## 3 Results and Discussions
### Effect of the Number of Impacts
A small amount of displacement is measured for the middle and upper parts of the large corrugation. As the number of impacts increases from five to ten, the radius of curvature decreases in the middle part of the large corrugation, with the corrugation deforming internally. After the fifth impact, the distortion of the butterfly wing shape at the knot part is clearly visible. As the number of impacts further increases, the butterfly wing shape becomes more distorted. Owing to the difficulty in measurement, any displacement of the knot part is not evaluated. The deformation of a large corrugation under the repeated impact in the lateral direction, however, can affect the deformation of the knot part; therefore, it is necessary to investigate the relationship between the two effects. Although the deformation of a large corrugation is accompanied by a distortion of the knot part, the deformation of a small corrugation did not distort the butterfly wing shape of the knot. However, as a small corrugation is deformed with repeated impacts, the affected zone caused by the deformation of the small corrugation is identical to the affected zone caused by the distorted butterfly wing shape of the knot of a large corrugation, which is illustrated in Figure 10.  
Figure 9: Deformed configuration of the L123 scenario specimen under (**a**) the first impact loading, (**b**) the fifth impact loading and (**c**) the tenth impact loading, and of the S123 scenario specimen under (**d**) the first impact loading, (**e**) the fifth impact loading, and (**f**) the tenth impact loading. As an impact is repeatedly applied to the large and small corrugation specimens, the hinge-shaped part connecting the large corrugation part and bottom sheet acts as a fulcrum and can generate leverage. Therefore, it is expected that the deformation of a large corrugation will affect the bottom part of the primary barrier, as shown in Figure 11. However, in the present study, no deformation is observed at the erection plate under the primary barrier, owing to the thickness of the steel plate. This phenomenon is noticeably observed in the case of specimens with large and small corrugations that have been flattened by impact. Though a corrugation lies horizontally, the lifted shape of the upper part of the bottom sheet in the primary barrier clearly shows that the hinge-shaped part plays the role of a fulcrum connecting the large corrugation and bottom sheet. The vertical displacement of the bottom sheet is not measured owing to its complexity. Figure 12a,b display the displacements of the large and small corrugation specimens in terms of the number of impacts. In Figure 12a, the effect of the number of impacts on the displacement of the large corrugations is evaluated. The displacements of the small corrugations under the repeated impacts between 3 to 10 times are analyzed as shown in Figure 12b. In the S230, S282, and S412 scenarios, the additional impact tests terminate at the fourth, third, and third impacts, respectively, owing to the decrease in the impact-effective area. The repeated impacts are applied up to ten times in the L123 and S123 scenarios. In the case of a large corrugation specimen, the displacement increment from the first to the fifth impact is more than from the sixth to the tenth for all the scenarios. Thus, the corrugation becomes less deformed under repeated impacts. According to previous research on the  
Figure 11: Schematic diagram of hinge-shaped part playing a role of a fulcrum in primary barrier. Figure 10: Affected area at the knot part distorted by (**a**) deformation of large corrugation and (**b**) deformation of small corrugation. effects of a repeated impact [27] when the same amount of impact is repeatedly applied, the amount of deformation decreases. Figure 12c,d represent the displacement increment per number of impacts according to the scenario for the large and small corrugation specimens.