## 3 Mechanical Properties
### Influence of Local Random Pitting Rate  
Using the methods proposed in Section 2, local random pitting models in which \(\lambda=25\) were employed, and the ultimate strength \(F_{u}\) of the models was calculated with the method in Section 3.1. Figure 12 shows the deformation modes of the models where local pitting was randomly generated in regions 1-8. Considering the randomness of the size and the location of the pits, the lower limit of ultimate strength in a 95% confidence interval was taken as \(F_{u}\) of the specimens corresponding to each pitting corrosion rate \(P_{0}\) and corrosion region. Figure 13 shows the regression results of the variation trend of \(F_{u}\) using the least-squares method with variable \(P_{0}\) in different corrosion regions.  
Figure 10: Deformation of the specimen. (**a**) \(\lambda=12.22\), (1) Experiment [20], (2) [PERSON]’s FEM result [20], and (3) FEM result of this paper; (**b**) \(\lambda=25\), (1) [PERSON]’s FEM result [20], and (2) FEM result of this paper.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(\lambda\) & \begin{tabular}{c} **Experimental** \\ **Result** \\ \end{tabular} & \begin{tabular}{c} **[PERSON]’s FE** \\ **Result** \\ \end{tabular} & \begin{tabular}{c} **FEM Result** \\ \end{tabular} & \
\begin{tabular}{c} **Error 1** \\ \end{tabular} &
\begin{tabular}{c} **Error 2** \\ \end{tabular} \\ \hline
12.22 & 2424 & 2384.3 & 2446.1 & 0.90\% & 2.60\% \\
25 & - & 2315.6 & 2237.2 & - & 3.40\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: The ultimate strength of the specimen.  
Figure 12: Deformation modes of models.  
For regions 1-3, \(F_{u}\) decreased linearly with the increase in pitting corrosion rate \(P_{0}\). The minimum value of \(R^{2}\) was 0.979, which showed an apparent linear trend. The slope of the fitted results took values in the range [\(-22.9\), \(-19.1\)] with a mean value of \(-21.1\), and the intercept took values in the range [\(2209.6\), \(2298.9\)] with a mean value of \(2248.7\). The variation coefficients \(\gamma_{s}=9.1\%\) and \(\gamma_{i}=2.0\%\) indicated a high trend consistency of \(F_{u}\) with \(P_{0}\) in regions 1-3. The difference between the mean value of the intercept and the \(F_{u}\) of the non-pitting specimen was 0.5%, which showed the rationality of the fitting result. For region 4, \(F_{u}\) decreased linearly with the increase in \(P_{0}\), with \(R^{2}=0.970\). The difference between the mean value of the intercept and the \(F_{u}\) of non-pitting specimen was 1.0%.  
Figure 13: Ultimate strength with variable \(P_{0}\) in different corrosion regions.  
For region 5, \(F_{u}\) showed a secondary parabolic decreasing trend with the increase in \(P_{0}\), with \(R^{2}\) = 0.995. For regions 6-8, \(F_{u}\) remained constant when corrosion rate \(P_{0}\) < 8%, which meant the low \(R_{0}\) had little effect on \(P_{u}\) when pitting occurred in regions 6-8. \(F_{u}\) decreased linearly with the increase in \(P_{0}\) when \(P_{0}\) > 8%, with \(R^{2}\) > 0.987. The slope of the fitted results took values in the range [\(-\)29.1, \(-\)27.6] with a mean value of \(-\)28.3, and the intercept took value in the range [2442.2, 2458.5] with a mean value of 2450.3. The variation coefficients \(\gamma_{s}\) = 2.7% and \(\gamma_{i}\) = 0.3% showed a high trend consistency. This paper holds that the decrease in the ultimate strength of the specimen was caused by the stress concentration and section weakening caused by pitting corrosion. With the increase in pitting corrosion rate, the influence of stress concentration on ultimate strength was less sensitive than that of section weakening.