Slip lines versus shear bands: two competing localization modes

International audienceGranular materials can develop two different failure modes following their initial density and the loading path: a localized mode, generally characterized by shear bands, and a diffuse mode without any macroscopic localized bands. In fact detailed analyses of experiments by tomography, of finite element computations and more recently of discrete element models (DEM) simulations show that two different localized objects co-exist at two different scales: a network of meso-slip lines appearing from the very beginning of deviatoric loading and a few macro-shear bands when approaching limit stress states. This paper aims to compare and analyze these two localized patterns, based on DEM results obtained for a given class of loading paths applied to several 2D numerical granular samples with different initial densities. It is first shown that void ratios and granular meso-structures are the same inside shear bands and for the whole samples in case of diffuse failure. Then slip lines and shear bands are shown to be distinct localized objects. Eventually, the questions of why and how the network of meso-slip lines is sometimes bifurcating into a set of few macro-shear bands are discussed