## 3 Damage Analysis  
### Analysis of Impact Process  
A high-speed camera (frame rate 160 kfps) is used to capture the dynamic process of the intersection of the spherical fragments with the structure, as shown in Figure 8.  
Situation 1: The fragment is incident from the bottom skin composite side at an incidence angle of 45\({}^{\circ}\), and the measured fragment exit velocity is 2327 m/s. The moment the fragment intersects with the structure, a strong photo-thermal phenomenon is generated at the impact location, resulting in local overexposure of the high-velocity camera. Subsequently, the fragment penetrates the structure and forms a debris cloud on the outside of the structure, accompanied by a splash of massive fiber. As the invasion progresses, the composite laminate forms a bulge. The rapid rupture of the bulge forms a debris cloud, which is mainly composed of broken small-diameter carbon fiber particles. The outer debris cloud at the incident end expands radially along the vertical direction of fragment incidence, and its main components are bulk fiber debris and fragment metal particles, while the inner debris cloud at the incident end expands radially along the direction of fragment incidence, and its main components are small-diameter carbon fiber debris groups.  
Situation 2: The dynamic intersection process of the fragment and structure in Situation 2 is similar to that shown in Situation 1. Due to the change in the direction of incidence, there is a difference in the expansion pattern of the debris cloud between the two. In Situation 2, the difference between the inner and outer debris cloud's highlight phenomena is more obvious, again due to the difference in their main components. The small-diameter carbon fiber debris group has a black main layer with dense spatial distribution and strong light absorption, and the photo-thermal phenomena captured by the high-velocity camera in the inner part of the incident section are weak.  
Situation 3: The fragment intruded from the top skin Al alloy side, and the photo-thermal phenomena were stronger both inside and outside. In Situation 1/2, when the projectile intrudes from the composite side, unlike the metal side, the carbon fiber breaks up to generate a large amount of dusty debris cloud, and the debris cloud is dominated by the carbon fiber debris, which will cover the entire high-velocity camera capture area, and the radial velocity is larger than the incident velocity during the expansion of the debris cloud. It is because the composite is made out of anisotropic material that when the impact direction is perpendicular to the fiber layup, the fiber is susceptible to shear fracture, and its normal strength is much lower than that of the metal material, resulting in a difference in the morphology of the bulge formed in the impact process, and therefore, the morphology of debris cloud diffusion formed in the final bulge rupture is also different.  
Figure 8: Impact process of test piece of composite/Al alloy lapping frame structure.  
The shock photo-thermal phenomena in Situation 1/2 are not as obvious as in Situation 3, and the main component of the firelight is the high-temperature metal fragments. Due to the violent friction between steel spherical fragments and Al alloys during the impact penetration process, the temperature is extremely high, resulting in the appearance of small metallic debris and the release of part of the heat accumulated by friction in the pattern of luminescence.