Experimentally validated DEM modelling of granular materials under simple shear testing
Simple shear testing’s ability in simulating realistic in situ conditions makes it an effective element-level testing method to study the response of granular materials under uni-directional and multi-directional loading. DEM modelling provides complementary information such as contact networks and particle movements, that is underlying, but not observed directly from the experiments. In this study, a number of uni-directional and bi-directional experimental and DEM simple shear tests were conducted to provide a database for studying the response of granular materials, as well as for developing DEM simulation techniques. The physical specimens consisted of 60,000 poly-dispersed steel spheres. The material properties and sample setup in DEM were closely matched to those in the experiments, to ensure the models perform in good agreement with the physical counterparts in terms of the macroscopic response.
Different boundary types were evaluated and compared regarding the shear transmission ability and computational efficiency. Flat boundaries showed not only a lower shear transmission ability, but also insufficient engagement of particle movements and rotations. The application of ribbed and pyramid-shaped projections on boundaries resulted in pronounced improvement. Boundaries with pyramid-shaped projections were eventually used for the DEM models because of the close match to the experimental results as well as the applicability in bi-directional tests.
A detailed analysis of the sample preparation process was conducted. An artificially assigned low interparticle friction coefficient throughout the entire consolidation stage resulted in an unrealistic macroscopic and microscopic initial response, the effects of which were gradually erased with shearing. In contrast, switching the interparticle friction coefficient back to the actual value before the application of any vertical stress generated dense specimens with a more realistic initial response.
The DEM bi-directional simple shear models successfully captured the trend of macroscopic response in physical tests. A larger angle of change of shearing directions resulted in greater reductions in the shear stress and a more contractive response. Inclined force chains could be observed in the vertical plane that paralleled the direction of shearing. Contact fabrics tended to orient at around 45o from the horizontal direction at large strain. The change of the direction of shearing caused the rearrangement of the force chains and contact fabrics. For specimens with the same density, a larger angle between the two shearing directions resulted in a larger number of downward-moving particles. For the same loading conditions, loose specimens exhibited greater volume reduction at the change point of shearing direction.