Analysis And Optimization Of Viscoelastic Damping Structures In Vibration Noise Reduction

Noise and vibration reduction of mechanical systems has been the focus of research, not only for its omnipresence, but also its impact to engineering. Visco-elastic material has good structural damping characteristics, which can play a significant role in the design of machinery with noise and vibration problems. The use of visco-elastic damping material in structures has been extensively studied. However, the elastic modulus and loss factor of visco- elastic material usually change with frequency, which will cause difficulties in the calculation of structure dynamics. In this thesis, the frequency-dependent properties of the visco-elastic material in the design of free layer damping as well as constrained layer damping structures, are replaced by constants, which is proved to give small errors. An procedure for the optimization of location of visco-elastic treatments in structures is presented. After determining the visco-elastic material constants used in the scaled model of a marine equipment, the visco-elastic damping effect in reducing structural noise is validated. The main contents of this thesis include:Chapter I: The research on visco-elastic damping structure and especially the optimization and computation methods about visco-elastic damping layer and dynamics are reviewed.Chapter II: Discussed are two kinds of models for the visco-elastic material: the standard rheological models and fractional derivative model, and also the frequency- dependent characteristics: elastic modulus and loss factor. The creep curves are obtained by the material creep testing and fitted by the above two models. According to the parameters of the two models, the frequency-dependent properties of materials are calculated. By comparing the results, the frequency-dependent properties described by the fractional derivative model are selected.Chapter III: The theoretical basis for design and calculation is discussed. Using the conversion relationships between the material damping ratio and material loss factor, the structural modal damping ratio is calculated by the material loss factors. Also presented is the power spectrum analysis method on the finite element model as well as the modal combination calculation, which has a significant role in the structure vibration and noise reduction. The objective function for the structural vibration and noise reduction as well as the program flow chart in ANSYS for frequency-dependent analysis is given is given.Chapter IV: The calculation and experiment are conducted on the scaled model of a real machine. Modal calculations on the constrained layer damping structure and the free layer damping structure are given by taking into account the frequency-dependent visco- elastic material properties. In the frequency band of analysis, the constant values instead of the frequency-dependent ones are determined by comparing natural frequencies, modal damping ratios and mode shapes. On the finite element model, the adhering positions for the damping layer are found according to the results of modal combination calculation. Harmonic response analysis and acoustic calculations for different damping structures as well as experiments on the scaled model are conducted and the measured acceleration and acoustic responses are compared with the calculated results to verify the correctness of the calculation method.Chapter V: The finite element model of a machine is built and calculations on vibration and noise reduction by visco-elastic damping are given. Optimization of the constrained damping layer structure and modal combination calculation are conducted, which gives a complete layout of the constrained damping layer. For two models that have respectively no damping and constrained damping layer, harmonic responses and acoustics with finite element method-acoustic boundary element method are analyzed, which verifies the effect of the visco-elastic constrained damping layer in the reduction of vibration and noise of the machine structure.Chapter VI: Conclusions of this thesis are summarized and further work is prospected.