## 3 Experimental Investigation
### Experimental Set-Ups  
For the present investigation, three different experimental set-ups are used separately from each other to prevent the measuring devices of the different set-ups from influencing each other. First, measurements with a SLDV are performed to visualize the vibration modes of the debonded face layer, identify its corresponding resonance frequencies and to compare the amplitudes caused by different excitation sources. Second, experiments for temperature-based damage identification are performed to examine the applicability of vibration-based thermography for face layer debonding detection and identification. The temperature monitoring is done via an IR-camera and surface temperature sensors. For excitation a piezoelectric stack actuator is used. Third, the applicability and potential of the novel vibration-based thermal health monitoring method is experimentally demonstrated by means of an array of surface temperature sensors and a PWAS (for excitation) permanently bonded to the considered structure. The first experimental set-up is depicted in Figure 3. It consists of the different excitation sources (piezoelectric stack actuator type Piezosystem jena PSt 1000/25/40 and adhesively bonded PWAS transducer type PI ceramic PIC255), the SLDV (Polytech PSV-500-HV) and the structure under investigation. The circular shaped PWAS transducer has a diameter of 25 mm and a thickness of 1 mm. The signal-to-noise level of the measurements is increased by amplifying the voltage excitation signal of the function generator (Tektronix AFG3022C) with the help of a linear power amplifier(AE Technon 7224). To identify the LDR frequency of the debonded face layer, (i.e., the damage) experimentally, the out-of-plane velocity at 16 SLDV scan points located horizontally (\(x\)-direction) along a line across the damage (with 2 mm spacing, which sufficiently resolves the considered 1 st axisymmetric flexural mode shape) is analyzed. The SLDV uses a sampling rate of 250 kHz. The time signals are averaged 26 times for each scan point to increase the signal quality. The experimental set-ups for the demonstration of the applicability of the vibration-based thermography for face layer debonding detection and identification and the investigation of its SHM potential are shown in Figure 4. In the first step, the sandwich structure is excited by the piezoelectric stack actuator (setup demonstration LDR for face layer debonding). The position of the piezoelectric stack actuator is selected to provide the most efficient excitation of the damage. For this purpose, measurements were made with the SLDV with varying piezo stack actuator positions and the amplitudes were compared. The position with the highest amplitude response at constant excitation voltage was selected. To demonstrate the SHM potential of the presented method, the glued PWAS transducer is used for excitation (setup demonstration of SHM potential; piezoelectric stack actuator was removed). The bonded PWAS transducer was positioned along the centerline of the structure to maintain symmetry for possible future experiments. Further care was taken to ensure that there was enough distance between the damage and the PWAS so that the temperature rise of the exciting PWAS would not affect the temperature sensor results. The temperature monitoring was performed by an array of four surface temperature sensors (CeLaGo TS-LM03) and an IR-camera (Micro-Epsilon TIM Connect T450), which monitors a surface area of approximately 109 mm by 83 mm in \(x\)-direction and \(y\)-direction, respectively. The four surface temperature sensors are arranged in a rectangular pattern with a side length of 45 mm on the sandwich face layer. One of the sensors is directly located on top of the debonded area of the face layer, see Figures 2-4.