Force chain evolution is key to shear band formation and failure of granular materials.  Force chains are quasi-linear, particle chains through which above average contact forces are transmitted within a deforming granular medium [1,2]. These chains may be only a few grains in length, or they may extend for hundreds of grain diameters.  Physical experiments have shown that force chains form to resist deformation, and that these “columnar structures” of particles preferentially align in the direction of maximum compressive stress in the system (the direction where the materials “feels” the greatest compression).  Force chains exist amidst a network of weak particles (i.e. particles bearing below average contact forces).  Under continued axial compression, the force chain columns ultimately buckle under lateral confinement from their weak neighbours.  Force chain buckling is a highly nonaffine mode of deformation.  Mounting experimental evidence suggests that this confined buckling event is responsible for the formation of shear bands – the key mechanism for failure in granular materials [3].  While earthquakes are recognizably highly complex processes, slip events in idealised earthquake cycles on tectonic plates (i.e. a cycle of successive earthquakes on a fault known also as “stick-slip”) have also been recently linked to force chain buckling [4,5].

In the simulation shown here, several experimentally observed nonaffine modes that emerge within a deforming granular material have been captured by our measure of nonaffine deformation (e.g. [6]) Particles coloured RED have undergone high nonaffine deformation relative to their first ring neighbours; BLUE particles sustained very low or zero nonaffine deformation. These nonaffine modes include rattlers, microbands and the shear band [7].  A rattler is a particle which rattles inside its cage of first ring neighbours.  Microbands are criss-crossing planes where particle sliding takes place.  The shear band is a band within which force chains fail by buckling.  Once fully developed, the shear band splits the material into two regions that can move (or slide) past each other in rigid body motion; at this point the material can no longer support load and is said to have failed.  David Kirszenblat’s poster on this topic can be found in http://www.mgm.ms.unimelb.edu.au/ugrad-index.php.