Micromechanical observation of kinematics of sheared circular discs

Particle rolling is an essential microscopic mechanism that governs macroscopic behavior. This study conducts biaxial shearing tests on bi-dispersed circular discs at different confining pressures. A novel 2D image analysis technique is employed to measure the rolling of all the particles. It is observed that most of the particles exhibit significant rolling during shearing. Rollings are normally distributed in clockwise and counterclockwise directions, and the net rolling of the granular assembly is almost zero. Generally, the rolling of a particle is accompanied by its neighboring particles opposite rolling in a similar magnitude. In some cases, a group of particles is observed to exhibit rolling in the same direction, accompanied by another opposite rolling group in the neighboring regions. Particles inside the shear band tend to show significant rolling. The rolling rate is prominent at the beginning of the shearing and gradually decreases towards the end. Small particles exhibit significantly higher rotations, while larger particles are relatively resistant to rolling. Small particles work as ball bearings between two big particles, reducing the shear strength of the granular materials. The experimental data obtained in this study can be used to perform detailed validation of numerical models to simulate realistic granular behavior such as DEM.