## 3 Damage Analysis
### Composite Damage Analysis  
In Situation 1/2, the damage morphology of the structural composite top skin and the stringer intrusion by the high-velocity spherical fragments are shown in Figures 9 and 10, respectively. The damage broken hole is mainly ellipsoidal under the 45\({}^{\circ}\) oblique impact of the fragment in Situation 1. In Situation 2, under the positive impact of fragment 0\({}^{\circ}\), the damage broken hole is mainly in the pattern of regular spherical rupture. In Situation 1/2, there is fiber spalling on the outer surface of the incidence. The stringer part is thin and shows a band-like broken hole under the shearing effect in both Situation 1/2. In Situation 3, the fragment is incident from the Al alloy metal side. According to Figure 11, the fragment formed a debris cloud consisting of a large number of metal particles when it penetrated the top skin. The debris cloud still has a high kinetic energy, and the secondary damage formed at the bottom skin, which is mainly in the pattern of small-diameter broken holes and surface spalling, is widely distributed. The test piece composite part was scanned by CT, and the damage morphology feature images of the composite test piece were obtained for each directional interface, layer by layer, and then the 3D view of the damage of the test piece composite part under each working condition was obtained by image rendering, as shown in Figure 12.  
Figure 10: Typical damage mode of composite stringer. Figure 9: Typical shear damage mode of composite bottom skin. [PERSON] pointed out that the damage aperture of thick plates decreases slightly along the thickness direction and then increases rapidly, while the aperture of thin plates expands in the shape of a circular truncated cone, and [PERSON] et al. used the combined area of a cylindrical and circular truncated cone [7; 8; 9]. In the early stage of projectile penetration, the front side of the laminate would generate a shear-plugging hole similar to the shape of the projectile contact area, where the matrix material is crushed and loses its support to the fiber, and shear failure occurs between the fiber and the surrounding fiber due to the presence of a large velocity gradient, which is called the shear failure zone (A). With the continuation of the penetration process, the projectile velocity decreases, the target plate bends, and with the continuous expansion of the bending deformation, fiber tensile failure occurs first in the outermost layer of the back side of the impact, and the tensile failure expands from the outer layer to the back side with a certain crack inclination angle and produces delamination, forming a fiber tensile failure zone behind the shear failure zone (B). Meanwhile, there is a small delamination damage zone around the shear failure zone/fiber tensile failure zone (C). In this test, the delamination zone of laminate caused by the high-velocity impact of spherical fragments is limited, and the fiber in the zone is mainly recoverable deformation without significant fiber damage. The projection of the laminate damage area in the direction of fragment incidence shows different morphological features in different damage areas, as shown in Figure 14. Herein, the position of the blue line is the position of the broken hole section in the vertical plane. On the upper surface of the shear failure zone, the main body of the broken hole section is circular. The laminate-free surface is strongly impacted by spherical fragments, and there are more striated fiber-stripping areas around the circular rupture holes. As the penetration progresses, the breach section inside the shear failure zone is mainly regular circular, and the area of the spalling zone decreases. At the shear failure zone/fiber tensile failure zone intersection, the breach section has common features of both. The shape of the breach section is transformed from a shear failure-oriented circle to a fiber tensile failure-oriented square.