Stress concentrations in granular materials under complex and narrow confinement

The influence of boundary geometry on force transmission through granular matter is not well understood, but valuable for a variety of engineering applications. In this study, we investigate how various confinement geometries bias the force and contact networks of monocrystalline ruby spheres during uniaxial compression. These confinement geometries take the form of compression platens possessing ridges of various angles, including flat platens for comparison, and smooth cylindrical side walls. We experimentally resolve the particle packing structure and contact network via X-ray tomography and individual particle strains via high energy X-ray diffraction microscopy (HEDM). The experiments validate a set of DEM simulations that indicate increasing levels of stress concentration towards the center of the sample with increasing ridge angles. The topology of the force transmission networks was analyzed to understand the source of the stress concentration. Additional simulations reveal a spatially uniform stress distribution with frictionless particles, suggesting the importance of shear jamming when localizing stress.