## 1 Introduction
It was shown that the wave mode shifted to antisymmetric Lamb wave in the presence of debonding. [PERSON] et al. developed a multi-level network of actuator-receiver sensors and used leaky guided waves to detect and measure the size of multiple debonded zones [19]. It is worth highlighting the fact the complex image fusion methodology that is required to superimpose point measurements and reconstruct the debonded zones. The latter disadvantage led to the proposal of an alternative wave-based methodology in this study--namely, passive Acoustic Emission inspection. Acoustic emission (AE) has a long-history in the monitoring of delamination phenomena in concrete structural elements retrofitted with fiber-reinforced external layering [21]. In a pioneering study, [PERSON] et al. correlated the delaminations at high load levels with the release of high-magnitude AE [22]. [PERSON] et al. defined the delamination onset based on AE b-value analysis and tracked the fracture snap-back instability by AE localized event distribution [23]. Interfacial debonding onset and propagation on hybrid concrete hollow TRC beams has been effectively reported by [PERSON] et al. by following the shift in the AE hits rate, the active AE events source location, and the changes in AE wave features [24; 25]. Nowadays, AE wave features shift (i.e., rise time jump in values as the stress is released in debonded zones) has been established as a damage indicator to discriminate fiber reinforcement delamination and pull-out from other synchronous-active damage modes (i.e., concrete cracking, steel rebars plastic deformation, etc.) [26]. In a similar study, [PERSON] et al. reported AE hits of a longer duration and lower amplitude as the pull-out of the concrete reinforcement evolves and slipping leads to the wearing of the interface surface [27]. To summarize, the extended literature on the application of AE as a debonding/delamination indicator has shown that:  
* The AE intensity and signal strength indicate the transition from matrix micro-cracking to the reinforcement debonding onset [26], and beyond that moment AEintensity rise tracks the progressive debonding stages (i.e., adhesion, friction, mechanical interlocking) [28]. * The AE wave features that describe the early part of the waveform, such as the rise time (RT) and the RA value, can indicate the presence of shear-dominated debonding actions [29]. * The shifts in the frequency range can be an indication of interfacial debonding and slippage, however no confident pattern has been reported among the different frequency indices (initiation/average/peak frequency) [22; 30]. In this paper, a comparative study is performed between a _reference_ (REF) sandwich-insulated panel and one at which early AE activity is proven to occur due to _internal_ skin-core _interfacial debonding_ (IID). The panels are manufactured in an identical way and tested in incremental bending, simulating in-service wind loads. AE continuously monitors the damage progress and the findings fully characterize the interfacial debonding. Digital Image Correlation (DIC), a full-field optical inspection technique, is applied to validate the AE observations and to provide complementary evidence of debonding. In the literature, DIC out-of-plane displacement tracking has been effectively applied in the work of [PERSON] et al. to detect the buckling of thin-walled TRC-skin sandwich-insulated panels under compression [31]. [PERSON] et al. measured the shear deformation of different foam materials designed as core elements on sandwich panels tested in bending [32]. For the first time in the literature, an integrated inspection methodology combines the AE and DIC outcome, localizes and measures the premature interfacial debonding, and also distinguishes this detrimental effect from other damage sources (i.e., cracking, pull-out). The detection of premature interfacial debonding appears to be of utmost importance for the structure's integrity and long-term durability.