## 3 Results and Discussions
### Effect of the Number of Impacts  
The effect of repeated impacts on the primary barrier was investigated by applying up to ten impacts on each specimen. The reaction force was measured each time, and the deformed shape of the primary barrier was evaluated at the first, fifth, and tenth impacts. Figure 8 displays the reaction force profiles of the large and small corrugation specimen in the 123 scenario. As the drop object falls freely, the impactor hits the specimen and an impact load is transferred to the specimen that is measured by the load sensor located at the bottom of the impact test jig. The reaction force exhibits a negative profile because the impactor acts in the downward vertical direction. In the reaction force-time history of the first impact, the longest duration and lowest peak reaction force are observed. A long duration suggests that the contact time between the impactor and corrugation was long. As shown in the deformed configuration of the above specimens, the middle part of the convex corrugation is deformed inward under the first impact. When the impact is repeated five times, the duration decreases and peak reaction force increases. In the case of a large corrugation, after the sixth impact, the peak reaction force decreases once and then converges to a particular value, which may be affected by the release process of the primary barrier from the impact test jig to capture photographs of the deformed specimens. In addition, the release process also affects the characteristic of the reaction force profile. Similar reaction force profiles are observed from the fourth to fifth impact on both the large and small corrugation specimens. After the release process, however, the shape of the reaction force profile from the fifth to tenth impact is different from the profile from the fourth to fifth impact. Figure 7: Comparison of displacement between large and small corrugation in accordance with impact energy. In the case of a small corrugation specimen, a slight increase in the reaction force is observed. This is a different result from that of a large corrugation specimen for which the reaction force did not show any further increase. It appears that the contrasting tendencies of the peak reaction forces of the large corrugation and small corrugation specimens result from the differences in the structural complexity. A large corrugation has reinforcing ribs to increase the buckling strength and connect with the butterfly wing shape part of the knot so that under repeated impacts a large corrugation becomes increasingly less deformed. An extensively deformed large corrugation undergoes strain hardening, leading to a dominant elastic behavior. However, in the case of small corrugation specimens, only the smallest corrugations gradually deform. This can be explained as a pseudo-shakedown phenomenon. The extent of permanent deformation of the steel-plate structure exhibiting plastic behavior under repeated impact loads of the same amount decreases, and the elastic behavior of the structure becomes dominant. This is because the amount of elastic strain energy absorbed by the structure increases as the structure exhibits a plastic behavior [27; 28; 29]. Thus, the additional permanent deformation decreases, and the peak reaction force continues to increase as the duration of the reaction force-time history  
Figure 8: Reaction force time history of L123 scenario (**a**) from the first to the fifth impact loading and (**b**) from the sixth to tenth impact loading, and S123 scenario (**c**) from the first to the fifth impact loading and (**d**) from the sixth to tenth impact loading. decreases owing to the impulse conservation. Such results were also obtained in previous research studies [29; 30]. Figure 9 illustrates the deformation configuration of the specimens with large and small corrugations under the first, fifth, and tenth impacts of L123 and S123 scenarios. As shown in Figure 9, the large corrugation specimen with an outward concave shape is deformed into an inner recess under the first impact.