Simulation of multi-axial compaction of granular media from loose to high relative densities

A multi-particle finite element model (MPFEM) was used to explore the densification yield surfaces of a collection of monosize particles in 2-D. Individual particles discretized with a finite element mesh allow for a full description of the contact mechanics and the local and global particle kinematics. Compaction modes ranging from hydrostatic to that of high shear are studied for various levels of interparticle friction. Isodensity curves in stress-space during densification are shown to take the equivalent shape of a cap and cone model whose shear failure line has a slope that is a function of interparticle friction. The mechanical response predicted by this model is softer than the one predicted by the discrete element method (DEM) model of Redanz and Fleck (2001. Acta Mater. 49 (2001) 4325) suggesting that the stiffer response of DEM is the result of simplifications on interparticle contact behavior. Deviation from affine motion, rearrangement, and particle rotation are shown to be significant and to depend on interparticle friction and macroscopic stress triaxiality. Local deviatoric stresses and equivalent plastic strains are present during the formation of stress chains at low relative densities indicating that contact deformation can occur early in densification. The combined effect of particle movements with early contact indentation leads to a softer global response to externally applied strains than other analytical or computation models. This work illustrates that the MPFEM model is useful in describing and understanding the interparticle behavior and its effect on microscopic and macroscopic response for various strain histories.