## 4 Results
### Finite Element Simulation Results  
During a lightning strike, the Joule heat generated by the conduction current flowing through the laminate will cause a continuous rise in the CFRP temperature [35]. A temperature contour exceeding a specific threshold value is used to characterize the damage area in each layer [16]. The threshold value usually adopts the decomposition temperature of the resin. When the temperature exceeds this temperature, the area is considered to be the decomposition damage area. According to Table 1, the decomposition temperature of the resin is set to 300 \({}^{\circ}\)C [29]. Therefore, the area surrounded by the 300 \({}^{\circ}\)C temperature contour in the FEA represents the same effect as the dark-red area (decomposition damage) under the C-scan. Figure 8(a1-a3) are the temperature profiles of the specimen under component C in the conduction current + heat flux, conduction current, and heat flux simulation models, respectively. Figure 8(a1,a2) indicate that the decomposition damage shape and size of the laminates are similar to each other and that the damage shape is circular. Such decomposition damage shapes, which are similar to the dark-red area in Figure 7a, do not reflect the anisotropy of the CFRP laminates because component C has a long duration, providing Joule heat in sufficient time to spread in all directions. The first layer (0\({}^{\circ}\) direction) and the second layer (90\({}^{\circ}\) direction) are two orthogonal layers, and their temperature distributions are shown in the upper-right corner and lower-right corner of Figure 8(a1), respectively. The decomposition damage shapes of the two layers are almost identical. Figure 7: Ultrasonic C-scan results of specimens after lightning strikes: (**a**) component C; (**b**) component D.  
In addition, the temperature of the top of the fastener (in the center of Figure 8(a1)) exceeds 3000 \({}^{\circ}\)C after the combination of the conduction current and the heat flux. This temperature is much higher than the melting temperature of the fastener and causes severe ablation damage to the fastener. Figure 8(a3) shows the temperature distribution when using the heat flux model. In this model, the damage to the fastener is more pronounced than the heat flux model. The heat flux model is more similar to the heat flux model than the heat flux model. Figure 8: Temperature profiles of specimens under different simulation models and different lightning current components: component C in the (**a1**) conduction current + heat flux model, (**a2**) conduction current model, and (**a3**) heat flux model; component D in the (**b1**) conduction current + heat flux model, (**b2**) conduction current model, and (**b3**) heat flux model. laminate caused by heat flux is approximately 2.5 mm, and the temperature of the fastener is similar to that in the conduction current + heat flux model, which means that the ablation damage of the fastener under the action of component C has an important relationship with the heat flux. Figure 9(a1-a3) are the cross-sectional temperature profiles of the specimen under component C in three simulation models. The heat diffusion can be inferred from the temperature gradient of the fasteners in Figure 9(a1,a3); heat is transferred to the CFRP laminate along with the fastener, causing the material temperature around the fastener to increase. In Figure 9(a2), the temperature of the fastener is far less than 300 \({}^{\circ}\)C and the decomposition damage occurs in a small area around the fastener. Because the damage occurs in the conduction current model, it is completely caused by the Joule heat generated by the conduction current.