## 4 Experimental Results
### Tensile Tests of Unconditioned and Aged Specimens  
The experimental results related to the tensile properties for a total number of 20 unconditioned reference specimens are shown in Table 3. As it can been observed the standard deviation and  
Figure 5: Fatigue testing (**a**) Fatigue testing machine; (**b**) GFRP specimen under test. Figure 6: Test protocol for fatigue cycles with different amplitudes. coefficient of variation are within a range of about 10% for the elastic modulus and about 20% for the tensile strength. This can be considered compatible with the experimental error. The typical failure modes and details of fiber breakage are reported in Figure 7.  
In all cases the tensile failure of the fibers was accompanied by an intra-laminar cracking occurred in the polymeric matrix that developed along the direction of the applied load. The breakage of the glass fibers occurred in different positions, even if the complete failure was mainly located in those regions nearby the nodal junctions. The stress-elongation behavior is linear or pseudo-linear up to failure, as shown in Figure 8. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen** & \begin{tabular}{c} **Young’s Modulus** \\ **E (GPa)** \\ \end{tabular} & \begin{tabular}{c} **Ultimate Load** \\ **P\({}_{\text{ult}}\) (KN)** \\ \end{tabular} & \begin{tabular}{c} **Tensile Strength** \\ **\({}_{\text{ult}}\) (MPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Elongation at** \\ **Failure (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Cross Section** \\ **(mm\({}^{2}\))** \\ \end{tabular} \\ \hline  
Average Value & 29.48 & 4.67 & 505.14 & 1.76 & 9.2 \\ Standard & 3.00 & 0.91 & 98.03 & 0.37 & - \\ Deviation & & & & & \\ COV (\%) & 10.19 & 19.41 & 19.41 & 21.04 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Tensile tests results for unconditioned specimens. Figure 7: Typical tensile failures of GFRP specimens. (**a**) and (**b**) near the joint; (**c**) tensile failure; (**d**) detail of the specimen’s section after tensile failure. Aged specimens subjected to alkaline environment exhibited the same failure modes and the same linear behavior in terms of stress-strain curves. The elastic modulus and ultimate tensile properties are shown in Table 4.  
Also in this case the standard deviation and coefficient of variation values were maintained within the range of the experimental error. From a comparison between aged and reference specimens no reduction was found in terms of elastic modulus, while a slight reduction of 18% was found in terms of tensile strength. This can be attributed to the weakening of the fibers that were put in contact with the alkaline ions, and at the same time to the damage of the matrix that was not totally impermeable to the diffusion of the harsh aqueous solution. The diffusion of alkaline ions and the breaking of the Si-O-Si bond, which is the main structure of the glass fiber, causes the damage of the fibers. The degradation mechanisms affecting glass fibers are different and combined. A filament corrosion due to OH\({}^{-}\) ions may occur, moreover a sort of static fatigue can cause the breaking due to the growth of existing surface defects, related to the OH\({}^{-}\) ions attack, furthermore a growth of densification products due to the precipitation of crystals in the interstices between the filaments of the fiber may cause a localized damage. Static fatigue failure is the most common mode. In general, fiberglass damage is closely related to the chemical composition of the alkaline solution to which it is subject. As for the matrix, it is important that it does not undergo degradation in such a way as to protect and transmit the loads to the fibers. However, the matrix, if subjected to alkaline solution, may undergo plasticization and/or swelling due to its diffusion inside, resulting in a reduction in the matrix's ability to perform its role. After the performance of this testing