## 6 Discussion
For possible applications in an industrial environment such as an aircraft hangar, each technology requires further studies to clarify some individual aspects which will be discussed in the following. For all technologies, a comparison study which includes a digital in parallel to a manual inspection by a certified aircraft technician is advisable. This allows the establishment of validated thresholds that are crucial for the automated classification of fasteners. Besides this, every technique requires some specific fine-tuning prior to further commercial exploration. The SLDV measurements show great potential and enable the distinction between loose, damaged, and intact rivets. However, the coupling of the piezoelectric transducer patch to the fuselage has to be reversible and at the same time provide a reliable connection. As discussed previously, the fundamental LDR resonance frequency of rivets might serve as a parameter that allows a classification of intact and damaged rivets. However, for this, an extended study with sufficient statistics has to establish thresholds that can be used for this classification. Another aspect for further studies is to strive to a fully contactless setting with an air-coupled excitation. This enables fast and automated inspection procedures that can possibly be performed fully remotely by for example, a drone. The presented ULT results do not show sufficient clarity for industrial applications which might be tackled with some additional post-processing and advanced visualization. By increasing the vibrational power, increased heating of defected fasteners should be targeted in further experiments. This might be realized with a higher input voltage at the piezoelectric transducer as well as an optimization of the exiting actuator which does not necessarily have to be a piezoelectric patch. Hence, it can be said that the presented experimental setup for ULT not easily able to detect loose rivets and provides only limited clarity regarding the detection of damaged rivets. The most innovative measurement technique in this paper is IPHWT. This technique requires further studies addressing the varying SNR with increasing distance to the heat source and a possible contactless, sequential multi-point excitation approach. Additionally, an advanced combination of the two post-processing methods that reveal different physical properties of the specimen should be investigated. By doing so, the missing distinction \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Detect Fasteners**} & \multicolumn{2}{c}{**Detection**} & \multicolumn{2}{c}{**Costs**} \\ \hline
**Method** & **Intact** & **Damaged** & **Loose** & **Clarity** & **Time** & **Money** \\ \hline SLDV & yes & yes & yes & good & slow & high \\ ULT & no & yes & no & poor & medium & medium \\ IPHWT & yes \({}^{*}\) & yes \({}^{*}\) & yes & medium & fast & medium \\ \hline \hline \end{tabular}
\end{table} 
Table 1: This table summarizes the key aspects for each individual inspection technique as well as the respective detection clarity and cost in time and money. \({}^{*}\) As described before, IPHWT detects intact and damaged rivets but does not enable the distinction between the two. between damaged and intact rivets might be realized and the method would become very effective and economically attractive.