## 5 Conclusions  
The present paper presents a novel vibration-based thermal health monitoring method for face layer debonding detection in aerospace sandwich structures. The novel approach extends the vibration-based thermography NDT method to a continuous and on-board SHM method with high potential for detection, localization and quantification of face layer debondings in composite sandwich structures. The method employs well known NDT methods for SHM, by replacing the large and heavy excitation sources (e.g., shaker or piezoelectric stack actuator) and temperature sensing devices (IR-camera) by small and lightweight PWAS transducers and surface temperature sensors, respectively. To demonstrate the applicability and potential of the novel method several experiments, with a component level sandwich structure and different excitation sources (PWAS transducer and piezoelectric stack actuator) were performed. First, the basics of the LDR concept and the vibration-based thermography NDT approach was demonstrated for identification of sandwich face layer debonding by means of an IR-camera and a piezoelectric stack actuator for excitation. Additionally, the proposed use of surface temperature sensors was validated by the IR-camera results. Second, the applicability of the novel thermal health monitoring method was experimentally demonstrated by using the surface temperature sensors and replacing the piezoelectric stack actuator by a PWAS transducer. The results of the surface temperature sensors show clearly the heating of the debonded face layer of approx. 1 inch diameter when excited and the cooling down to the initial temperature when not excited. The presented method shows highest potential for continuous and on-board damage detection, due to the high sensitivity and the low noise level of the surface temperature sensors compared to an IR-camera. Nonetheless, it should be noted that the presented method was demonstrated for laboratory conditions only. The application of this novel method in large sandwich structures requires modifications and improvements of the technique, for example, by using a broadband excitation to identify the local damage frequency or by using more suitable temperature sensors. Nevertheless, the paper showed that it is possible to detect a relevant damage of approx. 1 inch diameter by using lightweight on-board sensors and actuators in a possible SHM set-up. A further advantage of the presented novel method is that it enables a multi-sensor approach, as the PWAS transducers used for excitation could also be used for other SHM methods, (e.g., guided waves or electromechanical impedance method).  
Conceptualization, T.B., C.K. and M.S.; methodology, T.B.; investigation, T.B.; writing--original draft preparation, T.B.; writing--review and editing, T.B., C.K. and M.S.; visualization, T.B.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Christian Doppler Research Association, the Austrian Federal Ministry for Digital and Economic Affairs, and the National Foundation for Research, Technology and Development.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.