## 5 Results
### SLDV  
As described in Section 3.2, the analysis of the SLDV data results in one image that depends on the frequency filtering of the calculated FFT for each point. Figure 7a shows  
Figure 5: The measurement of area I (rotated by 90\({}^{\circ}\)) with the IPHWT results in two images based on the covariance analysis (**a**) and local gradient method (**b**). Combining both methods results in an image in which green corresponds to the covariance analysis and red corresponds to the local gradient method (**c**). Additionally, the identified heating area is indicated in black at the top left region.  
Figure 6: IPHWT of area II with the resulting covariance analysis (**a**) and local gradient method (**b**). In (**c**), both outputs are composed and green corresponds to the covariance analysis and red to the local gradient method. Additionally, the identified heating area is indicated in black at the top right region.  
the map of coherence of each measurement point with the input signal where a threshold of COH \(>\) 0.8 separates the red and green points. Figure 7b-f shows the RMS of the FFT of area I in some representative frequency bands. A detailed discussion about the calculation of the coherence can be found in Reference [40]. Each image indicates different features to a different extent. The most dominant spot in all six images is rivet 0 that appears red in the coherence as well as in all frequency bands. This indicates the low coherence and low RMS of the loose rivet respectively. The other eight rivets are not visible in the coherence representation and are somehow visible in the different frequency bands. In the lowest frequency band, rivet 1 and 3 show resonances and are visible as blue spots corresponding indicating a high vibration amplitude. In Figure 7d, a stripe-like artifact between 10 cm to 15 cm can be seen. The high frequency band with frequencies from 280 kHz to 330 kHz also reveals the intact rivets.  
The vibration analysis of area II is shown in Figure 8. As stated above, the nut of the screw 5 is loose. However, the screw this still tight in the surrounding plate and no loose parts are visible from the front side. Hence, the analysis of the coherence does not show any features and is not shown here. This can also be conducted from the fact that no or only very small clustered red areas are visible in the frequency filtered RMS images which indicates a good coupling between the piezoelectric transducer patch and all points in this field of view. As in area I, the different frequency bands reveal different features that resonate at different frequencies. At 15 kHz to 65 kHz, screw 5 with the loose nut is visible. However, also the other screws, which built a tight connection between the two plates, are visible due to the bright areas around them. As for the rivets, a second vibration mode of screw 5 can be seen at higher frequencies (115 kHz to 165 kHz) where the outer edge and the cross slot in the screw head are visible. At high frequencies above 260 kHz, all screws become visible and even a sixth feature, a rivet at the right of screw 5 shows some resonance and confirms the characteristic observed in area I.  
Figure 7: The coherence analysis (**a**) and band pass filtered and normalized RMS for different frequencies for area II (**b–f**) of the fuselage panel. The color bar at the bottom refers only to (**f**) as it is adapted for each filter according to the description above.