## 2 Experiment and Testing Procedure  
The aircraft composite-reinforced panel is shown in Figure 1. The thickness of the composite panel and the T-type stiffener was 3.6 mm and 24 ply. The length and width of the composite panel were 500 and 250 mm, respectively. The width and height of the stiffener were 50 and 38 mm, respectively. The lay-ups of composite-reinforced panel are shown in Figure 2, and details are given in Table 1. The thickness of the left and right T-type stiffeners was 1.8 mm at 12 ply. The reinforced core was filled into the R zone. The J-116B film adhesive was located between the composite panel and the T-type stiffener. Three types of lightning protection structures for composite-reinforced panel are shown in Figure 3. The composite-reinforced panel without protection was used as a benchmark specimen. For composite panels with full- and local-sprayed aluminum coatings, a 0.2 mm aluminum layer thickness was considered. For the composite panel with a copper mesh filling, a 0.2 mm copper mesh thickness was considered, and the space between meshes was 0.1 mm. According to SAE standards [15; 16; 17], the lightning strike experiment was performed by continuously loading impulse electrical current waveforms such as A, B, C, and D to composite-reinforced panels, and the distance between test specimens and electrode was set to 50 mm. A, B, C, and D current waveforms are shown in Figure 4, and their parameters are given in Table 2. Metal strips were placed on the sides of composite specimens, and their ends were linked with ground wire so as to ensure a good ground connection. The atmospheric pressure was 1.013 \(\times\) 10\({}^{5}\) Pa, the temperature was 25 \({}^{\circ}\)C, and the humidity was 79%. The ultrasonic C-scanning technique shown in Figure 5 is based on the propagation of the high frequency sound wave. The probe is capable of both transmitting and receiving high frequency sound waves, scanning the specimen surface on a certain path, and internal damage information regarding the composite specimen can be obtained by the reflected wave. Figure 3: Three types of lightning protection structures for composite-reinforced panel: (**a**) the local-sprayed aluminum coating; (**b**) the full-sprayed aluminum coating; (**c**) the full-embedded copper mesh filling. (Unit: mm). The composite-reinforced panels were immersed in water. The damage pattern and degree was detected by a full digital ultrasonic C-scanning system (UPK-T36) with a 5 MHz detector at room temperature. The detector was toward the composite panel and backward stiffener. The specimen surface was selected as the initial scanning location, and each specimen was scanned twice. Firstly, the composite-reinforced panel was scanned entirely in order to roughly distinguish the lightning damage condition. Secondly, further damage conditions were scanned, and the initial scanning point was 0.96 mm, and the scanning thickness was 2.64 mm. The scanning resolution ratio was 0.5 mm, and the range symmetrically involves the main damage region of the composite specimen, the length of which was 390 mm with a 175 m/s velocity and the width of which was 200 mm with a 100 m/s velocity. The scanning blind zone was considered in order to neglect zones on the front detector by setting the delay time of the echo, where invalid echoes may be disturbed to deal with the real echo when its intensity reaches a certain extent. The initial delay time was 75 \(\upmu\)s and the scanning time margin was 10 \(\upmu\)s along the height direction when the detector time was 0, which indicates that echo data within a 75 and 85 \(\upmu\)s delay time are valid, while those with a delay time between 0 and 75 \(\upmu\)s or greater than 85 \(\upmu\)s are invalid. The initial scanning threshold was a 50% amplitude