Rock fractures developed in the Class II post-peak unloading

Abstract

In this study, post-peak tests were conducted on sandstone specimens subjected to uniaxial compression under two different loading conditions: axial strain control and lateral strain control. The fractures developed in both loading regimes were investigated. Under axial strain control, Class I post-peak stress–strain curves (with negative slope) were obtained until the complete failure of the specimens. The observed macroscopic fractures have mixed orientations: sub-parallel and inclined to the axial loading direction. Under lateral strain control, the specimens were tested until different residual post-peak stresses; the tests yielded consistent Class II post-peak stress–strain curves (with positive slope). Interestingly, the tested specimens were found to be relatively intact, with only non-penetrating fractures observed on the lateral surfaces. X-ray CT scanning revealed that the subvertical macroscopic fractures were developed close to the lateral surfaces of the specimens in the post-peak stages, while the internal central parts remained visually undamaged. Analysis of longitudinal and shear wave velocities measured in the axial direction of sandstone specimens before and after post-peak tests under lateral strain control provided evidence of internal fracture (crack) development parallel to the loading axis in the internal central parts of the tested specimens. Concentration of these small fractures (cracks) is high, but it only marginally increases as the loading proceeds in post-peak region from 90% to 70% of the peak stress. This suggests that the Class II post-peak regime obtained under lateral strain control cannot be explained by pure unloading; also, the Class II post-peak unloading is mainly produced by the development of the large subvertical tensile fractures identified in the CT scans, rather than by the growth of these small fractures (cracks).