## 5 Results and Discussion
### Deformation Measurement of the Liner
#### 5.2.2 Strain Field Distribution by DIC  
The measurements taken using the DIC method were conducted in parallel to the measurements with FBG sensors while pumping air into the liner. The deformation field analyzed using the 3D-DIC system included a fragment of the dome and a fragment of the cylindrical part. The recorded deformation fields, \(\varepsilon_{xx}\) and \(\varepsilon_{yy}\), at 1.0 bar are presented in  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location/Sensor** & **1** & **2** & **3** & **4** \\
**Number** & & & & \\ \hline CC & 5460 \(\mu\varepsilon\) & 5530 \(\mu\varepsilon\) & 7080 \(\mu\varepsilon\) & - \\ \hline LC & 480 \(\mu\varepsilon\) & 250 \(\mu\varepsilon\) & 740 \(\mu\varepsilon\) & - \\ \hline CD & 300 \(\mu\varepsilon\) & 270 \(\mu\varepsilon\) & 470 \(\mu\varepsilon\) & \(-\)20 \(\mu\varepsilon\) \\ \hline LD & 2770 \(\mu\varepsilon\) & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Strain registered by FBG sensors for 1-bar internal pressure.  
Figure 11: Deformation registered with selected FBG sensors. (**a**) Over time and (**b**) over pressure. Arrows show the direction of pressure increasing/decreasing.  
Strain in the circumferential direction, \(\varepsilon_{xx}\). (Figure 12a), had the highest value of around 7000 \(\mu\)\(\varepsilon\) on the cylinder part, closest to the middle of the liner (the bottom part of the presented photos). The circumferential strain of a dome was significantly smaller (\(<\)1000 \(\mu\)\(\varepsilon\)). Measurement shown area with negative circumferential strain in the dome area (amplitude up to 100 \(\mu\)\(\varepsilon\)). Strain in longitudinal direction \(\varepsilon_{yy}\) (Figure 12b) had maximum values on the dome (about 3500 \(\mu\)\(\varepsilon\)).  
In Figure 12, three FBG sensors are visible, which allows validating both methods. Results of FEM (bottom of Figure 12) are in accordance with the DIC results for both \(\varepsilon_{xx}\) and \(\varepsilon_{yy}\) directions.  
The observation of compression in the circumferential direction was confirmed using FEA. Deformation of the liner from DIC and FEA along the generating line (marked in Figure 12) is shown in Figure 13.  
Figure 13 shows good correlation between results from DIC and FEM for both \(\varepsilon_{xx}\)\(\varepsilon_{yy}\) and direction. Compression in circumferential direction was visible for both methods. The area of this behavior is present on the dome, close to the cylinder. Displacement around this area has almost two times lower deformation as the maximum deformation of cylinder and boss. It means that this behavior can be interpreted as the presence of the liner's collapse, which can potentially be a weak spot during vessel operation.  
Figure 12: Deformation field in the circumferential and longitudinal direction at 1-bar pressure; (**a**) results of DIC tests and (**b**) FEM calculation.  
Figure 12: Along with the strain field obtained from the DIC method, the results from FEM are shown.  
It is worth noticing the visible non-homogeneity of the strain field. Some regions, even on the same cross-section, show non-uniform strain, which is attributed to thickness variations. Sensors attached to the surface of a liner could be placed in the areas where local strain non-homogeneity occurred. As a consequence, average strain measured by a sensor could be different in some local spots. This can be an explanation for the differences in the measurement results in Table 2.