(70db) Micro-Level Validation of Discrete Element Simulations with Experiments: a Focus on Force Propagation

While typical computer simulations of particulate materials are validated and calibrated against experiment, this validation is usually performed at the macro-level, using material properties that are readily available from experiments (for example, stresses measured from boundaries, void ratio or density distributions). Up until recently, the micro-level individual contact forces between particles could not be easily obtained from experiment, and hence, validation at this scale has not been performed. However, recent breakthroughs have been made whereby each individual vectorial contact force (both normal and tangential components) has been measured for a large system of 2500 photoelastic disks [1]. This invaluable microstructural information thus provides a unique opportunity for validation at the most fundamental level with discrete element simulations, which are based on calculating contact forces between particles and using these to determine particle dynamics.

Results are presented from a discrete element model that has been validated against photoelastic disk experiments at the level of interparticle contact forces, in two dimensions. Simulation results are compared against experimental measurements and excellent agreement is found. In particular: (i) probability distributions of the normal and tangential components of contact forces, (ii) distributions of contact orientation, (iii) angular variation of mean normal force, and (iv) the spatial structure of the contact force network obtained for fundamental benchmark tests (pure shear, isotropic compression) are in excellent agreement [2].

Further, it is vital to understand not only the distributions and average behaviours of these interparticle contact forces, but also the spatial distribution of the contact force network. Since forces are only transmitted via these interparticle contacts, the resulting stress field is highly heterogeneous. Specifically, a ramified complex force network develops that undergoes rapid changes in branch morphology during deformation. Therefore, we take the analysis of interparticle forces one step further by focusing on the nature of the contact force network, and more precisely, on the so-called ?force chains? that are the main pathways of force transmission.

Specifically, a quantitative definition and characterisation of a ?force chain? as a meso-scale property of a particulate material will be presented [3]. Using this, analysis of the evolution and propagation of force chains - from inception to collapse - will be performed. Various properties of individual chains (e.g. length, ?strength') will also be analysed as a first step towards establishing the link from the micro (particle) level to meso (force chain) level and finally, to the macroscopic response of the material.

This will serve as the foundation for the development of models that will incorporate more complicating aspects of particulate materials (e.g. shape distribution, cohesion).

[1] T. S. Majmudar and R. P. Behringer. Contact force measurements and stress-induced anisotropy in granular materials. Nature, 435:1079-1082, 2005.

[2] H. Hafez. 600-311 Research Report: A discrete element simulation of a 2D assembly of bi-disperse circular disks subjected to pure shear. The University of Melbourne, 24 June 2005.

[3] J. F. Peters, M. Muthuswamy, J. Wibowo and A. Tordesillas. Characterization of force chains in granular material. Phys. Rev. E. (in press).