Experimental investigation of cyclically sheared granular particles with direct particle tracking

We discuss experiments on dense packing of granular beads that are cyclically sheared quasi-statically between parallel walls under constant pressure boundary conditions. The particle positions inside the shear cell are tracked over several cycles in three dimensions using particle index-matching imaging technique. The total volume fraction of the particles φ in the cell is observed to increase slowly over thousands of cycles from φ ∼ 0.59 to φ ∼ 0.63, while even slower growth in volume fraction is observed in the bulk away from boundaries. We illustrate with internal images that the difference arises due to inhomogeneity of packing with ordered regions developing progressively from the boundaries. We then focus in the bulk where the packing is uniformly disordered, and find that a linear bulk strain is observed within the first half of a cycle, which is reversed in the second half of the cycle. We present analysis of the trajectories of the particles within a shear cycle as well as over several cycles. We find anisotropic fluctuations relative to shear gradient within a cycle. However, homogeneous growth of mean square displacement when fluctuations are examined average over a cycle. The rate of growth is significantly lower leading us to hypothesize that granular matter under cyclic shear show reversible as well as irreversible or plastic response for small enough strain amplitude.