By means of contact dynamics simulations, we investigate the mechanical strength and the internal structure of granular materials composed of adhesive nonconvex aggregates with increasing nonconvexity. The issue is how the shape nonconvexity will affect the macroscopic Coulomb cohesion emerging from particle interaction with a given bond number. We show that the nonconvexity amplifies considerably the macroscopic cohesion that increases with the degree of nonconvexity. We also found that the Coulomb cohesion evaluated from different shear tests, strongly depends on the boundary conditions due to the heterogeneity of either the strains or stresses inside the material. The largest cohesion is obtained for nearly homogeneous deformation at the beginning of axial compression without confinement (simple compression test). In the residual state, during the biaxial shear tests, the samples characterized by a unique level of nonconvexity, develop a heterogeneous structure strongly marked by shear bands localization (movie eta 5). Because of this, they flow in coherent block larger than particles sizes. Furthermore these samples show different schemes of strain localization, which lead to fluctuating values of the apparent cohesion as a function of the nonconvexity. On the contrary, by considering uniaxial test without the lateral walls, we found that the unconfined yield stress and the resultant apparent cohesion increases monotically with the nonconvexity, but an heterogeneous stress distribution occurs in the different packings (movies eta 4 and eta 7). A local analysis of the granular structure, shows that the more nonconvex are the aggregates the more heterogeneous are the distribution of the local packing fraction.
Biaxial compression test of dense cohesive granular sample composed of nonconvex aggregates (eta=0.5). (black and red lines are respectively tensile and compressive forces, their width is proportional to stress magnitude)
Simple compression test of dense cohesive granular sample composed of nonconvex aggregates (eta=0.4)
Simple compression test of dense cohesive granular sample composed of nonconvex aggregates (eta=0.7)