## 3 Results and Discussion
### Mode I Fracture Toughness  
Table 3 presents granite's mode I fracture toughness values as measured through MTB and CTB tests conducted under high-temperature conditions. These results indicated that cyclic loading has a pronounced and detrimental impact on granite's mode I fracture toughness. Notably, the maximum reduction in fracture toughness is substantial, reaching 18.25%, 16.29%, 27.81%, and 27.14% at the real-time temperatures of 25 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 400 \({}^{\circ}\)C, respectively. Figure 7: Evolution of granite’s elastic stiffness with the number of cycles at different increased temperatures. (**a**) 25 \({}^{\circ}\)C; (**b**) 200 \({}^{\circ}\)C; (**c**) 300 \({}^{\circ}\)C; (**d**) 400 \({}^{\circ}\)C.  
The fluctuations in mode I fracture toughness at different high temperatures concerning the upper load limit are depicted in Figure 8. Notably, the fracture toughness demonstrates an ascending pattern as the upper load limit increases at the same temperature. This can be attributed to the memory effect and the damage traits exhibited by the rock during cyclic loading. As such, recent research has confirmed that acoustic emission energy tends to be higher near the upper load limit during acoustic emission tests, as observed by [PERSON] et al. [56]. This observation proves that significant fatigue damage occurs near the upper load limit. Consequently, the sample is more susceptible to failure near this limit. In this regard, Sample 3 failed after 136 cycles, a notable departure from the failure cycles of Samples 1 and 2, occurring at 300 \({}^{\circ}\)C and 85% of the load limit. Not only do such variations align with the randomness and heterogeneity of granite, but they are consistent with observations in engineering scenarios. The results presented above clearly indicate that the upper load limit has a particular effect on the fracture initiation pressure of the reservoir. Specifically, when the upper load limit surpasses the damage threshold, the initiation pressure increases with the increase of the upper load limit. Therefore, lower load limits can be considered for cyclic hydraulic fracturing, offering a potential opportunity to reduce the demands on mechanical equipment. In theory, an increase in the upper load limit of cyclic loading should result in elevated CVD and more significant accumulated plastic deformation in each cycle, ultimately leading to a gradual decrease in the number of cycles needed to cause sample failure. However, the observed reduction in the required number of cycles is not monotonically reduced in this study, as presented in Table 3. This variation may be attributed to the heterogeneity of the granite samples, causing differences between the theoretical upper load limit and the actual values in the cyclic load tests. Another contributing factor could be the relatively minor changes in the upper load limit across the four levels, which may have resulted in a lack of visible patterns between the number of cycles needed to induce sample failure and the upper load limit. Further in-depth investigations are recommended to gain a more comprehensive understanding of the influence of the upper load limit on the number of cycles required for failure. The relationship between mode I fracture toughness and increasing temperature under different upper load limit conditions is visually presented in Figure 9. Notably, at 400 \({}^{\circ}\)C, all samples experience failure after cyclic loading. As the temperature decreases to 300 \({}^{\circ}\)C, samples under 80%, 85%, and 90% load limits fail, whereas in the range between 25 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, only samples subjected to 85% and 90% load limits ultimately fail. These observations collectively indicate a reduction in mode I fracture toughness as the temperature increases. Figure 8: Evolution of mode I fracture toughness with upper load limit at different temperature settings.