International audienceNumerical simulations, by the discrete element method (DEM), of a model granular assembly, made of spherical balls, are used to investigate the influence of a small amount of an interstitial wetting liquid, forming capillary bridges between adjacent particles, on two basic aspects of granular material rheology: (1) the plastic response in isotropic compression, and (2) the critical state under monotonic shear strain, and its generalization to steady, inertial flow. Tensile strength F0=πΓa, in contacts between beads of diameter aa joined by a small meniscus of a liquid with surface tension Γ, introduces a new force scale and a new dimensionless control parameter, P∗=a2P/F0, for grains of diameter a under confining stress P. Under low P*, as cohesion dominates, capillary cohesion may stabilize very loose structures. Upon increasing pressure P in isotropic compression, such structures gradually collapse. The resulting irreversible compaction is well described by the classical linear relation between logP* and void ratio in some range, until a dense structure forms that retains its stability without cohesion as confinement dominates for large P*. In steady shear flow, with uniform velocity gradient γ˙=∂v1/∂x2γ˙=∂v1/∂x2 under normal stress P=σ22, the apparent internal friction coefficient, which is defined as μ∗=σ12/σ22, depends on P* and inertial number (reduced shear rate) I=γ˙√m/aP, and so does solid fraction Φ. The material exhibits, as P* decreases, a strongly enhanced resistance to shear (larger μ*). In the quasistatic limit, for I→0, it is roughly predicted by a simple effective pressure assumption by which the capillary forces are deemed equivalent to an isotropic pressure increase applied to the dry material as long as P*≥1, while the yield criterion approximately assumes the Mohr-Coulomb form. At lower P*, such models tend to break down as liquid bonding, causing connected clusters to survive over significant strain intervals, strongly influences the microstructure. Systematic shear banding is observed at very small P

The behavior of model granular materials (glass beads) wetted by a small quantity of liquid forming capillary bridges is studied by one-dimensional compression test combined with Xray computed tomography (XRCT) observation. Special attention is paid to obtain very loose initial states (initial void ratio of about 2.30) stabilized by capillary cohesion. XRCT-based analyses involve spherical particle detection adapted to relatively low-resolution images, which enable heterogeneities to be visualized and microstructural information to be collected. This study on an ideal material provides an insight into the macroscopic compression behavior of wet granular materials based on the microstructural change, such as pore distance distribution, coordination number of contacts, coordination number of neighbors, number of contacts per grain.