## 4 Discussion on Comparison of NDT Techniques
However, it is observed that the guided wave contact-to-contact method has high variation in defect sizing for different defects: while defect number 3 has been sized with less than 1% error, others show more than 20% relative error. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**NDT Technique** & **Value** & **Defect \#1** & **Defect \#2** & **Defect \#3** & **Defect \#4** & **Defect \#5** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Immersion \\ Through-Transmission \\ \end{tabular} } & Size, mm & 12.48 & 12.61 & 13.28 & 13.35 & 13.00 \\ \cline{2-6}  & Error, mm & 0.22 & 0.09 & 0.58 & 0.65 & 0.30 \\ \cline{2-6}  & Error, \% & 1.70 & 0.66 & 4.60 & 5.17 & 2.38 \\ \hline \multirow{3}{*}{\begin{tabular}{c} Immersion \\ Pulse-Echo \\ \end{tabular} } & Size, mm & 13.83 & 12.78 & 13.04 & 14.25 & 12.89 \\ \cline{2-6}  & Error, mm & 1.13 & 0.08 & 0.34 & 1.55 & 0.19 \\ \cline{2-6}  & Error, \% & 8.92 & 0.62 & 2.65 & 12.21 & 1.46 \\ \hline \multirow{3}{*}{\begin{tabular}{c} Scanning Acoustic \\ Microscopy (SAM) \\ \end{tabular} } & Size, mm & 13.19 & 12.91 & 13.23 & 13.85 & 12.32 \\ \cline{2-6}  & Error, mm & 0.49 & 0.21 & 0.53 & 1.15 & 0.38 \\ \cline{2-6}  & Error, \% & 3.93 & 1.69 & 4.24 & 9.05 & 2.95 \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} Air-Coupled \\ Through-Transmission \\ \end{tabular} } & Size, mm & - & - & 13.18 & 16.03 & 15.29 \\ \cline{2-6}  & Error, mm & - & - & 0.48 & 3.33 & 2.59 \\ \cline{2-6}  & Error, \% & - & - & 3.78 & 26.19 & 20.36 \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Guided Waves \\ Air-Coupled to \\ Contact \\ \end{tabular} } & Size, mm & - & 16.98 & 21.46 & 14.86 & - \\ \cline{2-6}  & Error, mm & - & 4.28 & 8.26 & 2.16 & - \\ \cline{2-6}  & Error, \% & - & 33.70 & 69.01 & 17.00 & - \\ \cline{2-6}  & Size, mm & - & 4.82 & 12.65 & 15.48 & 9.61 \\ \cline{2-6}  & Error, mm & - & 7.88 & 0.05 & 2.78 & 3.09 \\ \cline{2-6}  & Error, \% & - & 62.03 & 0.39 & 21.95 & 24.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The measured defect dimensions and relative errors for different ultrasonic NDT techniques over five debonding defects. Figure 20: Decibel drop method (\(-6\) dB and \(-12\) dB) to measure the defect size for different ultrasonic NDT techniques: (**a**) immersion ultrasonic through-transmission; (**b**) immersion ultrasonic pulse-echo; (**c**) scanning acoustic microscopy; (**d**) air coupled ultrasonic testing through-transmission; (**e**) guided wave ultrasonic testing with air-coupled to contact; (**f**) guided wave ultrasonic testing with the contact-to-contact method. While the performance in defect detection and defect sizing of the immersion ultrasonic techniques including SAM is significantly higher than of the air-coupled and contact NDT techniques; the applicability aspects should be discussed. Immersion ultrasonic NDT applications are limited because structures being under water makes them susceptible to corrosion. Additionally, open-wound defects, such as impact damages, are not recommended to undergo immersion testing. In the case of SAM, while the inspection time is shorter, any irregularity in structure geometry such as curved surfaces significantly affects the inspection performance. While air-coupled through-transmission system provides a clear visualization of the inner structure, the amplitude loss in the air makes the experimental set-up very sensitive to structure boundaries and environmental error. Hence, the experiments require higher averaging which leads to a time cost, nevertheless air-coupled experiments would be preferred in an industrial set-up. Furthermore, guided wave (GW) inspection gives us the possibility to inspect large bonded structures in a short time. GW air-coupled to contact transducer set up shows that only one side contact to the bonded structure might be enough to determine debonding defects. When compared to the air-coupled to contact transducer guided waves, two-contact transducers method can give results with significantly higher amplitude and lower transmission losses because of better coupling.