Bond anisotropy and cohesion of wet granular materials

We analyse the Coulomb cohesion of wet granular materials and its relationship with the distribution of capillary bonds between particles. We show that, within a harmonic representation of the bond and force states, the shear strength can be expressed as a state equation in terms of internal anisotropy parameters. This formulation involves a dependence of the shear strength on loading direction and leads to the fragile behaviour of granular materials. The contact dynamics simulations of a wet material, in which a capillary force law is prescribed, are in excellent agreement with the predictions of this model. We find that the fragile character decreases as the local adhesion is increased or the mean stress is decreased.