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Correlative Optical and X-Ray Imaging of Strain Evolution During Double-Torsion Fracture Toughness Measurements in Shale

Chandler, MR; Fauchille, A-L; Kim, HK; Ma, L; Mecklenburgh, J; Rizzo, R; Mostafavi, M; ... Lee, P; + view all (2018) Correlative Optical and X-Ray Imaging of Strain Evolution During Double-Torsion Fracture Toughness Measurements in Shale. Journal of Geophysical Research: Solid Earth , 123 (12) pp. 10517-10533. 10.1029/2018JB016568. Green open access

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Abstract

Mode‐I Fracture Toughness, K_{IC}, was measured in six shale materials using the double‐torsion technique. During loading, crack propagation was imaged both using twin optical cameras, and with fast X‐ray radiograph acquisition. Samples of Bowland, Haynesville, Kimmeridge, Mancos, Middlecliff, and Whitby shales were tested in a range of orientations. The measured fracture toughness values were found to be in good agreement with existing literature values. The two imaging techniques improve our understanding of local conditions around the fracture‐tip, through in situ correlation of mechanical data, inelastic zone size, and fracture‐tip velocity. The optical Digital Image Correlation technique proved useful as a means of determining the validity of individual experiments, by identifying experiments during which strains had developed in the two “rigid” specimen halves. Strain maps determined through Digital Image Correlation of the optical images suggest that the scale of the inelastic zone is an order of magnitude smaller than the classically used approximation suggests. This smaller damage region suggests a narrower region of enhanced permeability around artificially generated fractures in shales. The resolvable crack‐tip was tracked using radiograph data and found to travel at a velocity around 470 μm/s during failure, with little variation in speed between materials and orientations. Fracture pathways in the bedding parallel orientations were observed to deviate from linearity, commonly following layer boundaries. This suggests that while a fracture traveling parallel to bedding may travel at a similar speed to a bedding perpendicular fracture, it may have a more tortuous pathway, and therefore access a larger surface area.

Type: Article
Title: Correlative Optical and X-Ray Imaging of Strain Evolution During Double-Torsion Fracture Toughness Measurements in Shale
Open access status: An open access version is available from UCL Discovery
DOI: 10.1029/2018JB016568
Publisher version: https://doi.org/10.1029/2018JB016568
Language: English
Additional information: © 2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/).
Keywords: shale, fracture, fracture mechanics, radiography, digital image correlation
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10077548
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