The Effect of Temperature and Heating–Cooling Cycles on Mode I, Mode II and Mixed-Mode I-II Fracture Toughness of Sandstone

In the case of explosions and fires, the rocks undergo a cycle of heating and cooling. For that, first, they are exposed to considerable heat and then cooled after extinguishing the fire. Temperature variations and subsequent contraction and expansion affect the physical and mechanical properties of rocks. Through two series of tests, the effects of temperature and the number of heating-cooling cycles on the mode I, mode II and the effective mixed-mode I-II fracture toughness of Lushan sandstone were investigated. In the first series, the effect of temperature was studied in a heating–cooling cycle at ambient temperature (۲۵°C) and ۶۰, ۱۵۰, ۲۰۰, ۳۰۰, ۵۰۰, and ۷۰۰°C. The highest and lowest mode I, mode II and the effective mixed-mode I-II fracture toughness were, observed at ۱۵۰ and ۷۰۰°C, respectively. In the second series of tests, the effect of the number of heating–cooling cycles was investigated on the mode I, mode II and the effective mixed-mode I-II fracture toughness of sandstone specimens at ۱۵۰°C (for hydraulic fracturing modeling) and a crack inclination angle of ۴۵°. According to the results, the mode I, mode II and the effective mixed-mode I-II fracture toughness increased in the first cycle and decreased with increasing the number of heating–cooling cycles. As the crack inclination increased, the effective mixed-mode I-II fracture toughness of the sandstone specimens increased. The mode II fracture toughness increased up to a crack inclination angle of ۴۵° and then decreased. Moreover, the mode of fracture changes from opening mode (mode I) at the crack inclination angle of zero degree to mixed mode (tension-shear) at the crack inclination angle of less than ۲۸.۸°. The mode of fracture changes from tensile-shear to compression-shear at the crack inclination angle of greater than ۲۸.۸°.