Meso-structure evolution in a 2D granular material during biaxial loading

Multi-scale approaches of constitutive modeling require an intermediate scale linking the variables in macroscopic scale (incremental stress and strain) to variables in microscopic scale (contact force and contact displacement). In this paper, we introduce a mesoscopic scale, in which the granular material is tessellated into small loops by contact network. Then numerical biaxial tests from different initial states by DEM modeling, is performed to investigate how the meso-structure (mesoscopic loops) evolves along the drained biaxial loading path. Results suggest that the procedure of the biaxial test is accompanied with the exchange between small, dense structures and big, loose structures. The macroscopic dilatancy primarily originates from this exchange. In dense and intermediate specimens the meso-structure evolution is found not to be consistent with the evolution of the macroscopic volumetric strain during contractancy phases. This inconsistency has led to interpret the elastic and the plastic parts of the volumetric strain from a meso-scale viewpoint. It is shown that the initial contractancy in dense and intermediate specimens is largely an elastic process, which is highly dependent on elastic parameters of the material.