## 4 Results and Discussion  
### Local Damage Vibration Results  
Figure 5b shows the dynamic responses in the frequency range of interest measured at the debonding center (cf. Figure 5a) by means of the SLDV (experimental setup depicted in Figure 3) for both, piezoelectric stack actuator and PWAS excitation, excited with a periodic chirp signal from 1 to 100 kHz. The first resonance frequencies \(f_{0}\) of the debonded area can be identified for both excitations as highest amplitudes in the corresponding spectra shown in Figure 5b. The peaks occur at 5.641 kHz and 5.562 kHz for PWAS and piezoelectric stack actuator excitation, respectively. Although the frequencies differ slightly, the modal shapes of the first eigenmode, as shown in Figure 5a, are found to be almost identical. The difference in the first resonant frequency can be explained by the increasing contact non-linearity for the piezoelectric stack actuator compared to the PWAS excitation [18; 32]. Nevertheless, this effect was neglected in this study, and the same resonance frequency was selected for both excitations. Figure 5a shows the first bending mode of the considered face layer debonding excited with the PWAS.  
Additional resonance frequencies of the debonded area are identified for PWAS excitation at \(f_{1}=11.438\) kHz and \(f_{2}=17.828\) kHz for the second and third bending eigenmodes, respectively. Nonetheless, for the present investigation only the first eigenmode is considered. To analyze the differences in the dynamic response between the excitation with the piezoelectric stack actuator and the PWAS transducer, which have a significant effect on the heating of the excited damage, the measurement results are analyzed in the center of debonding (cf. Figure 5a). The excitation is a sinusoidal voltage signal with the frequency of the first debonding resonance \(f_{0}\), resulting from the experiments with the PWAS used as excitation source. The voltage signal amplitude is selected as 40 V for the piezoelectric stack actuator and 150 V for the PWAS transducer. Figure 6 shows the resulting frequency spectra of the dynamic velocity response measured by the SLDV. The maximum velocity response amplitudes of the damage caused by the excitation with the piezoelectric stack actuator is 338.42 mm/s. The response amplitude resulting by the excitation with the PWAS is 75.64 mm/s, which is approximately 4.5 times smaller than the response to the piezoelectric stack actuator. A summary of the maximum dynamic out-of-plane velocity response for the considered excitation sources is presented in Table 1. Although the excitation voltage is smaller, the piezoelectric stack actuator introduces much more power (current is much higher) to the sandwich structure, and thus, the resulting velocity responses amplitude of the debonded area is much higher compared to the excitation with the PWAS transducer. As expected, both frequency spectra show the highest response  
Figure 5: Illustration of dynamic responses of the debonded area to a broadband excitation via the piezoelectric wafer active sensors (PWAS) by the scanning laser Doppler vibrometer (SLDV) results. (**a**) Eigenmode of debonded face layer at the first damage resonance frequency \(f_{0}\). (**b**) Velocity amplitude of dynamic response at the SLDV scan point at the center of the debonding for both excitations.  
at the first harmonic. Furthermore, strong higher harmonic response to the sinusoidal excitation can be observed. This is an expected result due to excitation signal distortion of the amplifier and contact acoustic non-linearity between debonded face layer and sandwich core. For the temperature-based damage detection experiments with the IR-camera and the surface temperature sensors, the excitation signal parameters according to Table 1 are used.