Experimental study of failure in a granular material

Solid amorphous materials, when submitted to large enough stresses, display localization of the deformation. This localization can take several forms from local plastic events to shear-bands formation. I will present recent experimental results showing how those processes occur in a granular material submitted to a biaxial test.

Our observations show that failure in a sheared granular material is a multi-scale process both on a spatial point of view and on strain increments point of view. When a small strain increment is used to define the deformation field, plasticity occurs in the form of local rearrangements. Those local events can be interpreted as elementary contributions building up in a macroscopic plastic flow on larger strain increments. Correlation between the rearrangements can be evidenced and a transient self-organization of the flow at intermediate strain increments is observed with the formation of intermittent micro-bands. The coupling between the rearrangements is due to the elastic redistribution of the stress when a plastic event takes place. This structuration of the plastic flow corresponds to the fluctuating part of the plastic field and is clearly of a different nature from the final, permanent, shear-band which is a feature of the mean plastic field, defined on large strain increments. We have shown that the shear band emerges through a bifurcation of the mean plastic field. This bifurcation is linked to the emergence of an angle linked to the internal friction of the material which is clearly distinct from the orientation of the anisotropic elastic coupling. I will discuss the interpretation of our observations in the framework of current theoretical models of the plasticity of granular materials.