On the metrology of interparticle contacts in sand from x-ray tomography images

In the mechanics of granular materials, interparticle contacts play a major role. These have been historically difficult to study experimentally, but the advent of x-ray microtomography allows the identification of all the thousands of individual particles needed for representative mechanical testing. This paper studies the metrology of detecting interparticle contacts and measuring their orientation from such images. Using synthetic images of spheres and high-resolution tomographies of two very different granular materials (spherical and very angular) as ground truths, we find that these measurements are far from trivial. For example, if a physically correct threshold is used to separate particles from pores there is a systematic over-detection of contacts. We propose a method of improvement that is effective for non-angular particles. When contact orientations are measured from the pixels that make up the contact area, standard watershed approaches make significant systematic errors. We confirm and build upon previous results showing the improvement in orientation measurement using a refined notion of particle separation. Building on this solid basis, future work should focus on a link between contact topology and measurement error, as well as evaluating the use of local surface normals for orientation measurement.