## 4 Conclusions  
This research undertook NDT inspection of BVID in ACS which has aluminum as a facet and with Nomex material as a sandwich core using UGW. The damage regions were successfully analyzed with the help of the A0 wave mode. The Lamb wave interactions and impact damage were visualized and detected. The frequency of excitation at 150 kHz was chosen after studying different frequency ranges. The results from the SLDV appropriately identified the damage location. This yielded a good analysis of the wave pattern and mode changes in the damage region. The Lamb wave propagation and reflection from the damage were identified with SLDV and shown in Figure 2. The SLDV full-field results identified the two different forms of BVID damage and the small hole with the help of UGW. Experimentation performed previously at our laboratory with SLDV identified debonding damage with an area of 0.5 cm in honeycomb structures with 15 kHz, and BVID with a diameter of 0.9 cm in CFRP structures with 150 kHz. The spectral elemental code was used to analyze similar conditions numerically. The results obtained identified the damage in a similar time period as the experimental case, as shown in Figure 3b. We used a simplified numerical model. The glue layer between the core and skin was not modeled, and no adhesive lever was included between the aluminum skin and the piezoelectric transducer. We identify these factors as the main reasons for the differences between the numerical and experimental signals. The elliptical signal processing technique works using the methodology of visualization of the wave reflection site. The interaction of the wave with damage creates a higher energy reflected zone. In this method, six different cases were studied, as shown in Table 7. Case 1 corresponds to the result obtained from experimental studies (SLDV) and cases 2-4 are subcases of case 1. Case 5 corresponds to the result obtained for SEM data and the scanning points placement is similar to case 1. Case 6 provided the best result in identifying the damage location for SEM data, and case 4 for the SLDV data. Both cases correspond to the same sensor placement. Thus, by using the elliptical method for signal processing, the data obtained from SLDV and SEM was successfully used to locate the damaged region. In summary, experimental and numerical case studies were carried out to understand the wave propagation phenomenon and to locate the damage with sensor network arrangements. **Author Contributions:** Conceptualization, K.B., S.S. and P.F. ; methodology, K.B. and P.F. ; software, K.B. and P.F. ; validation, K.B. and P.F. ; formal analysis, K.B. ; investigation, K.B. and P.F. ; resources, P.H.M. and S.S.; data curation, K.B. and P.F. ; writing--original draft preparation, K.B. and S.S.; writing--review and editing, K.B., P.F., P.H.M. and S.S.; visualization, K.B., S.S. and P.H.M. ; supervision, P.H.M. and S.S.; project administration, P.F., P.H.M. and S.S.; funding acquisition, P.F., S.S. and P.H.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by National Science Center, Poland under SONATA BIS project with grant agreement no. 2016/22/E/STS/00068, PRELUDIUM project with the grant no.UMO-2018/31/N/ST8/02865, This research was also supported by the Research Foundation-Flanders (FWO) Belgium under grant agreement no. FWO.3E0.2019.0102.01 in the frame of FWO MSCA fellowship. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Numerical**} & **Damage Map** & **Coordinates of** & **Error in Estimation** \\  & **Maximum Location** & **Damage Center (cm, cm)** & **(cm)** \\ \hline Case 5 & (29.8, 27.7) & (33.0, 25.0) & 4.10 \\ Case 6 & (31.9, 26.0) & (33.0, 25.0) & 1.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Error estimation of numerical cases.