## 5 Damage Detection and Localization
### Surface-Mounted Artificial Damage  
Figure 9 illustrates the placement of the weighted Blu Tack, which simulates surface-mounted artificial damage, on the surface of the 2 mm thick panel. The weighted Blu Tack is positioned in a designated location, typically off-center, to mimic the presence of localized damage on the composite surface. This configuration allows for the investigation of damage detection and localization using the correlation coefficient and the delay-and-sum (DAS) algorithm for panels with different thicknesses, including 2 mm, 4 mm, and 9 mm. The correlation coefficient and the DAS algorithm are applied to analyze the differences between the baseline (undamaged) UGW signals and the current UGW signals obtained from the damaged panels. These techniques enable the detection and localization of surface-mounted artificial damage, providing insights into the effectiveness of the damage detection methods across different panel thicknesses. By studying the results obtained from the correlation coefficient and the DAS algorithm, the researchers can evaluate the impact of panel thickness on the accuracy and reliability of damage detection and localization for surface-mounted artificial damage. This analysis contrib  
Figure 9: Photo of a weighted Blu Tack placed on the surface of the 2 mm thick panel. understanding of the thickness effects on the performance of structural health monitoring techniques in composite structures. Figures 10-15 present the results of damage detections and localizations for the panel thicknesses of 2 mm, 4 mm, and 9 mm at 50 kHz and 250 kHz, respectively. These figures also include the surface-mounted results for comparison. For the 4 mm panel, it is important to note that the embedded circuits of PZT transducer 1 shown in Figure 1 became disconnected during the manufacturing process. As a result, only three embedded PZT transducers were available for damage detection and localizations in this panel. Regarding the 9 mm panel, the much higher amplitude of crosstalk introduced interference in the first wave packet of the measured signal for pair paths 1-3/3-1 and 2-4/4-2 shown in Figure 1 at 50 kHz. This interference made it difficult to accurately identify the time-of-arrival of the measured signals for the aforementioned pair paths. Consequently, the correlation coefficient and DAS algorithm based on the time-of-arrival information could not be effectively used for damage detection and localization at 50 kHz. Figure 10: Detections of surface-mounted artificial damage for the 2 mm thick panel at (**a**) 50 kHz and (**b**) 250 kHz using embedded PZT transducers and at (**c**) 50 kHz and (**d**) 250 kHz using surface-mounted PZT transducers. The presented results highlight the outcomes of the damage detection and localization process using the correlation coefficient and DAS algorithm for different panel thicknesses. The comparisons with the surface-mounted results offer insights into the performance differences between embedded and surface-mounted PZT transducers for damage detection and localization. By examining these results, the researchers can evaluate the efficacy and  
Figure 11: Detections of surface-mounted artificial damage for the 4 mm thick panel at **(a)** 50 kHz and **(b)** 250 kHz using embedded PZT transducers and at **(c)** 50 kHz and **(d)** 250 kHz using surface-mounted PZT transducers. Figure 12: Detections of surface-mounted artificial damage for the 9 mm thick panel at **(a)** 250 kHz using embedded PZT transducers and at **(b)** 50 kHz and **(c)** 250 kHz using surface-mounted PZT transducers. limitations of the damage detection techniques across different panel thicknesses, contributing to the advancement of structural health monitoring methods for composite structures.