### Experimental Results
#### 3.2.2 Blast-Loaded Plates with Three FSP Impacts  
In this section, a more detailed insight on the response of plates with three FSP impacts is gained by assessing the plates' shape during deformation. The result of the tests of configurations 3 and 4 are given in Figures 18 and 19. Out of the four blast tests on plates with multiple ballistic impact perforations, only one resulted in cracks and no total fracture was observed. Figure 18 illustrates the result of the permanent deformation and the out-of-plane displacement of tests 2 and 3 of configuration 3. As a first observation,  
Figure 16: Configuration 2, test 2: **(a)** 3D full-field out-of-plane displacement of permanent deformation of blast-loaded aluminum plate previously perforated using a single 12.7 mm FSP; **(b)** out-of-plane displacement history of different points placed at different distances from the center of the impact on the plate. Figure 17: Illustration of the major principal strains for blast-loaded aluminum plate (configuration 2 test 2). the deformation response is shown to follow trends similar to those previously found in Figure 16. Only slight differences are seen when comparing the deformation profiles at maximum displacement. The findings show that the maximum out-of-plane displacement of blast-loaded plates subjected to a single or three FSP impact are comparable. Therefore, as long as the impacts are located far enough from one another (such as in configuration 3), their locations and their numbers do not influence the dynamic behavior of the blast-loaded specimens. Figure 19 presents an image of the target plate captured by one of the high-speed cameras during testing, showing the damage for configuration 4. A line can be observed between the first and the third impact and a lighter one between the second and third impact. A zoomed-in image of the three enlarged holes highlights the location of crack tips and the crack between the first and the third hole. The crack appears as a discontinuity in the displacement field. It is clear from the DIC results that there is a loss of information between impacts 1 and 3; therefore, the line corresponds to full crack formation on the plate. On the contrary, there is only an initiation of crack formation between impacts 2 and 3. Because the fireball passes through the perforated hole in the target plate, determining crack propagation within the initial milliseconds following the explosion is challenging. Figure 20 shows high-speed images of the crack propagation. It depicts that the crack initiates at the extremity of the hole created by the first FSP impact and starts to propagate in the direction of the third hole until it reaches its extremity (Video S4). The initiation of crack formation between impacts 2 and 3 is caused by the strain concentration in the area between the two holes as shown in Figure 21. During blast loading, the plate experiences high internal stresses at maximum deformation. The paint coating has elongated more than the plate and was not able to follow the deformation, leading to a reflection of the aluminum underneath the paint. The reflection is captured by the high-speed camera as a lighter line. Figure 18: Plane view of permanent deformation of blast-loaded aluminum plates previously perforated using three 7.62 mm FSPs (tests 2 and 3 of configuration 3). The figure illustrates the full-field displacement of the plates’ centers. Figure 21 provides an illustration of the major principal strains measured by DIC for blast-loaded aluminum plate (configuration 3, test 2 and configuration, 4 test 1). In the figure, a clear zone of strain concentration is visible between impacts 1 and 3 and impacts 2 and 3. Indeed, for the major principal strains, a clear concentration is observed in configuration 4. In conclusion, if a fracture occurs in the plate, the pre-damage by the FSP impacts influences the global deformation after the blast loading. The experimental findings show that hole spacing has an impact on the plate's damage mechanism.