DEFORMATION MODES FOR ASSEMBLIES OF FRICTIONLESS POLYDISPERSE SPHERES N. Kumar, O. I. Imole, V. Magnanimo and S. Luding Multi-Scale Mechanics, TS, CTW, University of Twente P.O. Box 217, 7500 AE Enschede, Netherlands Email: n.kumar@utwente.nl, Tel.: +31 53 489 3061 Keywords: DEM; uniaxial deformation; pure shear deformation; deviatoric stress and strain Abstract: The challenge of dealing with cohesive powders during storage, handling and transport are widely known in the process and pharmaceutical industries. Simulations with the discrete element method (DEM) provide further insight into the local microstructure of bulk materials. In this work, the DEM approach is presented to investigate the flow behavior of granular systems subjected to different modes of deformations. When uniaxial compression is applied of frictionless, polydisperse spheres above jamming (transition from fluid-like state to solid-like state), the evolution of coordination number (average number of contacts per particle) and pressure as functions of the volume fraction are, astonishingly, identical to results obtained for purely isotropic compression. Analytical predictions for the evolution of pressure and coordination number under isotropic strain can thus be separated from different deformation modes, as applied in this study. After two different modes of volume-conserving deviatoric shear, the results still compare quite well with results for purely isotropic compression. The difference between the two deviatoric modes and uniaxial deformation is examined with respect to the anisotropic stress response as a function of deviatoric strain. 1. Introduction Dense granular materials are complex systems, which show unique material properties, different from classical fluids or solids. This involves phenomena like dilatancy, shear band formation, history dependence, jamming, and yield stress, among others. A full understanding of the flow and deformation behavior, especially for fine, cohesive powders, still remains a challenging problem. It has been shown [1] that isotropic and deviatoric deformation modes are pure modes, while the uniaxial deformation test derives from the superposition of an isotropic and a deviatoric test. In this work, various deformations paths for aggregates of polydisperse packings of non-frictional particles are modeled using the DEM simulation approach. The evolution of coordination number, deviatoric stress, and pressure (isotropic stress) are reported as functions of isotropic and deviatoric strain for different deformation modes and material parameters in Ref. [8]. 2. Simulation procedure The Discrete Element Method (DEM) [3] can be used to perform simulations in a triaxial box. One great advantage of the triaxial box is the possibility of realizing different deformation modes with a single test experiment (element test) applying direct control of stress or strain [1]. For the sake of simplicity, the contact model used in this work is the linear visco-elastic normal force   =  +   (1) where  is the spring stiffness, the contact viscosity parameter,  the overlap between particles and   the relative velocity in the normal direction. In order to reduce dynamical effects and shorten relaxation times, an artificial background viscous dissipation force   = −    proportional to the moving particle i with velocity   is added, resembling the damping due to a background medium.