## 3 Results and Discussion
### NMR T\({}_{2}\) Spectrum and Pore Distribution Structure Analysis  
The NMR transverse relaxation time (T\({}_{2}\)) reflects changes in the rock pore structure [48,49]. In this study, granite pore size distribution is categorized into three types as follows: micropore (r < 10 \(\upmu\)m), mesopore (10 \(\upmu\)m \(\leq\) r < 100 \(\upmu\)m), and macropore (r \(\geq\)100 \(\upmu\)m). Figure 7a presents the T\({}_{2}\) curve of the sample after various freeze-thaw cycles. From left to right, two main peaks appear in the curve, representing micropores and mesopores. The area of the second peak accounts for over 85%. As the number of freeze-thaw cycles increases, the T\({}_{2}\) curves shift to the right, and the peak value of the second wave increases by 11.11% and 22.61%, respectively. After 80 freeze-thaw cycles, a third peak appears, but its amplitude is weaker than that of the first and second peaks. The proportion of micropores and mesopores gradually decreases, while the proportion of macropores gradually increases. After 80 freeze-thaw cycles, additional macropores appear (Figure 7b).  
Figure 8 shows the NMR T\({}_{2}\) curves of fractured granite before and after fatigue load failure. After failure, the T\({}_{2}\) curves shift to the right, and the spectral areas increase by  
Figure 6: The relationship between mass and compressive wave velocity with freeze–thaw cycles.  
Figure 7: T\({}_{2}\) spectral characteristics and pore size distribution of rocks under different freeze–thaw cycles. (**a**) T\({}_{2}\) spectral curve. (**b**) Pore size distribution.  
101.3%, 61.7%, and 80.7%. The curves for 40 and 80 freeze-thaw cycles change from a two-peak to a three-peak structure, and the signal amplitudes of the peaks increase significantly. This indicates that the internal pores of the fractured granite, after freeze-thaw cycles, continuously initiate, develop, and connect during the fatigue load failure process, forming additional macropores and penetrating cracks, ultimately leading to the sample's failure.  
According to fractal theory, if the internal pores of rocks are assumed to be composed of a series of capillary bundles, their fractal dimension can be characterized by the NMR T\({}_{2}\) transverse relaxation time and volume fraction [50]. The fractal dimensions of the three pore types in granite are shown in Table 2. The freeze-thaw cycle alters the pore size distribution range of granite at different scales. As the number of freeze-thaw cycles increases, the fractal dimension of small and medium pores decreases, while that of large pores gradually increases (Figure 9). This indicates that the proportion of medium and small pores in granite gradually decreases, whereas the proportion of large pores gradually increases. The freeze-thaw process promotes the development of micropores in granite, significantly deteriorating its microstructure.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Status**} & \multirow{2}{*}{**F-T Cycles**} & \multicolumn{3}{c}{**Micropore**} & \multicolumn{3}{c}{**Mesopore**} & \multicolumn{3}{c}{**Macropore**} \\  & & \multicolumn{3}{c}{**(r \(<\) 10 \(\mu\)m)**} & \multicolumn{3}{c}{**(10 \(\mu\)m \(\leq\) r \(<\) 100 \(\mu\)m)**} & \multicolumn{3}{c}{**(r \(\geq\) 100 \(\mu\)m)**} \\ \cline{3-10}  & & **T\({}_{2a}\)/ms** & **T\({}_{2b}\)/ms** & **D\({}_{b}\)** & **T\({}_{2c}\)/ms** & **T\({}_{2d}\)/ms** & **D\({}_{b}\)** & **T\({}_{2e}\)/ms** & **T\({}_{2f}\)/ms** & **D\({}_{b}\)** \\ \hline \multirow{3}{*}{Integrity} & 0 & 1.383 & 9.659 & 2.123 & 10.353 & 95.477 & 2.728 & 102.341 & 821.434 & 2.375 \\  & 20 & 1.589 & 9.659 & 1.983 & 10.353 & 95.477 & 2.712 & 102.341 & 943.788 & 2.467 \\  & 40 & 1.956 & 9.659 & 1.820 & 10.353 & 95.4