## 7 Conclusions  
This research included a total of 25 constant amplitude fatigue experiments to study the fatigue performance of M24 twisted-shear high-strength bolts. The design method for constant amplitude fatigue based on the nominal stress was established. In order to evaluate the fatigue performance of M24 twisted-shear high-strength bolts exactly, comparisons between fatigue test data obtained from M24 high-strength bolts of grades 8.8 and 10.9 with a large hexagon head under 20-40% pretension force and M20 bolts under full pretension force were completed. The following conclusions could be drawn based on the findings from the above experiments.  
1. Fatigue failure of all M24 twisted-shear high-strength bolts in the test occurred at the root of the high-strength bolt thread, and among them most bolts were damaged at the first circle of the thread between the nut and the bolt, where there was a serious stress concentration, and the fatigue gap effect was significant.
2. The constant amplitude fatigue S-N curve was obtained by test data fitting and the corresponding S-N curve equation of M24 twisted-shear high-strength bolts with the stress range as the design parameter, and the fitting correlation coefficient of the test data was about 0.8. Under the guaranteed rate of 97.72%, the allowable stress amplitude of 5 million fatigue life is about 167.0 MPa; that of 2 million fatigue life is about 181.0 MPa.
3. According to the fatigue S-N curve fitting formula, the value of the fatigue design parameter is 11.419, and the value of the fatigue design parameter C is 12.106 \(\times\) 10\({}^{31}\). It can be seen that when the fatigue life of the bolt is 5\(\times\)10\({}^{6}\), the value of normal amplitude fatigue limit is 167.0 MP, and when the fatigue life of the bolt is 2 \(\times\) 10\({}^{6}\), the value of the allowable stress amplitude is 187.0 MP.
4. It is basically feasible to estimate the fatigue life of high-strength bolts by using the Paris formula, which can generally reflect the changing trend of fatigue life.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**The Type of Bol**} & \multicolumn{1}{c}{**Equation**} & \multicolumn{1}{c}{**The Stress Amplitude/MP (2 Million**} \\  & & **Fatigue Life; the Reliability Level of** & **97.72\%)** \\ \hline
10.9-grade M24 twisted-shear bolt & \(\lg N=32.365-11.419\lg\Delta\sigma-2\times 0.141\) & 167 \\
8.8-grade M24 & \(\lg N=14.697-4.188\lg\Delta\sigma-2\times 0.106\) & 90 \\
10.9-grade M24 & \(\lg N=13.821-3.720\lg\Delta\sigma-2\times 0.056\) & 98 \\ M20 twisted-shear bolt & \(\lg N=27.023-8.348\lg\Delta\sigma-2\times 0.1535\) & 278 \\ \hline \hline \end{tabular}
\end{table}
Table 16: Constant amplitude fatigue S-N curve equation of M24 and M20 high-strength bolts with large hexagon head.  
M24 twisted-shear high-strength bolts with 100% pretension force had better performance in the specified stress range. Under the 97.72% survival probability, the fatigue strength of M24 pretension force high-strength bolts was 1.86 times and 1.71 times that of the 8.8-grade M24 and 10.9-grade M24 large hexagonal head bolts, respectively.
6. By comparing the test results of M24 and M20 bolts under full pretension, it is seen that when the diameter of the bolt decreases, the fatigue strength of the bolt will reduce.