## 6 Results and Discussion  
### Influence of the Bolts' Pretension on the Fatigue Strength  
In existing fatigue performance research of M24 high-strength bolts [13], with the help of an MTS fatigue test machine, a normal amplitude fatigue test on 21 M24 high-strength large hexagon head bolts that were of 8.8 grade and made of 35 K steel was carried out. The torque value of the pretension force put on the bolt in the test was from 230 to 360 N\(\cdot\)m. Test loading stress ratio \(R\) was equal to 0.5, \(\sigma_{\text{max}}\) was from 180 to 460, increasing by 10 MP every time to 300 MP and adding 20 MP per level from 300 MP. \(\sigma_{\text{min}}\) was from 90 to 150, increasing by 5 MP to 150 MP, and adding 20 MP per level from 150 MP. The constant stress amplitude fatigue of M24 high-strength large hexagon head bolts for fabricated steel structures was compared. The results of the constant amplitude fatigue test are shown in Table 13.  
In existing fatigue performance research of M24 high-strength bolts [14], the constant amplitude fatigue test of 14 M24 high-strength hexagon head bolts with the help of an MTS fatigue test machine was completed, in which the material (ML20 MnTiB) and strength grade (10.9) of the bolts were exactly the same as those of the test in this paper. The torque value of the pretension force put on the bolt in the test was from 200 to 300 N\(\cdot\)m, and the test loading stress ratio \(R\) was equal to 0.64. The results of normal amplitude fatigue tests in the literature are listed in Table 14.  
In order to compare the constant amplitude fatigue performance of M24 high-strength bolts under different conditions, the relationship between the stress range and fatigue life was investigated. Taking the nominal stress amplitude as the parameter, the power regression curve and S-N curve were established in the same coordinate system, as shown in Figures 10 and 11. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{4}{c}{**Stress/MPa**} & \multicolumn{2}{c}{**Stress Ampli-**} & \multicolumn{1}{c}{**Fatigue**} \\  & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & **Life/Time** \\ \cline{2-6}  & \(\sigma_{max}\) & \(\sigma_{min}\) & \(R\) &
\begin{tabular}{c} **The Torque Value of the** \\ **Pretension Force/(N\(\cdot\)m)** \\ \end{tabular} & \(\Delta\sigma\) & \(N\) \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 13: Constant amplitude fatigue test data of M24 high-strength bolts with large hexagon head. Combined with the data in the fatigue test process analysis in Tables 5-8 and Tables 12-14, taking the stress amplitude as the parameter, the constant amplitude power regression curve \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{3}{c}{**Stress/MPa**} & \multicolumn{2}{c}{**Stress**} & \multicolumn{1}{c}{**Fatigue**} \\  & \(\sigma_{max}\) & \(\sigma_{min}\) & \(\sigma_{m}\) & \(R\) & \(\Delta\sigma\) & \(N\) \\ \hline
1 & 1000 & 640 & 820 & 0.64 & 360 & 21,500 \\
2 & 1133 & 725 & 929 & 0.64 & 408 & 12,800 \\
3 & 430 & 278 & 354 & 0.64 & 155 & 411,700 \\
4 & 450 & 288 & 369 & 0.64 & 162 & 319,000 \\
5 & 470 & 300.8 & 385.4 & 0.64 & 169.2 & 306,900 \\
6 & 490 & 313.6 & 401.8 & 0.64 & 176.4 & 299,200 \\
7 & 800 & 512 & 656 & 0.64 & 288 & 46,000 \\
8 & 650 & 416 & 533 & 0.64 & 234 & 113,000 \\
9 & 600 & 384 & 492 & 0.64 & 216 & 159,300 \\
10 & 550 & 352 & 451 & 0.64 & 198 & 179,200 \\
11 & 400 & 256 & 328 & 0.64 & 144 & 668,200 \\
12 & 350 & 224 & 287 & 0.64 & 126 & 1,305,600 \\
13 & 370 & 236.8 & 303.4 & 0.64 & 133.2 & 732,700 \\
14 & 360 & 230.4 & 295.2 & 0.64 & 129.6 & 915,600 \\ \hline \end{tabular}
\end{table} 
Table 14: Constant amplitude fatigue test data of M24 high-strength bolts with large hexagon head. Figure 11: Comparing fatigue test results of three bolts. Figure 10: Comparing fatigue test results of three bolts.