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Cohesive powders form agglomerates that can be very porous. Hence they are also very fragile. Consider a process of complete fragmentation on a characteristic length scale , where the fragments are subsequently allowed to settle under gravity. If this fragmentation-reagglomeration cycle is repeated sufficiently often, the powder develops a fractal substructure with robust statistical properties. The structural evolution is discussed for two different models: The first one is an off-lattice model, in which a fragment does not stick to the surface of other fragments that have already settled, but rolls down until it finds a locally stable position. The second one is a simpler lattice model, in which a fragment sticks at first contact with the agglomerate of fragments that have already settled. Results for the fragment size distribution are shown as well. One can distinguish scale invariant dust and fragments of a characteristic size. Their role in the process of structure formation will be addressed.

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Abstract. We introduce a visco-elastic contact model for DEM simulations, which takes caking into account and investigate the impacts of this time dependent inter particle cohesion force on the bulk behavior under biaxial deformation. Starting from the assumption that the cohesion force between two particles develops on a characteristic timescale t c , we show, that two regimes can be identified. If t c is small compared to the shear rateγ, full cohesion is reached within the typical contact duration. The cohesion strength remains homogeneous throughout the sample. However, if t c γ crystallization bridges at fluctuating contacts never fully recover. Heterogeneous cohesion forces and granules are the consequence.