Particle-Scale Insight into Deformation Noncoaxiality of Granular Materials

This paper explores the mechanism of deformation noncoaxiality from the particle scale. A multiscale investigation has been carried out with the particle-scale information obtained from discrete element simulation on granular materials. The specimens were prepared anisotropically and sheared in various loading directions. The deformation noncoaxiality, i.e., noncoincidence between the principal stress direction and the principal strain increment direction, was observed. The directional statistical theory has been used to study the anisotropies in material fabric and particle interactions, and to characterize them in terms of direction tensors. Based on the stress-force-fabric relationship, the stress direction was determined from these direction tensors. In monotonic loading, it was observed that the force anisotropy is always coaxial with the loading direction, i.e., the strain increment direction, while the fabric anisotropy only gradually approaches the loading direction. This noncoincidence between the fabric anisotropy and the strain increment direction is found to be the main cause of deformation noncoaxiality. An expression to calculate the degree of noncoaxiality from the ratio and the direction deviation between the fabric and force anisotropies has been proposed.