Shear strength and micro-descriptors of bidisperse ellipsoids under different loading paths

The paper examines the macroscopic and microscopic behaviors of a granular material, systems of bidisperse ellipsoids. The larger ellipsoids of these systems have an axial length ratio (major axial length to minor axial length) of 1.5 and the shorter ellipsoids have an axial length ratio of 1.2. The weights of these two kinds are identical in each sample. Samples are subjected to various confining pressures ranging from 30 to 3000 kPa. Then they are loaded following three stress paths including axial extension condition, triaxial extension condition, and axial compression condition. These drained tests are stopped till the critical state is reached. A linear critical state line in the void ratio and mean stress space was observed for each stress path. The slopes of these critical state lines are different. The critical state lines for the axial extension and triaxial extension tests are above that of the axial compression tests. We also examined different microscopic descriptors at peak and critical states. They are based on the micro-structural tensor of contact normals or normal contact forces. Excellent relationship was found between these descriptors and the peak shear strength in the compression tests. Good relationship was observed for the extension tests. The relationships between these descriptors and the critical shear strength are fair for the compression tests. However, these descriptors do not capture the trends of axial extension tests in critical state.