# Reliability Study on FRP Composites Exposed to Wet-Dry Cycles
## 2 Materials and Methods  
### Specim Preparation  
A total of 48 FRP samples, including 24 CFRP samples and 24 GFRP samples, were designed as shown in Table 1. The CFRP samples were cut from a uni-directional sheet with a unit weight of 300 g/m\({}^{2}\) made of carbon fibers. The nominal thickness, length and width of the CFRP were 0.111 mm, 230 mm and 15 mm, respectively, as shown in Figure 1. The GFRP samples were cut from a glass fiber sheet with a unit weight of 600 g/m\({}^{2}\) and the nominal thickness was 0.169 mm. The length and width of the GFRP were the same with the CFRP samples. The type of glass was E-Glass. WSX epoxy resin was used as the bond layer, which is a two-part epoxy saturant mixed resin A and hardener B in a 4:1 weight ratio. The mechanical properties of the WSX resin provided by the manufacturer show that its tensile strength, Young's modulus, compressive strength, flexural strength and elongation are more than 40 MPa, 2.5 GPa, 70 MPa, 50 MPa and 1.5%, respectively. These mechanical properties of resin are fully satisfied with the requirements of the code named: Safety appraisal code for application of engineering structural strengthening materials GB 50728-2011 [27], that limits the minimum values of 30 MPa, 2.5 MPa, 65 MPa, 45 MPa and 1.2%, respectively. The prepared epoxy resin had a pot life of 45 min and a full cure time of 7 d at 25 \({}^{\circ}\)C from the production statement. The glass transition temperature \(T_{\text{g}}\) was about 105 \({}^{\circ}\)C. The resin will have viscous flow properties when the service temperature exceeds \(T_{\text{g}}\) and the micro-mechanical viscoelastic properties of FRP composite materials will need to be considered [28; 29]. In the test period, the range of the water temperature and ambient temperature (test temperature) is 0 \({}^{\circ}\)C-33 \({}^{\circ}\)C, which is far less than \(T_{\text{g}}\). Therefore, the effect of temperature on the mechanical properties of FRP is neglected in this study. The ends of the FRP samples are attached to a reinforcing aluminum gasket to apply the load. The width, length and thickness of the aluminous gasket are 15 mm, 50 mm, and 2 mm. The aluminum surfaces need to be roughened to ensure a good fit with FRP before bonding. At the same time, the aluminum gasket must be chamfered to reduce the shear stress and peeling stress near the end of the test piece, thus preventing the specimen from breaking near the end. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Wet-Dry**} & \multirow{2}{*}{**Numbers**} & \multicolumn{2}{c}{**Nominal**} & \multicolumn{2}{c}{**Ultimate**} & \multicolumn{1}{c}{**Young’s**} \\  & & **Cycles/days** & & **Thickness/mm** & **Strength/MPa** & **Modulus/MPa** & **Elongation/\%** \\ \hline CFRP-0 & 0 & 6 & 0.111 & 3947 & 2.36 \(\times\) 10\({}^{5}\) & 1.67 \\ CFRP-90 & 90 & 6 & 0.111 & 3765 & 2.40 \(\times\) 10\({}^{5}\) & 1.57 \\ CFRP-180 & 180 & 6 & 0.111 & 3614 & 2.40 \(\times\) 10\({}^{5}\) & 1.51 \\ CFRP-360 & 360 & 6 & 0.111 & 3579 & 2.34 \(\times\) 10\({}^{5}\) & 1.53 \\ GFRP-0 & 0 & 6 & 0.169 & 2003 & \(9.71\times\) 10\({}^{4}\) & 2.06 \\ GFRP-90 & 90 & 6 & 0.169 & 1880 & \(9.67\times\) 10\({}^{4}\) & 1.94 \\ GFRP-180 & 180 & 6 & 0.169 & 1841 & \(9.52\times\) 10\({}^{4}\) & 1.93 \\ GFRP-360 & 360 & 6 & 0.169 & 1763 & \(9.38\times\) 10\({}^{4}\) & 1.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The properties of FRP under different wet-dry cycling exposures. The prepared FRP samples were fully immersed in a sink filled with 5% NaCl water, which simulated seawater, for 12 h and then accelerative blow-dried for another 12 h per cycle. The wet-dry cycle is achieved by using a water pump to pump water from one sink to another, and pumping the water back to the first tank 12 h later. The blow-dried conditions were used to increase the flow of wind speed to accelerative specimens drying. For reference, 90, 180, and 360 wet-dry cycles were considered.