(112a) Flow behaviour of ultrafine cohesive powder: a 3D-“view” from inside

Discrete element method (DEM) simulations will be presented in 2 and in 3 dimensions with respect to modelling the standardised shear testers like Jenike shear cell or biaxial box. The main impact is made on modelling the flow behaviour of commercially widely used ultrafine cohesive powders (TiO₂, CaCO₃) and the influence of hysteretic contact models describing the microscopic particle-particle interaction behaviour with the load-dependent contact adhesion. The atomic force microscope (AFM)-based measurements of adhesion and friction forces between two particles are used to verify the contact behaviour on the micro-level. Therefore, the macroscopic dynamic behaviour of cohesive powder flow can be "microscopically" investigated and understood. Reference experiments with the Jenike shear cell coupled with volumetric strain measurements by triangulating laser displacement sensors will be shown and discussed as well. Comparison will be made between the simulations and experiments.

As an example of initial configuration one can see the model of classical translational shear cell, developed by Jenike (1964) (Fig. 1). In order to limit the CPU-time, only a small three-dimensional (3D) element of the real shear cell is simulated. Fig. 1 shows the model (top view) for about 3000 spherical particles with mechanical properties of titanium dioxide and diameter of about 9±1 mm, placed in the given volume up to the preset porosity (here 0.4). The network of the shear forces arising between the particles in contact could be seen in the lower part of the figure instead of the balls. The upper wall (shear lid) is stress controlled. During shearing, the particle reorganization causes the incremental change of the reaction force F_N, and in order to keep the normal stress constant the height of the shear lid is changed correspondingly. The normal stress s=F_N/A (A is the area of the shear ring) and the horizontal shear rate of the upper ring are preset.

Fig. 1: The model system for the simulations (orange lines in the lower part of the system show the contact shear forces,

with line thickness proportional to force value)

Resulting forces acting on each particle, are determined by means of applying a contact model (force-displacement law) for a particle contact in the particle system. These values are used then for the force and momentum balances (motion laws) in order to update the whole system. As the direct ?macro?-response of this ?micro?-interactions, the values of the reaction force (shear force F_S) are obtained, which acts on the shear ring. Furthermore, the corresponding shear stresses t=F_S/A are calculated and used to describe the flow behaviour of the material.