# Experimental Analysis of a Novel GFRP Stiffened Pipe
## 4 Results and Discussion  
### Stiffness Analysis  
In this paper, 3% displacement of the inner diameter was selected for calculating the initial stiffness of the pipe specimens. Considering the large dimensions of the pipe structures, three specimens for each type of pipe were used for the compression tests to demonstrate the repeatability of their mechanical behavior. The load-displacement (L-D) curves of DN400 mm and DN500 mm are presented in Figures 8 and 9, respectively.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Pipe** & **Pipe Diameter** & **Pipe Middle** & **I** & **Elastic** & **Module (Pa)** & **Stiffness (Pa)** \\
**Thickness (m)** & **(m)** & **Diameter (m)** & **(m\({}^{3}\))** & **(m\({}^{3}\))** & **Module (Pa)** & **Stiffness (Pa)** \\ \hline
0.00844 & 0.4 & 0.40844 & 5.01 \(\times\) 10\({}^{-8}\) & 2.2 \(\times\) 10\({}^{10}\) & 16,176.48 \\
0.00897 & 0.5 & 0.50897 & 6.01 \(\times\) 10\({}^{-8}\) & 2.2 \(\times\) 10\({}^{10}\) & 10,035.57 \\ \hline \end{tabular}
\end{table}
Table 4: Theoretical initial ring stiffness for the plain GFRP pipes.  
Figure 7: Locations of the strain gauge rosettes.  
Figure 8: Load-displacement response of DN400 pipes obtained by the plain type (P1, P2, and P3) and the outer stiffened type (OS1, OS2, and OS3), respectively.  
Figure 9: Load-displacement response of DN500 pipes obtained by the plain type (P1, P2, and P3) and the outer stiffened type (OS1, OS2, and OS3), respectively.  
As depicted in Figure 8, when the loading displacement reached 3% of the inner diameter (12 mm), the loads applied to the outer stiffened pipes were greater than those on the plain pipes. This phenomenon was continuously present before the loading displacement reached 30 mm, indicating that the stiffened pipes had better resistance to flatwise compressive loads. As shown in Figure 9, when the loading displacement reached 3% of the inner diameter (15 mm), there was no significant difference between the load applied to the outer stiffened pipes and the plain pipes. This phenomenon was apparent until the loading displacement reached 75 mm, suggesting that there was no enhancement on pipe stiffness brought about by the inner stiffened pipes. To evaluate quantitively the effects of stiffened structures on resistance to flatwise compressive loads, the ring stiffnesses (\(s_{0}\)) of the tested DN400 and DN500 pipe specimens were calculated by Equation (1), listed respectively in Table 5.  
According to the standard, the tested stiffness for pipes should be no smaller than the theoretical stiffness for the plain type of pipes, i.e., 16,176.48 Pa for DN400 pipes and 10,035.57 Pa for DN500 pipes. As shown in Table 5, the tested stiffness of each specimen was greater than the theoretical, meaning that all the specimens met the stiffness requirements. Comparing the stiffnesses of the plain pipes with those of the outer stiffened ones, we can see that the latter increased by an average of 38.7%. As presented in Table 5, the average ring stiffness of each tested specimen was slightly greater than the theoretical, meaning that all the specimens met the stiffness requirements. Comparing the stiffnesses of the plain pipes with those of the inner stiffened ones, it was observed that the latter were larger than the former by an average of 16.7%, and the latter increased (16.7%) much smaller than the former (38.7%) did. As shown in Table 3, with the same weight of fiber used, the average thicknesses for DN400 specimens of the stiffened and non-stiffened pipes were 11.25 mm and 8.42 mm, respectively, with a difference between the wall thicknesses of 2.83 mm. Those for DN500 specimens were 9.83 mm and 8.97 mm, respectively, with a thickness difference of 0.86 mm. According to Equations (2) and (3), the ring stiffness is proportional to the third power of wall thickness, therefore, the difference between ring stiffness of stiffened and non-stiffened pipes was greater for the D400 specimens than for the D500 specimens.