## 2 Materials and Methods
### Experimental Setup and Inspection Methods  
Pulsed thermographic tests have been conducted with long pulses in the experimental laboratory located in the Engineering Faculty of the University of Salento (Lecce, Italy). Thermographic set-up means both the equipment used to test and the appropriate arrangement of specimen position and excitation are influent on results [19]. The equipment consists of an array of four halogen lamps, each of 1000 W, controlled by a signal generator with single square wave form of amplitude set to maximum lamp power and period calculated on the base of established heating time in following tables to synchronize thermal pulse and recording acquisition; a FLIR 7500M IR camera (FLIR Systems, Inc., Wilsonville, OR, U.S.), with a FPA cooled detector, endowed with NETD 25 mK In-Sb sensor and image resolution of 320 \(\times\) 256 pixels; a custom processing software. Previously cited experimental campaigns defined optimal set-up possibilities for a better characterization of defects on similar CFRP plates [18; 20], suitable thermographic parameters in terms of heating times in the range 12-40 s (capable to better identify defect depths in 2-6 mm range with low power halogen lamps thermal input), experimental configuration distance, frame rate acquisition, and lamp configuration. For NDT, the specimen's surface is positioned at 0.5 m from lamps and an expanded polystyrene support is applied because of thermal insulation avoids board effects, performing acquisitions based on previous experimental experience [18]; Figure 3 shows the optimal setup of the thermographic \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Part** \\ **Component** \\ \end{tabular} & \begin{tabular}{c} **Sub-Part** \\ **Component** \\ \end{tabular} & \begin{tabular}{c} **Material** \\ **Material** \\ \end{tabular} & \begin{tabular}{c} **Lamina** \\ **Thickness (mm)** \\ \end{tabular} & \begin{tabular}{c} **N\({}^{\circ}\)** \\ **Plies** \\ \end{tabular} & \multicolumn{2}{c}{**Stacking Sequence**} \\ \hline \multirow{2}{*}{**Skin**} & \(\begin{array}{c}\text{Skin}\,1\\ \text{Skin}\,2\end{array}\) & \begin{tabular}{c} CFRP \\ CFRP \\ \end{tabular} & 0.186 & 20 & \(\begin{array}{c}[45/90/-45/-45/45/90/0/-45/45/01\,_{\text{S}}\\ \text{24}\) \\ \end{array}\) \\ \hline \multirow{2}{*}{**Stinger**} & \(\begin{array}{c}\text{CFRP}\\ \end{array}\) & \
\begin{tabular}{c} 0.186 \\ 0.186 \\ \end{tabular} & 24 & \(\begin{array}{c}[45/90/0/0/-45/45/45/90/0/-45/45/0]\,_{\text{S}}\\ \text{24}\) \\ \end{tabular} \\ \hline \multirow{2}{*}{**Rib**} & \(\begin{array}{c}\text{CFRP}\\ \end{array}\) &
\begin{tabular}{c} 0.208 \\ \end{tabular} & 12 & \(\begin{array}{c}[45/0/-45/90/45/0]\,_{\text{S}}\\ \text{24}\) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Part components for multi-stringer element. Figure 2: **(a)** CFRP multi stringer multi-stringer component’s geometry and **(b)** and artificial damage geometry. apparatus used for all tests, to ensure acceptable uniform heating of the exposed surface, 12 total tests are conducted: 6 on Plate I (three for both sides) and 6 on Plate II (three for both sides), selecting different heating times (Table 4). For the multi-stringer component, preliminary ultrasonic inspections are conducted with phased array technology, which generally provides the pulses controlled excitation (e.g., amplitude and delay) of single elements in a multi-element probe, as opposed to single-element probes for conventional UT [21]. The excitation of piezo-composite elements can generate a focused ultrasonic beam with the possibility to modify parameters such as beam and angle, focal distance through software. The phased array system uses the physical principle of multiple progressive sound waves, varying the time between a series of ultrasonic pulses [22].