**4. Conclusions**

In this study, the possibility of identifying debonding damage of the glass epoxy composite plates was proposed using the EMI technique. Three experiments were conducted with the 0.6 mm thick composite plate attached to another composite plate with varying thickness of 0.2 mm, 0.4 mm, and 0.6 mm. The PZT transducer was attached to a metal disc before attachment to the composite structure to create a large impedance signature peak, which was necessary for the study. Twenty impedance signatures were acquired for each of the three test specimens where 10 signatures were acquired from the undamaged part of the composite plate and the other 10 signatures from the debonding part of the composite plate. Here, the PZT-metal transducer was randomly placed on the test specimen each time for measuring a signature. The reason for random placement of the sensor was necessary as this is an important issue to be solved for the EMI technique to be used for practical application. For general inspection of composite components or structures, it would be virtually impossible to attach the sensor onto the exact same spot every time. From the experiments in this study, the random placement of the PZT-metal transducer caused the impedance signature to change. However, the change was more severe when subjected to debonding as the resonance peak amplitude increased with a shift towards the left direction.

Using the statistical metrics RMSD and CCD, a simple guideline was proposed based on the findings acquired from the study for identifying the debonding depth of composite structures. By measuring multiple impedance signatures, averaged RMSD and CCD values between 30% and 40% meant debonding has occurred at 0.2 mm depth. Secondly, averaged RMSD over 30% and CCD of 50% indicated debonding damage at 0.4 mm depth and lastly, averaged values of RMSD and CCD between 10% and 20% implied that debonding has occurred at 0.6 mm depth. Since this guideline was proposed based on the experiments conducted in this work, the future work will consist of testing out this idea with real debonding damage case scenarios to bring the EMI technique a step closer for real field applications.

**Author Contributions:** W.S.N. is the principal author for the research involved in writing the manuscript and analyzing data; J.B. contributed in conducting experiments and acquiring data.

**Funding:** The research was supported by a gran<sup>t</sup> from "Development of infrastructure technology for hyper speed transportation system (20190140-001)" funded by Korea Institute of Civil engineering and building Technology (KICT), Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
