## 3 Results and Discussion  
Figure 5 shows the typical stress-strain curves of the PET fiber bundle specimens at different strain rates. The curves exhibit a bilinear stress-strain relationship and a rapid drop from the peak stress to zero, which indicates a brittle fracture of the PET fiber bundle. The tensile strength, the elastic modulus, the failure strain, and the toughness are sensitive to the strain rate. When the strain rate increased from 1/600 to 160 s\({}^{-1}\), there was an increase in the tensile strength, which can be attributed to the fact that there was not enough time for the defects of filaments in the fiber bundle to develop at a high strain rate [45]. The initial elastic modulus and the second elastic modulus (i.e., slopes of the two portions of the stress-strain curve) increased with an increase in the strain rate from 1/600 to 160 s\({}^{-1}\). Conversely, there was a decrease in the failure strain and the toughness with the increasing strain rate from 1/600 to 160 s\({}^{-1}\). Figure 6 shows the failure modes of the PET fiber bundles at different strain rates. At a low strain rate (e.g., 1/600 s\({}^{-1}\)), the fracture surface of the fiber bundle had a chaotic appearance with different fracture positions of filaments. At a high strain rate (e.g., 160 s\({}^{-1}\)), the fiber bundle ruptured with a relatively trim fractography. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Type of** \\ **Test** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **D \(\times\) H** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Nominal** \\ **Thickness** \\ **of FRP** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Volume** \\ **Fraction** \\ **of Fiber** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Tensile** \\ **Strength** \\ **of FRP** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Number** \\ **of FRP** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Impact** \\ **of** \\ **Strength** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Unconfined** \\ **Concrete** \\ **Strength** \\ **Specimen** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Number of** \\ **Identical** \\ **Specimen** \\ \end{tabular} } \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Drop-weight \\ test \\ \end{tabular} } & 95 \(\times\) 285 & - & - & - & - & - & 2 & 58.5 & 2 \\  & 95 \(\times\) 285 & AFRP & 0.169 & 19\% & 3732 & 1 & 2 & 58.5 & 2 \\  & 95 \(\times\) 285 & PET FRP & 0.841 & 31\% & 751 & 1 & 2 & 58.5 & 2 \\  & 95 \(\times\) 285 & PET FRP & 0.841 & 31\% & 751 & 1 & 3 & 58.5 & 2 \\  & 95 \(\times\) 285 & PET FRP & 0.841 & 31\% & 751 & 1 & 4 & 58.5 & 2 \\  & 95 \(\times\) 285 & PET FRP & 0.841 & 31\% & 751 & 1 & 5 & 58.5 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of specimens for the drop-weight impact test. Figure 4: Drop-weight test setup. Figure 7a shows the failure mode of one layer of the CFRP-confined concrete at a strain rate of 218 s\({}^{-1}\) from SHPB tests. At a strain rate of about 220 s\({}^{-1}\), one layer of the CFRP-confined concrete was smashed together with the external FRP rupture, whereas, due to its very large rupture strain, two layers of the PET FRP-confined concrete could not be destroyed (Figure 7b). Therefore, to investigate the failure mechanism of the concrete confined with the PET FRP, the specimen was repeatedly impacted under the same impact velocity until failure (Figure 7b). As shown in Figure 7b, only a small piece of failure is visible, but the failure mechanism of the PET FRP-confined concrete is not affected by the fracture of the fracture. Figure 5: Tensile stress–strain curves of the PET fiber bundle at different strain rates. Figure 6: SEM images of failure modes at the strain rates of 1/600 s\({}^{-1}\) (**a**); 40 s\({}^{-1}\) (**b**); 80 s\({}^{-1}\) (**c**); 120 s\({}^{-1}\) (**d**); 160 s\({}^{-1}\) (**e**).