A novel three-dimensional contact model for granulates incorporating rolling and twisting resistances

This paper presents a new three-dimensional (3D) contact model incorporating rolling and twisting resistances at inter-particle contact, which can be introduced into the discrete element method (DEM) to simulate the mechanical behavior of particulates. In this model, two spheres were assumed to physically interact over a circular contact area, where an infinite number of normal spring-dashpot-divider elements and tangential spring-dashpot-slider elements were continuously distributed. The model consists of four interactions, in normal/tangential/rolling/twisting direction, with physically-based stiffness, peak resistance and damping coefficient in each direction. The two main features of the model are that (1) the contact behavior was physically derived and (2) only two additional parameters, shape parameter β (linking the contact area radius and particle size) and local crushing parameter ζ_c (describing local contact crushing resistance) were introduced when compared with the standard 3D DEM. The new model was implemented into the 3D DEM code and used to simulate conventional triaxial and plane-strain compression (CTC and PSC) tests to examine its ability to capture the quasi-static behavior of sands. The numerical results show that the strength of the DEM material obtained in CTC tests increases with the parameter β and is within the experimentally observed typical strength range of sand (30-40°). Rolling and twisting resistances can lead to increased dilation in volume. Consistent with previous studies, the high level of out-of-plane confinement observed in the PSC tests can greatly increase the strength and produce a strain-softening response. A unique critical state line (CSL) was identified in the CTC tests, where the intercept and slope on the e-lg. p plane increased with β. Comparison with previous experimental data confirms that the shape parameter β can be well correlated to a statistical measure of real particle shape. The new model is also compared with previous DEM models.