## 3 Results and Discussion
### Microscopic Failure Mechanism Analysis  
As illustrated in Figure 16, which presents SEM images of coarse- and fine-grained granite under varying cooling conditions, it is evident that, at ambient temperature, the mineral crystals within both coarse- and fine-grained granite are predominantly intact, exhibiting smooth surfaces and firmly bound mineral grains. Some pores demonstrate a propensity to expand, and, in general, there is a paucity of intragranular cracks and fragments in the stressed regions. However, when the heat treatment temperature reaches 200 \({}^{\circ}\)C, the mineral crystals within both coarse and fine granite remain intact with smooth surfaces. The microcrack types are mainly transgranular and intragranular cracks, and the width of the cracks in the rock is relatively small. This is because at this stage the temperature is low and the thermal stress generated is minimal. The process of evaporation of free and bound water within the rock results in enhanced particle contacts, with thermal hardening dominating, leading to a significant increase in the shear strength and cohesion of the rock (Figure 17a). The application of a cooling shock, in the form of liquid nitrogen, results in the formation of numerous micro and small pores on the surface of the granite fracture. This phenomenon occurs due to the lower temperature of the liquid nitrogen, which, upon contact with the high-temperature rock, generates greater thermal stress [49]. As the temperature of the heat treatment increases from 200 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, the integrity of the granite fracture surface gradually decreases, with both the number and width of cracks increasing. Transgranular cracking gradually becomes the dominant type of cracking  
Figure 15: Pore distribution map of granite under different cooling methods: (**a**) coarse-grained granite after high-temperature water-cooled impact, (**b**) coarse-grained granite after high-temperature liquid-nitrogen-cooled impact, (**c**) fine-grained granite after high-temperature water-cooled impact, (**d**) fine-grained granite after high-temperature liquid-nitrogen-cooled impact. in the rock, and at this stage the volume of the mineral grains inside the granite expands due to the high temperature, increasing the stress between the grains (Figure 17b). The formation of small cracks at the periphery of the grains, coupled with the evaporation of internal water, results in the deterioration of the mineral structure, with certain minerals undergoing decomposition and oxidation. The process of thermal damage escalates, leading to a progressive and continuous deterioration of the rock's physical properties. At a temperature of 400 \({}^{\circ}\)C, the fracture surface of fine-grained granite transitions from smooth to rough [50], a consequence of the uncoordinated deformation of mineral particles, which serves to reduce the original cementation between these particles [45]. In contrast, the fracture surface of coarse-grained granite becomes rough at 500 \({}^{\circ}\)C, a phenomenon that may be attributed to the disparate initial levels of cementation between the two granite types. Based on the findings presented in Sections 3.2.1 and 3.3 of this study, an inverse relationship was observed between fracture surface roughness and the rock brittleness index. Specifically, smoother fracture surfaces were associated with higher brittleness indices, whereas an increase in fracture surface roughness corresponded to a progressive reduction in rock brittleness [50]. Figure 16: SEM images of granite under different cooling methods (500\(\times\)): (**a**) coarse-grained granite after high-temperature water-cooled impact, (**b**) coarse-grained granite after high-temperature liquid-nitrogen-cooled impact, (**c**) fine-grained granite after high-temperature water-cooled impact; (**d**) fine-grained granite after high-temperature liquid-nitrogen-cooled impact.