Effects of grain shape on packing and dilatancy of sheared granular materials

Abstract

A granular material exposed to shear shows a variety of unique phenomena: Reynolds dilatancy, positional order and orientational order effects may compete in the shear zone. We study granular packing consisting of macroscopic prolate, oblate and spherical grains and compare their behaviour. X-ray tomography is used to determine the particle positions and orientations in a cylindrical split bottom shear cell. Packing densities and the arrangements of individual particles in the shear zone are evaluated. For anisometric particles, we observe the competition of two opposite effects. On the one hand, the sheared granules are dilated, on the other hand the particles reorient and align with respect to the streamlines. Even though aligned cylinders in principle may achieve higher packing densities, this alignment compensates for the effect of dilatancy only partially. The complex rearrangements lead to a depression of the surface above the well oriented region while neighbouring parts still show the effect of dilation in the form of heaps. For grains with isotropic shapes, the surface remains rather flat. Perfect monodisperse spheres crystallize in the shear zone, whereby positional order partially overcompensates dilatancy effects. However, even slight deviations from the ideal monodisperse sphere shape inhibit crystallization.

Effective friction of granular flows made of non-spherical particles

“…It has been observed, however, that in 3D numerical systems of identical spheres crystallization may happen after sufficiently long shear [41] or shaking [42]. Experiments of sheared identical spheres also displayed ordering, where chains of particles formed along the streamlines, and neighboring chains were arranged on a triangular lattice in the perpendicular plane [36].…”

“…Finally, these constitutive laws have been incorporated to models for a diphasic description of sediment transport [27][28][29][30]. Several recent studies investigated the behevior of elongated grains under shear [31][32][33][34][35][36][37]. These studies mostly focused on quasistatic flows [31,32] or on the orientational distribution of particles as a function of grain elongation and shear rate [33][34][35][36].…”

“…Several recent studies investigated the behevior of elongated grains under shear [31][32][33][34][35][36][37]. These studies mostly focused on quasistatic flows [31,32] or on the orientational distribution of particles as a function of grain elongation and shear rate [33][34][35][36]. Stresses were analyzed as a function of particle shape for glued-spheres and true cylinders [38].…”

Local Origin of Global Contact Numbers in Frictional Ellipsoid Packings

“…Frictional ellipsoids used in experiments [13][14][15][16] exhibit a number of differences to the frictionless ellipsoids often studied numerically [17][18][19][20][21][22][23]. The latter have been found to form packings with less than the number of contacts required for isostaticity, which is defined as having enough constraints to block all degrees of freedom of the particles [17,21,24].…”

Non-spherical granular flows down inclined chutes

“…However, the appropriate description of systems composed of non-spherical particles is of significant importance for practical applications such as the handling of rocks, rice, wheat or tablets. Thus, there is currently and increasing interest in the behavior of non-spherical grains both experimentally [14][15][16][17] and numerically [18,19]. The goal of this study is to extend existing theories of frictional avalanching flows of spherical particles in steady state situations, to the case of non-spherical grains.…”