## 3 Stress Test and Results
### Test Methods  
Five specimens are used for the static tensile test and the average maximum tensile strength of the material is \(\sigma_{\text{b}}=1100\) MPa. Then, every five test pieces are used together to carry out the fatigue test under different stress amplitudes. The test conditions are: symmetrical cycle R = \(-1\), sine wave loading, and a frequency of 10 Hz. The average number of cycles for each group of experiments is shown in Table 2.  
According to the data in Table 2, the fitting function of the material S-N curve is obtained, in Figure 4 (\(S=(2.318\times 10^{4})\times N^{-0.002028}-2.208\times 10^{4}\)). The S-N curve is used \begin{table}
\begin{tabular}{c c} \hline
**Element** & **Content** \\ \hline C & 0.15–0.21\% \\ Si & 0.40\% \\ Cr & 1.50–1.80\% \\ Mn & 0.50–0.90\% \\ Ni & 1.40–1.70\% \\ P & \textless{}0.035\% \\ Mo & 0.25–0.35\% \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of 18 CrNiMo7-6. According to the obtained S-N curve, the two-stage loading test plan is designed as follows: take the stress value at the inflection point of the curve 520 MPa, for high-low load application conditions; firstly, cycle \(n_{1}\) times with 520 MPa stress (\(N_{f1}\)\(\approx\) 240,000); and then apply a 420 MPa stress (\(N_{f2}\)\(\approx\) 3,200,000) cycle until breaking (corresponding number of cycles \(n_{2}\)). For low-high load conditions, after 420 MPa stress cycle \(n_{1}\) times, apply a 520 MPa stress cycle until breaking (corresponding number of cycles \(n_{2}\)). Every five test pieces are a group, and the average data from the test is shown in Table 3.  
In the sequence of load application, the first high-stress load will have a greater impact on the overall damage as the data in Table 3. In high-low loading, the total damage value \(n_{1}\)/\(N_{f1}\)+\(n_{2}\)/\(N_{f2}\) is less than 1; in low-high loading, the total damage value \(n_{1}\)/\(N_{f1}\)+\(n_{2}\)/\(N_{f2}\) is greater than 1. Unlike in the linear damage accumulation model, the total damage value in the cumulative model is always equal to 1. In order to further analyze the tension-compression fatigue fracture mechanism, the fatigue fracture specimen was sliced and then analyzed by the JSM-5610 LV scanning electron microscope, as shown in Figures 5 and 6. After binarization, the black pixels are in the form of dimples. According to statistics, the pixels in Figure 7 occupy the total selection area of S1 (0.921%), and the pixels in Figure 8 occupy the total selection area of S2 (9.79%). S2/S1 = 10.63. High-low loading significantly promotes the formation of dimples. Binarize the middle part of Figures 5 and 6 to obtain Figures 7 and 8. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Two-Stage Loading**} & \multicolumn{5}{c}{**Test Value**} \\ \cline{2-6}  & \(n_{1}\) & \(n_{1}\)/\(N_{f1}\) & \(n_{2}\) & \(n_{2}\)/\(N_{f2}\) & \(n_{1}\)/\(N_{f1}\)+\(n_{2}\)/\(N_{f2}\) \\ \hline \multirow{4}{*}{520–420} & 60,000 & 0.25 & 1,480,000 & 0.463 & 0.7125 \\ \cline{2-6}  & 120,000 & 0.50 & 850,000 & 0.266 & 0.766 \\ \cline{2-6}  & 156,000 & 0.65 & 324,800 & 0.1015 & 0.7515 \\ \hline \multirow{4}{*}{420–520} & 800,000 & 0.25 & 216,000 & 0.90 & 1.15 \\ \cline{2-6}  & 1,350,000 & 0.42 & 177,600 & 0.74 & 1.16 \\ \cline{1-1} \cline{2-6}  & 2,000,000 & 0.63 & 144,000 & 0.60 & 1.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: 18 CrNiMo7-6 two-stage loading test data. Figure 4: S–N curve. Figure 8: High-low loading fracture surface (520-420 MPa, \(n_{1}\)/\(N_{1}\) = 0.25) binarization. Figure 6: High–low load fracture surface (520–420 MPa, \(n_{1}\)/\(N_{1}\) = 0.25). Figure 7: Low–high load fracture surface (420–520 MPa, \(n_{1}\)/\(N_{1}\) = 0.25) binarization. under high-low loading. The formation of dimples under the repeated action of normal stress accelerates the formation of microscopic voids caused by plastic deformation of the material in a small range during the high-low loading process.