  The strain-rate sensitivity of confined granular materials has been widely
explored, with most findings exhibiting rate-strengthening behaviors. This
study, however, reveals a distinct rate-softening behavior across a certain
strain rate range based on triaxial tests on particle clusters of various
materials with different surface properties, particle sizes, shapes, and
stiffness. This softening effect is especially pronounced in the case of common
rice particles. By examining the behavior of rice particles under different
confining pressure and surface conditions, and directly measuring the
frictional coefficient across various loading rates, we find that the reduction
in surface frictional coefficient with the increasing strain rate predominantly
contributes to this rate-softening behavior. This conclusion is validated by
results from Finite Element Method (FEM) simulations. Additionally, we identify
confining pressure as a critical factor regulating the normal stress between
particles, and thereby enhancing frictional behavior. Rheometer tests reveal
that the shear modulus exhibits a similar rate-softening trend. This study of
rate-softening behavior in granular materials enhances our understanding of the
mechanisms during their deformation under confining pressure. It also suggests
that local inter-particle tribology significantly impacts overall granular
behavior.
