(38d) Meso-Scale Modelling and Measuring Bulk Cohesion in Surface-Wet Granular Assemblies

The cohesive properties of powder materials have a predominant effect on their flow behaviour. The bulk mechanical properties such as unconfined yield strength, tensile strength as well as bulk cohesion are very important for the reliable design of bulk storage and handling equipment. These parameters are all interlinked through the powder's failure criterion that can be represented , for example, by the yield locus, the conical failure surface in the principal stress space or the Roscoe failure surface.

The paper will discuss and evaluate against experimental measurements , the results of 3-D DEM simulations of that have been performed to investigate the internal tensile strength and shear strength of fine, cohesive granular materials. Inter-particle cohesion is taken into account by modelling liquid bridges in pendular state. The influence of particle surface roughness is considered by a minimum separation distance of the particles with respect to liquid bridges. Agglomerates of mono-sized spheres have been formed to measure the stress caused by the liquid bridges.

A good general agreement with Rumpf's equation is demonstrable and divergences may be explained by the existence of stretched bridges; see [1]. Spheres and more complex particle shapes have been used for shear and tensile test simulations.

By means of simulated tensile tests, it could be shown that even when the ultimate tensile strength is reached, elastic body contacts still exist within the granular assembly; supporting the external tensile load. For comparison, shear cell tests are performed using the Schulze annular shear cell and various internal yield loci are obtained for different materials. The experimental results are compared with DEM simulation results and good agreement is reported between experimental values of the bulk cohesion and tensile strength those obtained from the simulations; [2].

References

[1] Groger T., Tuzun U. and Heyes D.M. (2003) , Powder Technology, 133, p203. [2] Tuzun U., Baxter J. and Heyes D.M. (2004) Phil. Trans. R. Soc. London A , 362, p1931.