## 1 Introduction  
Fiber reinforced polymers (FRP) sandwich structures are increasingly utilized for a wide range of lightweight applications, due to their big beneficial in plate strength-to-weight and stiffness-to-weight ratios compared to metal and monolithic structures. Consequently, many applications can be found in the aerospace, the automotive and the wind energy industry. However, the major disadvantages of using FRP sandwich composites are the high initial cost due to the expensive manufacturing of molds and the complex failure mechanisms. The latter reduces the lightweight potential because of the high safety factors, since failure prediction is not reliable so far. In order to use the full lightweight potential of FRP material and to guarantee the integrity of composite structures at the same time new technologies, like structural health monitoring (SHM) are introduced. SHM is the continuous monitoring of mechanical structures during operation by using sensors to detect and identify damages [1]. As a consequence, damage tolerant structures can be exchanged or repaired before catastrophic failures. Furthermore, SHMmight allow a condition-based maintenance of mechanical structures in the future, and thus, has the potential to reduce down times and maintenance costs dramatically. SHM methods utilize specific measurable physical effects of damages. They can be classified into static and dynamic methods, depending if static or dynamic physical effects are evaluated [1]. Well known dynamic methods are, for example, the electro-mechanical impedance [2; 3] and the guided waves method [4; 5]. Typical static methods are the electrical impedance tomography [6] or strain-based methods [7; 8; 9]. Thermal effects are also used for SHM, but there has not been much research considering these effects. Only a few publications present methods and strategies for the use of thermal effects under the term thermal health monitoring for SHM. A thermal health monitoring concept that uses fiber optical sensors (FOS) embedded in a composite structure as temperature sensor was published by [PERSON] et al. [10; 11]. It applies FOS sensors to measure changes in the thermal conductivity of a specimen due to a damage. The heating of the specimen was performed by a flash lamp chamber. The main motivation of the presented work is to discuss a novel vibration-based thermal health monitoring method as a possible SHM method to detect and identify face layer debondings in aerospace sandwich structures. Therefore, the present study considers a simplified composite sandwich structure derived from an aircraft spoiler of a large civil aircraft, as shown in Figure 1. Face layer debonding is a critical internal damage mechanism of such sandwich structures [12; 13]. This damage mechanism can be triggered by impact events like bird strike or tool drop. Also small manufacturing defects which grow during operational live time can lead to a critical face layer debonding. Therefore, the focus of the present paper is the continuous on-board monitoring of face layer debonding in sandwich structures. Nowadays, reliable face-core bonding and quality control during manufacturing by a non-destructive testing method (NDT) are employed for avoiding face layer debonding. For composite delamination, the well known vibration-based thermography can be used [14; 15]. This NDT method is the basis for the presented SHM method, it works on the same principle. Vibrations cause friction between separated face layers and the sandwich core, which results in heat generation and an increase in local temperatures [14]. This technique is often combined with the concept of local defect resonance (LDR) [16; 17; 18; 19; 20], which represents the most efficient way to excite the damaged area. This can be realized if the LDR frequency and the excitation frequency of the ultrasonic transducer match. Nonetheless, the commonly used shakers and the external IR-cameras limit this technique to a ground-based system.