Avalanche Instability and Friction Mobilization : A Two-Phase Stability Limit

摘要

The evolution of 2D granular piles driven towards the stability limit by gravitational loading has been numerically investigated using the non smooth contact dynamic method. During the evolution, despite an intrinsic gradient loading effect, the texture can be classically analyzed in terms of a strong and a weak contact force networks with characteristic stress contributions. The system evolution is also characterized by intermittent dynamical events, driven by local contact slip instabilities, which lead to reorganize the granular packing and the texture. This can be described in term of an order parameter related to the density of critical contacts denned as the relative number of contacts where the friction is fully mobilized. A statistical analysis show spatio-temporal correlations of the critical contacts which are organized spatially as clusters. The clusters are localized domains of low pressure which can not sustain shear stress increments and appear as fluid (or plastic) bubbles. The mean size of the clusters exhibits a power-law divergence as the system approaches the stability limit which is reminiscent of a percolation process. These results, together with the analysis of the granular texture, suggest that granular material can be analyzed as a bi-phasic material and the destabilization process as second order transition.