Study on mixed-mode I+Ⅱ fracture properties of red sandstone under thermal-acid conditions

This study aims to explore the influence of sulfuric acid solution under different conditions of temperature (25 °C, 60 °C) and pH value (1, 3) on the mechanical properties and deterioration mechanism of sandstone. The apparent change characteristics of sandstone under diverse treatment conditions were delineated through systematic analysis, and a thorough exploration was conducted into the evolution patterns of peak load and the degree of deterioration in fracture toughness within the sandstone. The digital image correlation (DIC) technique was employed to determine the length of the fracture process zone in sandstone, followed by a comparative analysis with the theoretical value. Meanwhile, by combining the theoretical values and the DIC measurement data, the fracture initiation angle of the sandstone specimens was effectively predicted, and the GMTS criterion was adopted to improve the prediction accuracy. Through X-ray diffraction (XRD) analysis, the mineral composition of sandstone was revealed, and the possible chemical reactions and their deterioration mechanisms were discussed. Further, the deterioration degree of sandstone was analyzed in detail from a microscopic perspective using nuclear magnetic resonance (NMR) technology. The research results show that white substances formed on the surface of sandstone, and its peak load and fracture toughness significantly decreased; under different treatment conditions, there are differences in the length of the fracture process zone of sandstone; the GMTS criterion calculated using the Schmidt model can predict the fracture initiation angle of sandstone well; the internal pore structure of sandstone changed, with the proportion of macropores increasing, and the total porosity showing an upward trend.