## 3 Experimental Result Analysis
### Deformation Characteristics  
#### 3.3.1 Elastic Modulus  
Figure 7 presents the relationship between the elastic modulus of granite and the number of cycles at different temperatures. The dashed segments represent the real-time elastic modulus obtained from uniaxial compression tests at high temperatures. The cyclic loading and unloading modulus of elasticity was calculated using the same method as the uniaxial compression test. Specifically, the slope of the linear segment corresponding to 40-60% of peak stress in each loading cycle was taken as the modulus of elasticity [38]. Under both constant and variable lower limit cyclic loading conditions, the elastic modulus exhibited a three-stage trend as the number of cycles increased. In the first stage (approximately before 4-6 cycles), the specimens experienced low stress levels, causing the native pores and cracks within the rock to compress continuously, resulting in a rapid increase in the elastic modulus. Under the constant lower limit condition, the elastic modulus increased by 14.67, 15.20, 12.02, and 8.87 GPa from 0 to 6 cycles at different temperatures. Under the variable lower limit condition, the increases were 16.96, 20.71, 16.29, and 10.13 GPa, respectively. In the second stage, increasing axial stress caused the initial cracks in the specimens to densify. However, new cracks also formed during unloading, reducing the effect of pore closure and slowing the increase in elastic modulus. In the third stage, during the later phase of cyclic loading and unloading, damage progressively accumulates, gradually reducing the specimen's bearing capacity and stiffness, ultimately leading to failure. Comparing Figure 7a,b, each specimen underwent no more than 20 loading cycles before failure, and the elastic modulus under the variable lower limit condition exhibits noticeable fluctuations during loading, unlike the smooth and gradual changes observed under the constant lower limit condition. This fluctuation is attributed to the small-amplitude stress loading amplitude in variable lower limit loading, which prevents sufficient elastic recovery before the next cycle, leading to short-term oscillations in the elastic modulus. The influence of loading stage and temperature varied between the two cycling methods. During the initial loading stage, the elastic modulus increases more rapidly under the variable lower limit cycle than under the constant lower limit cycle at all temperatures. At this stage, the elastic modulus under uniaxial compression test can be reached faster by the variable lower limit cycle than by the constant lower limit cycle. This suggests that small-amplitude stress loading accelerates primary pore closure, resulting in a rapid increase in elastic modulus. In the later stage of cyclic loading, the elastic modulus remains similar under both cycling methods at room temperature. Under the same number of cycles, the elastic modulus under variable lower limit cycling is generally higher than that under constant lower limit cycling. This indicates that, at high temperatures, specimens  
Figure 7: Variation curves of loaded elastic modulus with the number of cycles at different temperatures: (**a**) Constant lower limit cycle. (**b**) Variable lower limit cycle. subjected to variable lower limit cycling exhibit greater resistance to deformation and damage. In the uniaxial constant lower limit cycling test, the elastic modulus decreases with increasing real-time temperature. This occurs because damage accumulation and microcrack expansion progress more slowly under constant lower limit cycling due to the fixed lower limit stress. However, at high temperatures, thermal expansion and crack propagation gradually reduce the elastic modulus. In the variable lower limit cycling test, the elastic modulus first increases from 25 \({}^{\circ}\)C to 200 \({}^{\circ}\)C and then decreases beyond 200 \({}^{\circ}\)C. This trend occurs because the increasing lower limit stress in variable lower limit cycling allows rock specimens to experience higher stress values.