reduced pressures P * >> 1, externally imposed forces dominate the adhesive strength, the relevant force scale is Pa 2 [9]. The main objective of the present paper is the study of the effect of a gradual compression, starting from cohesion-dominated loose states at small P * , and ending in confinement-dominated denser states at large P * .…”

“…In this study, the results are priorly shown in the conventional compression -decompression curves in soil mechanics e-log(P * ), in which e = 1/ -1. Three regimes can be classified in the compression curve, depending on the range of the pressure P * [9]. A first regime, thereafter called regime I, is observed in the range of low reduced pressures P * .…”

“…The simplified Hertz-MindlinDeresiewicz force model is used for calculating the contacts between grains, relating the elastic normal force F H N to the normal deflection h, as described in [11,9].…”

“…Yet, cohesive granular materials, especially in loose/very loose states, have less frequently been investigated by numerical simulation than cohesionless ones. Based on the 2D model for cohesive powders of Gilabert et al [1,2], a 3D model [9] is developed to investigate the mechanical behavior of a model bonded granular soils (glass beads with capillary bonds in the pendular state). Results are presented for simulations of isotropic compression cycles, both for the macroscopic behavior (plastic compression curve) and the evolution of the arrangement of microstructure upon growing of pressure.…”