Application of micro-CT and digital volume correlation for investigating the macro-/mesoscopic failure mechanism of shale under uniaxial loading: insights into fracture behavior relevant to shale gas recovery

To study the correlation between the mesoscopic damage evolution and macroscopic failure characteristics of anisotropic shale, an in situ high-resolution micro-computed tomography (micro-CT) was used to conduct a uniaxial loading experiment with real-time scanning on Carboniferous shale from the eastern Qaidam Basin. The subvoxel displacement field of each specimen was calculated based on the correlation coefficient interpolation of the image subset with the digital volume correlation method, and the high-precision strain field was obtained to evaluate the deformation localization characteristics of shale specimens with low and high bedding inclination angles during loading. The research results show that the stable cracks expansion is caused by the synergistic effect of tension and shear. However, the unstable cracks expansion in low bedding inclination angle shale is controlled by tension and shear, whereas in high bedding inclination angle shale, it is dominated by tension. The evolution of the axial strain field of the low bedding inclination angle shale confirms the compaction of the bedding defects, strengthening bedding planes and inhibiting the formation of cracks along bedding. Conversely, high bedding inclination angle shale experiences concentrated tensile and shear strains due to damage to original bedding defects, leading to rapid strain increase and localized strain band formation consistent with subsequent splitting failure. The strain localization can predict the development location of cracks before they become macroscopically visible in CT images.