# Sewater Effect on Fatigue Behaviour of Notched Carbon/Epoxy Laminates
## 3 Results  
Fatigue tests at constant load amplitude were performed in tension (R = 0.1), for which the mean load was inputted by software, and the maximum and minimum axial displacements were constantly monitored. In terms of fatigue test results, the S-N diagram was the main tool used to analyse them. This was based on the stress-life approach and represented the fatigue life dependence on stress. In this study, the total separation of the specimens was adopted as the failure criterion. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline \multicolumn{6}{l}{0-day immersion} \\ \hline \(\sigma_{\text{a}}\) (MPa) & 216.0 & 228.4 & 252.9 & 261.1 & 277.4 & 308.6 \\ \hline N\({}_{\text{f}}\) (cycle) & 3,325,233 & 1,177,063 & 374,299 & 138,655 & 62,318 & 18,267 \\ \hline \multicolumn{6}{l}{30-days immersion (natural sea water)} \\ \hline \(\sigma_{\text{a}}\) (MPa) & 221.1 & 244.9 & 252.9 & 261.1 & 277.8 & 293.0 \\ \hline N\({}_{\text{f}}\) (cycle) & 231,596 & 1,065,134 & 373,244 & 10,953 & 2337 & 8521 \\ \hline \multicolumn{6}{l}{30-day immersion (artificial sea water)} \\ \hline \(\sigma_{\text{a}}\) (MPa) & 212.0 & 228.3 & 244.5 & 261.0 & 270.7 & 277.2 \\ \hline N\({}_{\text{f}}\) (cycle) & 3,166,016 & 42,577 & 259,469 & 161,689 & 2282 & 3241 \\ \hline \multicolumn{6}{l}{60-day immersion (natural sea water)} \\ \hline \(\sigma_{\text{a}}\) (MPa) & 212.0 & 228.4 & 244.9 & 261 & 277.2 & 293.9 \\ \hline N\({}_{\text{f}}\) (cycle) & 3,113,244 & 135,405 & 75,449 & 2965 & 11,265 & 1849 \\ \hline \multicolumn{6}{l}{60-day immersion (artificial sea water)} \\ \hline \(\sigma_{\text{a}}\) (MPa) & 212.0 & 228.5 & 244.8 & 260.9 & 277.3 & 309.9 \\ \hline N\({}_{\text{f}}\) (cycle) & 3,243,975 & 3535 & 648,411 & 24,927 & 7396 & 2151 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of fatigue tests. Figure 2: Experimental apparatus used in the uniaxial stress-controlled fatigue tests. The results obtained for the different test conditions are shown in Figure 3, in terms of stress amplitude versus the number of cycles to failure for the several studied cases, namely 0-day immersion (Figure 3a), 30-day immersion in natural seawater (Figure 3b), 30-day immersion in artificial seawater (Figure 3c), 60-day immersion in natural seawater (Figure 3d), and 60-day immersion in artificial seawater (Figure 3e). The mean curves obtained by linear relationship on log-log scales (via the least squares method) of the experimental results are also superimposed. Regardless of the scatter observed, which is typical of fatigue tests, similar behaviour was observed for all curves and, as expected, the fatigue life increased for lower stress amplitude values. Figure 3: S–N curve for the series: (**a**) without seawater immersion, (**b**) 30-day immersion in natural seawater, (**c**) 30-day immersion in artificial seawater, (**d**) 60-day immersion in natural seawater, and (**e**) 60-day immersion in artificial seawater. From all these curves, it is possible to evaluate the effect of exposure time and the type of seawater (natural and artificial seawater) on the fatigue life, comparing the mean curves obtained by the linear relationship of the results obtained. Therefore, this comparison is shown in Figure 4 in terms of exposure time and in Figure 5 in terms of seawater type. From these figures (Figures 4 and 5), it is possible to conclude that immersion in seawater affects fatigue strength. However, considering the scatter observed in Figure 3, it is impossible to report any effect of the immersion time for the analysed values (30 and 60 days), and the type of seawater studied (natural and artificial). Therefore, this allows us to conclude that artificial seawater can replace natural seawater because both lead to similar results in terms of fatigue life for the tested conditions. On the other hand, a more detailed analysis shows that in longer lives, for example, the effect of immersion in seawater can be neglected because all specimens have similar fatigue lives.