## 3 Results and Discussion  
For the four different types of aircraft carbon fiber/epoxy composite-reinforced panels, the surface damage pattern and size due to lightning strikes is shown in Figure 6. The composite panel without protection was the most serious, caused by fiber fracture and matrix cracking, resin vaporization, delamination, etc. Lightning damage propagated significantly along the fiber direction. For lightning protection structures, lightning strikes can result in the melting of metal covers, and composite damage is inevitable. The melting of the metal coating was propagated outward from the attachment point of the lightning arc. The surface damage area of four different types of composite-reinforced panels is given in Table 4. It can be seen that the surface damage area of the lightning protection structures was larger than that of structures without protection, and the local-sprayed aluminum coating had the largest damage area. Although the metal covers were damaged, the composite-reinforced panels underneath were protected from lightning strikes, and composite damage was decreased to some extent. Therefore, the internal lightning damage state of composite structures under metal covers needs to be further evaluated by non-destructive testing method such as ultrasonic C-scanning in our present study. Damage echo amplitude, echo location, and location data of the second scanning is shown in Figures 7-10 for four different composite panels--those without protection, with a full-sprayed aluminum coating, with a local-sprayed aluminum coating, and with a full-embedded copper mesh filling, respectively. Delamination damage is developed with a ladder type along the thickness direction due to substantial lightning Joule heat, thermal shock, internal explosion, etc. The higher percentage is, the larger echo the amplitude is and the lower the absorbed energy of the reflected wave is. This implies a lower occurrence probability of delamination damage or internal composite-reinforced panel flaws. The white zone in the echo amplitude figures implies that echo amplitude was beyond the scanning height range or dissipated completely due to the complex damage pattern such as the fluffed damage, which is in accordance with the black zone in the echo location figures. Damage location data were analyzed corresponding to the selected section of the test specimen, presented in yellow cross curves. Part of the central lightning attachment zone is shown in white and red in Figure 7a. Vast surface carbon fibers were fractured or sublimated, and appeared to have unconstrained conditions, and an epoxy resin matrix had a carbonized decomposition based on visible detection. The white and red zones distributed along the 45 degree fiber direction on the top lamina and a narrow fiber fracture belt ran throughout the entire specimen, which is symmetrical with respect to the attachment point of impulse electrical current. An approximate elliptical damage appeared irregularly around the white and red zones, and was mainly formed in the deterioration zone. Delamination damage is distinguished by abrupt color changes in Figure 7b. Damage location data in Figure 7c show that the specimen surface was slightly tilted and that the deterioration depth was about 2 mm apart from the top lamina. Approximate circular attachment damage appears irregularly in the central zone in Figure 8a, in which a small part is presented in white and the other in red. The reason may be that the thickness of the sprayed aluminum coating is not constant along the specimen surface due to the spraying craft limit, which results in a non-uniform electrical charge distribution. The isotropic aluminum layer was uniformly distributed, and electrical current was conducted along its surface. Comparison with visible detection shows that the aluminum layer was intensely melted and vaporized in the central zone. Many carbon fibers were broken up, and the epoxy resin matrix had carbonized decomposition in this zone.