Experimental studies on the effect of cyclic thermal shock and cooling methods on the mechanical properties and fracture behavior of prefabricated fissured sandstone

The construction of sandstone thermal storage systems and the evaluation of their long-term stability are of great significance to the development of geothermal energy. In this study, the effect of the number of thermal shocks and cooling methods on the mechanical properties and fracture behavior of sandstone containing a single prefabricated fissure was investigated. The experimental results show that as the number of thermal shocks increases, the peak stress and elastic modulus of the fissured sandstone gradually decrease, and show a very good exponential functional relationship. The first thermal shock weakens the mechanical properties of fissured sandstone most severely, and the deterioration effect of mechanical properties is significantly slowed down when the number of thermal shocks exceeds 4. In addition, compared with water cooling, the reduction of the mechanical properties of fissured sandstone by air cooling is lower. As the cooling water temperature decreases, the thermal shock effect of the fissured sandstone is more severe, resulting in more significant damage. An empirical formula considering the number of thermal shocks and cooling water temperature is proposed to characterize the damage to the fissured sandstone. The crack initiation of fissured sandstone under uniaxial compression subjected to different thermal shocks is dominated by tensile failure (wing crack). As the number of thermal shocks increases and the cooling water temperature decreases, the initial crack modes gradually change from T1 to T2, or a more complex pattern. Subsequently, the thermal shock process of fissured sandstone is simulated in COMSOL Multiphysics. The effect of heat transfer coefficient and cooling water temperature on the temperature field and stress field in fissured sandstone are discussed, and the reasons for the thermal cracking of the fissured sandstone under thermal shock are revealed.