Evolution of distributions and spatial correlations of single-particle forces and stresses during compression of ductile granular materials

Uniaxial compression of disordered packings of millimeter-sized ductile particles formed from microcrystalline cellulose is investigated experimentally, at compression pressures in the vicinity of the minimum pressure required to form a coherent compact. Distributions of normal forces and stresses exerted by individual particles on a confining wall are determined. Spatial force and stress correlations are investigated. The distribution of normal forces is found to narrow with increasing pressure, but no indication of a crossover to a Gaussian decay at high forces is observed. The distribution of normal stresses is found to be considerably more Gaussian in shape for all pressures investigated. This finding may be interpreted as resulting from a positive correlation between the area corresponding to each particle and the force it experienced during compression. Spatial force and stress correlations are observed for distances smaller than three particle diameters. The spatial stress correlations indicate that the mode of stress transmission changes when the compression pressure exceeds the minimum pressure required to form a coherent compact.