Three-dimensional experimental granular mechanics

Results are presented from a new method for experimental analysis of the mechanics of real granular materials in three dimensions. The method involves a combination of X-ray tomography and three-dimensional (3D) X-ray diffraction to provide the grain-scale structural evolution (grain positions and kinematics, grain-contact distributions and grain–grain contact fracture) simultaneously with the tensor strains of each of the individual grains, while performing mechanical testing on a granular assembly. Using this approach, force chains can be identified in three dimensions as continuous lines of load-carrying (strained) grains. Furthermore, the identified load distribution can be considered as a separation and organisation of ‘carriers’ (grains experiencing significant strain) and ‘non-carriers’ (grains carrying little or no load), which is characterised by the use of the Gini coefficient, borrowed from economic statistics. Correspondences are thus identified between the macroscopic stiffness of the granular assembly and both the evolution of granular strains and the distribution of the main load carriers.