## 6 Discussion  
As described before, this work aims to compare three NDT techniques that can detect and quantify loose and damaged rivets and nuts. Multiple measurements on one single specimen being a commercial aircraft fuselage panel allow a great principle comparison of the three techniques. Each technique has its advantages and disadvantages concerning the sensitivity, accuracy and applicability in an industrial environment like an aircraft hangar. These aspects will be discussed and compared in the following. The results obtained from the measurement of area I which contains a loose, as well as sheared rivets, underline the possibilities of inspections with SLDVs. In this area, the SLDV measurement is able to detect, visualize and distinguish loose, sheared and intact rivets. The loose rivet is badly coupled to the surrounding plate and appears as the red area in a broad frequency range (1 kHz to 350 kHz) associated with low vibration energy. Additionally, the visualization of the coherence between the input signal and the vibration highlights loose parts in the investigated area. At the same time, due to their reduced tightness, sheared and damaged rivets appear as resonances in the low frequency range below \(\leq\)200 kHz. Reference [41] proposes to call these resonances _local defect resonances_ (LDR). LDRs result from the fact that the presence of a defect leads to a local decrease in stiffness of a certain mass of the material in the area of the resonance. Reference [31] also introduces the so-called _effective rigidity_\(K_{eff}\) and _effective rigidity_\(M\) of a defect which determine its fundamental resonance frequency \(f_{0}\). A higher mass decreases the fundamental resonance frequency \(f_{0}\) and a higher stiffness increases \(f_{0}\). \[f_{0}=\frac{1}{2\pi}\sqrt{\frac{K_{eff}}{M}}. \tag{3}\]  
The LDR behavior of the damaged rivet is confirmed by a close-up at different frequencies which is shown in Figure 11. The different patterns correspond to the different vibrational modes of the system with a fundamental frequency \(25(1)\) kHz and higher harmonics at 51 kHz, 76 kHz, 89 kHz, 125 kHz and 169 kHz. Figure 10: The amplitude (**a**) and phase image (**b**) of area II for a frequency range of 110 kHz to 120 kHz and a periodicity of \(\tau=5\) s. The dark spot in both images corresponds to the screw with the loose nut. This is in agreement with the excitement of intact rivets at high frequencies above \(\geq\)200 kHz. For low as well as intact rivets, the connection between rivet and plate generates a weak spot in the plate-like structures which experiences vibrational excitement. The tighter the connection between rivet and plate, the higher the stiffness and LDR resonance frequency. This is also confirmed by the measurement results of area II (Figure 8) where the screw with the loose nut as well as the intact screws are visible. Again, the loose screw associated with a low stiffness is visible at low frequencies (15 kHz to 65 kHz and 115 kHz to 165 kHz) while the tight screws (high stiffness) are visible at high frequency band images (260 kHz to 310 kHz). Besides the different frequency range in which intact and damaged connections are visible, also the appearance differs in the two cases. For intact rivets, only a small area at the edge of the rivets becomes visible while the two damaged rivets show large area LDRs. A similar phenomenon can be seen at area II where intact screws show only small LDRs and the loose screw appears as a large area LDR. This can be explained with the increasing excitation frequency that comes with a shorter wavelength. The shorter the wavelength, the smaller LDRs can be exited and appear in the image. Especially the detection of intact rivets and screws should be highlighted as it is of big advantage in an industrial environment. Here, safety and reliability are crucial and the detection of intact parts serves as a evidence that the inspection tool works and is used in the correct way.