## 4 Results  
The mean tensile strength of the tested material was 815 MPa, while the mean yield strength at 0.2% plastic elongation was 745 MPa. The standard deviations of tensile strength and yield strength were 14 MPa and 50 MPa, respectively. Figure 6 presents the obtained engineering stress-strain curve for all specimens. It is worth mentioning that the scatter of stress increases after reaching maximum force. A similar effect (increased dispersion of test results after reaching maximum force) can be found in the paper [32] for DC04 low-carbon steel. Additionally, statistical analysis of the tensile test of S355 steel can be found in [33], where the standard deviation for 1089 specimens was 25.1 MPa and 25.4 MPa for tensile strength and yield strength, respectively. However, min. and max. values were 461-665 MPa for the tensile strength and 325-483 MPa for the yield strength. Figure 4: Testing equipment diagram used for the tests. Figure 5: The geometry of the specimens used for tensile tests. S355J2+C steel does not have a clear yield strength, which was expected for the material after plastic deformation such as rod drawing (more details about it can be found in [34; 35; 36]). The obtained values meet the requirements of PN-EN 10,277 [37], which are as follows: tensile strength 630 \(\pm\) 950 MPa and yield strength min. 520 MPa. The fatigue test results are shown in Figures 7-9. The solid lines show the characteristics of a 50% probability of failure. The dashed lines indicate characteristics for a confidence level of \(\alpha\) = 5%. The power value of determining the fatigue characteristics \(1-\beta\) according to Formula (7) and the error according to Formula (5) are shown in Table 3. A comparison of characteristics is shown in Figure 10. The power of the test was calculated to detect differences in mean values of 15%, while the standard deviation \(\sigma\) was taken for characteristics \(n\) = 30 of 0.183. Figure 6: Tensile engineering stress-strain curves for S355 J2+C. Figure 7: S–N characteristics for S355J2+C steel for 9 specimens. Figure 8: S-N characteristics for S355J2+C steel for 21 specimens. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Number of** & **Slop Coeff.** & **Intercept** & **Standard** & **Confidence** & **Probability of** & **Power of** & **Error** \\
**Specimens** \(n\) & \(m\) [\(-\)] & \(c\) [\(-\)] & **Dev.** \(\sigma\) [\(-\)] & **Level \(\alpha\) [\%]** & **Error \(\beta\) [\%]** & **Test \(M\) [\(-\)]** & \(\delta\) [\%]** \\ \hline
9 & \(-10.16\) & 32.2 & 0.108 & 5 & 62.7 & 0.372 & 6.3 \\
9 & \(-10.16\) & 32.2 & 0.108 & 10 & 49.2 & 0.508 & 6.3 \\
21 & \(-9.91\) & 31.3 & 0.175 & 5 & 12.2 & 0.737 & 5.0 \\
21 & \(-9.91\) & 31.3 & 0.175 & 10 & 16.7 & 0.833 & 5.0 \\
30 & \(-9.68\) & 30.8 & 0.183 & 5 & 12.2 & 0.878 & 3.8 \\
30 & \(-9.68\) & 30.8 & 0.183 & 10 & 6.8 & 0.932 & 3.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Value of the estimation error and the power of the statistical test of S-N characteristics. Figure 9: S-N characteristics for S355J2+C steel for 30 specimens. The first test was made on 21 specimens. At the beginning of the test, one specimen was tested on the stress amplitude level \(Sa\). Then, the preliminary slope coefficient \(m\) was determined. In the next step, stress level intervals were defined for the full test to determine the \(S\)-\(N\) curve equal to 45 MPa. The different slope coefficient \(m\) leads to different stress level intervals, which was discussed in the paper [38]. This is why, in Figure 8, there are few single specimens between stress amplitude levels. However, the test for nine specimens was made later, so stress levels were known. For this test, there are no single specimens between stress levels.