Evaluation of the Residual Capacity of a Submarine for Different Limit States with Various Initial Imperfection Models

The current design philosophy for submarine hulls, in the preliminary design stage, generally considers as governing limit states material yielding along with various buckling modes. It is common belief that, beyond the design pressure, material yielding of the shell plating should occur first, eventually followed by local buckling, while global buckling currently retains the highest safety factor. On the other hand, in the aeronautical field, in some cases structural components are designed in such a way that local instability may occur within the design loads, being the phenomena inside the material elastic range and not leading to a significant drop in term of stiffness. This paper is aimed at investigating the structural response beyond a set of selected limit states, using nonlinear FE method adopting different initial imperfection models, to provide the designers with new information useful for calibrating safety factors. It was found that both local and global buckling can be considered as ultimate limit states, with a significant sensitivity towards initial imperfection, while material yielding and tripping buckling of frames show a residual structural capacity. In conclusion, it was found that the occurrence of local buckling leads to similar sudden catastrophic consequences as global buckling, with the ultimate strength capacity highly affected by the initial imperfection shape and amplitude.