Experimental study of shear band formation: bifurcation and localization

We report the experimental observation of the bifurcation at the origin of localization of the deformation in a granular material submitted to uniaxial compression. We present a quantitative characterization of the heterogeneity in the strain field repartition allowing to evidence objectively the existence of a bifurcation initiating the shear bands formation process. We show that this bifurcation is supercritical and has no clear signature on the stress-strain curve. At the bifurcation, a symmetry breaking occurs characterized by the emergence of a well-defined orientation corresponding to the Mohr-Coulomb angle. Yet, plasticity is still diffuse and the shear band extension is of the order of the sample width. While loading proceeds, the shear band narrows until it reaches, after the peak of the stress-strain curve, a stationary width.