Numerical Investigation of Grain Fragmentation of a Granular Specimen under Plane Strain Compression

In this paper the influence of grain fragmentation on the change of the grain size distribution and an additional compaction of a cohesionless granular material was modeled in a simplified manner by introducing the mean grain diameter and the solid hardness as state variables into an extended micropolar hypoplastic model. The solid hardness was defined in the sense of a continuum description and was a key parameter for modeling the incremental stiffness. The reduction of the mean grain diameter and the solid hardness caused by grain fragmentation was described by evolution equations taking into account an increase of the mean pressure and a change of the stress deviator. Because in the extended model the critical void ratio also depended on the evolution of the solid hardness, a reduction of the critical void ratio caused by grain fragmentation also was taken into account. To investigate the effect of grain fragmentation on the mechanical response, numerical simulations of plane strain compression tests were carried out for two different types of tests. A biaxial compression test under unconstrained lateral deformation and a biaxial test of the Hambly type were simulated using the finite-element method. Particular attention was paid to the influence of the initial void ratio and the type of the lateral boundary conditions on the interaction between the evolution of strain localization and the reduction of the mean grain diameter. The numerical simulations showed that the reduction of the mean grain diameter was pronounced in zones of shear strain localization and it was larger for the initially dense specimen. The evolution of patterns of strain localization and of zones of intense reduction of the mean grain diameter strongly depended on the boundary condition of the test type.