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 - JOUR T1 - Cohesive granular materials composed of nonconvex particles JF - Physical Review E Y1 - 2013 A1 - Baptiste Saint-Cyr A1 - Farhang Radjaï A1 - Jean-Yves Delenne A1 - Philippe Sornay AB -The macroscopic cohesion of granular materials made up of sticky particles depends on the particle shapes. We address this issue by performing contact dynamics simulations of 2D packings of nonconvex aggregates. We find that the macroscopic cohesion is strongly dependent on the strain and stress inhomogeneities developing inside the material. The largest cohesion is obtained for nearly homogeneous deformation at the beginning of unconfined axial compression and it evolves linearly with nonconvexity. Interestingly, the aggregates in a sheared packing tend to form more contacts with fewer neighboring aggregates as the degree of nonconvexity increases. We also find that shearing leads either to an isotropic distribution of tensile contacts or to the same privileged direction as that of compressive contacts.

VL - 87 IS - 5 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 - 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 - 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

By means of contact dynamics simulations, we investigate the shear strength and internal structure of granular materials composed of two-dimensional nonconvex aggregates. We find that the packing fraction first grows as the nonconvexity is increased but declines at higher nonconvexity. This unmonotonic dependence reflects the competing effects of pore size reduction between convex borders of aggregates and gain in porosity at the nonconvex borders that are captured in a simple model fitting nicely the simulation data both in the isotropic and sheared packings. On the other hand, the internal angle of friction increases linearly with nonconvexity and saturates to a value independent of nonconvexity. We show that fabric anisotropy, force anisotropy, and friction mobilization, all enhanced by multiple contacts between aggregates, govern the observed increase of shear strength and its saturation with increasing nonconvexity. The main effect of interlocking is to dislocate frictional dissipation from the locked double and triple contacts between aggregates to the simple contacts between clusters of aggregates. This self-organization of particle motions allows the packing to keep a constant shear strength at high nonconvexity.

VL - 84 IS - 4 JO - Phys. Rev. E ER -