## 3 Results and Discussion
### Evolution of Elastic Stiffness  
Elastic stiffness is a crucial mechanical property of rocks, often represented by the load-displacement curve's slope, which describes how rocks evolve at elevated temperatures. In this study, the elastic stiffness during the loading stage is estimated based on the slope of the secant, which covers approximately 50% to 75% of the peak load at the corresponding temperature, an approach applied in previous studies [43; 50]. The variation in elastic stiffness of granite during the loading phase as a function of the number of cycles in the CTB tests is illustrated in Figure 7.  
As the number of cycles increases, the elastic stiffness of the granite initially rises, followed by a stabilization phase and, ultimately, a decline. Notably, the elastic stiffness of specific nonfailure samples does not exhibit a significant decrease. Figure 7a further illustrates that the elastic stiffness of these nonfailure samples undergoes a significant enhancement as the number of cycles progresses. It escalates rapidly during the initial loading stage, reaching its maximum value after a few cycles. The initial cycles significantly impact the elastic stiffness, contributing to approximately 70-80% of the total increase. This is attributed to the substantial compaction of primary fractures and microcracks during these initial loading cycles, resulting in a notable improvement in the sample's compactness. Consequently, the elastic stiffness of the samples experiences a remarkable increase during the second loading cycle.  
As the cyclic loading test progresses, the sample's internal structure undergoes restructuring, further enhancing the sample's compactness. During the intermediate phase of cyclic loading, the increase rate in elastic stiffness diminishes as the number of cycles rises. Ultimately, the elastic stiffness reaches a stable state with further increases in cycle numbers. This has been associated with increased friction between fissures, resulting from debris settling into the microcracks during loading, which induces stiffness augmentation [51; 52].  
Therefore, the initial loading cycles effectively boost the sample's elastic stiffness. However, as depicted in Figure 7, most samples have a subsequent reduction in elastic stiffness. During this phase, the internal microcracks and fatigue damage in the granite progressively accumulate with each cycle, resulting in increased plastic deformation and, consequently, a decline in elastic stiffness. Several studies have revealed that an augmentation in the upper load limit leads to a decline in the elastic stiffness of the tested specimens [31, 53]. This occurrence can be ascribed to the acceleration of crack development and the consequent premature initiation of the elastic stiffness degradation phase caused by the heightened upper load limit. The findings of the current experiment effectively substantiate this claim. Additionally, with an increase in temperature, there is a noticeable downward shift in the elastic stiffness curves, as illustrated in Figure 7. This shift indicates a decrease in the elastic stiffness of granite as the temperature rises. This observation aligns with the experimental findings of [PERSON] et al. [54]. Nevertheless, certain studies have discovered that the elastic stiffness of rocks exhibits an increase within the temperature range of 25-200 \({}^{\circ}\)C [55]. This finding further suggests the absence of a definitive pattern in the alteration of mechanical properties of rocks between 25 and 200 \({}^{\circ}\)C.