Dynamic mechanical properties and constitutive modeling of red sandstone after high-temperature treatment under different confining pressures: Micromechanical perspective

High-temperature environment poses a major challenge to deep underground engineering, and understanding mechanical behaviour of rock after thermal exposure is of clear engineering significance. Dynamic compression tests were performed on red sandstone specimens cooled from different temperatures under various confining pressures using a Split Hopkinson Pressure Bar system with confining pressure. Effects of temperature and confining pressure on failure mode and fractal characteristics of fragment size were analysed. Interfacial behaviour between mineral crystals and cementitious matrix was examined by polarised light microscopy, while evolution of internal pore structure after heat treatment was characterised by high-resolution CT scanning. Results show that red sandstone at room temperature contains only a small number of inherent microcracks. Once temperature exceeds 300 °C, rapid thermal decomposition of clay minerals and pore connectivity induced by crystal expansion lead to marked increases in porosity and pore-throat volume. Dynamic compressive strength changes little below 300 °C, but decreases significantly above this threshold. Under constant confining pressure, elastic modulus and reflected energy increase with temperature, absorbed energy decreases, and transmitted energy first increases and then decreases. Overall, dynamic peak strength is positively correlated with incident energy, transmitted energy, and confining pressure, but negatively correlated with temperature. Strain rate is strongly negatively correlated with reflected energy and positively correlated with fractal dimension of fragments. A three-dimensional viscoelastic-plastic constitutive model incorporating thermal degradation and strain-rate effect was further established, which captures temperature-dependent dynamic strength and plastic flow behaviour of red sandstone under confining pressure with good accuracy. This study provides theoretical basis and experimental support for stability analysis and safety assessment of deep engineering applications such as geothermal energy development and underground waste storage.