## 5 Results
### Strain Development
This is markedly different to the plateau-shape of the OBR-curve of the 10 cm sample. During Stage V, the OBR readings decrease and the OBR-curves develop a \"bathtub\"-shape. The average \"bottom\"-strain-value of this \"bathtub\" is for all samples between \(-0.08\%\) and \(-0.12\%\). During Stage VI (lower row in Figure 5), the strain values decrease further and reach values more than an order of magnitude lower then at the end of Stage V. For the 10 cm sample the shape of the strain curves is outlined in Figure 3. However, the strain along the length of the 60 cm vessel develops a distinct \"W\"-shape. On the one hand, the strain at the sides of the vessel develops approximately as for the 10 cm sample. On the other hand, the strain in the core area decreases much less and a plateau-like structure forms between ca. 100 mm and 450 mm. Both effects lead to the appearance of valley-like structures between ca. 18 mm and 50 mm (mirrored at the other side of mold). Figure 6 shows the final strain state of all the experiments stated in Table 1 after the the samples cooled down to room temperature. Table 3 summarizes the key characteristics of these curves. Due to the symmetry of the setup, all strain curves are in general mirrored at the middle axis. For all samples, the strain curve shows an initial negative gradient up to ca. 2 cm into the sample. The slope of this gradient is \(-0.061\%\) per millimeter for the 10 cm sample. On average, this value approximately doubles for the other samples. In the 10 cm sample, the strain along the length of the axis develops as expected and outlined in Figure 3. After the steep initial gradient over the first 2 cm, the strain decreases significantly less quickly to reach the \"bottom of the bathtub\" with an average value of \(-1.41\%\) in the middle area (core) of the sample. However, the strain in all samples with a length larger or equal to 20 cm develops markedly different. After the initial slope, a valley-like structure appears with a mean length of ca. 2 cm. In these valleys, the largest negative strain values occur which are remarkable equal for all  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Moid Length** & **1 st Gradient Slope** & **Length of Valley** & **Minimum Strain** & **2 nd Gradient Slope** & **2 nd Gradient Length** & **Plateau Mean-Strain Value** \\ \hline
10 cm & \(-0.061\%\)/mm & - & \(-1.43\%\) & - & - & \(-1.41\%\) \\
20 cm & \(-0.111\%\)/mm & 15 mm & \(-1.69\%\) & +0.018\%/mm & 35 mm & \(-0.97\%\) \\
40 cm & \(-0.142\%\)/mm & 35 mm & \(-1.71\%\) & +0.008\%/mm & 50 mm & \(-larger samples (cf. Table 3). With a mean value of \(-1.70\%\), it is comparable to the value for the \(10\,\mathrm{cm}\) sample. A second positive gradient follows this valley. This second gradient goes over a mean length of \(4.5\,\mathrm{cm}\) and exhibits a slope between \(+0.008\%\)/mm and \(+0.018\%\)/mm. Finally, the strain develops into a plateau-like structure with a mean strain value of \(0.96\%\). It is to be noted that the deterioration of the strain values towards the exit point of the fiber for the \(40\,\mathrm{cm}\) and the \(60\,\mathrm{cm}\) sample is a manifestation of the exceptional physical conditions the optical fiber is exposed to, as mentioned in Section 2. These conditions lead to microbending of the fiber, which in return results in a diminishing backscatter signal amplitude after the fiber enters the structure. The second steep strain gradient at the exit point of the fiber has a further reduction of the backscatter signal as result and no meaningful strain values can be determined afterwards from the raw data. This mechanism however is outside the scope of this paper and interested readers are referred to the discussion of the details of the applied method in [29]. However, since this affects the strain values just after it exits the structure, it has no influence on the general application of the measurement method.