Failure mechanism determination for industrial granules using a repeated compression test Willem J. Beekman a, * , Gabrie M.H. Meesters a,b,c , Todd Becker c , Alfred Gaertner c , Mark Gebert c , Brian Scarlett d a DSM Food Specialties, P.O. Box 1, 2600 MA Delft, The Netherlands b Delft University of Technology, P.O. Box 5054, 2600 GA Delft, The Netherlands c Genencor International, 925 Page Mill Road, Palo Alto, CA 94304, USA d Engineering Research Centre for Particle Science and Technology, University of Florida, USA Abstract This work describes the development of a particle compression test that allows direct and repeated application of the stress. The test is designed to quickly reduce the load on a granule during its incipient failure. By so doing, the breakage can be arrested and thus the process can be studied in detail. Experimental tests have been made on samples of industrial enzyme granules, which have a complex layered structure, and reproducible results have been obtained. The contribution of the various layers to the strength of the granule has been investigated, showing that the use of coating materials results in improved granule strength. The microstructure of the granule determines the failure mode of the granule. It is concluded that the failure mechanisms can be defined from tests on only a few granules as can assessment of the relative contribution of the layers and of the granule core to its strength. A measurement of the distribution of strength requires a larger, statistically representative, sample. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Strength; Compression; Failure; Fatigue; Granules; Coating 1. Introduction In the process industries, many products are available either as powders or, alternatively, as granules which may range in size from 10 Am to several millimeters. Granular products can be handled more safely and easily than pow- ders, for example, products containing enzymes. One prob- lem associated with such products is that they are subject to attrition, which gradually reduces the granules back to the original dust. We have already described a repeated impact test [1], which subjects the particle to multiple events from which the rate of attrition by surface erosion can be deter- mined. The particle may also suffer attrition by fracture when it is divided into several smaller parts by crack propagation. The fracture strength of a particle can also be measured by an impact test, but an alternative is to measure it by direct compression. In fact, the major difference between impact and compression tests is that the rate of strain is inevitably high during an impact, but can be controlled during com- pression. Impact tests and compression tests are complementary. The impact test assesses better the attrition and the com- pression test better the fracture. The impact test is more convenient and the compression is more accurate. The impact test chooses random points over the whole surface of the particle and the compression test follows the history of two particular points. The impact test measures the fracture behaviour as a function of the impact velocity and the compression test measures the applied stress directly. It is this last feature, which particularly prompted the development of the compression test that is described in this paper. The need was to compare the strength character- istics of complex particles, enzyme granules that may have several layers surrounding a central core. For this purpose, a detailed determination of the stress – strain characteristic of the granule during its failure is needed if the relative contribution of the core and the layers is to be differ- entiated. 0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0032-5910(02)00238-3 * Corresponding author. DSM-Gist, P.O. Box 1, 2600 MA Delft, The Netherlands. www.elsevier.com/locate/powtec Powder Technology 130 (2003) 367– 376