Stress fluctuations during monotonic loading of dense three-dimensional granular materials

The paper examines the sudden and irregular fluctuations of stress that are commonly observed in DEM and laboratory tests of slow monotonic loading of granular materials. The stresss fluctuations occur as stress-drops that are spaced in an irregular, random manner as the material is loaded, and occur at an average rate of about 0.05 drops per particle per one percent of strain. Each fluctuation is accompanied by a drop in the number of contacts and in the number of sliding contacts, a brief increase in the particles' kinetic energy, a reduction in the elastic energy of the contacts, and a reduction in bulk volume. Stress-drops are shown to originate within small regions of the larger assembly and are likely the result of a multi-slip mechanism. An advanced discrete element method (DEM) is used in the study, with non-convex non-spherical particles and an exact implementation of the Hertz-like Cattaneo-Mindlin contact model. The simulations reveal differences with the avalanche phenomenon of amorphous solids, in particular the different scalings of stress-drop magnitude with assembly size and drop frequency.