Introduction: To improve the low frequency isolation capability of vibration isolation system, a kind of electromagnetic quasi-zero-stiffness (QZS) vibration isolator was designed. Materials and methods: The negative stiffness electromagnetic spring paralleled with the linear positive stiffness spring to achieve the state of QZS. The nonlinear dynamic model of vibration isolator was established. Through the combination of theoretical formula and simulation analysis, the electromagnetic force expression was obtained. The amplitude-frequency responsecharacteristics and force transmissibility was solved by harmonic balance method, and the effects of system parameters on amplitude - frequency characteristics and transmissibility was analyzed. Conclusion: The results show that the new isolator has better performance than the linear system, and the decreased damping ratio and excitation amplitude make the effects of vibration isolation of system superior.

The vibration and sound radiation characteristics of the submarine pressure hull structure are important performance indicators for the acoustic design of the submarine structure. In this paper, the spectral element method is used to analyze the vibration of the shell structure. Combined with the periodic structure theory, the theoretical calculation method of the periodic ribbed shell structure is deduced, and the analytical method that can be used to calculate the vibration of the pressure shell structure is established. The feasibility of applying the spectral element method to the vibration calculation of the pressure shell structure is verified by comparing the analytical calculation and the finite element simulation. The vibration characteristics test experiment of the actual submarine pressured hull structure is carried out. The results show that the theoretical analysis is correct. Highlights: The vibration and sound radiation characteristics of the submarine pressure hull structure have an important influence on the radiated noise of the submarine. A theoretical analysis method of vibration characteristics of pressure hulled based on spectral element method and periodic structure theory is proposed. The experimental study on vibration characteristics and active control of line spectrum of the pressure hull is carried out.

The leakage of the ship’s pipeline system will bring great risks to the engine equipment and seriously threaten the vitality of the ship. In this paper, the pipeline leakage detection and localization research are carried out based on the vibration signal generated by pipeline leakage. First, the finite element model of the pipeline is constructed to obtain the variation law of the vibration signal when the pipeline leaks are carried out. Second, the vibration signal is processed based on the variational mode decomposition (VMD) and radial basis function (RBF) neural networks. The wavelet packet threshold noise reduction is conducted before signal decomposition to improve the signal-to-noise ratio. Then, the denoised signal is decomposed by VMD. The effective component is identified by analyzing the correlation coefficient between the component and the denoised signal. The center frequency and energy of the effective component are used as feature vector to train the RBF neural network to identify and locate leakage. Finally, a pipeline leakage test platform is built under laboratory conditions. After processing the data samples collected from the test, the RBF neural network is trained to identify and locate leaks. The test sample identification results show that the leak identification and localization method based on VMD-RBF has a high accuracy.

In order to suppress low frequency vibration more effectively, a quasi-zero stiffness vibration isolation device is designed by using five parallel linear springs. The stiffness characteristic of the system is analyzed, and the quasi-zero stiffness (QZS) condition of the system is given. The dynamic behavior analysis of the system is analyzed by harmonic balance method, and the force transmissibility is obtained. The effects of damping ratio and excitation force on the system transfer rate are analyzed. In order to verify the vibration isolation performance of QZS vibration system, a series of experiments were carried out. The experimental results show that the low frequency vibration isolation performance of the QZS vibration system is much better than that of the corresponding linear system.

Considering the force‐displacement–current curve and the intrinsic nonlinear characteristics of the electromagnetic force, the time‐lag characteristics of the adjustable high‐static‐low‐dynamic‐stiffness vibration isolation system are analyzed. The dynamic equations of the high‐static‐low‐dynamic‐stiffness system with time‐lag parameters are established. Utilizing the averaging method, an analysis of the system’s bifurcation behavior is conducted, yielding the critical conditions under which the time‐lag system undergoes Hopf bifurcation. The amplitude–frequency relationship of the main resonance response is derived through the application of the multiscale method. It is demonstrated that when the time‐lag time is zero, the maximum amplitude of the response of the system decreases as the coefficients of the time‐lag term increase. When the time‐lag is present, the relationship between the system’s resonance peak and the time‐lag is not monotonic. Furthermore, employing the multiscale method, the amplitude–‐frequency relationship of the 1/3 subharmonic resonance response is obtained, and the conditions for the existence of the 1/3 subharmonic resonance response are analyzed.