Study on couple stress and shear band development in granular media based on numerical simulation analyses

Abstract

From the reasonable accordance between the simulation and laboratory tests, it is concluded that the simulation test using the distinct element method can provide a powerful tool to simulate the micro- as well as macro-behavior of granular media. This is true, in particular, when the rotational resistance is introduced into the conventional one. Based on both the simulation and laboratory tests, we reached the following conclusions: One of the most important changes in microstructure, which takes place during a strain hardening process, is the formation of column-like structure growing parallel to the major principal stress direction. After failure, the column-like structure is reconstructed during a strain softening process by means of rolling, not sliding, at contact points so that a high gradient of particle rotation is generated, changing from negative to positive in a relatively narrow shear zone. Large voids appear in the shear band, and the resulting local void ratio can exceed the corresponding maximum one determined by a standard method. This fact strongly suggests that unique stress condition, which leads to such special microstructure, may develop in the shear band. In fact, couple stresses exist in a shear band in a manner consistent with the change of the particle rotation gradient from negative to positive. In spite of the presence of the couple stress, the stress tensor is nearly symmetric, indicating that the couple stress is very small in magnitude. The presence of the small couple stress still plays an important role in the development of microstructure in shear bands.