Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit (I similar to 10(-5)) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle., an increasing function of I, and solid fraction., a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for I similar to 0.2. Under growing I, the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10(-2). The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

VL - 4 UR - http://link.springer.com/10.1140/epje/i2018-11608-9 IS - 1 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Rheology of granular materials composed of crushable particles JF - The European Physical Journal E Y1 - 2018 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï KW - CONTACT DYNAMICS METHOD; DISCRETE ELEMENT METHOD; SHEAR BANDS; NUMERICAL-SIMULATION; BED COMMINUTION; FRAGMENTATION; BREAKAGE; ROCK; DEM; MODEL AB -We investigate sheared granular materials composed of crushable particles by means of contact dynamics simulations and the bonded-cell model for particle breakage. Each particle is paved by irregular cells interacting via cohesive forces. In each simulation, the ratio of the internal cohesion of particles to the confining pressure, the relative cohesion, is kept constant and the packing is subjected to biaxial shearing. The particles can break into two or more fragments when the internal cohesive forces are overcome by the action of compressive force chains between particles. The particle size distribution evolves during shear as the particles continue to break. We find that the breakage process is highly inhomogeneous both in the fragment sizes and their locations inside the packing. In particular, a number of large particles never break whereas a large number of particles are fully shattered. As a result, the packing keeps the memory of its initial particle size distribution, whereas a power-law distribution is observed for particles of intermediate size due to consecutive fragmentation events whereby the memory of the initial state is lost. Due to growing polydispersity, dense shear bands are formed inside the packings and the usual dilatant behavior is reduced or cancelled. Hence, the stress-strain curve no longer passes through a peak stress, and a progressive monotonic evolution towards a pseudo-steady state is observed instead. We find that the crushing rate is controlled by the confining pressure. We also show that the shear strength of the packing is well expressed in terms of contact anisotropies and force anisotropies. The force anisotropy increases while the contact orientation anisotropy declines for increasing internal cohesion of the particles. These two effects compensate each other so that the shear strength is nearly independent of the internal cohesion of particles.

VL - 41 UR - http://link.springer.com/10.1140/epje/i2018-11656-1 IS - 4 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Cohesive strength of iron ore granules JF - EPJ Web of Conferences Y1 - 2017 A1 - Contreras, Rafael Jaimes A1 - Berger, Nicolas A1 - Izard, Edouard A1 - Douce, Jean-François A1 - Koltsov, Alexey A1 - Jean-Yves Delenne A1 - Emilien Azéma A1 - Saeid Nezamabadi A1 - van Loo, Frédéric A1 - Roland Jean-Marc Pellenq A1 - Farhang Radjaï ED - Luding, S. AB -We present an experimental and numerical investigation of the mechanical strength of crude iron ore (Hematite) granules in which capillary bonds between primary particles are the source of internal cohesion. The strength is measured by subjecting the granules to vertical compression between two plates. We show that the behavior of the granules is ductile with a well-defined plastic threshold which increases with the amount of water. It is found that the compressive strength scales with capillary cohesion with a pre-factor that is nearly independent of size polydispersity for the investigated range of parameters but increases with friction coefficient between primary particles. This weak dependence may be attributed to the class of fine particles which, due to their large number, behaves as a cohesive matrix that controls the strength of the granule.

VL - 140 UR - http://www.epj-conferences.org/10.1051/epjconf/201714008020 IS - 10141146 JO - EPJ Web Conf. ER - TY - JOUR T1 - Three-dimensional bonded-cell model for grain fragmentation JF - Computational Particle Mechanics Y1 - 2017 A1 - Cantor, D A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï KW - Bonded-cell model; Fragmentation; Discrete element method; Contact dynamics method; Voronoi cell; Weibull statistics AB -We present a three-dimensional numerical method for the simulation of particle crushing in 3D. This model is capable of producing irregular angular fragments upon particle fragmentation while conserving the total volume. The particle is modeled as a cluster of rigid polyhedral cells generated by a Voronoi tessellation. The cells are bonded along their faces by a cohesive Tresca law with independent tensile and shear strengths and simulated by the contact dynamics method. Using this model, we analyze the mechanical response of a single particle subjected to diametral compression for varying number of cells, their degree of disorder, and intercell tensile and shear strength. In particular, we identify the functional dependence of particle strength on the intercell strengths. We find that two different regimes can be distinguished depending on whether intercell shear strength is below or above its tensile strength. In both regimes, we observe a power-law dependence of particle strength on both intercell strengths but with different exponents. The strong effect of intercell shear strength on the particle strength reflects an interlocking effect between cells. In fact, even at low tensile strength, the particle global strength can still considerably increase with intercell shear strength. We finally show that the Weibull statistics describes well the particle strength variability.

VL - 4 UR - http://link.springer.com/10.1007/s40571-016-0129-0 IS - 4 JO - Comp. Part. Mech. ER - TY - JOUR T1 - Binary mixtures of disks and elongated particles: Texture and mechanical properties JF - Physical Review E Y1 - 2016 A1 - Emilien Azéma A1 - Preechawuttipong, Itthichai A1 - Farhang Radjaï AB -We analyze the shear strength and microstructure of binary granular mixtures consisting of disks and elongated particles by varying systematically both the mixture ratio and degree of homogeneity (from homogeneous to fully segregated). The contact dynamics method is used for numerical simulations with rigid particles interacting by frictional contacts. A counterintuitive finding of this work is that the shear strength, packing fraction, and, at the microscopic scale, the fabric, force, and friction anisotropies of the contact network are all nearly independent of the degree of homogeneity. In other words, homogeneous mixtures have the same strength properties as segregated packings of the two particle shapes. In contrast, the shear strength increases with the proportion of elongated particles correlatively with the increase of the corresponding force and fabric anisotropies. By a detailed analysis of the contact network topology, we show that various contact types contribute differently to force transmission and friction mobilization.

VL - 94 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Scaling behaviour of cohesive granular flows JF - EPL (Europhysics Letters) Y1 - 2016 A1 - Berger, Nicolas A1 - Emilien Azéma A1 - Douce, Jean-François A1 - Farhang Radjaï AB -The shear strength of dense granular flows is generally described by an effective friction coefficient, ratio of shear to normal stress, as a function of the inertial number *I*. However, this ratio depends on the normal stress when the particles interact via both friction and adhesion forces, and in this sense it does not properly represent a Coulomb-like friction. For the same reason, it is not a unique function of *I*. We used extensive contact dynamics simulations to isolate the cohesive strength from the purely frictional strength in dense inertial flows for a broad range of shear rates and adhesion forces between particles. Remarkably, while the frictional part of the strength increases with *I*, the cohesive strength is found to be a decreasing function of *I*. We show that a single dimensionless parameter, combining interparticle adhesion with *I*, controls not only the cohesive strength but also the packing fraction and granular texture in inertial flows.

Particle degradation and fracture play an important role in natural granular flows and in many applications of granular materials. We analyze the fracture properties of two-dimensional disklike particles modeled as aggregates of rigid cells bonded along their sides by a cohesive Mohr-Coulomb law and simulated by the contact dynamics method. We show that the compressive strength scales with tensile strength between cells but depends also on the friction coefficient and a parameter describing cell shape distribution. The statistical scatter of compressive strength is well described by the Weibull distribution function with a shape parameter varying from 6 to 10 depending on cell shape distribution. We show that this distribution may be understood in terms of percolating critical intercellular contacts. We propose a random-walk model of critical contacts that leads to particle size dependence of the compressive strength in good agreement with our simulation data.

VL - 91 IS - 2 JO - Phys. Rev. E ER - TY - JOUR T1 - Effects of shape and size polydispersity on strength properties of granular materials. JF - Phys Rev E Stat Nonlin Soft Matter Phys Y1 - 2015 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï AB -By means of extensive contact dynamics simulations, we analyze the combined effects of polydispersity both in particle size and in particle shape, defined as the degree of shape irregularity, on the shear strength and microstructure of sheared granular materials composed of pentagonal particles. We find that the shear strength is independent of the size span, but unexpectedly, it declines with increasing shape polydispersity. At the same time, the solid fraction is an increasing function of both the size span and the shape polydispersity. Hence, the densest and loosest packings have the same shear strength. At the scale of the particles and their contacts, we analyze the connectivity of particles, force transmission, and friction mobilization as well as their anisotropies. We show that stronger forces are carried by larger particles and propped by an increasing number of small particles. The independence of shear strength with regard to size span is shown to be a consequence of contact network self-organization, with the falloff of contact anisotropy compensated by increasing force anisotropy.

VL - 91 IS - 3 ER - TY - Generic T1 - Evolution of particle size distributions in crushable granular materials T2 - 3rd International Symposium on Geomechanics from Micro to Macro Y1 - 2015 A1 - Duc-Hanh Nguyen A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Philippe Sornay ED - Kenichi Soga ED - Krishna Kumar ED - Giovanna Biscontin ED - Kuo, Matthew AB -

By means of the contact dynamics method together with a particle fracture model, in which the particles are cohesive aggregates of irreducible polygonal fragments, we investigate the evolution of particle size distribution in the process of uniaxial compaction of granular materials. The case of single particle breakup under compressive stress is used to test the method and the influence of discretization (number of irreducible fragments). We show that the breaking threshold of the granular assembly scales with the internal cohesion of the particles but it depends also on the initial size distribution and irregularity of polygonal particle shapes. The evolution of size distribution proceeds by consecutive periods of intense particle crushing, characterized by local shattering instability, and periods of little breaking activity. Starting with either monodisperse or power-law distribution of particle sizes, the latter evolves towards a broad distribution of the fragmented particles with a nearly power-law distribution in the range of intermediate particle sizes. Interestingly, a finite number of large particles survive despite ongoing crushing process due to the more homogeneous distribution of forces in the presence of small fragmented particles filling the pores between larger particles.

JF - 3rd International Symposium on Geomechanics from Micro to Macro PB - CRC Press CY - SEP 01-03-2014 Univ Cambridge, Cambridge, ENGLAND VL - Geomechanics from Micro to Macro SN - 978-1-138-02707-7 ER - TY - JOUR T1 - Internal friction and absence of dilatancy of packings of frictionless polygons JF - Physical Review E Y1 - 2015 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Jean-Noel Roux AB -By means of numerical simulations, we show that assemblies of frictionless rigid pentagons in slow shear flow possess an internal friction coefficient (equalto0.183±0.008 with our choice of moderately polydisperse grains) but no macroscopic dilatancy. In other words, despite side-side contacts tending to hinder relative particle rotations, the solidfraction under quasistatic shear coincides with that of isotropic random close packings of pentagonal particles. Properties of polygonal grains are thus similar to those of disks in that respect. We argue that continuous reshuffling of the force-bearing network leads to frequent collapsing events at the microscale, thereby causing the macroscopic dilatancy to vanish. Despite such rearrangements, the shear flow favors an anisotropic structure that is at the origin of the ability of the system to sustain shear stress.

VL - 91 IS - 1 JO - Phys. Rev. E ER - TY - Generic T1 - Numerical simulation of granular media composed with irregular polyhedral particles: effect of particles' angularity T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2015 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Dubois, Frédéric ED - Onate, E ED - Owen, DRJ KW - angularity KW - force transmission KW - Granular Materials KW - particle shape KW - texture AB -We use contact dynamic simulations to perform a systematic investigation of the effects of particles shape angularity on mechanicals response in sheared granular ma-terials. The particles are irregular polyhedra with varying numbers of face from spheres to "double pyramid" shape with a constant aspect ratio. We study the quasi-static behav-ior, structural and force anisotropies of several packings subjected to triaxial compression. An interesting finding is that the shear strength first increases with angularity up to a maximum value and then saturates as the particles become more angular. Analyzing the anisotropies induced by the angular distributions of contacts and forces orientations, we show that the saturation of the shear strength at higher angularities is a consequence of fall-off of the texture anisotropies compensated by an increase of the tangential force anisotropy. This is attributed to the fact that at higher angularity, particles are bet-ter connected (or surrounded) leading to an increase of friction mobilization in order to achieve the deformation. Moreover, the most angular particles also have very few sides so that, this effect is enhanced by the increase of the proportion of face-side and side-side contacts with angularity.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-01112373 ER - TY - Generic T1 - Rheology and Micromechanical Analysis of Granular Media Composed of Platy Particles: A Step Toward the DEM Simulation of Clayey Soils T2 - 15th Pan-American Conference on Soil Mechanics and Geotechnical Engineering (PCSMGE) / 8th South American Congress on Rock Mechanics (SCRM) Y1 - 2015 A1 - Boton, Mauricio A1 - Estrada, Nicolas A1 - Emilien Azéma ED - Manzanal, D ED - Sfriso, AO AB -This work was carried out in the framework of a larger project devoted

to the micro-mechanical modeling of clayey soils by means of discrete element

simulations. Here, we study the rheology and microstructure of granular media

composed of platy particles. The particles are three-dimensional square plates,

approximated as spheropolyhedra. Several samples composed of particles of

different levels of platyness (related to the ratio of length to thickness) were

numerically prepared and sheared up to large deformations. Specifically, we

revisit and analyze in detail an ordering phenomenon described briefly in a

previous paper [1] and investigate its consequences in terms of the alignment of

particles and cluster formation. We find that particle alignment is strongly

enhanced by the degree of platyness and leads to the formation of face-connected

clusters. Due to dynamics, this spontaneous clustering is a robust characteristic of

granular systems composed of platy particles even in the absence of attraction

forces.

JF - 15th Pan-American Conference on Soil Mechanics and Geotechnical Engineering (PCSMGE) / 8th South American Congress on Rock Mechanics (SCRM) CY - NOV 15-18 2015 Buenos Aires, ARGENTINA VL - FROM FUNDAMENTALS TO APPLICATIONS IN GEOTECHNICS ER - TY - JOUR T1 - Internal Structure of Inertial Granular Flows JF - Physical Review Letters Y1 - 2014 A1 - Emilien Azéma A1 - Farhang Radjaï AB -

We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient μ arises from three different parameters pertaining to the contact network and force transmission: (1) contact anisotropy, (2) force chain anisotropy, and (3) friction mobilization. Our extensive 3D numerical simulations reveal that μ increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I≃0.1 the force chains break into clusters immersed in a background “soup” of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of μ with I.

VL - 112 IS - 7 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Particle alignment and clustering in sheared granular materials composed of platy particles JF - The European Physical Journal E Y1 - 2014 A1 - Boton, Mauricio A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï AB -By means of molecular dynamics simulations, we investigate the texture and local ordering in sheared packings composed of cohesionless platy particles. The morphology of large packings of platy particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and anisotropic orientations of contacts between particles. We find that particle alignment is strongly enhanced by the degree of platyness and leads to the formation of face-connected clusters of exponentially decaying size. Interestingly, due to dynamics in continuous shearing, this ordering phenomenon emerges even in systems composed of particles of very low platyness differing only slightly from spherical shape. The number of clusters is an increasing function of platyness. However, at high platyness the proportion of face-face interactions is too low to allow for their percolation throughout the system.

VL - 37 IS - 11 JO - Eur. Phys. J. E ER - TY - Generic T1 - A benchmark for particle shape dependence T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Gael Combe A1 - Cécile Nouguier-Lehon A1 - Emilien Azéma A1 - Krzysztof Szarf A1 - Baptiste Saint-Cyr A1 - Marie Chaze A1 - Farhang Radjaï A1 - Pascal Villard A1 - Jean-Yves Delenne A1 - Vincent Richefeu A1 - Philippe Sornay A1 - Charles Voivret A1 - CEGEO Group ED - Yu, A ED - Dong, K ED - Yang, R AB -Particle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes. We show that the results may be analyzed in terms of a low-order shape parameter η describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of η with nearly the same trend for all shapes, the differences being of second order compared to η. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with η from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as η is further increased. Finally, the analysis of contact forces for the same value of η leads to very similar statistics regardless of our specific particle shapes.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - Comparison of the effects of rolling resistance and angularity in sheared granular media T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo ED - Yu, A ED - Dong, K ED - Yang, R KW - angularity KW - anisotropy KW - force distribution KW - Granular material KW - rolling resistance KW - Shear strength KW - solid fraction AB -In this paper, 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. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find 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.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842799 ER - TY - JOUR T1 - Packings of irregular polyhedral particles: Strength, structure, and effects of angularity JF - Physical Review E Y1 - 2013 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Dubois, Frédéric AB -We present a systematic numerical investigation of the shear strength and structure of granular packings composed of irregular polyhedral particles. The angularity of the particles is varied by increasing the number of faces from 8 (octahedronlike shape) to 596. We find that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 46 faces). At the same time, the packing fraction increases to a peak value but declines for more angular particles. We analyze the connectivity and anisotropy of the microstructure by considering both the contacts and branch vectors joining particle centers. The increase of the shear strength with angularity is shown to be due to a net increase of the fabric and force anisotropies but at higher particle angularity a rapid falloff of the fabric anisotropy is compensated by an increase of force anisotropy, leading thus to the saturation of shear strength.

VL - 8713 IS - 6 JO - Phys. Rev. E ER - TY - JOUR T1 - Quasistatic rheology and microstructural description of sheared granular materials composed of platy particles JF - Physical Review E Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio AB -This is the first paper of a series devoted to the micro-mechanical modeling of clayey soils, by means of discrete element simulations. We specifically focus here on the effect of the platy shape of particles by reducing the interactions between particles to mechanical contact forces (i.e., neither electrostatic repulsion nor van der Waals forces are taken into account). The particles are three-dimensional square plates, approximated as spheropolyhedra. Several samples composed of particles of different levels of platyness (related to the ratio of length to thickness) were numerically prepared and sheared up to large deformations. We analyzed the shear strength, packing fraction, orientation of the particles, connectivity, fabric of the interactions network, and interaction forces as functions of the platyness. We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. The principal underlying phenomenon is the alignment of particle faces along a particular direction. This ordering phenomenon, which emerges even for shapes that deviate only slightly from that of a sphere, enhances the ability of the packing to develop an anisotropic structure leading to large shear strength, especially as a consequence of the fabric and mobilization of friction forces. Moreover, the connectivity of the packings and their packing fraction also evolve with the platyness. In particular, the packing fraction evolves in a nonmonotonic fashion, as observed in other granular materials composed of elongated or angular particles.

VL - 87 IS - 3 JO - Phys. Rev. E ER - TY - JOUR T1 - Rheology of three-dimensional packings of aggregates: Microstructure and effects of nonconvexity JF - Physical Review E Y1 - 2013 A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Baptiste Saint-Cyr A1 - Jean-Yves Delenne A1 - Philippe Sornay AB -We use 3D contact dynamics simulations to analyze the rheological properties of granular materials composed of rigid aggregates. The aggregates are made from four overlapping spheres and described by a nonconvexity parameter depending on the relative positions of the spheres. The macroscopic and microstructural properties of several sheared packings are analyzed as a function of the degree of nonconvexity of the aggregates. We find that the internal angle of friction increases with nonconvexity. In contrast, the packing fraction increases first to a maximum value but declines as nonconvexity further increases. At high level of nonconvexity, the packings are looser but show a higher shear strength. At the microscopic scale, the fabric and force anisotropy, as well as friction mobilization are enhanced by multiple contacts between aggregates and interlocking, revealing thus the mechanical and geometrical origins of shear strength.

VL - 87 IS - 5 JO - Phys. Rev. E ER - TY - Generic T1 - Shear strength and microstructure of 3D assemblies of platy particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio ED - Yu, A ED - Dong, K ED - Yang, R KW - Clays KW - DEM KW - Microstructure KW - Shear strength AB -As a first step towards particle-scale modeling of clayey soils, we investigate the mechanical behavior and microstructure of assemblies of three-dimensional rectangular platy particles by means of the discrete element method. Several samples composed of particles of different levels of platyness (ratio of width to thickness) were numerically prepared and sheared to large deformations. We analyze the shear strength, packing fraction, connectivity, contact and force anisotropies, and mobilization of friction forces as functions of platyness.We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. This happens, in particular, because of the alignment of particle faces along a particular direction. Additionally, as observed for other granular materials with complex shapes, the packing fraction passes by a peak value before decreasing for larger values of platyness.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842787 ER - TY - Generic T1 - Shear strength, force distributions and friction mobilization in sheared packings composed of angular particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -In this paper, we explore the effect of particle shape angularity on the mechanical behavior of sheared granular packings. A first series of contact dynamics simulations is performed in 2D with regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. Then, in order to approach “idealized” angular particles, a second series of simulations is performed in 3D with irregular polyhedra with the number of faces ranging from 8 (octahedron-like) to 596. A counterintuitive finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 6 sides in 2D and 46 faces in 3D). A micromechanical analysis of force and contact orientations, all enhanced by face-face and face-side contacts, reveals that this increase is due to an increase of both contact and force anisotropies, and the saturation for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - Generic T1 - Effect of Particle Shape non-Convexity on the Rheology of Granular Media : 3D Contact Dynamics Simulations T2 - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) Y1 - 2012 A1 - Baptiste Saint-Cyr A1 - Emilien Azéma A1 - Jean-Yves Delenne A1 - Farhang Radjaï A1 - Philippe Sornay ED - Onate, E ED - Owen, DRJ KW - force transmission KW - Granular Materials KW - non-convexity KW - particle shape KW - texture AB -We analyze the effect of particle shape non-convexity on the quasi-static behavior of granular materials by means of contact dynamics simulations. The particles are regular aggregates of four overlapping spheres described by a nonconvexity parameter depending on the relative positions of the particles. Several packings are first submitted to isotropic compression without friction. We find that, as in 2D, the solid fraction of isotropic packings increases with non-convexity up to a maximum value and then declines to be nearly equal to that of a packing composed of only spheres. It is also remarkable that the coordination number increases quickly and saturates so that the packings composed of grains with a high level of nonconvexity are looser but more strongly connected. Then, the quasi-static behavior, structural and force anisotropies are analyzed by subjecting each packing to a triaxial compression. We find that the shear strength increases with non-convexity. We show that this increase results from the presence of multiple contacts between trimers leading to enhanced frictional interlocking.

JF - 2nd International Conference on Particle-Based Methods - Fundamentals and Applications (Particles) CY - OCT 26-28 2011 Barcelona, SPAIN VL - PARTICLE-BASED METHODS II: FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-00686453 ER - TY - JOUR T1 - Fabric evolution and accessible geometrical states in granular materials JF - Granular Matter Y1 - 2012 A1 - Farhang Radjaï A1 - Jean-Yves Delenne A1 - Emilien Azéma A1 - Stéphane Roux AB -We analyze the geometrical states of granular materials by means of a fabric tensor involving the coordination number and fabric anisotropy as the lowest-order descriptors of the contact network. In particular, we show that the fabric states in this representation are constrained by steric exclusions and the condition of mechanical equilibrium required in the quasi-static limit. A simple model, supported by numerical data, allows us to characterize the range of accessible fabric states and the joint evolution of fabric parameters. The critical state in this framework appears as a jammed state in the sense of a saturation of contact gain and loss along the principal strain-rate directions.

VL - 14 IS - 2 SI JO - Granular Matter ER - TY - JOUR T1 - Force chains and contact network topology in sheared packings of elongated particles JF - Physical Review E Y1 - 2012 A1 - Emilien Azéma A1 - Farhang Radjaï AB -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.

VL - 85 IS - 3 JO - Phys. Rev. E ER - TY - Generic T1 - Microscopic Origins of Shear Strength in Packings Composed of Elongated Particles T2 - 9th International Workshop on Bifurcation and Degradation in Geomaterials (IWBDG 2011) Y1 - 2012 A1 - Emilien Azéma A1 - Farhang Radjaï ED - Stéphane Bonelli ED - Dascalu, Cristian ED - François Nicot KW - Elongated particles KW - Fabric properties KW - force transmission KW - Harmonic decomposition AB -We investigate the rheology, force transmission and texture of granular materials composed of elongated particles by means of contact dynamics simulations. The particles have a rounded-cap rectangular (RCR) shape described by a single elongation parameter varying from 0 for a circular particle to 1 for an infinitely thin or long particle. We study the quasi-static behavior, structural and force anisotropies as a function of the elongation parameter for packings submitted to biaxial compression. The shear strength is found to increase linearly with this parameter whereas the solid fraction both at the initial isotropic state and in the critical state is nonmonotonous. We show that for these elongated particles a harmonic decomposition of the stress tensor provides a fairly good approximation of the internal state. Our data suggest that the increase of shear strength with reflects both enhanced friction mobilization and anisotropic particle orientation as the elongation of the particles increases.

JF - 9th International Workshop on Bifurcation and Degradation in Geomaterials (IWBDG 2011) CY - MAY 23-26 2011 Porquerolles, FRANCE VL - Springer Series in Geomechanics and Geoengineering - ADVANCES IN BIFURCATION AND DEGRADATION IN GEOMATERIALS UR - https://hal.archives-ouvertes.fr/hal-00686728 ER - TY - JOUR T1 - Nonlinear effects of particle shape angularity in sheared granular media JF - Physical Review E Y1 - 2012 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï AB -We analyze the effects of particle shape angularity on the macroscopic shear behavior and texture of granular packings simulated by means of the contact dynamics method. The particles are regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. The packings are analyzed in the steady shear state in terms of their shear strength, packing fraction, connectivity, and fabric and force anisotropies, as functions of the angularity. An interesting finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below six sides). In contrast, the packing fraction declines towards a constant value, so that the packings of more angular particles are looser but have higher shear strength. We show that the increase of the shear strength at low angularity is due to an increase of both contact and force anisotropies, and the saturation of the shear strength for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy. This transition reflects clearly the rather special geometrical properties of these highly angular shapes, implying that the stability of the packing relies strongly on the side-side contacts and the mobilization of friction forces.

VL - 86 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Particle shape dependence in 2D granular media JF - EPL (Europhysics Letters) Y1 - 2012 A1 - Baptiste Saint-Cyr A1 - Krzysztof Szarf A1 - Charles Voivret A1 - Emilien Azéma A1 - Vincent Richefeu A1 - Jean-Yves Delenne A1 - Gael Combe A1 - Cécile Nouguier-Lehon A1 - Pascal Villard A1 - Philippe Sornay A1 - Marie Chaze A1 - Farhang Radjaï AB -Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter

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.

VL - 84 IS - 1 JO - Phys. Rev. E ER -