## 3 Results and Discussion
### Automated IRT and PA Inspection on Stiffened Panel  
The stiffened panel was initially scanned by PPI-LT for rapid wide-area identification of damage location, after which PA inspection was pursued. This primmer task was undertaken with the IRT module that was integrated on the VRP, as seen in Figure 2b as a lab-scale demonstration of the envisioned fully automated NDT solution for rapid  
Figure 6: Detection of damage by rapid PPI-LT (thermograms, left column) and subsequent high-resolution characterization by PA imaging (right column) on flat laminates (9 layers) with estimated impact energies (**a**) 4.5 J (**b**) 5.5 J and a curved laminate with estimated impact (**c**) 4.5 J.  
and reliable damage inspection in aircraft. The thermographic assessment of damage locations is presented in the left-hand side part of Figure 7. Therein, all three impact damages of the stiffened panel are observed to have been positively identified by the PPI-LT technique. Moreover, the extent of damage appears to be greater in the skin region D1, in the absence of back face stiffening, and smaller in the stiffened area, D3. It is interesting to note that the thermographic signal that was acquired from the stiffened panel did not appear to be deteriorated, when compared to previous findings, as a consequence of the presence, 0.2 mm below the surface, of the copper mesh lighting-protection lamina. Such behavior can be rationalized by the consideration of the small LWM dimension of the mesh with respect to the scanned area, which may lead to a uniformly higher background thermographic signal throughout the specimen, practically masked within the phase scale of thermographs containing high-contrast damage areas. The damaged locations that were identified by IRT were subsequently processed by PA.  
Figure 7: Comparison of IRT and PA imaging for the impact damages (D1, D2, and D3) on the stiffened panel collected using the IRT and PA module on the robot. (**a**) Skin region (**b**) web (T junction) region and (**c**) boundary of the skin-flange region. Hence, in the next step, the PA module was integrated on to the vortex robot for automated scanning of the stiffened panel. Initially, the PA hardware was pre-loaded with a configuration file with all of the PA inspection parameters, as mentioned earlier; subsequently, PA hardware control was handed over to the robot. The robot interfaces with PA hardware through a custom-developed communication controller to control the PA data acquisition, storage, and water-pump operation for delivering water mist as couplant at the foot of the transducer. The robot then moves to the designated target area on the stiffened panel, around the damages (D1, D2, and D3), and it sequentially performs separate PA line scans. Each line-scans are of fixed 150 mm length and a width of 34.5 mm. The right-hand side graphs of Figure 7 represent the results of the assessment of impact damages on the stiffened panel performed by the PA module mounted on the VRP. The impact damage 'D1' on the skin region is well detected, as shown in Figure 5a. However, damage 'D2' exactly on the web region is not detected, but the damage 'D3' is partially detected. as shown in Figure 7. The thickness of the skin and flange region in the stiffened panel is 1.6 mm and 2.1 mm Figure 5b, c, respectively. At the T-junction region formed by the intersection of both the flanges, there is no backwall reflection of the ultrasonic waves due to the geometric curvature and web width, and, hence, the small damage 'D2' imaging was not successful. Furthermore, it was observed that, on the back-face of the panel, at the transition region between the skin and flange of the stringer, there is an accumulation of excess resin and, hence, at this region there is no reflection of the ultrasonic waves for a short width of 3-5 mm. Because to this reason, the damage 'D3' is partially masked, as shown in Figure 5c.