Multi-segment shell typical of underwater vehicles is currently important underwater equipment. Stealth performance is a key indicator to improve the working capacity of underwater vehicles. Low-noise design is an important means to improve stealthy performance. Accurate and effective identification of its structure Frequency Response Function (FRF) is the premise of low-noise design. However, the multi-section shell joint interface has many connection points, complex transmission path and typical nonlinear characteristics, which challenges the acquisition of its structural FRF. The Receptance Coupling Substructure Analysis (RCSA) is used to segment the multi-stage shell, which can realize the integration of finite element method and experimental method, and provide an effective way to obtain the FRF of multi-segment shell. At the same time, the multi-segment shell is a typical thin-wall structure, and its actual buckling process under impact is a nonlinear (large deformation) process. Therefore, the nonlinear joint interface is constructed based on linear connection, and the RCSA for nonlinear joint interface is studied. On the basis of accurately obtaining the FRF, it is of theoretical and application significance to study the low noise design of shell.

This paper proposes a difference discretization method (DDM) under a difference frame for the prediction of milling stability. In this method, the dynamic milling process is described by a delay differential equation (DDE) with two degrees of freedom rather than the traditional state-space form with a single discrete time delay. After discretization, only the velocity and acceleration in the DDE are approximated by the first- and second-order central difference for each smaller time interval, while the other items are kept unchanged. Then, the criterion for the optimal discretization interval number is put forward and derived based on the largest effective time interval (also called the critical time interval). The use of the critical time interval cannot only obtain sufficient accuracy, but also promotes as much efficiency as possible. Subsequently, a new DDM (NDDM) with varied discretization interval numbers as the milling rotating speeds is developed. Finally, the effectiveness of the proposed algorithm is demonstrated by using a benchmark example for a two-degrees-of- freedom milling model compared to the full discretization method (FDM) and the Hermite-interpolation full discretization method (HFDM). The results show that the proposed method has satisfactory stability charts and is able to increase the efficiency by 100% or more.

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