Abstract:
The non-coaxial behavior of sand is characterized by the misalignment between directions of the principal stress and the plastic strain rate. Researchers have systematically studied its mechanical mechanism through single shear tests and hollow cylinder torsional shear tests. Existing research indicates that an increment in shear stress induces plastic deformation along its direction, causing the principal stress and plastic strain rate directions to progressively align and become coaxial. At the microscopic scale, the non-coaxial behavior of sand originates from fabric anisotropy caused by non-uniform particle distribution. To account for anisotropy, conventional elastoplastic constitutive models typically introduce anisotropic parameters into the yield function, plastic potential function, critical state line, and dilatation equation. However, these parameters are often empirically derived and lack a theoretical basis. In contrast, multiscale micromechanical models directly incorporate the soil's fabric anisotropy in the stress averaging equation without any empirical anisotropic parameters required by conventional elastoplastic constitutive models. Using a rotated second-order deviatoric fabric tensor and a fabric evolution law, we employed the Chang-Hicher (CH) micromechanical model to simulate drained triaxial and hollow cylinder torsional shear tests under various loading directions. Simulations demonstrate that the stress ratio, volumetric strain, and fabric anisotropy of sand evolve toward the critical state at large strains, which validates the compatibility of the model with the anisotropic critical state theory. The strength and deformation behaviors of soils are inherently linked to the evolution of their fabric. For undrained triaxial experiments subjected to non-coaxial loading with the material fabric, the strength of sands reduces significantly at large strains. Subsequently, changes in the fabric result in more contacts aligned with the loading direction, thereby recovering the soil's strength. Compared to simulation results reported in previous studies, the CH model accurately captures the effect of loading direction on the mechanical behaviors of sands.