This study focuses on the pressure hull structures of deep-sea submersibles. These structures ensure the safety, stability, and continuous operation of vehicles in challenging underwater environments. The cylindrical pressure hull, known for its high spatial efficiency, robust load-bearing capacity, mature design theory, and ease of manufacturing, is widely adopted in deep-sea submersible applications. Given the pressing need to improve the buoyancy-to-weight ratios of underwater vehicles, the selection of materials for pressure hulls has become a critical aspect of submersible design. Pressure hull materials can be broadly categorized into metal and nonmetal materials. Titanium alloys, which are technically mature metals, are commonly used owing to their superior properties. However, metallic materials are susceptible to corrosion in seawater and can significantly increase the overall weight of underwater vehicles. Nonmetal materials, typically fiber-reinforced composite materials, offer advantages such as high specific strength, high specific modulus, corrosion resistance, and design flexibility. Hence, such materials are widely used in cylindrical pressure structures. However, the fabrication of pressure hulls using pure composite materials is challenging, and the low ductility of the composite materials is not conducive to the arrangement of internal equipment within the submersible hull. Metal–composite structures combine the strengths of both materials, as the metal layer provides excellent ductility and the external composite layer enhances safety and reduces overall weight. Thus, metal–composite structures have become a subject of extensive research. Moreover, metal-composite pressure hull structures exhibit superior strength under external pressure. This study specifically explores the structural and performance aspects of titanium alloys and composite pressure hulls. Moreover, this research employs modeling and finite element analyses to investigate the impacts of different layer thickness ratios of the metal and composite materials on the strength of the pressure hull. The finite element model incorporates three-dimensional solid modeling to simulate real loading conditions using precise boundary conditions. Additionally, this study involved the practical fabrication of composite pressure hulls using a wet winding process that was followed by underwater burst tests conducted alongside titanium alloy pressure hulls. The analysis included both buckling and strength analyses conducted using the finite element method. The buckling analysis was divided into linear and post-buckling analyses. In the linear buckling analysis, applying a pressure of 1 MPa to simulate the external surface of the pressure hull revealed that buckling primarily occurs in the cylindrical section, with both the composite and titanium alloy layers playing crucial roles. However, linear buckling analyses, which predict the theoretical buckling strengths of ideal elastic bodies, do not consider material nonlinearity or structural defects. The introduction of first-order modal displacements for the nonlinear buckling analysis indicates that the critical buckling load gradually increases with an increase in the number of composite layers. These results are attributable to the higher stiffness of the composite layers, which makes the buckling of the pressure hull more challenging. However, the critical buckling load decreases with a further increase in the number of composite layers. This decrease occurs because the large span of the two end caps makes the thickness variation significantly affect the stability, indicating that the titanium alloy layer should not be too thin. Overall, the addition of composite layers significantly enhances the critical buckling load of the pressure hull, which thereby emphasizes the importance of ensuring the appropriate thickness of the titanium alloy layer. Strength analyses utilize static/general analytical steps to simulate a 60 MPa external pressure environment. A material failure analysis employing the Hashin failure criterion indicated that composite pressure hulls exhibited higher strength and reduced weight under identical conditions. Similar to the buckling analysis results, the highest strength was achieved when the thicknesses of the composite and titanium alloy layers were comparable. However, an excessively thin metal layer reduces the overall hull strength. In underwater burst tests, the buckling results aligned well with the theoretical calculations for titanium alloy pressure hulls. However, the experimental results for the composite pressure hulls deviated significantly from the calculations, suggesting that the low bond strength between the titanium alloy and carbon fiber composite layers may lead to delamination and rapid failure under high pressure. Future studies should focus on addressing these issues. Furthermore, recognizing that pressure hulls under external pressure bear both axial and radial loads in composite layers, this study proposes the introduction of helical winding layers to replace circumferential winding layers. To fully exploit the performance advantages of carbon fiber composite materials, a MATLAB program was developed based on classical laminated plate theory. The program calculates the layer strength coefficients and optimizes the design of the helical layer angles and thickness distribution. The validation of the program’s efficiency and optimization results indicates that the layering approach achieves the highest strength within the 30°-40° angle range with a moderate ratio between the circumferential and helical layers. Finite element analysis results further validated the optimization results, thus confirming the effectiveness of introducing helical winding layers in enhancing the strength of pressure hull structures. This comprehensive study serves as a beacon in the field of deep-sea submersible technology by providing invaluable insights into the intricacies of pressure hull design. From theoretical analyses to practical applications and innovative design proposals, this research not only deepens our understanding of pressure hull behavior but also charts a course for future advancements in underwater vehicle technology. © 2024 Chinese Mechanical Engineering Society. All rights reserved.
