Influence of loading direction on the behavior of anisotropic granular materials

Due to material anisotropy, the behavior of a granular material is generally directional-dependent. The directions of stress and strain increment have been observed non-coincident even for the case of loading in fixed directions. This paper uses the discrete element method to explore the underlying mechanisms of this non-coaxiality. Numerical experiments have been carried out to simulate the monotonic behavior of granular materials with fixed strain increment directions, and to provide associated particle-scale information. An initially anisotropic specimen was prepared using a deposition method to investigate the effect of material anisotropy produced during sample preparation. A preloaded specimen was obtained by shearing the initially anisotropic specimen in the deposition direction and then unloading it to the isotropic stress state. In this way, we can investigate the impact of a previous loading history.Monotonic loadings with fixed strain increment directions were applied on the two samples. The samples were tested under loading in directions varying from vertical to horizontal at 15° intervals. Numerical results were found to be in qualitative agreement with laboratory observations. A lower stiffness and a more contractive behavior are observed when the loading direction deviates further away from the deposition direction/the previous loading direction. Special attention has been placed on the observed non-coaxiality between the principal directions of stress and strain increment. In particular, it was found that non-coaxiality is more significant in the preloaded specimen than in the initially anisotropic specimen due to the difference in the development of contact force anisotropy.