Research Of Vibration Isolation Characteristics Of Marine Sandwich Structures Based On Phononic Crystal Band Gap Characteristics

Vibration and noise problems are important factors affecting the safety and comfort of ships.Serious vibration and noise will not only affect the concealment of ships and the physical and mental health of personnel,but also affect the normal work of ships’ mechanical equipment,lead to fatigue damage of instruments and shorten service life.For the sources of ship vibration and noise,suitable vibration and noise reduction methods should be adopted to block the transmission of elastic waves,so as to reduce the harm brought by vibration and noise.The vibration problems of ships are characterised by many vibration sources,a wide range of frequencies,a mixture of low and high frequencies,and a predominance of low and medium frequencies.With conventional theoretical designs it is often difficult to achieve the requirements of wide vibration isolation bands and low frequencies.The phononic crystal structure has band gap characteristics,and within the band gap range,it can play a good role in suppressing the transmission of vibration,which has potential application value in the field of ship vibration control.And given the current research,how to solve the problem of ship vibration below 100 Hz is still the pain point of the moment.It would be of great significance if low frequency vibration control could be achieved,especially if a method could be proposed to isolate both low and high frequency vibrations.Bragg-type phononic crystals can isolate high-frequency vibrations and local resonance-type phononic crystals can isolate lowfrequency vibrations,so if the advantages of both can be combined,theoretically both lowfrequency and high-frequency vibration isolation can be achieved.In this thesis,a dualperiodic phononic crystal sandwich plate structure is investigated,which is expected to exploit both the Bragg scattering mechanism and the local resonance mechanism to generate low-frequency bandgaps,increase the number of bandgaps and better isolate flexural vibrations to achieve low-frequency and broad-frequency vibration isolation.This thesis combines numerical simulations with experimental verification to investigate the bandgap characteristics and vibration damping performance of marine phononic crystal sandwich panels.The main research elements and conclusions of this thesis are as follows:Firstly,the Bragg scattering S-shaped oscillator and the local resonance S-shaped oscillator are designed and analyzed for their band gap and vibration isolation characteristics,respectively.The results show that the energy band structure of the Bragg scattering S-shaped oscillator opens a wide band gap between 3500-6100 Hz,and the decay frequency domain of the vibration transmission curve corresponds to the band gap range;the energy band structure of the local resonance S-shaped oscillator opens a band gap between 55-90 Hz,and the decay frequency domain of the vibration transmission curve also corresponds to the band gap range.Secondly,a doubly periodic 1D phonon crystal structure with both 1D longitudinal periodicity and 1D transverse periodicity is constructed based on the study of Bragg scattering type S-vibrators and local resonance type S-vibrators.Analysis of its dispersion relation and frequency response function reveals a band gap with low frequency and wide range.Further analysis of the displacement field of the eigenmodes at the boundary of the band gap reveals the reason for the appearance of this low-frequency band gap.And it is shown that the more the number of periods of the S-shaped oscillator,the better the damping characteristics,and the decay amplitude of the band gap frequency band is increasing with the increase of the number of periods of the S-shaped oscillator.Thirdly,based on the study of the one-dimensional phonon crystal structure,the phonon crystal sandwich plate structure is studied,which has both longitudinal one-dimensional periodicity and transverse two-dimensional periodicity.By analyzing the energy band structure and transmission curves,the proposed phononic crystal sandwich structure has significant low-frequency flexural wave band gap and vibration isolation characteristics,and the energy band structure and transmission curves show consistent results.By adjusting the geometric parameters,the flexural vibration band gap frequency range of the marine phononic crystal sandwich structure can basically cover the range of 30-700 Hz.In order to further verify the accuracy and reliability of the numerical method,experimental verification of the vibration transmission characteristics of the phononic crystal sandwich panel structure was carried out.Firstly,quasi-static tensile tests are carried out on the FRP panel and the core material selected for the experiments to determine the material parameters of the selected material,and then the vibration transmission characteristics are tested to verify the vibration damping characteristics of the sandwich panel structure and the accuracy and reliability of the numerical simulation results.Finally,the damping characteristics of the marine phononic crystal sandwich structure are investigated with reference to the application of phononic crystal structures in the field of marine vibration damping.The results of the study show that the marine phononic crystal sandwich structure has strong adaptability and significant vibration isolation effects,whether it is applied to bulkheads,floating valve vibration isolation systems or vibration isolation platform structures.In summary,this work is an active exploration into the field of marine vibration damping.