## 4 Experimental Tests of the Polymer Liner
### DIC Method  
As an additional measurement method, a 3D DIC system was used. The test setup is shown in Figure 8. The liner, after the placement of the sensors, was covered with a speckle pattern. The surface around the FBG sensors was cleaned so that the paint would not influence the sensor readouts. DIC system analysis was performed on a one-sided view of half the liner (one dome and 1/3 part of the cylinder part, dashed line in Figure 8).  
The cameras for the DIC measurement system were mounted on a tripod, approximately 120 cm away from the vessel. The 3D DIC system was composed of two AVT Stingray 16 Mpx cameras, equipped with lenses with a focal length of 50 mm and a PC using  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Direction** & **Location** & **Short Name** & **Description** \\ \hline \multirow{3}{*}{Circumferential: C} & \multirow{3}{*}{Cylinder: C} & CC1 & Cylindrical part, circumferential, 130 mm from middle \\ \cline{3-4}  & & CC2 & Cylindrical part, circumferential, central \\ \cline{3-4}  & & CC3 & Cylindrical part, circumferential, symmetrical to CC1 \\ \hline \multirow{3}{*}{Longitudinal: L} & Dome: D & LD1 & Dome, radial \\ \cline{2-4}  & & LC1 & Cylindrical part, longitudinal, 190 mm from middle \\ \cline{3-4}  & & LC2 & Cylindrical part, longitudinal, central \\ \cline{3-4}  & & LC3 & Cylindrical part, longitudinal, symmetrical to LC1 \\ \hline \multirow{3}{*}{Circumferential: C} & \multirow{3}{*}{Dome: D} & CD1 & Dome, 7 mm from boss, circumferential, placed next to \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \cline{3-4}  & & & gas bubbles \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of the placement of optical strain sensors.  
Figure 8: Experimental setup for liner testing using the DIC method and FBG sensors. Dashed line shows the part of the liner inspected using the DIC method.  
Istra4D software. A typically obtained set of results from a single 3D DIC measurement was referenced to the Cartesian coordinate system and included the shape in the form of a cloud of points (x,y,z), two components of in-plane displacements (U(x,y), V(x,y)), and an out-of-plane displacement map (W(x,y)).  
The strain maps were calculated from the displacement maps in the 'xy' plane. Since the radii of curvature of a liner geometry were large and analysis was performed on a one-side view of the liner, it was assumed that \(\varepsilon_{xx}\) strain was strain along the circumferential direction, while \(\varepsilon_{yy}\) was in the longitudinal direction (along generation line). The validity of this approach was proven in [31].  
To compare the results of the 3D DIC measurements and the FBG measurements, virtual extensometers were used. They were located in the displacement maps at the FBG locations. Their strain was calculated as the average strain between the sensor attachment points.  
The experimental setup for the DIC test is presented in Figure 8. During the experiment, the liner was internally loaded with compressed air, with a pressure range from 0 to 1.3 bar. The pressure increase was controlled by a pressure gauge and a control valve.