Flow of wet granular materials: a numerical study

We simulate dense assemblies of frictional spherical grains in steady shear flow under controlled normal stress $P$ in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci, assumed isolated (pendular regime), and to attractive forces. The system behavior depends on two dimensionless control parameters: inertial number $I$ and reduced pressure $P^*=aP/(\pi\Gamma)$, comparing confining forces $\sim a^2P$ to meniscus tensile strength $F_0=\pi\Gamma a$, for grains of diameter $a$ joined by menisci with surface tension $\Gamma$. We pay special attention to the quasi-static limit of slow flow and observe systematic, enduring strain localization in some of the cohesion-dominated ($P^*\sim 0.1$) systems. Homogeneous steady flows are characterized by the dependence of internal friction coefficient $\mu^*$ and solid fraction $\Phi$ on $I$ and $P^*$. We record fairly small but not negligible normal stress differences and the moderate sensitivity of the system to saturation within the pendular regime. Capillary forces have a significant effect on the macroscopic behavior of the system, up to $P^*$ values of several units. The concept of effective pressure may be used to predict an order of magnitude for the strong increase of $\mu^*$ as $P^*$ decreases but such a crude approach is unable to account for the complex structural changes induced by capillary cohesion. Likewise, the Mohr-Coulomb criterion for pressure-dependent critical states is, at best, an approximation valid within a restricted range of pressures, with $P^*\ge 1$. At small enough $P^*$, large clusters of interacting grains form in slow flows, in which liquid bonds survive shear strains of several units. This affects the anisotropies associated to different interactions, and the shape of function $\mu^*(I)$, which departs more slowly from its quasistatic limit than in cohesionless systems.Comment: 20 pages, 29 figures with 39 subfigure

Annular shear of cohesionless granular materials: from inertial to quasistatic regime

Using discrete simulations, we investigate the behavior of a model granular material within an annular shear cell. Specifically, two-dimensional assemblies of disks are placed between two circular walls, the inner one rotating with prescribed angular velocity, while the outer one may expand or shrink and maintains a constant radial pressure. Focusing on steady state flows, we delineate in parameter space the range of applicability of the recently introduced constitutive laws for sheared granular materials (based on the inertial number). We discuss the two origins of the stronger strain rates observed near the inner boundary, the vicinity of the wall and the heteregeneous stress field in a Couette cell. Above a certain velocity, an inertial region develops near the inner wall, to which the known constitutive laws apply, with suitable corrections due to wall slip, for small enough stress gradients. Away from the inner wall, slow, apparently unbounded creep takes place in the nominally solid material, although its density and shear to normal stress ratio are on the jammed side of the critical values. In addition to rheological characterizations, our simulations provide microscopic information on the contact network and velocity fluctuations that is potentially useful to assess theoretical approaches

Influences des paramètres micromécaniques dans la simulation numérique discrète des matériaux granulaires : assemblage, déformation quasi-statique, écoulements

27 pagesWe review the influence of micromechanical parameters on the macroscopic mechanical behaviour of granular materials, as numerically simulated in discrete element approaches, both in quasistatic conditions an din dense flow. We insist in particular on the role of suitably defined dimensionless numbers apt to provide a classification of rheological regimes of quite general validity

Simulation numérique discrète et comportement mécanique des matériaux granulaires

Article in French. A free English translation is available on the web site of the journal : http://www.lcpc.fr/en/sources/blpc/index.phpInternational audienceAs a complementary tool to laboratory experiments, discrete numerical simulation, applied to granular materials, provides valuable information on the grain and contact scale microstructure, thereby enabling one to better understand the microscopic origin of macroscopic mechanical behaviours. We first introduce different simulation methods, which we regard as techniques for numerical experimentation, in connection with micromechanical models for intergranular contacts. We lay special emphasis on the important issue of sample representativity and stress the usefulness of dimensional analysis in the definition of relevant control parameters. We then apply this approach to two important rheological regimes of granular systems: solid-like, slowly strained granular materials, which might be ruled by elastoplastic constitutive relations; and liquid-like, dense granular flows, either confined or with a free surface, described by suitable friction and dilatancy law

Note sur une collection de serpents du Congo avec description d'une espèce nouvelle

#Philothamnus hughesi n.sp. est décrit de Gangalingolo au Congo. Cette espèce était précédemment confondue avec #P. hoplogaster. Son aire de répartition géographique s'étend du Cameroun au Gabon, à la RCA et au Nord du Zaïre. Six espèces sont rapportées pour la première fois du Congo : #Rhinotyphlops caecus, #Chamaelycus christyi, #Chamaelycus parkeri, #Philothamnus nitidus leveridgei, #Aparallactus modestus ubangensis et #Paranaja multifasciata anomala. Des exemplaires de deux espèces rares sont décrits : #Miodon fulvicollis et #Boulengerina christyi. (Résumé d'auteur

Un modèle référentiel pour analyser les pratiques cartographiques dans l’enseignement et la formation.

Cet article présente la construction d'un modèle référentiel destiné à analyser, à partir d'entretiens et d'observations, les pratiques cartographiques supposées innovantes de professeurs d'histoire-géographie de l'enseignement secondaire. Des références d'épistémologie de la géographie et de la cartographie sont sollicitées pour définir des postures d'enseignants vis-à-vis des rapports entre espace terrestre, espace cartographique, espace géographique et apprentissage.This paper presents the construction of a referential model aimed to analyse, from interviews and observations, the presumed innovative cartographical practices of high school history-geography teachers. Epistemological references of geography and cartography help to define teacher's attitudes towards the relationships between terrestrial space, cartographical space, geographical space and learning

Dynamic crack propagation with a variational phase-field model: limiting speed, crack branching and velocity-toughening mechanisms

International audienceWe address the simulation of dynamic crack propagation in brittle materials using a regularized phase-field description, which can also be interpreted as a damage-gradient model. Benefiting from a variational framework, the dynamic evolution of the mechanical fields are obtained as a succession of energy minimizations. We investigate the capacity of such a simple model to reproduce specific experimental features of dynamic in-plane fracture. These include the crack branching phenomenon as well as the existence of a limiting crack velocity below the Rayleigh wave speed for mode I propagation. Numerical results show that, when a crack accelerates , the damaged band tends to widen in a direction perpendicular to the propagation direction, before forming two distinct macroscopic branches. This transition from a single crack propagation to a branched configuration is described by a well-defined master-curve of the apparent fracture energy Γ as an increasing function of the crack velocity. This Γ(v) relationship can be associated, from a macroscopic point of view, with the well-known velocity-toughening mechanism. These results also support the existence of a critical value of the energy release rate associated with branching: a critical value of approximately 2Gc is observed i.e. the fracture energy contribution of two crack tips. Finally, our work demonstrates the efficiency of the phase-field approach to simulate crack propagation dynamics interacting with heterogeneities, revealing the complex interplay between heterogeneity patterns and branching mechanisms

Interface roughness effect on slow cyclic annular shear of granular materials

International audienceWe experimentally investigate the mechanical behaviour in cyclic shear of a granular material near a solid wall in a pressure controlled annular shear cell. The use of a model system (glass beads and saw-tooth shaped solid surface) enables the study of the influence of the wall roughness. After an initial shakedown procedure ensuring reproducible results in subsequent tests, wall shear stress S , volumetric variation Δ V , and the displacement field of the sample bottom surface, are recorded as functions of wall displacement. A dimensionless roughness parameter Rn is shown to control the interface response. The local grain-level or mesoscale behaviour is directly correlated to the global one on the scale of the whole sample