Coupled micro-computed tomography and micromechanical experiment on the failure of discontinuous porphyry rock

Abstract

Rock is a discontinuous, heterogeneous material with complex mechanical behaviour. The variable material properties create uncertainty in response to load, associated rock mass damage and communition. Traditional approaches in rock mechanics rely on laboratory tests on intact specimens and mapping of structures to characterise the material properties and make predictions of damage scales. This paper expands on conventional triaxial laboratory tests using in situ x-ray micro-computed tomography and digital volume correlation to continuously track strain field changes during the experiment. Within a defect-rich porphyry, high-resolution imaging at 9 μm shows the micromechanical ductile-brittle fracture processes where microscopic defects lead to failure in the rock. Our methodology allows simultaneous observation of stress, strain, and elastic properties, spatially linking stress-induced strain localisation to discontinuities and pores. We present an integrated analysis, combining strain field data with tomogram attenuation values, revealing micromechanical feature evolution from initial strain to failure and post-peak behaviour. A remarkable product of the analysis was the multiple datasets that complemented the illustration of the micromechanisms. We show micro and macro fracture closure mechanisms visible in tomograms, which can be mapped as negative volumetric strain and generate increased specimen stiffness. Another important observation was the progression of the shear zone from strain localisation, visualised as both positive and negative volumetric strain regions in a 3D point cloud with the meshed shear. Our study provides valuable insights into the mechanics of fractured rock through the stages to failure. Understanding these underlying mechanisms and the strain field evolution at the specimen scale can improve our understanding of rock mechanics.