## 5 Analysis of Experimental Results
### Effects of Depth on the Detectability of Delaminations  
Figure 10 shows the mean values of the SNR of ten different heating regimes. In the graph, the blue horizontal dotted line separates two regions, in which data points above this line represent detectable delaminations, while the other points under the line indicate undetectable delaminations.  
Overall, the same general tendency can be observed in all cases of heating time. SNR decreases quickly with the increase of depth, except for defects located at 8 cm depth, which indicate no tendency. It can be observed that the image contrast drops significantly against the depth of shallow delaminations, while it changes gradually with deep delaminations. For example, considering the 30 min heating regime, the SNR decreases by around 14.9 dB for the depth from 1 to 4 cm, whereas SNR is just 5.9 dB for the depth from (5 to 8) cm.  
On the other hand, shallower deteriorations show more clearly and easily than deeper ones. The heat flux reflects quickly back to the surface, and produces a high signal of contrast, whereas a larger volume of concentrated heat above a deeper delamination diffuses in many directions [39]. This leads to reducing the thermal contrast. For example, the SNR in the heating case of 30 min varies from approximately 25.3 to 2.3 dB, corresponding to the defect depths increasing from 1 to 6 cm.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\
\begin{tabular}{c} **Heating** \\ **Time** \\ **(min)** \\ \end{tabular} } & **BD4** & **BD3** & **BD2** & **BD1** & **FD1** & **FD2** & **FD3** & **FD4** \\  & **(Depth 1 cm)** & **(Depth 2 cm)** & **(Depth 3 cm)** & **(Depth 4 cm)** & **(Depth 5 cm)** & **(Depth 6 cm)** & **(Depth 7 cm)** & **(Depth 8 cm)** \\ \hline \multirow{2}{*}{
\begin{tabular}{c} 2 \\ 3 \\ 4 \\ 5 \\ 10 \\ 20 \\ 25 \\ 30 \\ 40 \\ \end{tabular} } & 12.88 & 1.90 & \(-5.62\) & \(-7.81\) & - & - & - & - \\  & 7.12 & 7.35 & \(-0.93\) & \(-6.11\) & - & - & - & - \\  & 18.48 & 7.05 & \(-0.52\) & \(-3.06\) & - & - & - & - \\  & 18.81 & 9.72 & 2.57 & \(-0.98\) & - & - & - & - \\  & 20.88 & 13.67 & 6.89 & 2.47 & - & - & - & - \\  & 22.42 & 18.38 & 12.53 & 5.18 & \(-4.65\) & \(-6.51\) & \(-8.19\) & \(-9.11\) \\  & 20 & 23.18 & 19.09 & 14.85 & 6.15 & \(0.67\) & \(-0.32\) & \(-2.82\) & \(-0.92\) \\  & 25 & 23.67 & 19.38 & 15.09 & 8.87 & 2.10 & \(1.62\) & \(-2.30\) & \(-1.97\) \\  & 30 & 25.34 & 19.69 & 15.92 & 10.41 & 3.86 & \(2.31\) & \(-2.38\) & \(-2.06\) \\  & 40 & - & - & - & - & \(7.08\) & \(4.94\) & \(17.78\) & \(-0.87\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: The mean value of SNR in heating regime cases.  
Figure 9: Partition of the undetectability and detectability of delaminations in the tests.  
Figure 11 shows the SNR during cooling duration. Fifth-order polynomial fit curves were applied to reduce the noise, and illustrate the tendencies more clearly. For example, four SNR curves of the delaminations from 1 to 4 cm in Figure 11 show the same tendency. After the heat sources are turned off, the SNR curves increase and reach a peak, before dropping. Figure 11 also demonstrates the effects of depth on the detectability of delamination by using the SNR. The peak value of the SNR indicates the best observation of subsurface delamination.