What triggers failure in frictional granular assemblies?

We examine numerically the yielding or failure of small granular packings subjected to an increasing deviatoric stress. As the load increases, the packing softens and the number of sliding contacts rises. When the packing fails, the kinetic energy starts to rise exponentially in time. It is always possible to identify a contact status change that triggers the collapse of the packing. Furthermore, by use of the stiffness matrix, we show that this change often causes a mechanical instability or a motion with neutral stability. In some cases, the status change provokes an oscillation and a second status change following shortly thereafter introduces an instability. Failure can also be considered from the perspective of energy flux: before failure, the energy injected by the load is stored as potential energy in the contacts. When this is no longer possible, failure occurs and the injected energy is converted to kinetic energy. However, the force disequilibrium then soon becomes the dominant energy source.