The effect of rotational degrees of freedom on the formation of deformation patterns in granular materials using digital image correlation

Abstract

The shear band formation and evolution is a predominant mechanism of deformation patterning in granular materials. Independent rotations of separate particles can affect the pattern formation by adding the effect of rotational degrees of freedom to the mechanism of instability. Measurement of independent grain rotations is technically challenging. We use a special experimental technique based on digital image correlation in order to recover both displacement and independent rotation fields in 2D physical models of granular material. In the model the particles are represented by smooth steel disks with speckles painted on them to enable the rotation reconstruction. Both mono-and polydispersed particle assemblies are used. We show that the average values of the angles of disk rotations are insignificantly different from zero. It means that the rotations are microscopic (at the scale of the grain size) and do not proliferate to the macroscopic scale. However the particle rotations exhibit mesoscopic clustering: monodispersed assemblies produce vertical columns of particles rotating the same direction; polydispersed assemblies 2D form clusters of particles with alternating rotations.