## 5 Discussion
### Influence of Temperature on \(K_{\text{IC}}\) and \(\sigma_{t}\)  
It can be seen from Figures 5 and 6 that temperature has a significant influence on the mode-I fracture toughness and tensile strength of the rock. In the temperature range 20-100 \({}^{\circ}\)C, the mode-I fracture toughness of sandstone increases slightly, showing a toughening effect, and the tensile strength is almost unchanged. In general, the mechanical properties of Sichuan sandstones are not significantly changed by heating to temperatures below 100 \({}^{\circ}\)C. Figures 10 and 11 respectively show the variation in the mode-I fracture toughness and in the tensile strength of different types of rocks with temperature. It is apparent from Figures 10 and 11 that there is significant scatter in the change trends in fracture toughness and tensile strength with temperature. As rock is exposed to higher temperatures, the tensile strength decreases first because microcracks are generated and then via plastic deformation [43]. A decrease in the fracture toughness of gabbro with heat-treatment to temperatures from 20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C is probably due to structural and mineralogical changes in the rock [44]. Heat-treating rocks to 100 \({}^{\circ}\)C resulted in reduced fracture toughness due to the development of tensile stress caused by the uncoordinated expansion of mineral particles in the rock and the consequent development of microcracks. Under the action of heat, the rock mineral particles gradually expand and the volume increases. For non-compact rocks, an increase in mineral particle volume reduces the volume of original voids and enhances extrusion between mineral particles, which in turn enhances the mechanical properties of the rock. Heating also causes free water in the rock to evaporate, and frictional resistance increases as the particles become dry, which also leads to an increase in mechanical properties. Of course, the generation of thermal cracks will also weaken the mechanical properties of the rock. Observing the SEM picture of Sichuan sandstone, rock samples at room temperature and 100 \({}^{\circ}\)C are  
Figure 9: SEM fractography of fractures in sandstone (the red box indicates fragmentation, and the arrow indicates a pore. (**a**) BD fracture (20 \({}^{\circ}\)C); (**b**) BD fracture (300 \({}^{\circ}\)C); (**c**) SCB fracture (300 \({}^{\circ}\)C); (**d**) SCB fracture (600 \({}^{\circ}\)C); (**e**) SCB fracture (600 \({}^{\circ}\)C); (**f**) BD fracture (600 \({}^{\circ}\)C). relatively intact, and pores and fissures are rarely seen. It can be seen that at 100 \({}^{\circ}\)C, the effect of thermal cracking is not dominant, and volume expansion is likely to be more dominant. Therefore, the mechanical properties of the rock will be slightly enhanced. At temperatures above 100 \({}^{\circ}\)C, the mode-I fracture toughness and tensile strength of the rock are reduced, mainly due to thermal cracking of the rock structure. As the temperature rises, the thermal expansion coefficient of the mineral particles changes; the thermal deformation is not coordinated, and cracks are generated. Thermal stress caused by an uneven temperature distribution also causes tensile cracks to form. After rock has been subjected to heat, the cracks formed inside it are still present after cooling and are irreversible [27]. It can be observed from the SEM image that exposure to temperatures above 100 \({}^{\circ}\)C increased the number of cracks and pores in the rock, which leads to the gradual weakening of its properties. The structural damage caused to the rock by the heat is a result of both physical and chemical reactions such as dehydration release of bound water from crystals, conversion between homogeneous polymorphs of quartz, mineral decomposition reactions, and thermal expansion.