## 3 Results and Discussion
### Fracture Surface Morphology  
During cyclic loading, the crack initially originates at the tip of the pre-existing crack and subsequently propagates along the loading direction toward the loading point. As shown in Figure 11, at temperatures below 200 \({}^{\circ}\)C, the granite surface exhibits a gray-white color. When the temperature rises, the sample's color transitions gradually from gray-blue to gray-white, accompanied by yellow spots on the surface. This change is attributed to the transformation of black mica-rich granite to yellow at elevated temperatures, primarily due to dehydration [59; 63]. The crack paths are detected using a camera photography system, as depicted in Figure 12. The crack deviation distance, which measures how far the crack path deviates from the pre-crack plane, is closely associated with the particle size and internal structure of the sample, as found by [PERSON] et al. [64]. In the temperature range of 25 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, the crack deviation distance is relatively small, and the crack path appears relatively straight, indicating apparent features of brittle fracture. However, as the temperature rises, the crack deviation distance and the tortuosity degree gradually increase. This is primarily due to the elevated temperature causing more significant thermal damage and the formation of transgranular cracks. The growing number of cracks and their unpredictable propagation direction contribute to increased crack path tortuosity. On the other hand, the decomposition of larger mineral particles significantly affects the crack path at elevated temperatures, as noted by [PERSON] et al. [65]. This analysis underscores that the impact of rising temperature on the crack path is more pronounced compared to cyclic loading. Figure 13 shows the microcrack morphology of the granite after MTB and CTB under high temperature using SEM (500\(\times\)), respectively. At 25 \({}^{\circ}\)C, as depicted in Figure 13a, a few secondary pores and primary cracks are visible on the fracture plane of the granite sample under MTB, indicating that the granite structure remains nearly intact and compact. It should be noted here that most main cracks are intercrystalline. This phenomenon aligns with the findings of [PERSON] et al. [66], who also observed numerous micropores on the failure surface following cyclic loading, a characteristic not detected in the present test. As the temperature reaches 200 \({}^{\circ}\)C, intergranular cracks develop on the fractured surface, which can be attributed to the differential expansion between various mineral grains at high temperatures, leading to higher stress concentration at the crack tip. Eventually, the concentrated stress causes the breakage of the mineral grains at the end of the crack, inducing an increase in the length and width of the intergranular crack, which is consistent with the findings of [PERSON] et al. [67]. Within the temperature range of 25 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, the  
-fracture morphology of the specimen exhibits overall integrity, with intercrystalline cracks predominantly present, indicative of a characteristic brittle fracture [22]. At 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, the differential expansion between mineral grains becomes more significant, and the prevalence of intergranular cracks increases considerably. As intergranular cracks develop, they tend to intersect and interconnect, leading to the loosening of mineral grains at these crack intersections [42; 68]. This process contributes to the structural degradation of the granite sample, where mineral grains are exfoliated, and larger pores and crystal fracture zones are formed. SEM imaging results reveal the presence of numerous microcracks and crystal fracture zones resulting from exposure to high temperatures, leading to the degradation of the rock's microstructure, which degrades the macro-mechanical properties of granite.