Effect of particle type on the shear behaviour of granular materials

Abstract

In discrete element method (DEM) simulations, multi-sphere (MS) clumped and convex particles are two main particle models that are used to study the mechanical behaviours of granular materials. Of interest is the evaluation of the effect of multiple contacts between clumped particles or single contacts between convex particles on the mechanical behaviours of granular materials. In this context, a series of drained triaxial compression tests were conducted on convex true (CT) ellipsoids and MS ellipsoids with aspect ratios (ARs) ranging from 1.0–2.0. The microscale results indicate that at a given AR, the critical friction angle φc changes with the particle type, whereas the peak friction angle φp is nearly independent of the particle type. The anisotropic analysis provides underlying mechanisms of the shear strength evolution from two perspectives. First, the anisotropies of granular materials are essential to shear strength as the deviatoric (q)-to-effective mean (p′) stress ratio can be expressed as the sum of the anisotropies, i.e., q/p′ ≈ 0.4ac+ 0.4an + 0.6at, where ac, an and at are the normal contact anisotropy, normal contact force anisotropy and tangential contact force anisotropy, respectively. For all samples, ac and an underpin the shear strength and are influenced by the particle type. The similar φp displayed by the CT and MS ellipsoids does not translate to similar an and ac but similar ac+an for the two particle types. In addition, owing to their larger ac+an, the CT ellipsoids have a higher φc than the MS ellipsoids. Second, there is a satisfactory linear relationship between q/p′ and ac within strong and non-sliding (sn) contacts acsn (i.e., q/p′ = kacsn), where k is the fitting parameter. Accordingly, in the peak state, the subtle difference in shear strength is attributed to the greater acsn in the CT ellipsoids than in the MS ellipsoids that is counteracted by the smaller k. However, in the critical state, the greater difference in acsn between the CT and MS ellipsoids is partially offset by the smaller difference in k, causing a higher φc in the CT ellipsoids than in the MS ellipsoids.