Evolution of Contact Forces, Fabric, and their Collective Behavior in Granular Media under Deformation: A DEM Study

A 2-dimensional model of a granular assembly, subjected to quasistatic loading conditions (i.e., biaxial compression and pure shear), is examined using the Discrete Element Method (DEM). A series of simulations is performed for dense assemblies of circular particles with varying polydispersity and mode of deformation. The evolution of the contact force distributions (magnitude and orientation) is analyzed and correlated with the development of strain-induced structural anisotropy. A statistical analysis of force chains and their evolution is also performed. The assemblies are found to undergo dilatation and strain-softening. This behavior is correlated with the underlying loss of contacts and increasing structural anisotropy. In the strain-hardening regime, an increase in the average length of the force chains is observed, although there is little change in the overall percentage of particles that constitute these chains. The onset of strain-localization signals a transition to fluid-like behavior and is correlated with the critical coordination number and the maximum degree of contact anisotropy. Evolution of the distribution of the normal contact forces also captures the increasing fragility of the system and the emergence of subsystems of strong and weak particle networks (i.e. particles with above average contact forces and below average contact forces, respectively). The mechanical properties of the granular assemblies are analyzed at the microscale (particle scale), mesoscale (longer range correlations spanning several particles like force chains) and macroscale (e.g. dilatation, strain-softening).