Résumé
The strength of granular materials is highly dependent on grain connectivity
(fabric), force transmission, and frictional mobilization at the particle
scale. Furthermore, these bulk properties are strongly dependent on the
geometry and history of loading. It is well established that anisotropy in
fabric and force transmission through a granular packing directly relates to
the bulk scale strength of the packing via the Stress-Force-Fabric (SFF)
relation. We have recently verified the validity of this framework for a broad
variety of loading histories and geometries in experimental granular packings,
using photoelastic disks to measure individual interparticle contact forces. By
tracking both particle positions and interparticle contact force vectors, we
mapped the anisotropy of the fabric and forces to the macroscale stress and
strain and found excellent agreement between the anisotropic particle-scale
measures and the macroscale responses in experiments. Here, we present an
analysis of the effect of strong spatial gradients (shear bands) using the SFF
framework in a sheared annular geometry, finding that there are strong
variations in contact orientation depending on the location within or outside
the shear band, even though the principal loading direction is uniform. This
highlights that the fabric connectivity significantly changes across the shear
band but does not contribute to the direct loading of the material. We
disentangle the effects of packing fraction gradients and boundary constraints
on the differences in fabric orientation.