Fracture characteristics and damage mechanisms of heat-treated sandstone with combined hole-joint flaws under uniaxial compression

Abstract

In underground coal gasification (UCG) and enhanced geothermal systems (EGS), deep rock masses are simultaneously subjected to in-situ stress and high temperatures. Furthermore, joint defects are commonly distributed within the working tunnels of UCG and the geothermal drillings. Hence, it is essential to investigate the fracture characteristics of rock with hole-joint combined flaw under heat treatment and monotonic loading conditions. X-ray diffraction and thermogravimetric tests showed free and structural water separation causes thermal damage in sandstone. The evaporation of free water reduces the distance between crystals and strengthens sandstone. The separation of structural water generates a large number of transgranular cracks, which greatly reduces the sandstone strength. This research performed uniaxial compression experiments to investigate the mechanical characteristics, failure behaviours, energy dissipation, damage constitutive relations, and precursory features of instability in heat-treated sandstone (25 °C to 800 °C) with hole-joint defect under monotonic loading conditions. The acoustic emission (AE) signals and digital image correlation (DIC) data were simultaneously recorded during the tests. The research outcomes reveal that with climbing temperatures, the peak strength of sandstone first rises and then drops, turning at 200 °C. The stiffness of the specimens gradually weakens with rising temperatures, while their ductility gradually enhances. The failure behaviours of sandstone subjected are divided into two modes: wing crack-dominated and anti-wing crack-dominated. As the temperature rises, the failure mechanism of the specimens tends to transition from anti-wing crack-dominated to wing crack-dominated. However, the heat treatment merely enhances the ratio of shear cracks without altering the tensile-dominated failure mode. It is found that rocks subjected to higher temperatures exhibit greater dissipated energy, higher degrees of fragmentation, and lower levels of damage severity λ_max. The damage constitutive model derived from energy evolution divides the damage cycle into four steps: a slow growth period, a silent period, a stepped growth period, and a rapid growth period. Finally, based on the critical slowing down (CSD) theory, the variance and autocorrelation coefficient curves of AE parameters were obtained. It was found that the gap between the critical point of the variance of AE parameters and the breaking point increased with rising temperature.