Identification of rolling resistance as a shape parameter in sheared granular media

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.Comment: Soumis a Physical Review E; Statistical, Nonlinear, and Soft Matter Physics http://link.aps.org/doi/10.1103/PhysRevE.84.01130

Force chains and contact network topology in packings of elongated particles

By means of contact dynamic simulations, we investigate the contact network topology and force chains in two-dimensional packings of elongated particles modeled by rounded-cap rectangles. The morphology of large packings of elongated particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and orientations of contacts between particles. We show that particle elongation affects force distributions and force/fabric anisotropy via various local structures allowed by steric exclusions and the requirement of force balance. As a result, the force distributions become increasingly broader as particles become more elongated. Interestingly, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system. The strongest force chains are carried by side/side contacts oriented along the principal stress direction.Comment: Soumis a Physical Review

Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods

We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed

Stress-strain behavior and geometrical properties of packings of elongated particles

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review

Shear strength properties of wet granular materials

We investigate shear strength properties of wet granular materials in the pendular state (i.e. the state where the liquid phase is discontinuous) as a function of water content. Sand and glass beads were wetted and tested in a direct shear cell and under various confining pressures. In parallel, we carried out three-dimensional molecular dynamics simulations by using an explicit equation expressing capillary force as a function of interparticle distance, water bridge volume and surface tension. We show that, due to the peculiar features of capillary interactions, the major influence of water content over the shear strength stems from the distribution of liquid bonds. This property results in shear strength saturation as a function of water content. We arrive at the same conclusion by a microscopic analysis of the shear strength. We propose a model that accounts for the capillary force, the granular texture and particle size polydispersity. We find fairly good agreement of the theoretical estimate of the shear strength with both experimental data and simulations. From numerical data, we analyze the connectivity and anisotropy of different classes of liquid bonds according to the sign and level of the normal force as well as the bond direction. We find that weak compressive bonds are almost isotropically distributed whereas strong compressive and tensile bonds have a pronounced anisotropy. The probability distribution function of normal forces is exponentially decreasing for strong compressive bonds, a decreasing power-law function over nearly one decade for weak compressive bonds and an increasing linear function in the range of tensile bonds. These features suggest that different bond classes do not play the same role with respect to the shear strength.Comment: 12 page

Force transmission in a packing of pentagonal particles

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit "strong" and "weak" force networks in the pentagon packing, but our simulation data provide also evidence for a large class of "very weak" forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zig-zag aspect and a decreasing exponential probability distribution as in a disk packing

Vibrational dynamics of confined granular material

By means of two-dimensional contact dynamics simulations, we analyze the vibrational dynamics of a confined granular layer in response to harmonic forcing. We use irregular polygonal grains allowing for strong variability of solid fraction. The system involves a jammed state separating passive (loading) and active (unloading) states. We show that an approximate expression of the packing resistance force as a function of the displacement of the free retaining wall from the jamming position provides a good description of the dynamics. We study in detail the scaling of displacements and velocities with loading parameters. In particular, we find that, for a wide range of frequencies, the data collapse by scaling the displacements with the inverse square of frequency, the inverse of the force amplitude and the square of gravity. Interestingly, compaction occurs during the extension of the packing, followed by decompaction in the contraction phase. We show that the mean compaction rate increases linearly with frequency up to a characteristic frequency and then it declines in inverse proportion to frequency. The characteristic frequency is interpreted in terms of the time required for the relaxation of the packing through collective grain rearrangements between two equilibrium states

Modélisation des matériaux granulaires cohésifs a particules non-convexes (Application à la compaction des poudres d'UO2)

On s'intéresse à la modélisation des matériaux granulaires composés d'agrégats non-convexes et cohésifs en vue d'application à la rhéologie des poudres d'UO2. L'influence du degré de non-convexité des particules est analysé en termes de grandeurs macroscopiques (frottement interne et cohésion de Coulomb) et de paramètres micro-mécaniques tels que l'anisotropie de la texture et la transmission des efforts. Il apparaît en particulier que la compacité évolue d'une manière complexe avec la non-convexité et que la résistance au cisaillement augmente mais sature sous l'effet d'imbrication entre agrégats. Des modèles simples sont introduits pour décrire ces comportements en termes de paramètres micro-mécaniques. De même, des études systématiques par cisaillement, compaction uniaxiale et compression simple montrent que la cohésion interne augmente avec la non-convexité mais est fortement contrôlée par les conditions aux limites et l'apparition de bandes de cisaillement ou de concentrations de contraintes.We model in this work granular materials composed of nonconvex and cohesive aggregates, in view of application to the rheology of UO2 powders. The effect of nonconvexity is analyzed in terms of bulk quantities (Coulomb internal friction and cohesion) and micro-mechanical parameters such as texture anisotropy and force transmission. In particular, we find that the packing fraction evolves in a complex manner with the shape nonconvexity and the shear strength increases but saturates due to interlocking between the aggregates. We introduce simple models to describe these features in terms of micro-mechanical parameters. Furthermore, a systematic investigation of shearing, uniaxial compaction and simple compression of cohesive packings show that bulk cohesion increases with non-convexity but is strongly influenced by the boundary conditions and shear bands or stress concentration.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Relations microstructure-fissuration-perméabilité dans les milieux granulaires cimentés.

Ce travail de thèse est consacré à l'étude de la corrélation entre la microstructure des matériaux granulaires cimentés, la morphologie des fissures qui peuvent y apparaître et leur perméabilité apparente. Une approche numérique par discrétisation sur réseau a permis de prendre en compte la structure polyphasique hétérogène de ces matériaux et d'étudier les mécanismes qui contrôlent leur comportement et leur rupture. Trois régimes de rupture ont été identifiés en fonction de la fraction volumique de la phase cimentaire et de l'adhésion aux interfaces inclusions/matrice cimentaire. Dans le cadre de la sûreté nucléaire, et dans une optique d'analyse d'étanchéité sur des bétons fissurés, une méthodologie de génération de microstructures représentatives du béton, combinant les fractions volumiques des phases et les distributions des tailles des grains, a été proposée. Cette méthodologie permet d'analyser le réseau de fissures qui peut y apparaitre sous chargement en fonction de la microstructure. En particulier, la tortuosité des fissures est analysée en fonction de la fraction volumique des inclusions et de la distribution de leurs diamètres. Enfin, la perméabilité apparente des des fissures a été étudiée par la simulation de l'écoulement d'un liquide par la methode Lattice Boltzmann. Une corrélation microstructure-tortuosité-perméabilité a été ainsi obtenue. Les outils de modélisation et d'analyse proposés sont suffisamment génériques pour permettre de prendre en compte la complexité microstructurelle d'autres types de matériaux polyphasiques et leurs évolutions.In this work, we investigate the relationships between the microstructure of cemented granular materials, morphology of cracks that may appear under tensile loading and their apparent permeability. In order to simulate the complexity of the multiphase heterogeneous structure of these materials, a numerical approach based on a lattice discretization was developed and used to study the mechanisms that control their behavior and rupture. Three distinct regimes of crack propagation were evidenced in terms of the combined influence of the matrix volume fraction and particle/matrix adherence. In the context of nuclear safety and in view of analyzing cracked concrete toughness, a methodology is proposed for generating representative microstructures with controlled phase volume fractions and particle size distributions. The cracks obtained under tensile loading are analyzed and a relationship is obtained between the microstructure of concrete and the tortuosity of the cracks. The permeability of cracked numerical microstructures was obtained by the simulation of liquid flow through the cracks by means of the Lattice Boltzmann method. Finally, a microstructure-tortuosity-permeability relation is proposed. The modeling and analysis tools developed in this work are generic enough to be applied to other complex multiphase heterogeneous materials.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF