## 3 Experimental Result Analysis  
### Stress-Strain Curve  
Figures 4 and 5 present the stress-strain curves for cyclic loading and unloading under real-time high-temperature conditions, with constant and variable lower limits, respectively. Compared to the uniaxial compression curves, the specimens exhibit greater overall deformation under cyclic loading and unloading conditions. However, the stress path did not alter the overall trend of the stress-strain curve, and the granite specimens still exhibited brittle failure characteristics.  
Figures 4 and 5 show that the outer envelope curve of each loading stage in both cyclic methods follows a similar trend to the conventional uniaxial compression curve. Each loading-unloading process intersects the upper stress point of the previous loading curve, indicating that granite specimens exhibit a deformation \"memory\" effect [27]. At 200-400 \({}^{\circ}\)C, the deviation between the outer envelope curves and the uniaxial compression curves in both cyclic methods increases (Figures 4b,c and 5b,c) but gradually converges at 600 \({}^{\circ}\)C (Figures 4d and 5d). Additionally, the loading curves in both cyclic methods do not follow the previous unloading path but instead lie above the unloading curve, forming a plastic hysteresis loop [28]. This phenomenon occurs because rock is a natural material that contains defects such as microcracks and pores, resulting in non-ideal elastic behavior. When the rock is loaded to a certain stress level and then unloaded, the unloading curve does not coincide with the original loading curve but lies below it. This mismatch, together with the adjacent reloading curves, forms a closed hysteresis loop [29]. As the number of cycles increases, the hysteresis loops in both cyclic modes gradually shift toward the strain axis, exhibiting a \"migration\" phenomenon. Simultaneously, the loop area expands, and significant strain accumulation occurs as failure approaches. Notably, the \"migration\" effect is more pronounced in the constant lower limit cycle, leading to a more distinct hysteresis  
Figure 4: Constant lower limit cyclic loading and unloading stress–strain curve: (**a**) 25 \({}^{\circ}\)C; (**b**) 200 \({}^{\circ}\)C; (**c**) 400 \({}^{\circ}\)C; and (**d**) 600 \({}^{\circ}\)C.  
loop. As the number of cycles in the constant lower limit test increases, internal damage in the rock samples intensifies, leading to a reduction in stiffness. The hysteresis area expands. However, the specimens retain sufficient elastic recovery time, and the deformation is mainly elastic. In the variable lower limit test, the loading and final unloading curves are nearly linear and overlap, forming a nearly closed hysteresis loop. As the number of cycles increases, microcrack propagation and damage occur in the rock samples, causing a slight increase in the hysteresis area. However, the loop remains nearly closed. Due to the relatively high lower limit of stress, the deformation that occurs is difficult to recover, leading to accelerated crack propagation and predominantly plastic deformation. The constant lower limit cycle is primarily associated with elastic deformation and exhibits minimal energy dissipation. In contrast, the variable lower limit cycle is characterized by plastic deformation and results in significant energy dissipation. This observation is consistent with the findings of subsequent studies on energy dissipation [30; 31]. With increasing temperature, the initially dense hysteresis loops gradually become more sparse. At 600 \({}^{\circ}\)C, individual loop curves become clearly distinguishable (Figures 3(d) and 3(d)). High-temperature exposure degrades the mechanical properties of granite, increasing plastic deformation under axial stress. Although the outer envelope curves of both cyclic loading methods follow similar trends, their offset from the uniaxial compression curve differs due to variations in stress paths. The variable lower limit cycle remains closer to the uniaxial compression curve at 200-600 \({}^{\circ}\)C.