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Correlative optical and X‐ray imaging of strain evolution during Double Torsion Fracture Toughness measurements in shale

DOI: 10.1029/2018JB016568 DOI Help

Authors: Michael R. Chandler (University of Manchester) , Anne-laure Fauchille (The University of Manchester; Research Complex at Harwell; Ecole Centrale de Nantes) , Ho Kyeom Kim (University of Bristol; The Open University) , Lin Ma (The University of Manchester) , Julian Mecklenburgh (The University of Manchester) , Roberto Rizzo (The University of Manchester; Research Complex at Harwell) , Mahmoud Mostafavi (University of Bristol) , Sebastian Marussi (Research Complex at Harwell) , Robert Atwood (Diamond Light Source) , Steven May (The University of Manchester) , Mohammed Azeem (Research Complex at Harwell; University of Leicester; University College London) , Ernie Rutter (The University of Manchester) , Kevin Taylor (The University of Manchester) , Peter Lee (Research Complex at Harwell; University College London)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of Geophysical Research: Solid Earth

State: Published (Approved)
Published: December 2018
Diamond Proposal Number(s): 13824

Abstract: Mode‐I Fracture Toughness, KIc, 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 (DIC) 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 DIC 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 470μm.s−1 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 travelling 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.

Journal Keywords: Shale; Fracture; Fracture Mechanics; Radiography; Digital Image Correlation

Subject Areas: Earth Science

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing

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