Static friction and arch formation in granular materials

We report experiments in two and three dimensions that show that the flow mode of a granulate confined in a container strongly depends upon preparation. After a shock or a compressive stress, the granulate can flow freely or exhibit fragmentation resulting from what we call ''vaults hardening.'' We analyze this effect in the framework of a classical triangular bead pattern where the central particle is submitted to a vertical load. The model includes the indetermination of static friction forces and a spring that mimics the elastic tension in a chain of deformable particles. During reversal of gravity, we show that there is a locking mechanism that maximizes the tension of the spring. Then, the magnitude of friction forces is also maximum and may be large enough to prevent any motion of the central particle. This work can be looked upon as an approach to the more general problem of the stability of contact chains in a granulate.