Particle shape dependence in granular media

CEGEO Group

This study has been done by the CEGEO collaborative group (scale change in geomaterials) see below for information about the authors.Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter η describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and non- convexity, η is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of η with nearly the same trend for all shapes, the differences appearing thus to be 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. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of η.

Movie of compression tests of a granular sample composed of polygonal particles

Normal contact forces

Second invariant of stress tensor

Movie of the force-bearing particles at η = 0.7 and normal forces represented by the thickness of the segments joining the particle centers to the application point of the force. The color level for the particles is proportional to the orientation of the major particle axis for the particles with at least one side/side contact. The particles having no side/side contacts are in gray. The strong and weak forces are in black and red, respectively