## 5 Summary and Conclusions  
In this paper, a destroyed flat cover made of 18 Ni (350) used in an adjustable ballast tank during high pressure testing was analyzed. To find out the cause of the damage, the mechanical properties of the material used in the flat cover were re-examined by sampling and testing of the broken parts. Non-metallic inclusions analysis, microstructure analysis, and fracture surface analysis were conducted to find out the possible fracture cause. Finite element analysis based on fracture mechanics to understand the ultimate cause of destruction was conducted. The following conclusions and recommendations can be made:  
Figure 17: Stress intensity factor along crack front with increasing external pressure.  
Figure 18: Stress intensity factor at the surface points and the deepest point.  
(1) Maraging steels are expected to still have high toughness when improving their ultra-high strength level. 18 Ni (250) grade is currently an acceptable candidate material for deep-sea pressure hulls, which has achieved satisfactory application experience. To obtain better weight-to-displacement ratio, the designers applied 18 Ni (350) for an adjustable ballast tanks used in a deep-sea submersible. The strength of 18 Ni (350) is higher than that of 18 Ni (250), however its toughness is lower. In the current design guideline for deep-sea pressure hulls, the requirements for material toughness have not been emphasized. The trial of applying 18 Ni (350) was made in the present study but unexpected failure of the flat cover occurred when load was relatively small.  
(2) Non-metal inclusions analysis, microstructure analysis, and fracture surface analysis showed that the original state of the material meets the design requirements with fewer kinds of non-metallic inclusions. The material has low impact toughness, which is one of the reasons for the cracking of the flat cover during the test.  
(3) From the design point of view, it is suggested to optimize the transition from right angle to chamfer at variable cross-sections, and to increase the thickness of the outer edge liner of the cover appropriately, which will reduce the possibility of damage. During processing, the surface finish of structural parts should be ensured, especially the root surface of the variable cross-section, so as to avoid surface processing defects.  
(4) The reliability of deep-sea pressure hulls, especially the pressure hulls of manned submersibles, guarantees the safety of personnel and equipment. There must be attention paid to the comprehensive performance of materials. From the point of view of material selection, the application of 18 Ni (350) maraging steel to the pressured structure in deep-sea environments is a risky choice. On the other hand, the 18 Ni series of maraging steels have broad application prospects in deep-sea environments, so improving the toughness and plasticity together with increasing the strength level will be an important research direction [23].  
The individual contributions are specified as follows: conceptualization, F.W. and W.C.; methodology, F.W. and J.Z.; software, M.W.; validation, M.W., S.Z., G.T., and Z.J.; formal analysis, F.W.; investigation, S.Z.; resources, Z.J.; data curation, M.W. and G.T.; writing--original draft preparation, F.W.; writing--review and editing, W.C.; visualization, F.W.; supervision, W.C.; project administration, F.W. and W.C.; funding acquisition, F.W. and W.C.  
This research was funded by the General Program of National Natural Science Foundation of China, grant number 51679133, the State Key Program of National Natural Science of China, grant number 51439004, and the \"Construction of a Leading Innovation Team\" project by the Hangzhou Municipal government.  
The authors declare no conflict of interest.