Digital volume correlation applied to compaction of granular materials

Compaction of powders and granular materials is an important process used to manufacture products including pharmaceuticals, ceramics, metals and explosives. In order to ensure mechanical integrity of the final product, an understanding of the compaction process, with particular reference to the homogeneity of the compacted bed, is required. In particular, it is necessary to have an improved understanding of the physics of the process, which can only be achieved through appropriate experimental measurements. X-ray tomography offers the opportunity to make full-field measurements of displacements in-situ: without removing the specimens from the die. This allows researchers to better understand the processes that take place at different stages in the compaction, and to more rigorously test numerical models. In this paper, we present data obtained using Digital Volume Correlation (DVC) to measure displacements in a compacted bed of sugar, with data obtained using X-ray microtomograpy. Instead of adding tracer particles, the natural, random, microstructure of the bed is used as the pattern for the volume correlation. The displacement data obtained are then used to calculate strain fields in the bed. A novel Finite Element (FE) based smoothing technique is applied to robustly smooth the data, allowing accurate and continuous strains to be calculated. These strains are compared to those calculated from the unsmoothed data. The paper presents details of both the DVC and FE smoothing routines. (C) 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of H.D. Espinosa and F. Hild.