## 3 Results and Discussion
### Depth Estimation  
As mentioned before (see Section 2.3.5), the sole requirement to retrieve the defect depth from PPT is the knowledge of the thermal diffusivity and the determination of the blind frequency. The thermal diffusivity for all specimens was estimated as described in the precedent section. The blind frequencies were obtained as follows.  
Figure 11(a) presents an example of the estimation of the \(f_{b}\) for defect 1 in sample S45. This graph shows the phase vs. frequency profiles obtained from raw data (blue and red dots corresponding to sound and defect areas, respectively), and the phase vs. frequency profiles obtained from TSR data (blue and red solid lines). The \(f_{b}\) is determined as the first crossing point (starting at \(f\) = 0 Hz) between the defect and sound phase from TSR profiles, i.e., \(f_{b}\) = 5.481 Hz. Replacing this and the measured thermal diffusivity value from Table 1 (i.e., \(\alpha\) = 4.935 \(\times\) 10\({}^{-7}\) m\({}^{2}\)/s for sample S45) into Equation (5): \(z_{\rm est}\) = 0.3024 mm, which is relatively close to the actual \(z\) = 0.287 mm.  
The depth of defect 2 in sample S45 was estimated in a similar way. Figure 11(b) shows the phase vs. frequency profiles. The blind frequency, in this case, is \(f_{b}\) = 0.1667 Hz, and the estimated depth is \(z_{\rm est}\) = 1.734 mm, also close to the actual \(z\) = 1.7 mm. The depth of defect 3 could not be estimated since it was not detected as seen in Table 2.  
Figure 11: Calculation of the thermal diffusivity by flash method and partial times (sample S44, front face): (**a**) raw data (points) and fitting (dotted line) result using the Savitzky–Golay filter; and (**b**) pixel-by-pixel diffusivity map.  
The same procedure was followed to estimate the depth at different locations inside the damaged area of the impacted samples. Figure 13 shows the results for the impacted specimen 55. The phase profiles can be seen in Figure 13a, from which, the blind frequencies are extracted for each ROI. ROI\({}_{1}\) is located at the left near the center of the plate as seen in Figure 13b, which presents a phasegram at a very low frequency (\(f\) = 0.02 Hz) showing the deepest features (i.e., ROI\({}_{1}\)). As the frequency increases, the phasegrams show more and more shallow features as seen in Figure 13c-e.  
The blind frequency of ROI\({}_{1}\) can be obtained from the phase profiles in Figure 13a, i.e., \(f_{b}\) = 0.1875 Hz. The depth can then be estimated from Equation (7) using this value and the thermal diffusivity reported in Table 1, which gives \(z_{\text{est}}\) = 1.65 mm. Similarly, depths for the other ROIs were estimated as 1.49, 1.1, and 0.65 mm for ROI\({}_{2}\), ROI\({}_{3}\), and ROI\({}_{4}\), respectively, as shown in Figure 13c-e.  
Finally, impact damage on all four samples can be seen in Figure 14. In Figure 14a, the damaged area is zoomed-in and rotated 90\({}^{\circ}\) with respect to the white rectangle shown in Figure 14b to be compared to Figure 13b. Both images show roughly the same area. The PT result in Figure 13b shows evidently more details (e.g., composite fibers) given the higher spatial resolution and sensitivity of the cooled camera, but the damage appears with higher contrast in the LST result presented in Figure 14b. This is because the sample was heated much longer by LST than for PT.  
Figure 12: Determination of the blind frequencies for depth estimation with the phase, plate S45 inspected: (**a**) defect 1 (depth = 0.287 mm); and (**b**) defect 2 (depth = 1.7 mm).  
Figure 14b shows a visual (subjective) comparison of the degree of damage for increasing impact energies. For instance, 10 J impacts (samples S5 and S9) show similar gray levels. Nevertheless, gray levels at the damaged areas increase with the impact energy as clearly seen when comparing energies of 10, 12, and 15 J.