Contact anisotropy and coordination number for a granular assembly:a comparison between DEM simulation and theory

We study an ideal granular aggregate consisting of elastic spherical particles, isotropic in stress and anisotropic in the contact network. Because of the contact anisotropy, a confining pressure applied at zero deviatoric stress, produces shear strain as well as volume strain. Our goal is to predict the coordination number k, the average number of contacts per particle, and the magnitude of the contact anisotropy ɛ, from knowledge of the elastic moduli of the aggregate. We do this through a theoretical model based upon the well known effective medium theory. However, rather than focusing on the moduli, we consider their ratios over the moduli of an equivalent isotropic state. We observe good agreement between numerical simulation and theory

Bedforms Produced on a Particle Bed by Vertical Oscillations of a Plate

We describe a new mechanism that produces bedforms and characterize the conditions under which it operates. The mechanism is associated with pressure gradients generated in a fluid saturated particle bed by a plate oscillating in the water above it. These vertical pressure gradients cause oscillatory bed failure. This facilitates particle displacement in its interior and transport at and near its surface that contribute to the formation of a heap under the plate. Flows over erodible beds generally cause shear stresses on the bed and these induce bed failure. Failure driven by pressure gradients is different from this. We report on bedforms in a bed of glass beads associated with such fluctuating pressure gradients. We measure the development of the profiles of heaps as a function of time and determine the tangential and normal motion of areas on the beds surface and estimate the depth of penetration of the tangential transport. The measurements compare favorably with a simple model that describes the onset of failure due to oscillations in pressure

Micromechanical behavior of a random aggregate of particles

Dottorato di ricerca in ingegneria delle strutture. 12. ciclo. Tutori James Thomas Jenkins, Luigi Gambarotta e Alfredo SollazzoConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Yield loci for an anisotropic granular assembly

7 pagesInternational audienceYield loci of a granular material are derived in case of triaxial compression carried out at constant pressure. The theory is based upon a simple micromechanical model in which particles move according to an average, homogeneous deformation. We show how the presence of an inherent anisotropy in the aggregate (typical of laboratory samples due to depositional processes) produces a deviation of the yield loci in the stress space from the expected Mohr-Coulomb prediction. That is, when the compaction pressure in an anisotropic aggregate is increased, irreversibility associated with sliding between particles occurs and this will influence the yield function in the subsequent triaxial test. Numerical simulations support the theoretical result

Localization in an anisotropic planar aggregate of spheres

We present a micro-mechanical model that is able to predict localization in a sheared planar aggregate of spheres. We assume a non-linear contact law between interacting particles that deform differently from an affine deformation. Equilibrium determines this deviation. The aggregate is isotropically compressed and then sheared so anisotropy develops because of contacts deletion and a non-linear contact law. Because of anisotropy and fluctuations in particles deformation, the resulting macroscopic stiffness tensor, which relates increments in the average stress with increments in the average strain, is characterized by a lack of major symmetry, Aijkl ≠ Aklij. At given shear strain and coordination number it is possible to detemine a plane over which discontinuity in strain occurs; this is identified as localization

Failure in granular materials based on acoustic tensor: a numerical analysis

We investigate localization in granular material with the support of numerical simulations based upon DEM (Distinct Element Method). Localization is associated with a discontinuity in a component of the incremental strain over a plane surface through the condition of the determinant of the acoustic tensor to be zero. DEM simulations are carried out on an aggregate of elastic frictional spheres, initially isotropically compressed and then sheared at constant pressure p0. The components of the stiffness tensor are evaluated numerically in stressed states along the triaxial test and employed to evaluate the acoustic tensor in order to predict localization. This occurs in the pre-peak region, where the aggregate hardens under the circumstance to be incrementally frictionless: it is a regime in which the tangential force does not change as the deformation proceedes and, consequently, the deviatoric stress varies only with the normal component of the contact force

Localization in an anisotropic planar aggregate of spheres

We present a micro-mechanical model that is able to predict localization in a sheared planar aggregate of spheres. We assume a non-linear contact law between interacting particles that deform differently from an affine deformation. Equilibrium determines this deviation. The aggregate is isotropically compressed and then sheared so anisotropy develops because of contacts deletion and a non-linear contact law. Because of anisotropy and fluctuations in particles deformation, the resulting macroscopic stiffness tensor, which relates increments in the average stress with increments in the average strain, is characterized by a lack of major symmetry, Aijkl ≠ Aklij. At given shear strain and coordination number it is possible to detemine a plane over which discontinuity in strain occurs; this is identified as localization