## 3 Experimental Result Analysis
### Peak Strength  
Figure 6 presents the relationship between peak strength and temperature under three different loading methods. The figure shows that, at 25 \({}^{\circ}\)C, the strengths under cyclic loading and unloading exceed those under uniaxial compression. This suggests that cyclic loading enhances the rock's mechanical properties by raising its crack damage  
Figure 5: Variable 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.  
threshold, making it harder for pre-existing closed cracks to reopen [32]. Furthermore, the strength in the variable lower limit cycling mode exceeds that in the constant lower limit mode, indicating that a higher stress lower limit enhances the cyclic strengthening effect. At 200 \({}^{\circ}\)C, the thermal strengthening effect on granite becomes evident. Compared to room temperature, the specimen strength increased to 171.64 MPa under uniaxial compression (14.87% increase), 167.87 MPa under the constant lower limit (8.58% increase), and 177.63 MPa under the variable lower limit (13.23% increase). The constant lower limit method yielded the lowest strength. This phenomenon can be attributed to the differential thermal expansion of granite minerals, particularly quartz and feldspar, which partially fill pre-existing microcracks. Additionally, the adsorbed water within the rock evaporates or contracts, leading to tighter intergranular contact and, consequently, an increase in peak strength. Thermal hardening effects have begun to emerge [28]. As temperature increases, thermal damage begins to outweigh the thermohardening effect [33]. At 400 \({}^{\circ}\)C, the strength under uniaxial compression dropped significantly to 134.09 MPa, falling below its room-temperature value. High-temperature-induced thermal cracks increased specimen fragility; this is consistent with [PERSON] et al. [34]. Although the strength under cyclic loading also declined (to 158.18 and 160.30 MPa), it remained above the room-temperature level. At this stage, cyclic loading mitigated thermal degradation, suppressing microcrack propagation and helping maintain granite's strength. At 600 \({}^{\circ}\)C, thermal damage dominates, causing the strengths under all three loading conditions to drop sharply to similar levels: 78.81, 84.57, and 75.87 MPa, and the reductions were 41.23%, 46.54%, and 52.67%, respectively. This indicates that thermal deterioration intensifies significantly between 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, and fatigue effects begin to manifest at this stage [35]. At this stage, the strength in the variable lower limit cycling mode falls slightly below that of the uniaxial compression test. This indicates that, under the effect of high temperature, the effect of the variable lower limit cycle on the specimen may change from strengthening to fatigue damage [36]; this is consistent with [PERSON] et al. [37]. When combined with the findings of this study, it suggests that elevated temperatures initiate thermal cracking, while cyclic loading further promotes the formation of pores and microcracks due to fatigue. Ultimately, under temperature-stress coupling, this leads to cumulative fatigue damage and a notable reduction in mechanical strength. At 600 \({}^{\circ}\)C, the specimens subjected to the variable lower limit cycle exhibited lower strength than those under the constant lower limit cycle. Because the higher lower stress limit in the variable lower limit cycle continuously subjects the specimens to greater stress, accelerating crack propagation and causing increased damage and weakening of the rock. In contrast, the constant lower limit cycle has a lower stress limit of 0, allowing for sufficient elastic recovery time, which results in more stable crack growth and slower strength degradation.  
Figure 6: Peak strength of specimens at different temperatures and loading modes.