## 3 Results and Discussion  
### Macroscopic Damage Characteristics of Fractured Granite under Freeze-Thaw Cycles  
Figure 3 shows the macroscopic surface changes in the fractured granite following various freeze-thaw cycles. After 40 freeze-thaw cycles, small free particles and new micro-cracks appeared on the specimen's surface, showing no obvious damage. After 80 cycles, these cracks increased and became clearly visible. They formed a network at stress concentrations and structural weak points. Block-like peeling occurred at the specimen's edges and the tips of prefabricated cracks. Fractured granite exhibits more pronounced damage compared with intact granite. Figure 4 shows the granite slices after different freeze-thaw cycles, indicating that internal damage in the granite continues to accumulate and the pore structure becomes increasingly developed.  
The morphology of the fracture surface around pre-existing cracks in the granite was observed using a scanning electron microscope (SEM) after various freeze-thaw cycles. The SEM image with zero freeze-thaw cycles shows complete and smooth granite crystals with only a few primary cracks, as depicted in Figure 5a. After 40 freeze-thaw cycles, surface cracks begin to form on the granite crystals and the number of transgranular cracks increases significantly, accompanied by some fine crystal exfoliation, as illustrated in Figure 5b. After 80 freeze-thaw cycles, the crystals undergo further cracking, with transgranular cracks expanding, gradually developing, and connecting. The crystal surface becomes rough, accompanied by the exfoliation of large crystals, as depicted in Figure 5c.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Parameters** \\ \hline Equipment size & 1200 \(\times\) 1000 \(\times\) 1870 mm (long \(\times\) wide \(\times\) high) \\ Equipment weight & 3500 kg \\ Frame stiffness & 6 MN/mm \\ Maximum axial pressure & 1500 kN \\ Maximum confining pressure & 50 MPa \\ Dynamic load waveform & oblique wave, sine wave, triangular wave, and square wave \\ Dynamic load frequency & 0.001–1 Hz \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the RMT-201 rock mechanics test system.  
After the freeze-thaw cycle, both the mass and longitudinal wave velocity of the sample progressively decrease (Figure 6). This indicates that after the saturated freeze-thaw cycle, the surface cuttings of the double-fractured granite exfoliate, and internal pores continuously develop because of frost heave force. Consequently, the degree of  
Figure 4: Microscopic damage to granite slices after different freeze–thaw cycles. (**a**) F-T 0 cycles. (**b**) F-T 40 cycles. (**c**) F-T 80 cycles.  
Figure 5: SEM image of granite after different freeze–thaw cycles. (**a**) F-T 0 cycles. (**b**) F-T 40 cycles. (**c**) F-T 80 cycles.  
Figure 3: Apparent changes in samples after different freeze–thaw cycles. (**a**) Intact granite. (**b**) Fractured granite.  
damage gradually increases, resulting in a macroscopic decrease in both the sample's mass and longitudinal wave velocity. The freeze-thaw process induces a \"liquid-solid\" phase transition, which weakens the original pore structure of rocks and leads to the accumulation of meso-damage. The presence of prefabricated cracks induces stress concentration, thereby accelerating the rock damage process.