## 2 Materials and Methods
### Mechanical Testing  
The test fixture, illustrated in Figure 3 and produced using a lathe machine, consists of two semicircular metal plates (D-blocks) connected to the upper and lower arms of the fixture with pins. During testing, the bending moment generated at the junction of the split-disk test fixtures primarily affects the apparent tensile strength rather than the true tensile strength of each test [13]. Consequently, the test fixture was specifically designed to minimize the influence of this bending moment. The experimental tests were carried out using the Walter Bai LF300 Walter+Bai AG, (Lohningen, Switzerland) universal testing machine with the capacity of 300 kN, as shown in Figure 4, at a constant strain rate of 3 mm/min. All tests were performed at room temperature. The data were continuously monitored by the computer and data acquisition system until the specimen attained its ultimate tensile strength (UTS) and subsequently fractured. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Test no.** & **Temperature, \({}^{\circ}\)C** & **Environment** \\ \hline
1 & 50 & saltwater \\
2 & 50 & alkaline solution \\
3 & 20 & saltwater \\
4 & 20 & alkaline solution \\
5 & 50 & air \\
6 & 20 & air \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Testing conditions. Figure 2: Samples immersed at high temperature. Figure 4: Circumferential tensile testing. Figure 3: Split-disk tensile test system. The ultimate tensile strength was determined with the formula (1) [20]:  
\[\sigma=\frac{F}{2\cdot A}=\frac{F}{2\cdot b\cdot t} \tag{1}\]  
where \(F\)--measured force, N; \(A\)--section area in the calibrated area, mm\({}^{2}\); \(b\)--width in the calibrated area, mm; \(t\)--pipe wall thickness, mm. The tensile testing machine measured the force, \(F\), necessary for the calculation of the tensile strength in the circumferential direction. The procedure followed during the split-disk tests involved several key steps to ensure accurate measurements and results. Firstly, the reduced section dimensions of the specimens were measured using digital calipers. Thickness measurements were taken at four points on each specimen, two of which were located in the gauge sections, while the width of the reduced sections was also recorded. These measurements allowed for the calculation of the reduced section areas, using the minimum thickness and width values. Next, the specimens were mounted onto the split-disk test fixture, ensuring that the reduced sections were aligned with the split in the fixture. Once mounted, the testing machine was set to a constant loading rate, and the test was initiated. During the test, load and strain data were continuously recorded until the specimen failed. These data were then used to generate stress-strain curves for each specimen. After the tests, the ultimate hoop tensile strength and hoop tensile modulus values were calculated by averaging the measurements from all specimens in each test group. Standard deviations were also computed and reported to reflect the variability within the test groups, providing a comprehensive understanding of the material's performance under the tested conditions. To determine the Poisson's ratio for the investigated samples, strain gauges were attached to the surface of each specimen in two directions to measure both longitudinal (\(\varepsilon_{x}\)) and transverse strains (\(\varepsilon_{y}\)), as illustrated in Figure 5a. The strain gauges used had a gauge factor of \(k=2.19\) and a resistance of 350 \(\Omega\), with data recorded using the MGCplus acquisition system (Figure 5b). Figure 5: Strain gauge measurements: (**a**) strain gauge connection; (**b**) data record. The uncertainty range for each measured property was calculated using the standard deviation of the measured values. For each experimental condition, three measurements were taken, and the standard deviation was determined to quantify variability. This approach provides a statistical representation of the uncertainty associated with the measurements.