## 3 Results and Discussion  
### Results of Manual IRT and PA Inspection on Coupons and Laminates  
The synergistic PPI-LT/PA strategy minimizes the timescale that is required for reliable aircraft inspection by the preferential application of PA inspection, for thorough damage characterization, only to areas previously identified with damage presence by rapid wide-area IRT. Under this concept, PPI-LT and PA were used in succession to fast scan the composite and indicate areas that require further PA inspection. Figures 4-6 report pairs of successive results of IRT-identified and PA-quantified damaged areas on 9- and 18-layer composite coupons and laminates, respectively, for different levels of impact energy. All of the thermograms (figures on the left-hand side) are in the phase domain and they include relevant length and phase scale bars; the bars in the first thermogram of each subset are common for the rest, in the same subset. The C-scan image's horizontal scale represents the scan-axis, along which the C-scans were recorded with a resolution of 1 mm. The vertical scale represents the index resolution of 0.6 mm, which is also the pitch between the PA transducer elements. The grayscale color bar that is shown in the C-scan images represents ToF of the maximum peak echo of the received signal from the material's entire thickness except the dead-zone. The inverse 'gray' color-pallet with custom color-bar facilitates straightforward damage discrimination using an image processing routine in 'Imagej' software. The calculated damage area provides a close estimation of the extent of the damage. Figure 4 compares the infrared thermograms and PA images from nine-layer composite coupons impacted with energies of 2.5 J (a), 4 J (b), 8 J (c), and 12 J (d). IRT and by PA also capture damage in composite coupons of 18-layers in Figure 5, for impact energies of 8 J (a), 12 J (b), and 20 J (c). In Figure 6, thermographic and subsequent PA investigation is presented for composite laminates that are impacted with different energies. By observation of the thermographic results that are presented in Figures 4-6, it is concluded that damage is captured with exceptional precision by the developed PPI-LT approach and sensor in all types of specimens. It is interesting to note that the technique not only assesses the presence of damage, but it can represent its main geometrical features, such as shape and orientation, as documented by the high contrast area corresponding to damage in \(-45^{\circ}\) laminas in the thermograms. In certain cases, such as Figure 5b,c, the barely visible dent produced at the site of impaction is also visible (dark circular mark at the damage center). The increasing extent of damage with increasing impact energy is efficiently assessed by IRT. Hence, it entails that the developed PPI-LT approach and sensor can not only identify damage presence, but can also provide its basic dilatational characteristics. Figure 4: Detection of damage by rapid PPI-LT (thermograms, left column) and subsequent high-resolution characterization by PA imaging (right column) for nine-layer coupons with different impacted energies: (**a**) 2.5 J, (**b**) 4 J, (**c**) 8 J, and (**d**) 12 J. Observing PA C-scan images of impact damages that are presented in Figures 4-6, the following general observations can be derived: the impact damage size increases with impact energy, and damage tends to propagate along the 45\({}^{\circ}\) to fiber direction. In all coupons, the impact damage shows large sub-surface damage, with little or no external indication of the impact. Hence, it is vital to carry out detailed NDT inspections on the composite aircraft's external surface during heavy maintenance. In all coupons and laminates, IRT imaging shows the good detectability of damage, but the damage area is consistently smaller in size when compared to C-scan.