Revisiting localized deformation in sand with complex systems

Complex systems techniques are used to analyse X-ray micro-CT measurements of grain kinematics in Hostun sand under triaxial compression. Network nodes with the least mean shortest path length to all other nodes, or highest relative closeness centrality, reside in the region where the persistent shear band ultimately develops. This trend, whereby a group of grains distinguishes themselves from the rest in the sample, remarkably manifests from the onset of loading. The shear band's boundaries and thickness, evident from the network communities' borders and essentially constant mean size, provide corroborating evidence of early detection of strain localization. Our findings raise the possibility that the formation and the location of the persistent shear band may be decided in the nascent stages of loading, well before peak shear stress. Grain-scale digital image correlation strain measurements and statistical tests confirm the results are robust. Moreover, the trends are unambiguously reproduced in a discrete element simulation of plane strain compression.