## 6 Discussion
In such conditions, the inspection of riveted joints will show both, intact and damaged rivets. Hence, the detection of intact rivets serves as a cross-check of the measurement and the likelihood of false negatives is reduced. Also the ULT measurement of area I reveals the damaged rivet. However, as this technique is based on the heat production due to rubbing and clapping of vibrational exited parts, the loose rivet which experiences no vibrational excitation is not detected. While the SLDV measurement covers a broad frequency spectrum in one measurement, the experimental setting only allows ULT measurements of frequency bands with a bandwidth of 10 kHz. As the LDR frequencies of damaged rivets are a priori unknown, the scanning of a broad frequency band coming with multiple measurements is required. Such a measurement procedure scanning wide frequency ranges could be automatized for industrial applications but will always increase the inspection time compared to small frequency range measurements. As described before, the high heat conductivity and heat capacity of aluminium is an additional challenge for this technique. Even if lock-in processing leads to a high SNR of the measurement, the defect rivets and screws are barely visible. This could be improved by more sensitive recording IR cameras or an increase of the vibrational excitation which leads to an increase in heat production. Both could potentially be realized with at higher costs due to more expensive equipment. The presented experiment is performed in laboratory conditions with an IR camera setting that has an NEDT \(=35\) mK. Taking into account that an industrial environment implies a lot of noise increasing factors like surface reflections, hot or cold air flow and other heat radiating apparatus, it is unlikely to achieve a better SNR while keeping an economical benefit. Hence, an increase of heat production by a higher vibrational excitation should  
Figure 11: Different vibration modes of the middle left rivet (area I) for the fundamental frequency (**a**) 25(1) kHz and higher harmonics (**b**–**f**) 51 kHz, 76 kHz, 89 kHz, 125 kHz and 169 kHz. Additional vibration patterns are shown in Reference [40]. be the aim for further developments. This includes a profound coupling of the exiting piezoelectric transducer to the specimen as well as the proper selection of piezoelectric transducer patch and amplifier. However, even with an improved excitation setup and a sufficient measurement SNR, one critical aspect being the in-service reliability check remains. As discussed earlier, the ULT technique only detects damaged rivets and is not sensitive to intact or loose rivets. Therefore, hardware problems for example, with the coupling of the piezoelectric waver to the specimen easily result in false negative results in which present defects may not be detected. To minimize the risk of false negatives, some kind of calibration during the measurement at the specimen under test is needed to indicate a proper inspection. A removable artificial defect being attached to the specimen could serve this purpose. In this case, measurement is only classified as valid if the artificial defect is visible in the result. IPHWT visualizes loose, damaged and intact rivets as well as tight and loose screws. As the method relies on the in-plane heat flow and its disturbance due to loose or damaged fasteners, no direct information of the tightness of a fastener is acquired. Without a more profound study including some statistics, it is difficult to distinguish between damaged and intact rivets. In comparison to the SLDV measurement, this method is much faster and the full-field character allows the inspection of large areas in one measurement. The maximum size of the inspected area is dependent on the heat source and the thermal material properties. In the first order, the heatwave generated by a point-like source has a cubic decay with distance from the heat source.