## 4 Results and Discussion
### Temperature-Based Damage Identification
#### 4.2.2 Potential of Vibration-Based Thermal Health Monitoring  
The evaluation of the potential of the novel thermal health monitoring method is done by temperature difference measurements at the debonded and bonded face layer by the surface temperature sensors while being excited by a PWAS transducer (cf. Figure 3). In this experiment, each excitation and non-excitation phase in a cycle was extended to two minutes to overcome the smaller dynamic response (compared to the piezoelectric stack actuator; cf. Figure 6), and correspondingly expected slower and smaller heating of the damage. The excitation cycle is again three times repeated and the results depicted in Figure 8b show the normalized measured surface temperature difference \(\Delta T_{A}\) and \(\Delta T_{B}\) over time. The temperature differences \(\Delta T_{A}\) and \(\Delta T_{B}\) show clearly a temperature increase at the debonding during the excitation phase and a temperature decrease back to the initial temperature when not excited. To verify the results of the surface temperature sensors by the thermography results, the same spatial averaging procedure as described in Section 4.2.1 is used. The difference \(\Delta\overline{T}_{k}\) of the averaged temperature data at the debonded face layer area and at the bonded area are again calculated and compared to the surface temperature sensor output \(\Delta T_{A}\) and \(\Delta T_{B}\), see in Figure 8b. Thermography and surface temperature sensors show clearly the same behavior, that is, heating of the debonding when excited and cooling down to the initial temperature when not excited. Nevertheless,  
Figure 8: (**a**) Shows the normalized temperature profile of the averaged difference of the pristine and damaged area \(\Delta\overline{T}_{k}\) resulting of the IR-images compared to the results of the surface temperature sensors \(\Delta\overline{T}_{A}\) and \(\Delta\overline{T}_{B}\) at the experiment, in which the piezoelectric stack actuator is used as excitation source, whereas (**b**) shows the results of the experiment with the PWAS bonded on the structure used as excitation source. The excitation trigger signal is schematically shown whether the excitation is on (1), or off (0).  
the thermography results, although being averaged, are very noisy compared to the surface sensor results. The maximum temperature difference increase \(\Delta T_{B}\) measured by the surface sensors at the debonded face layer is approximately 0.05 K at a very good signal-to-noise level. This temperature increase is much lower than the temperature increase caused by the excitation via the piezoelectric stack actuator due to the lower excitation power. The temperature difference measured by the surface sensors at the bonded sandwich area do not show any reaction to the excitation state. Moreover, the temperature difference increase presented in Figure 8b shows convergence to a static value for excitation with the PWAS transducer. This behavior would indicate that the heating has reached a maximum for the given excitation, which was not given for the excitation with the piezoelectric stack actuator (cf. Figure 8a).  
In summary the applicability of the presented novel thermal health monitoring method for sandwich debonding is demonstrated for the considered sandwich structure equipped with small and lightweight temperature sensors and a conventional adhesively bonded PWAS transducer as excitation source. Furthermore, the very high excitation voltage and duration that was used with the PWAS transducer could be reduced for the given setup due to high sensitivity of the used temperature sensors and the very good signal-to-noise ratio. However, it should be mentioned that these results may vary with different setups of sensors, excitation sources, structures, and damage. Nevertheless, with a sufficiently dense array of surface temperature sensors and PWAS transducers for excitation the novel thermal health monitoring method is capable of detecting, localizing and quantifying debonding of sandwich face layers.