A micromechanical study of shear-weakening characteristics of granular flow in a torsional shear cell

This study employs Discrete Element Method (DEM) simulations to investigate the micromechanical characteristics of granular materials subjected to shear in a torsional shear cell across various flow regimes, including quasi-static, transitional, and grain-inertial states. Two simulation configurations with slightly different initial volume fractions (packing densities) were compared to elucidate both force chain evolutions and macroscopic rheological responses of granular flow. The observed macroscopic stress responses, particularly shear-weakening phenomenon, were further interpreted using modified analytical constitutive laws. This approach successfully captured the trends and yielded quantitative macroscopic parameters that are sensitive to initial packing density. By further tracking the evolution of force chain properties such as length, number, curvature, anisotropy, and load distribution, we aim to establish connections between microscopic force chain characteristics and macroscopic rheological behavior like shear-weakening observed in the transitional regime. The results reveal distinct evolutionary patterns in force chain characteristics across different flow regimes and between the two simulation configurations. Notably, specific parameters such as opposing trends in force chain anisotropy and different variations in force chain numbers between two configurations can serve as microscopic indicators for identifying more noticeable macroscopic shear-weakening phenomenon. Conversely, parameters such as force chain curvature exhibited a reduced dependence on packing density and interparticle stress, indicating that they are more reflective of intrinsic structural reorganization patterns. This research enhances our understanding of the micromechanical origins of granular rheology and establishes connections between macroscopic constitutive parameters and microscopic force chain indicators.