Résumé
The bulk behaviour of granular materials is tied to its mesoscale and
particle-scale features: strength properties arise from the buildup of
various anisotropic structures at the particle-scale induced by grain
connectivity, force transmission, and frictional mobilization. More
fundamentally, these anisotropic structures work collectively to define
features like the bulk friction coefficient and the stress tensor at the
macroscale and can be explained by the Stress-Force-Fabric (SFF)
relationship stemming from the microscale arrangement of the forces and
fabric. Although the SFF relation has been extensively verified by
discrete numerical simulations, a laboratory realization has remained
elusive due to the challenge of measuring both normal and frictional
contact forces. In this study, we analyze experiments performed on a
photoelastic granular system under four different loading conditions:
uniaxial compression, isotropic compression, pure shear, and
annular shear. During these experiments, we record particle locations,
contacts, and normal and frictional forces to measure the particle-scale
response to progressing strain. We experimentally assess the
Stress-Force-Fabric (SFF) relation across multiple loading conditions in a
2D photoelastic granular system. We track microscale measures like the
packing fraction, average coordination number, and average normal force,
along with anisotropic distributions of contacts and forces. We then
connect these particle-scale anisotropies to bulk behavior using the SFF
relation, which provides compact expressions for both the stress tensor
and the bulk friction coefficient in terms of fabric and force
anisotropies. Our results demonstrate that these expressions accurately
capture the bulk stress and friction across different loading histories,
validating the predictive power of the SFF framework. Additionally, we
test the assumption that contact and force anisotropies contribute equally
to load transmission in our granular packings and show that this
assumption is sufficient at large strain values and can be applied to
areas like rock mechanics, soft colloids, or cellular tissue where force
information is inaccessible.