Single and bulk compressions of soft granules: Experimental study and DEM evaluation

The characterisation of mechanical properties of particulate solids (e.g. yield pressure and failure strength) is often done by an analysis of bulk compression. However, the relationship between bulk compression behaviour and the physical and mechanical parameters of single particles is not well understood. In this paper the deformation and failure behaviour of soft synthetic detergent granules is investigated experimentally using single granule and bulk compression analyses. Furthermore, the bulk compression of soft spheres, with the properties, which are the same as those of single granules used here, is simulated by distinct element method (DEM) and the predictions are compared with the experimental results. Both the experimental and simulation analyses are for a range of low pressures, less than 30 kPa, with the aim of characterising the yield pressure (limiting contact pressure) of the individual granules using the bulk compression models of Heckel, Kawakita and Lüdde and Adams and co-workers. It is shown that the ratio of the yield pressure of single granules to the Heckel parameter for this type of granules is less than one, and is about 0.5. Furthermore, a relationship is developed among the above three models of bulk compression, in which the above ratio is related to parameters such as the inter-particle and particle wall friction coefficients, initial porosity of the bed and lateral to axial pressure ratio of the particles in the bed. Therefore the use of bulk compression method to infer single particle properties should be made with great caution.