## 3 Experimental Result Analysis
### Microscopic Damage Characteristics  
To study the effect of high temperature and cycling mode on the internal microstructure of granite specimens, scanning electron microscopy (SEM) was used to observe the micro-morphology of the granite specimen sections after real-time high-temperature cyclic loading and unloading tests at 500\(\times\) magnification. Figure 12 presents SEM images of granite specimens under varying temperature and cycling mode conditions. At 25 \({}^{\circ}\)C, the microstructures of the specimens in both loading modes were relatively intact, with minimal spalling of rock debris (Figure 12a,e). At this temperature, the grains in the specimens are closely packed, with a more complete microstructure. Cracks are primarily intracrystalline, existing within the mineral grains. These primary cracks cause localized damage, with limited crack propagation. Under variable lower limit loading, the specimens exhibited more intracrystalline cracks and a lower degree of crystal integrity compared to constant lower limit loading. At 200 \({}^{\circ}\)C, the specimens primarily exhibited transgranular and intergranular cracks, with a striated slip band appearing under variable lower limit  
Figure 11: Variable lower limit-graded cyclic loading and unloading energy density and dissipation ratio curves with number of cycles: (**a**) Input energy density. (**b**) Elastic energy density. (**c**) Dissipated energy density. (**d**) Energy dissipation ratio. loading (Figure 12f). This behavior is due to the increased strength of granite under this loading mode. High temperatures cause the internal mineral particles to slip more easily under stress. Simultaneously, high temperatures cause some initial microcracks to close, resulting in polygonal cracks around the mineral particles that have not fully formed closed polygons. As the temperature increases to 400 \({}^{\circ}\)C, cracks in the specimens are dominated by intergranular cracks, accompanied by transgranular cracks. Locally closed polygonal cracks appear under variable lower limit loading, with more transgranular and intergranular cracks than under constant lower limit loading (Figure 12g). At 600 \({}^{\circ}\)C, both cyclic loading and unloading lead to widespread transgranular and intergranular cracks, with many crystals splitting into smaller fragments. The fracture surfaces of granite exhibit a significant increase in fracture severity, along with a noticeable rise in both the number and width of cracks. Crystals under variable lower limit loading and unloading also exhibit localized holes from large cracks (Figure 12d,h). Figure 12: SEM images of granite samples at different temperatures and loading modes (500\(\times\)): (**a**) Constant lower limit 25 \({}^{\circ}\)C. (**b**) Constant lower limit 200 \({}^{\circ}\)C. (**c**) Constant lower limit 400 \({}^{\circ}\)C. (**d**) Constant lower limit 600 \({}^{\circ}\)C. (**e**) Variable lower limit 25 \({}^{\circ}\)C. (**f**) Variable lower limit 200 \({}^{\circ}\)C. (**g**) Variable lower limit 400 \({}^{\circ}\)C. (**h**) Variable lower limit 600 \({}^{\circ}\)C.  
As shown in Figure 12, most intergranular cracks occur between quartz and feldspar grains at temperatures between 200 \({}^{\circ}\)C and 400 \({}^{\circ}\)C [42]. This occurs because quartz has a high thermal expansion coefficient, significantly different from other minerals. Under high temperatures, mismatched deformation at quartz boundaries generates local thermal stresses, leading to cracking at crystal junctions [43]. Transgranular cracks primarily develop within feldspar grains due to their lower strength. During testing, feldspar experiences both mechanical stress and thermal expansion, increasing the likelihood of crack initiation and propagation. At 600 \({}^{\circ}\)C, mica decomposition and the quartz \(\alpha\)-\(\beta\) phase transition cause a sudden volume increase in certain mineral components [39], leading to extensive microcrack formation.