Catching failure in the act: mapping fracture initiation and spreadingusing X-ray tomography

Description

We are using high-resolution X-ray computed tomography to understand the circumstances behind sudden rockfailure, caused by the onset of a brittle cascade. Using a uniaxial press, we have deformed a series of 5 cm diametercores of magnesite and gabbro just up to, and beyond, the point of failure, and imaged them using high-resolutionX-ray computed tomography (CT). Fractures occur across a range of scales, and thus some will be above andsome below the resolution of a CT system. To maximize the range we observe, we use subvolume scanning toimage the centers of the cores at higher resolution, along with whole-sample scans when 3D context is useful.We measure fracture apertures at sub-voxel resolution using partial-volume calculations, which we recently addedto the Blob3D software created at the University of Texas. The algorithm uses a grid of parallel traverses near-normal to the fracture to reduce uncertainty in the calculation and determine 3D orientation, allowing us to bothmap orientation distribution and correct for apparent-thickness effects. Using these capabilities, we find evidencefor the onset of brittle behavior in the pre-failure sample as a series of sparse micro-cracks, on the order of 2mm long and 5-20μm wide at their centers, without an obvious preferred orientation in the uniform magnesitesample. The failed sample with the same lithology contains larger master fractures spanning the core that reflectthe final failure, but also a web of fractures encompassing the range of initial orientations. The mode of occurrenceof these secondary fractures thus both predates and postdates catastrophic failure, and its topological configurationis influenced by the rock fabric, or lack thereof.

Period	9 Apr 2019
Event title	EGU General Assembly 2019: EGU 2019
Event type	Conference
Location	Vienna, AustriaShow on map