/pdf/high-resolution-x-ray-computed-tomography-in-geosciences-a-3sj7249knj.pdf

High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications

X-ray computer tomography (CT) is fast becoming an accepted tool within the materials science community for the acquisition of 3D images. Here the authors review the current state of the art as CT transforms from a qualitative diagnostic tool to a quantitative one. Our review considers first the image acquisition process, including the use of iterative reconstruction strategies suited to specific segmentation tasks and emerging methods that provide more insight (e.g. fast and high resolution imaging, crystallite (grain) imaging) than conventional attenuation based tomography. Methods and shortcomings of CT are examined for the quantification of 3D volumetric data to extract key topological parameters such as phase fractions, phase contiguity, and damage levels as well as density variations. As a non-destructive technique, CT is an ideal means of following structural development over time via time lapse sequences of 3D images (sometimes called 3D movies or 4D imaging). This includes information nee...

https://www.tandfonline.com/doi/pdf/10.1179/1743280413Y.0000000023

Quantitative X-ray tomography

Recent developments in the application of x-ray micro-tomography in laboratory geomechanics have allowed all the individual grains of sand in a test sample to be seen and identified uniquely in 3D. Combining such imaging capabilities with experiments carried out “in situ” within an imaging set-up has led to the possibility of directly observing the mechanisms of deformation as they happen. The challenge has thus become extracting pertinent, quantified information from these rich time-lapse 3D images to elucidate the mechanics at play. This paper presents a new approach (ID-Track) for the quantification of individual grain kinematics (displacements and rotations) of large quantities of sand grains (tens of thousands) in a test sample undergoing loading. With ID-Track, grains are tracked between images based on some geometrical feature(s) that allow their unique identification and matching between images. This differs from Digital Image Correlation (DIC), which makes measurements by recognising patterns between images. Since ID-Track does not use the image of a grain for tracking, it is significantly faster than DIC. The technique is detailed in the paper, and is shown to be fast and simple, giving good measurements of displacements, but suffering in the measurement of rotations when compared with Discrete DIC. Subsequently, results are presented from successful applications of ID-track to triaxial tests on two quite different sands: the angular Hostun sand and the rounded Caicos Ooids. This reveals details on the performance of the technique for different grain shapes and insight into the differences in the grain-scale mechanisms occurring in these two sands as they exhibit strain localisation under triaxial loading.

/pdf/grain-scale-experimental-investigation-of-localised-j1jdnn1kze.pdf

Grain-scale experimental investigation of localised deformation in sand: a discrete particle tracking approach

As a carrier of deformation information, the speckle pattern, or more exactly the random intensity distributions, which could be naturally occurred or artificially fabricated onto test samples’ surface, plays an indispensable role in digital image correlation (DIC). It is now well recognized that the accuracy and precision in DIC measurements not only rely on correlation algorithms, but also depend highly on the quality of the speckle pattern. Considering the huge diversity in test materials, spatial scales and experimental conditions, speckle pattern fabrication could be a challenging issue facing DIC practitioners. To obtain good speckle patterns suitable for DIC measurements, some key issues of fabrication methods and quality assessment of speckle patterns must be well addressed. To this end, this review systematically presents the speckle pattern classification and fabrication techniques for various samples and scales, as well as some typical quality assessment metrics.

A Review of Speckle Pattern Fabrication and Assessment for Digital Image Correlation

The concept of the critical state in granular soils needs to make proper reference to the fabric structure that develops at critical state. This study identifies a unique property associated with the fabric structure relative to the stresses at critical state. A unique relationship between the mean effective stress and a fabric anisotropy parameter, K, defined by the first joint invariant of the deviatoric stress tensor and the deviatoric fabric tensor, is found at critical state, and is path-independent. Numerical simulations using the discrete-element method under different loading conditions and intermediate principal stress ratios identify a unique power law for this relationship. Based on the findings, a new definition of critical state for granular media is proposed. In addition to the conditions of constant stress and unique void ratio required by the conventional critical state concept, the new definition imposes the additional constraint that K reaches a unique value at critical state. A unique s...

/pdf/unique-critical-state-characteristics-in-granular-media-afx69f4wkh.pdf

Unique critical state characteristics in granular media considering fabric anisotropy

The objective of this work was to observe and quantify the onset and evolution of localised deformation processes in sand with grain-scale resolution. The key element of the proposed approach is combining state-of-the-art X-ray micro tomography imaging with three-dimensional volumetric digital image correlation techniques. This allows not only the grain-scale details of a deforming sand specimen to be viewed, but also, and more importantly, the evolving three-dimensional displacement and strain fields throughout loading to be assessed. X-ray imaging and digital image correlation have been in the past applied individually to study sand deformation, but the combination of these two methods to study the kinematics of shear band formation at the grain scale is the first novel aspect of this work. Moreover, the authors have developed a completely original grain-scale volumetric digital image correlation method that permits the characterisation of the full kinematics (i.e. three-dimensional displacements and rotations) of all the individual sand grains in a specimen. The results obtained using the discrete volumetric digital image correlation confirm the importance of grain rotations associated with strain localisation.

Discrete and continuum analysis of localised deformation in sand using X-ray mu CT and volumetric digital image correlation

The recent development of e cient 3D imaging tools such as X-Rays com- puted microtomography combined with the extension to volumetric images of Digital Image Correlation (DIC) techniques provide new insights on the analysis of materials and structures. Among many other possible fields of application, geomaterials are good candidates for such investigations, owing to their relative transparency to X-rays and the presence in many samples of a natural contrast suitable for deformation mapping. How- ever, these materials often deform discontinuously at microscale, for instance in the form of the development of a networks of microcracks. Discontinuity is even the dominant rule in granular-type materials such as sand in which the contribution to overall deformation of the microcontinuous phenomena -elastic strains inside grains- are negligible. To investi- gate deformation at the scale of these discontinuous mechanisms, specific DIC algorithms are required, which override the assumption of continuity of the transformation at the scale of the correlation windows. The recent so-called Discrete-DIC procedure (Hall et al, 2010) is a possible answer. We recall here its general principles and focus on its potential accuracy, from both theoretical and practical points of view. We show that the position and the rotation of individual grains with an average diameter of 500 m can be determined from images recorded with a laboratory microCT scanner, with a 15 m voxel size, with an accuracy of the order of 1 m and 0,1 degree, respectively.

/pdf/discrete-volumetric-digital-image-correlation-for-the-3rqimw41z2.pdf

Discrete volumetric digital image correlation for the investigation of granular type media at microscale: accuracy assessment

We present a comparison of two approaches for 3D volumetric digital image correlation that were developed independently and in different scientific contexts. One approach is that of CorrelManu3D, which was developed in the framework of solid mechanics for x-ray tomography images. The other approach, called TomoWarp, is derived from a geophysics approach for the analysis of images of subsurface reservoirs. Both the approaches have been applied to quantify deformation and damage in diverse geomaterials submitted to mechanical loading based on x-ray tomography images. We compare the approaches adopted by the two codes in their application to the analysis of images of a clay-rock submitted to triaxial loading with confinement. We find that, in this example, the two approaches give different quantifications of the localised deformation, probably due to the difference in the sub-voxel refinement used. Mots-clefs : correlation d’images numerique ; geomateriaux ; tomographie a rayons X

http://documents.irevues.inist.fr/bitstream/handle/2042/15803/CFM2007-1186.pdf?sequence=1

Deux approches de la corrélation 3D d'images volumiques comparées sur des données de tomographie à rayons X

Les techniques de correlation d'image numerique ont recemment ete etendues afin de pouvoir analyser des sequences d'images tridimensionnelles, obtenues par exemple par microtomographie X. La resolution et la resolution spatiale des champs de deformation mesures dependent directement de la repartition, de la taille et du contraste local des marqueurs utilises pour realiser l'appariement des images. Lorsqu'il est present, le contraste naturel issu des heterogeneites intrinseques au materiau peut directement fournir ces marqueurs [1]. Lorsque celui-ci fait defaut, des marqueurs supplementaires peuvent etre ajoutes, mais leur repartition spatiale est difficile a maitriser [2]. Dans les deux cas, le contraste local peut etre fortement non-uniforme, contrairement a ce qui peut etre obtenu avec un mouchetis de peinture en correlation d'image de surface. Habituellement le calcul du critere de ressemblance est realise sur des sous-domaines espaces regulierement. Cette discretisation spatiale homogene facilite le post-traitement des donnees mais ne permet pas toujours une exploitation optimale du signal. En effet, les domaines de correlation ne sont a priori pas centres sur les marqueurs presentant le meilleur contraste, ce qui peut conduire a des erreurs de mesure. Il est propose ici de determiner la position des points de mesure (centre des domaines de correlation) en fonction de plusieurs criteres quantifiant la qualite du signal present dans l'image. Les premieres applications seront realisees sur le materiau presente dans [2] : des essais de traction sur une eprouvette entaillee en aluminium marque par des inclusions de cuivre, puis sur le meme materiau renforce avec de larges particules spheriques de Zircone-Silice. Enfin, la deformation d'un alliage d'Aluminium-Cuivre en cours de solidification [3] sera analysee malgre un contraste local peu abondant dans la structure. [1] Bay ,Smith ,Fyhrie ,Saad, Digital volume correlation : three-dimensional strain mapping using X-ray tomography, Experimental Mechanics, 1999. [2] Bornert ,Chaix ,Doumalin ,Dupre ,Fournel, Jeulin, Maire, Moreaud, Moulinec, Mesure tridimensionnelle de champs cinematiques par imagerie volumique pour l'analyse des materiaux et des structures. Instrumentation, Mesure, Metrologie, 2004. [3] Limodin, Salvo, Suery, Dimichiel, In-situ investigation by X-Ray tomography of the overall and local microstructural changes occurring during partial remelting of an Al-15.8wt%Cu alloy, Acta Materialia, 2007.

https://hal.archives-ouvertes.fr/hal-00173747/document

Yannick Pannier

Papers

Discrete and continuum analysis of localised deformation in sand using X-ray mu CT and volumetric digital image correlation

Discrete volumetric digital image correlation for the investigation of granular type media at microscale: accuracy assessment

Deux approches de la corrélation 3D d'images volumiques comparées sur des données de tomographie à rayons X

Mesure 3d de champs cinématiques dans le cas d'un contraste non uniformément réparti.

Automatic segmentation and fibre orientation estimation from low resolution X-ray computed tomography images of 3D woven composites