## 3 Results and Discussion
### Analysis of the Blow-Off Impulse
But the pressure is not as large as when the implosion effect starts, the maximum pressure is 2.846 \(\times\) 10\({}^{8}\) N at this moment. The blow-off impulse effect is completed when time is equal to 2 \(\upmu\)s, and the maximum damage is reached at this point. A large concave pit is formed around the blow-off impulse area and the depth of the concave pit is about 1.088 mm, as shown in Figure 12. Additionally, it also can be seen from Figure 12 that the element deletion phenomenon also occurs in the T stripper under the action of the blow-off impulse, indicating that the shock wave may cause some inner damage that cannot be seen on the surface. The element bulge phenomenon appears around the edges of element failure and the bottom of the concave pit is uneven. It can be concluded that the blow-off impulse effect not only causes external damage in the benchmark skin but also causes internal damage in the T stripper. Figure 11: Pressure contours in several typical moments. (**a**) \(t\) = 0.099 \(\upmu\)s; (**b**) \(t\) = 0.4 \(\upmu\)s; (**c**) \(t\) = 1.5 \(\upmu\)s; (**d**) \(t\) = 2 \(\upmu\)s.  
Figure 12: Damage forms in the cross-section of the composite reinforced panel. Figure 13 presents the pressure contours in blow-off impulse elements at the beginning, middle, and end of the calculation, respectively. Figure 14 presents the pressure change of blow-off impulse elements at different moments. It can be seen from Figures 13 and 14 that the pressure increases sharply when the explosion begins and the maximum pressure is 4.181 \(\times\) 10\({}^{9}\) N. The pressure then decreases sharply and the decrease-rate becomes slow and eventually tends to balance. However, the volume of blow-off impulse elements expands rapidly in the process of the explosion, then the volume reaches the maximum when the blow-off impulse analysis is completed, which can reflect the bulge phenomenon around the attachment area of lightning strike. Figure 15 presents the overall pressure contours of the composite reinforced panel after the blow-off impulse analysis is completed. It can be seen that an obvious bulge phenomenon appears around the blow-off impulse area, and the element failure mode around the blow-off impulse area is similar to that around the attachment area of lightning strike. Therefore, the blow-off impulse effect can reflect the damage forms of the composite reinforced panel induced by lightning strike and should be considered. Additionally, the pressure around the blow-off impulse area presents as isotropic rather than anisotropic, which agrees well with the fact that the mechanical properties of composite materials present as anisotropic in low pressure and isotropic in high pressure [25; 26]. Figure 16 presents the von Mises stress and pressure in the T stripper. It can be seen that there is no element deletion on the surface of the T stripper, but some internal elements are deleted as shown in Figure 12. This indicates that the blow-off impulse effect has a serious influence on the center areas in the top six layers of the benchmark skin, while the influences on the T stripper and other areas are relatively less serious. Figure 14: Pressure change of blow-off impulse elements. Figure 8:0 Pressure contours in blow-off impulse elements. (**a**) \(t\) = 0.0096 \(\upmu\)s; (**b**) \(t\) = 1 \(\upmu\)s; (**c**) \(t\) = 2 \(\upmu\)s.  
Mesh quality and element dimensions have a great influence on the calculation results. In order to study the influence of mesh quality on calculation results, the damage degree of a composite reinforced panel with five kinds of element dimensions are compared. Calculation results for different element dimensions are given in Figure 8:1 It can be seen that the error will increase with the increase of element dimensions. Although failure area errors change greatly with the increase of the element dimension, damage depth errors change little. However, there will be no blow-off impulse elements around the attachment area if the element dimension is larger than 10.0 mm in length direction.