## 8 Multiple Impacts of Space Debris Particles
There are no detached microscopic grains behind the barrier since the back side is made of steel. A heterogeneous screen with a direct gradient of steel has sufficient resistance to the particle flow impact. Since the front side of the screen is made of pure A356 alloy, it is natural that the initial stage of screen deformation resembles that of a layered screen. However, as the process advances into the depth of the screen, the resistance to deformation increases due to the inclusion of 316L steel. Therefore, the main damages and fractures of the screen occur on the front side, and the exit of waves to the back side of the screen does not lead to spall phenomena since the back side of the screen consists of pure 316L steel. However, the damage to the aluminum matrix is present throughout the volume of the screen (see Figure 15c). As all three screens have practically equal chances of protecting an important element of the spacecraft from both a single SD particle and a particle stream, let us consider the capability of the protective screens to withstand a large SD particle twice the diameter at the same impact velocity. Thus, the mass of the impacting SD particle equals eight masses of the particle used in the experiments and calculations described above. Figure 16 shows the comparison of the results of the calculations of the impact of a large SD particle on the screens. All selected screens have through holes. Screen #5, made of the volumetrically reinforced composite, has a hole close to a cylindrical shape. Large fragments are the fragments of reinforcing steel, while small fragments are the fragments of the aluminum matrix. The velocity of the large fragments in the head part of the barrier stream reaches 380-400 m/s, while that of the small ones reaches 450-475 m/s. On the periphery of the through cavity, stratifications of the reinforcement and matrix are noticeable due to the large deformations (see Figure 16a). Figure 15: Cross-sectional view of the screens after the impact of a 8 SD particle flow: (**a**) screen #5; (**b**) screen #4; (**c**) screen #7. The hole in the two-layer screen #4 has a more complex structure since the front aluminum plate received the main energy of the impacting SD particle, which led to the significant deformation of the material and subsequent damage and fracture. The rear steel plate of the screen was subjected to a weakened impact, which resulted in its bending. The stratification of the plates also allowed the shattered aluminum fragments to move radially, reducing the impact on the steel plate, so the final hole had small dimensions and was only in the central area. The steel damage was present in a small vicinity of the through hole. The fragment velocities in the cloud behind the screen were the same as above (see Figure 16b). Screen #7 is the heterogeneous screen with a direct distribution of steel in the aluminum matrix and also has a through hole due to the impact of a large SD particle. The front side of the screen suffered significant damage due to the release of strong compression waves on the periphery of the initially formed crater, resulting in the detachment of sufficiently thick layers. Since the screen strength increases with the depth of penetration, a layer with large deflections is formed near the rear surface, and the cavity narrows. Further, the residual mass of the impacting SD particle breaks through the thin steel layer, forming a plug. The velocity of both large and small fragments is approximately 360-380 m/s (see Figure 16c).