Micromechanics of granular material response during load reversals: Combined DEM and experimental study

The use of ideal granular materials with regular, simple geometries (e.g. steel spheres) allows an accurate geometrical representation of physical test specimens to be made in DEM simulations. Physical tests on these materials can then be used to validate DEM models and these DEM models can be confidently used to develop insight into the micro-scale interactions driving the macro-scale response observed in the laboratory. A novel approach to simulating triaxial tests with DEM using circumferential periodic boundaries has been developed by the authors. In a previous study this approach was validated analytically and by considering a series of laboratory monotonic triaxial tests on specimens of uniform and non-uniform steel spheres. The current paper extends the earlier research of the authors by simulating the response of specimens of about 15,000 steel spheres subject to load-unload cycles in quasi-static triaxial tests. In general, good agreement was attained between the physical tests and the DEM simulations. Following a description of the simulation and testing approach adopted, the results of the DEM simulation are used to explore the particle-scale mechanics during the load reversals. The micro-scale analyses considered both the magnitude and orientation of the contact forces as well as the motion of the particles during the load-unload cycles. These micro-scale analyses revealed that the relatively stiff, almost elastic macro-scale response observed in the load-unload cycles is underlain by a particle-scale response involving a substantial redistribution of the contact forces without a significant disturbance to the contact force network. (C) 2009 Elsevier B.V. All rights reserved.