Cyclic Features of Polygon-shaped Materials through DEM Simulations

Numerical simulations of cyclic biaxial tests were performed on samples with polygonal particles having aspect ratios (AR) equal to 1 up to 3 to better understand the cyclic behaviour of granular materials. The samples are created by deposit under gravity leading to the existence of an inherent anisotropy. Nevertheless, for the sample with AR equal to 1, numerical tests were also performed from an initial isotropic state which is generally the reference state considered for the modelling of granular soils. The evolution of both global and local variables was analysed: stress ratio and void ratio, fabric and anisotropy of normal and tangential forces on the other hand. The numerical tests show that in the range of studied deformations, the initial fabric is never erased during cycling, even for isometric particles for which inherent fabric is generally low. Generally very different behaviours are observed for materials with AR equal to 1 and equal to 3 during cycling. The reorientation of principal axes of anisotropy is the key concept explaining the discrepancy of behaviour. The evolution of the characteristic state, state of maximum volumetric deformation is analysed through cycling. In this work, the characteristic state which uniqueness is subjected to controversy is found different from the critical state. It can be written from the critical state which is unique for a given material in terms of global variables (the void ratio) and local variables related to the fabric.