## 5 Damage Detection and Localization
### Impact Damage  
To investigate the thickness effects on the detection of real barely visible impact damage (BVID) in composites using embedded ultrasonic guided waves (UGWs), impact tests were conducted on composite panels with thicknesses of 2 mm, 4 mm, and 9 mm. Figure 14: Localizations 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 (where the “\(\Circur\)” is the position for real damage and the “\(\times\)” is the position for predicted damage). Figure 15: Localizations 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 (where the “\(\Circur\)” is the position for real damage and the “\(\times\)” is the position for predicted damage). The impact tests were performed using a drop tower, specifically the INSTRON CEAST 9350, as shown in Figure 16. The impact position for each panel was the same as that used for the surface-mounted artificial damage in Section 5.1. The impact energy was initially set at a relatively low level and increased incrementally by 2 J for each impact test. After each impact, the impact area was inspected using a DolphiCam C-scan (CF08) to identify the resulting damage. The impact energy was increased until a suitable damage area was detected and captured by the C-scan. The impactor used in the tests was a hemisphere with a diameter of 20 mm. The load cell used for the 2 mm thick panel was a 22 kN load cell with an additional 3 kg mass, while a 90 kN load cell with the same additional mass was used for the 4 mm and 9 mm thick panels. The impact energy and velocity values used for each panel were as follows: 15 J and 3.53 m/s for the 2 mm thick panel, 20 J and 2.72 m/s for the 4 mm thick panel, and 57 J and 4.59 m/s for the 9 mm thick panel, respectively. These impact tests provide the basis for evaluating the detection and characterization of real BVID in composite panels of different thicknesses using embedded UGWs. The impact parameters were carefully chosen to achieve suitable damage areas for subsequent analysis and assessment. Figure 17 illustrates the results of the non-destructive inspection (NDI) using the DolphiCam C-scan (CF08) for the impact damage on each composite panel. The following observations can be made based on the figure:  
1. For the 2 mm thick panel (Figure 17a), the detected damage area was approximately 176 mm\({}^{2}\), and the damage thickness was measured to be around 1.2 mm from the impact surface. Additionally, surface cracks were visible on the panel after the impact test. 2. In the case of the 4 mm thick panel (Figure 17b), the detected damage area was approximately 452 mm\({}^{2}\). The horizontal and vertical B-scans indicated a damage thickness of approximately 3 mm from the impact surface. Similar to the 2 mm thick panel, surface cracks were also observed. 3. For the 9 mm thick panel (Figure 17c), the detected area of the impact damage was around 254 mm\({}^{2}\). The horizontal and vertical B-scans revealed a damage thickness of approximately 2 mm from the impact surface. Surface cracks were also visible on this panel following the impact test. Figure 16: INSTRON CEAST 9350 drop tower for the impact test. These NDI results provide visual evidence of the impact damage inflicted on each composite panel. The DolphiCam C-scan successfully captured the extent and location of the damage, as well as the presence of surface cracks. These findings will serve as a basis for further analysis and comparison with the results obtained from the embedded UGW measurements for damage detection and localization. In this section, the correlation coefficient and the delay-and-sum (DAS) algorithm were employed to detect and localize the impact damage for composite panels with thicknesses of 2 mm, 4 mm, and 9 mm.