# Sewater Effect on Fatigue Behaviour of Notched Carbon/Epoxy Laminates
## 3 Results
This evidences that the impact of the open holes (and respective stress concentration induced) is superior to the effect of exposure to seawater. However, in short lives, the impact of immersion into seawater significantly affects the fatigue strength. For example, considering a fatigue life of \(10^{4}\) cycles, the stress amplitude was 1.2 times higher for dry laminates (control samples) than those immersed in seawater. These results agree with the studies developed by [PERSON] et al. (2016), in which the effect of water on the fatigue strength was dependent on the stress level, as in the  
Figure 4: Effect of immersion time on the fatigue life for: (**a**) natural seawater and (**b**) artificial seawater. Figure 5: Effect of seawater type on fatigue life for: (**a**) 30-day immersion; (**b**) 60-day immersion. present study. For example, the wet fatigue strength was 20-25% lower at high stresses, and at high cycles, the wet fatigue strength was 8% lower [36]. [PERSON] et al. [11] found a decrease in tensile strength of about 20% to 40%. Still, no significant effect was observed in terms of elastic modulus, which was explained by the lower interfacial adhesion that was assessed by the ILSS tests. [PERSON] et al. [3] observed, for bending fatigue tests, a significant decrease of the fatigue life due to the combination of edge effect and capillary effect. [PERSON] and [PERSON] [37], for example, observed that the fatigue life of polymeric composites dramatically decreases, by up to 85%, when cyclic loading is applied under immersion conditions, showing a very significant loss of mechanical strength under cyclic loading due to exposure to seawater during the stress cycles. Finally, [PERSON] et al. [38] observed that the moisture diffusivity in CFRP composites immersed in tap water or seawater reveals negligible differences at the same temperature. Moreover, the damage degradation of the fibre/matrix interface in seawater is more significant than in tap water. According to what is reported in the literature and observed in this study, it is possible to conclude that the decrease of fatigue life is a consequence of the seawater absorption, promoting both matrix cracking and the matrix/fibre interface. To consolidate this evidence, Figure 6 shows the water uptake curves as a function of the immersion time for both solutions (natural and artificial seawater). The percentage weight of water absorbed was evaluated considering the following equation:  
\[Water\ absorption\ (\%)=\left(\frac{m_{W}-m_{D}}{m_{D}}\right)\times 100 \tag{1}\]  
where \(m_{W}\) is the wet weight and \(m_{D}\) is the dry weight. It was possible to observe that the specimens absorbed water slowly, and the weight remained almost constant after 30 days of immersion; therefore, this shows that saturation had been reached. On the other hand, this figure also denotes no difference between natural and artificial water absorption. Hence, these results support those described above and suggest that fatigue life is similar for natural and artificial seawater. Moreover, the analysed immersion time (30 and 60 days) was irrelevant because the samples reached saturation after 30 days of immersion. According to [PERSON] [39], diffusive and/or capillary processes are the main mechanisms by which moisture can penetrate into polymeric composites. Consequently, in terms of the matrix, reversible and irreversible changes occur in the mechanical properties of thermoset polymers due to water absorption. Plasticisation and swelling are among the most important consequences. On the one hand, the former generally induces plastic deformation and a decrease in the glass transition temperature (Tg). On the other hand,  
Figure 6: Weight of water absorbed by the composite during seawater immersion ageing. the latter is related to the differential strain created by the expansion force exerted by the liquid during the stretching of polymeric chains [40].