Response of Granular Materials to Shear: Origins of Shear Jamming, Particle Dynamics, and Effects of Particle Properties

Abstract

Granular materials under shear are common in nature and industry. Previous results show changes of system behaviors when friction is added and particle shapes are varied, e.g. shear jamming for frictional grains. Understanding these changes depends on characterization of deformation induced by shear. However, previous studies mainly focus on yielding processes and are locally symmetric, e.g. shear transformation zones (STZ's). Besides, the grain scale explanation is lacking. In this thesis, I study the shear response of granular materials with various particle properties in two dimension, utilizing a novel setup that suppresses shear banding. Particles made of photoelastic materials can reveal inter-particle contact forces and be customized to have different friction and shapes. I propose novel minimum structures, trimers and branches, that account for shear jamming. These structures are locally asymmetric, which is contrary to STZ's. Systems with three different friction coefficients $\mu$ are studied: $0.15, 0.7$ and one higher than $1.7$. Shear jamming is still observed for the lowest $\mu$ studied, with the lowest value of packing fraction $\phi$ for shear jamming, $\phi_S$, increasing as $\mu$ decreases. Furthermore, these systems for all $\mu$ show abnormal diffusion under cyclic shear. The diffusion exponents show transitions as $\phi$ increases, with a $\mu$-dependent onset $\phi$. This behavior is consistent with the non-affine displacements under linear shear. In addition, systems composed of ellipses exhibit novel structural and mechanical responses different from disks, e.g., nematic ordering and local density variability under shear.