## 3 Results and Discussion  
### Load-Displacement Curve  
Figure 5 illustrates the load-displacement curves for CTB tests conducted on granite at 400 \({}^{\circ}\)C, each with varying upper load limits. To differentiate between the fracture load of granite samples in MTB and CTB tests, it is essential to clarify that the peak load refers to the fracture load in MTB tests, while the damage load refers to the fracture load in CTB tests. Notably, a significant memory effect is observed in the granite's behavior during the cyclic loading. The load-displacement curves of the first cycle closely resemble those observed in MTB tests. Nonetheless, as the number of cycles increases, even when the upper load limit is below the peak load observed in MTB tests, the microcracks within the sample gradually connect, resulting in sample failure. Notably, at 400 \({}^{\circ}\)C, in comparison to the peak load in MTB tests, the damage load of samples subjected to CTB tests with upper load limits of 75%, 80%, 85%, and 90% experiences a reduction of 27.14%, 19.38%, 19.55%, and 13.36%, respectively, expressed as the percentage reduction of the CTB damage load to the peak load in MTB tests at the same temperature. Across different temperature settings. i.e., 25 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C, the test outcomes consistently demonstrate similar patterns, with the damage load in CTB tests increasing as the upper load limit rises, as summarized in Table 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & **Peak Load** & **CTB Upper Load** & **CTB Lower** \\  & **of MTB Tests (N)** & **Limit** & **Limit Load (N)** \\ \hline \multirow{4}{*}{25} & \multirow{4}{*}{P\({}_{\text{max}(25\text{\text{\textminus}})}=1992\)} & 75\% P\({}_{\text{max}(25\text{\textminus}})\) & \multirow{4}{*}{150} \\  & & 80\% P\({}_{\text{max}(25\text{\textminus}})\) & \\  & & 85\% P\({}_{\text{max}  
Figure 5: Load–displacement curves for CTB tests on granite at 400 \({}^{\circ}\)C under various upper load limits, with the MTB results provided as a reference. (**a**) 75%; (**b**) 80%; (**c**) 85%; (**d**) 90%.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{T/(\({}^{\circ}\)C)} & \multirow{2}{*}{\
\begin{tabular}{c} \(\mathbf{MTB}\)\(K_{\text{lc}}\)/ \\ (\(\mathbf{MPa}\cdot\mathbf{m}^{1/2}\)) \\ \end{tabular} } & \multirow{2}{*}{**ULL/(\%)**} & \multicolumn{4}{c}{**Number of Cycles \({}^{\ast}\)**} & \multirow{2}{*}{
\begin{tabular}{c} \(\mathbf{CTB}\)\(K_{\text{lc}}\)/ \\ (\(\mathbf{MPa}\cdot\mathbf{m}^{1/2}\)) \\ \end{tabular} } & \multirow{2}{*}{**DA/(\%)**} \\ \cline{5-8}  & & & & **Sample 1** & **Sample 2** & **Sample 3** & \\ \hline \multirow{4}{*}{25} & \multirow{4}{*}{1.52} & 75 & 200 & 200 & 200 & - & - \\  & & 80 & 200 & 200 & 200 & - & - \\  & & 85 & 61 & 71 & 1.24 & 18.25 \\  & & 90 & 11 & 36 & 13 & 1.28 & 14.19 \\  & & 75 & 200 & 200 & 200 & - & - \\
200 & 1.27 & 80 & 200 & 200 & 200 & - & - \\  & & 85 & 27 & 16 & 12 & 1.06 & 16.29 \\  & & 90 & 8 & 23 & 15 & 1.06 & 16.23 \\  & & 75 & 200 & 200 & 200 & - & - \\  & & 80 & 29 & 30 & 9 & 0.80 & 27.81 \\
300 & 1.11 & 85 & 5 & 49 & 136 & 0.91 & 19.68 \\  & & 90 & 33 & 15 & 6 & 0.98 & 14.46 \\  & & 75 & 8 & 11 & 17 & 0.77 & 27.14 \\
400 & 1.05 & 80 & 18 & 3 & 4 & 0.85 & 19.38 \\  & & 85 & 13 & 12 & 6 & 0.84 & 19.55 \\  & & 90 & 4 & 3 & 4 & 0.82 & 13.36 \\ \hline \hline \end{tabular} Note: MTB K\({}_{\text{lc}}\)—average mode I fracture toughness from MTB tests; ULL—upper load limit; CTB \(K_{\text{lc}}\)—average mode I fracture toughness from CTB tests. DA—decreasing amplitude (DA \(=\frac{\text{MTB}_{\text{lc}}\text{-}\text{CTB}_{\text{lc}}}{\text{MTB}_{ \text{lc}}}\times 100\%\)). \({}^{\ast}\) The maximum cycle number is 200.  
\end{table}
Table 3: Mode I fracture toughness acquired during CTB tests.