Divergence Instability and Diffuse Failure in Granular Media

The question of divergence instability and diffuse failure, which is a common failure mode for granular materials like soils, is tackled here from an experimental, theoretical and numerical perspective. The necessary condition of instability as given by the loss of definite-positiveness of the elasto-plastic matrix (the so-called 'second order work criterion') is considered to analyze some experimental tests leading to diffuse failure, the link between kinetic energy and second order work is established, a discrete element method is used to simulate these failure conditions and eventually a 3D bifurcation analysis is performed with incrementally piecewise and non-linear elasto-plastic relationships. Essentially in relation with the non-associate character of the plastic strains of granular materials, a whole bifurcation domain in the stress space emerges from these analyses, leading to novel views of failure for non-associative materials.