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 micro tomography 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.

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Numerous studies have shown that the fabric of granular materials plays a fundamental role in its macroscopic behaviour. Due to technical limitations, this fabric remained inaccessible in real experiments until recently when x-ray tomography became accessible. However, determining the fabric from tomographic images is relatively challenging, due to various inherent imaging properties. Triaxial experiments on natural sands are chosen to investigate the contact fabric evolution. Two different observation windows in the specimen are chosen for the contact fabric analysis: one inside and another one outside a shear band. Individual contact orientations are measured using advanced image analysis approaches within these windows. The fabric is then statistically captured using a second order tensor and the evolution of its anisotropy is related to the macroscopic behaviour.

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Abstract. X-ray μ computed tomography (CT) made three-dimensional in-situ imaging of natural granular materials possible. Previous work using x-ray μCT and triaxial compression tests has studied the 3D kinematics of individual grains during shear banding [1]. This works aims to supplement these measurements of kinematics with the measurement of different fabric entities, such as particle or contact orientations. It was found that the individual orientations of the different fabric entities pick up on the forming and the direction of the evolving shear band. The evolution of the anisotropy of the bulk of orientations corresponds to the macroscopic behaviour during the shearing test.

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The soil response in triaxial compression tests, that are commonly treated as element tests, is known to be inhomogeneous. Several studies have revealed the localisation of deformation throughout the whole specimen by digital image correlation techniques on X-ray tomographies. The fabric of a soil specimen has so far only been studied on complete specimens as a bulk measurement or in chosen subsets. In this contribution, we present a study on the spatial and temporal distribution of the fabric throughout one Hostun sand sample in triaxial compression. Therefore, we calibrated the minimum representative element size first for three chosen fabric variables considering three different criteria. By distributing the elements in a regular grid over the specimen, we are able to clearly identify the onset of the localisation in terms of void ratio, coordination number and contact fabric anisotropy. Spatially and temporally the contact fabric variables precede the void ratio changes as they are much more sensitive to small changes. Graphical Abstract