## 6 Discussion
A low temperature profile results in a low SNR for both analysis methods and results in a decreasing SNR for larger distances to the heat source. The maximum surface temperature increase depends on the injected energy and the heat capacity of the specimen. As the latter is fixed in industrial applications such as the in-service aircraft inspection, further investigations should focus on the optimization of the stimulating heat source. Although a contactless heat excitation as proposed by Reference [35] has some advantages, this study uses a Peltier element for the local heating of the specimen. The drawback of a Peltier element in comparison to a Laser-excitation is the required access to the surface and its spatial dimension. The latter is much larger than for the focused laser spot used by Reference [35] (\(\sim\)0.35 mm) which reduces the point-like behavior of the excitation and the resulting heat dispersion. However, three advantages of the Peltier element outweigh its aforementioned disadvantages. Firstly, Peltier elements are very cheap while being easy and safe to operate which avoids the safety restrictions coming with the use of (high-power) lasers. This point plays an outstanding role for applications in industrial environments like daily aircraft maintenance. Secondly, the thermal energy induced into the specimen by the Peltier element is orders of magnitudes larger than the thermal energy of the laser. This has several benefits like a higher SNR of the data, a possibly large distance from the heat source to the crack and the option to thermally excite thick plates. And thirdly, Peltier elements come with the positive side effect of cooling after switching off. Peltier elements also called the thermoelectric heat pumps, transfer the heat energy from one to the other side. After switching off, the two sides of the element strive for thermal equilibrium which results in a cooling of the hot side. This phenomenon influences the shape of the thermal wave which shows a steep negative slope after the switch-off of the Peltier element. A sequential multi-point measurement with a relocated heat source could make some of the aforementioned advantages redundant (e.g., large inspection area). A contactless heat source would simplify the technical realization and hence encourage such a measurement procedure. Contactless heat sources are used in most fields of NDT such as flash thermography or Eddy current thermography. Even if most established heating methods come with a more complex hardware setup than a power source and an easy to handle Peltier element, such adaptations could be easily realized. Another aspect that should be taken into account for the comparison of the different technologies is a potential fusion of them. One hybrid solution could include the IPHWT and ULT measurement as both are based on thermal videos. In such a case, only the excitation unit which is a relatively cheap part of the setup needs to be modified, while the thermal camera can be used for both techniques simultaneously. Table 1 summarizes the key aspects of the three different technologies being the feasibility to detect intact, loose and damaged fasteners as well as an estimate of the costs in time and money. In addition, Table 1 lists a qualitative estimate of the clarity of detection which includes the interpretation complexity as well as how explicit the measurement data is. The listed costs rating is based on an approximate estimation and only indicates the costs relative to each other. The same holds for the inspection time which highly depends on the hardware setting, the measurement accuracy and procedures and only provides a tendency for each technique. It should be highlighted that these statements are based on the experiments and results presented in this work. The presented results show the feasibility of detecting and qualifying loose, damaged and intact fasteners in aircraft fuselage lab-joints with state-of-the-art NDT techniques.