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Ando E.,Grenoble Institute of Technology | Viggiani G.,Grenoble Institute of Technology | Hall S.A.,Lund University | Hall S.A.,European Spallation Source AB | Desrues J.,Grenoble Institute of Technology
Geotechnique Letters | Year: 2013

Combining x-ray tomography and three-dimensional (3D) image analysis has finally opened the way for experimental micro-(geo)mechanics, allowing access to different scales of interest. When these correspond to a scale that has been imaged at high spatial resolution, high-quality measurements can be obtained (e.g. 3D displacements and rotations of individual grains of sand sample under load). However, there are issues when the scale of interest is smaller, for example the characterisation of grain-to-grain contacts (their orientations and evolution) or production of fines by grain breakage. This paper presents a short selection of new grain-scale measurements obtained using existing techniques. The challenges associated with smaller scale measurements on the same images are also discussed through a few examples from ongoing work.

Vitone C.,University of Bari | Cotecchia F.,University of Bari | Viggiani G.,Grenoble Institute of Technology | Hall S.A.,Grenoble Institute of Technology | And 2 more authors.
International Journal for Numerical and Analytical Methods in Geomechanics | Year: 2013

This article presents the developments of an ongoing research aimed at modelling the influence of fissuring on the behaviour of clays. In particular, it recalls the main results of an extensive laboratory investigation on a fissured bentonite clay from the south of Italy and presents the data of a new investigation on the evolution with shearing of the strain fields developing within the clay, resulting from Digital Image Correlation (DIC). Element test results are analysed in the framework of continuum mechanics and linked to the clay fissuring features, once characterised using the Fissuring IDentity (F-ID) chart. This article compares the bentonite behaviour with that of other fissured clays of different F-IDs, highlighting the common behavioural features. Thereafter, the soil response at the macro level is related to the DIC-derived strain fields evolving within the clay with loading. For this purpose, DIC was successfully used to investigate the deformation processes active in the fissured clay and the sources of the localisation phenomena. DIC is shown to provide indications of the extent to which highly to medium fissured clays element test results can be of use to model the clay behaviour according to continuum mechanics. © 2012 John Wiley & Sons, Ltd.

Ainsworth R.,Royal Holloway, University of London | Molloy S.,European Spallation Source AB
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2014

The European Spallation Source (ESS) linac will consist of three families of superconducting RF cavities to accelerate protons to a final beam energy of 2.5 GeV for collision with the target. Beam induced HOMs in these cavities may drive the beam unstable and increase the cryogenic load, severely limiting the operation of the linac. The effect of these modes on the beam quality is investigated in detail using a numerical code dedicated to beam-HOM interaction. © 2012 Elsevier B.V.

Hall S.A.,Lund University | Hall S.A.,European Spallation Source AB | Desrues J.,Laboratoire | Viggiani G.,Laboratoire | And 2 more authors.
Procedia IUTAM | Year: 2012

This paper outlines some recent advances in the full-field experimental characterisation of the mechanics of granular geomaterials (in particular, sands) using a range of methods that provide characterisation at different scales, from the sample-scale down to the inter-and intra-grain scale. The techniques used are "full-field" approaches involving in-situ x-ray micro-tomography, 3D-volumetric digital image analysis/correlation and grain ID-tracking, in-situ 3D x-ray diffraction and in-situ, spatially-resolved neutron diffraction. These methods provide new data on the mechanics of sand at different scales, including continuum measures of strain, porosity, and fabric plus discrete measures of particle kinematics and force transmission. The results of such measurements might be used to advance higher-order continuum theories, and provide the necessary input parameters, or to calibrate discrete grain-scale simulations of sand behaviour to explore loading paths that are inaccessible in the laboratory. © 2012 Published by Elsevier B.V.

Ando E.,Grenoble Institute of Technology | Hall S.A.,Lund University | Hall S.A.,European Spallation Source AB | Viggiani G.,Grenoble Institute of Technology | And 2 more authors.
Geotechnique Letters | Year: 2012

Strain localisation plays a key role in the deformation of granular materials. Such localisation involves bands of just a few grains wide, which dominate the material's macroscopic response. This grain-scale phenomenon presents challenges for continuum modelling, which is the rationale behind models that explicitly take micro-scales into account. These in turn require micro-scale experimental analysis. In this work, X-ray tomography is used to image a small sample of oolitic sand while it deforms under triaxial compression. Grains are followed with a technique combining recent developments in image correlation and particle tracking. From these rich data, the evolution of the material in a subvolume of a thousand grains inside the sample (which contains 53 000 grains) is presented. The subvolume is chosen to lie inside the shear band that appears at the sample scale. Three-dimensional (3D) grain kinematics are analysed in three increments: the beginning of the test, the peak of the sample's macroscopic axial stress response and the residual stress state. When the sample's deformation is homogeneous (increment one) or fully localised (increment three), the kinematics of the grains in the subvolume appear to be representative of the kinematics occurring at the sample scale, allowing micro-mechanical observations to be made. In the transition from homogeneous to localised deformation (increment two), however, the scale of observation requires a zoom out of the subvolume to the sample scale in order to capture the complex mechanisms at play.

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