During the granulation process, granules formed are a complex matrix of powder partially saturated with liquid binder whose saturation and porosity are continuously changing as a result of granule compaction. The granules are in motion, and their dynamic mechanical properties during collisions with other granules and the granulation equipment are important in determining the rate and extent of granule consolidation, growth and breakage.

Iveson and coworkers1-3 characterised the granule dynamic yield strength by the peak flow stress during unconfined uniaxial compression at strain rates comparable to those experienced by granules in a granulator. This information has been used as the basis for developing coalescence criteria and a granule growth regime map. In this analysis, typically the granule is assumed to a perfect elastic-plastic material with a strain rate dependent yield stress. In fact, real granule behaviour is much more complex.

This paper extends the Iveson's work in two areas. Firstly, the effect of granule saturation and porosity on peak flow stress is studied. Varying saturation between 50% and 90% can change the peak flow stress by as much as an order of magnitude at high strain rates. Secondly, the change from brittle to plastic behaviour is studied using diametrical experiments. Experiments cover a wide range of powder and binder properties to give granules with widely varying mechanical properties. The implication of the results for predicting granule behaviour, especially wet granule breakage is discussed.