By means of the contact dynamics method together with a particle fracture model, in which the particles are cohesive aggregates of irreducible polygonal fragments, we investigate the evolution of particle size distribution in the process of uniaxial compaction of granular materials. The case of single particle breakup under compressive stress is used to test the method and the influence of discretization (number of irreducible fragments). We show that the breaking threshold of the granular assembly scales with the internal cohesion of the particles but it depends also on the initial size distribution and irregularity of polygonal particle shapes. The evolution of size distribution proceeds by consecutive periods of intense particle crushing, characterized by local shattering instability, and periods of little breaking activity. Starting with either monodisperse or power- law distribution of particle sizes, the latter evolves towards a broad distribution of the fragmented particles with a nearly power-law distribution in the range of intermediate particle sizes. Interestingly, a finite number of large particles survive despite ongoing crushing process due to the more homogeneous distribution of forces in the presence of small fragmented particles filling the pores between larger particles.

By means of three-dimensional Contact Dynamics simulations, we inves- tigate the microstructural properties of sheared granular packings by varying sys- tematically the inertial number I from the quasi-static to the collisional regime. A detailed analysis of the connectivity of the particles reveals that the transition be- tween the regimes is correlated to profound changes of the nature of the contact net- work. In particular, we show that the increase of internal friction angle with inertial number is mainly a consequence of contact anisotropy. As the inertial number in- creases, the role of force chains decreases and that of friction mobilization increases as long as percolating force chains are present in the system. A detailed analysis of the spatial correlations of floating particles, i.e., particles without bearing con- tacts, reveals the occurrence of “fluidized” volumes in the dense flow regime, which increase in size and coalesce at the approach of the collisional regime.

The compaction of powders depends both on grain rearangements and grain breakage. We introduce a grain fracture model prescribed in the framework of the contact dynamics method for the simulation of uniaxial compaction. We find that the grain size reduction is highly heterogeneous as a consequence of inhomogeneous stress transmission as observed in real grinding processes or in natural degradation of geomaterials. Even under high stresses, a significant fraction of grains survive whereas many grains are fully shattered. The grain size distribution tends to a power- law distribution with increasing size span. We analyze the progressive evolution of compressibility during compaction as well as the effect of grain shape and size distribution.

We investigate the rheology, force transmission and texture of granular materials composed of elongated particles by means of contact dynamics simula- tions. The particles have a rounded-cap rectangular (RCR) shape described by a single elongation parameter varying from 0 for a circular particle to 1 for an in- finitely thin or long particle. We study the quasi-static behavior, structural and force anisotropies as a function of the elongation parameter for packings submited to biax- ial compression. The shear strength is found to increase linearly with this parameter whereas the solid fraction both at the initial isotropic state and in the critical state is nonmonotonous. We show that for these elongated particles a harmonic decomposi- tion of the stress tensor provides a fairly good approximattion of the internal state. Our data suggest that the increase of shear strength with reflects both enhanced friction mobilization and anisotropic particle orientation as the elongation of the particles increases.

We use numerical simulations to investigate force and stress transmission in cohesive granular media covering a wide class of materials encountered in nature and industrial processing. We consider both liquid bridges and solid bridges between particles. The liquid bridges will be treated by implementing a capillary force law within a debonding distance between particles and simulated by the Discrete Element Method (DEM). The solid bridges or more generally the binding matrix is treated by means of the Lattice Element Method (LEM) based on a lattice-type discretization of the particles and matrix. Our data indicate that the exponential fall-off of strong compressive forces is a generic feature of both cohesive and noncohesive granular media. Interestingly, the tensile forces exhibit a similar decreasing exponential distribution, suggesting that this form basically reflects granular disorder. This is consistent with the finding that not only the contact forces but also the stress components in the bulk of the particles and matrix, accessible from LEM simulations, show a exponential fall-off. We also find that the distribution of weak compressive forces is sensitive to packing anisotropy, particle shape and particle size distribution. In the case of wet packings, we analyze the self-equilibrated forces induced by liquid bonds and show that the positive and negative particle pressures form a bi-percolating structure.