## 5 Conclusions  
To investigate the mechanisms and rules of SHGs' sagging damage when subjected to a near-field underwater explosion bubble, the CEL two-way fluid-structure interaction was adopted, verifying the model's accuracy. Afterward, the responses of 11 SHGs subjected to the near-field underwater explosion were simulated. Then, the plastic hinge lines and size of the bend zone were investigated, and the defensive effect of transverse bulkheads was analyzed. Finally, the equivalent moment of inertia considering distant transverse bulkheads was constructed to evaluate the overall longitudinal bend resistance. The main conclusions can are summarized as follows:  
(1) The finite element model of SHG for near water surface and near-field underwater explosion based on the CEL method is verified by comparison with the literature experiment. The calculation results agree with the experimental data and show consistency regarding the crease pattern on the side plates. This indicates that the computing method can be used for further related work. Figure 15: Weight coefficient of transverse bulkhead. (2) Regarding sagging damage, the overall bend in the SHGs is composed of multiple plastic hinge lines on the hull plates and mainly depends on the local folding of the side plates. For the bending mode where the creases on side plates are W-shaped, a bend comprises ten plastic hinge lines: eight on the side plates and two on the bottom plates. The transverse bulkhead longitudinal relative position and load strength affect the bend position, size, and quantity. (3) For the sagging damage of the W-shaped creases, the formula (2.43 times the height of the side plate) for estimating the length of the potential bend zone of the SHGs is given by the regression method. This formula mainly applies to predictions regarding the sagging damage area of the most basic hull plate configuration (opening, no additional structure) subjected to near-field underwater explosions. The error of this formula is less than 5% for cases in this paper, which can provide a reference for the anti-impact work of ships. In practice, the creases are not sharp but have a certain curvature, which will expand the core deformation zone. Therefore, it is recommended to multiply the appropriate safety factor when using the formula for estimation. (4) When facing the near-field underwater explosion load, the transverse bulkhead, not only on the corresponding section but also at a certain distance, has an important influence on the total longitudinal strength. When located in the bend zone, the transverse bulkheads can enhance the overall bend resistance by increasing the local strength and limiting the development of creases on the side plates. On the other hand, when located outside the bend zone, the transverse bulkhead has a non-negligible influence, affecting the deflection results in this paper by 94.6%. However, this does not always play a positive role. (5) The equivalent moment of inertia is established, and the weight coefficient functions of transverse bulkheads at different positions are preliminarily given. The equivalent moment of inertia can appropriately reflect the influence of transverse bulkheads at different positions on the overall bend resistance when the near-field underwater explosion bubble causes sagging damage. The establishment of the weight coefficient is based on the SHG results in the paper, and its applicability to different ship types requires further investigation. It should be noted that the \"W\" plastic hinge lines mode that was constructed and the prediction formula are mainly based on an open box hull girder structure used in this paper, which is a foundation form. Evolutionary modes of \"W\" or other modes may occur when there are additional structures, but the potential bend zone length should be less than the W-shaped zone. Conceptualization, W.Z. and Z.D. ; data curation, Y.G. and Y.Z. ; formal analysis, Y.G. ; investigation, Y.G. and W.Z. ; methodology, Y.G. and W.Z. ; project administration, W.Z. and Z.D.