Study On The Effects Of Environmental Load On The Structural Acoustic Performance Of Plate And Shell Structures With Elastic Boundary Conditions

The vibration and noise issues in the ship and marine structures,aerospace,architecture,and other engineering fields have always been the focus of domestic and foreign scholars.The actual engineering structures usually have complex boundary conditions and suffer from various complex loads,the severe environmental loads will even cause the structural damage,which will affect the dynamic performance and acoustic performance of the actual structure.Therefore,it is very important to establish an actual physical model to predict the dynamic and acoustic performance of the structure accurately.In addition,how to realize broadband vibration and noise reduction of lightweight structure is also a research focus for researchers at home and abroad.The bandgap characteristic of periodic structure which prevents the elastic wave propagation in a specific frequency provides a new idea for the vibration and noise reduction of the structure.However,the connection between cell units is assumed to be rigid fixed at present,which is not conducive to the practical application of locally resonant bandgap structure,and it is of great importance in both theory and practical to design light weight structure which has low sound radiation and excellent sound insulation performance.Aiming at the above problems,the calculation models of the effects of environmental load(static pressure,hydrostatic pressure thermal load etc.)on the acoustic performance of plate and shell structures with elastic boundary conditions are established in this dissertation,which take the force load,thermal load and plate,shell structure in naval ship engineering as the research object,the effects of elastic boundary condition(or elastic connection),force load,thermal load and damage on acoustic performance are analyzed.In addition,a calculation model of plate-type locally resonant bandgap structure based on elastic connection of cell units is established,the effects of elastic connection stiffness of cell unit on band gap and dynamic performance of locally resonant plate structure are discussed,and the design method of wide band gap based on damping ratios of locally resonant structure and multifrequency resonance is proposed.The main contents and achievements of this dissertation are as follows:(1)Considering that the stiffness of elastic foundation,boundary stiffness and elastic connection stiffness are included into the overall stiffness matrix,the vibration and acoustic radiation calculation models of stiffened plate and stiffened plate-cylindrical shell coupled structures are established,the effects of the stiffness of elastic foundation,boundary stiffness and elastic connection stiffness on the vibration and acoustic performance of plate and shell structures are discussed.The results show that in the transition process from free boundary to simply supported boundary and to rigid fixed boundary,there are two stages in which the natural frequencies increase sharply.At the same time,the boundary support stiffness is an important parameter that affects the radiation efficiency of the structure,‘soft’boundary is beneficial to reduce the radiation efficiency of the structure.When the support boundary is large enough,the radiation efficiency of the stiffened plate changes little when the elastic foundation stiffness increases.However,increasing the stiffness of the elastic foundation can reduce the mean square velocity of the structure,reduce the low-frequency radiated noise of the structure,and at the same time,the elastic range and elastic position of the structure may affect the reduced amplitude of the radiated sound power of the structure.In addition,the modes of the coupled structures can be divided into the single mode and the coupled mode of multiple structures.When the stiffness of the connection increases,the mean square velocity curve,and the radiated sound power curve of the coupled structures all move towards high frequency and the resonant peaks increase,while the number of resonance frequencies decreases.(2)The effects of in-plane force on structural vibration and acoustic radiation performance are studied,then the plane waves with multiple random incident angles,amplitudes and phase angles are introduced to simulate the reverb sound field,and a sound insulation calculation model of stiffened plate under static pressure is established,which can be used to modify the low frequency sound insulation performance of structures.On this basis,acoustic radiation performance calculation model of stiffened plate-cylindrical shell coupled structures under static(water)pressure is established.The model allows the structure to have arbitrarily complex elastic boundary and the elastic connection between the structures,and it also takes into account the stress stiffness caused by the static(water)pressure and give its explicit expression.In addition,the vibration and sound radiation calculation model of plate structure with arbitrary boundary conditions under local thermal load is also established,the effects of boundary conditions,heating position and heating area on critical temperature,vibration frequency and acoustic radiation of plate structure are studied,which improve the accuracy of the acoustic radiation performance of the complex structure in the actual environment.(3)The calculation model of locally resonant bandgap structure based on elastic connection of unite cell is established,the effects of elastic connection stiffness of unite cell on bandgap and dynamic performance of locally resonant structure are studied.The numerical results show that there is a range of elastic connection stiffness within which the bandgap frequencies and elastic wave suppression degree of locally resonant structure increase rapidly when the connection stiffness of unit cell increases.When the connection stiffness of unit cell continues to increase,the band gap frequencies and the suppression degree of elastic wave remain basically unchanged.This discovery widens the application scope of vibration and noise control using locally resonant structures.In addition,the low frequency band gap formed by the single-band gap locally resonant structure is usually relatively narrow and the sound insulation dip is prone to occur near the band gap.To overcome these shortcomings,the locally resonant multi-bandgap structure model is firstly established,and the Floquet-Bloch method is used to study the band gap performance.Then the acoustic calculation model of the periodic plate structure of the locally resonant multi-bandgap plate is established,the effects of damping ratios of locally resonant structure on the acoustic performance of multi-bandgap periodic plate structure are studied.Finally,a design method of low frequency wide band gap based on damping ratios of locally resonant structure and multi-frequency resonance is proposed.The method not only achieves wider band excellent low-frequency sound insulation than the original structure,but also eliminates the sound insulation dip near the band gap,which provides reference for the design of light weight,low sound radiation and excellent low-frequency sound insulation performance of the actual structures.(4)Based on the anisotropic damage constitutive relation of composite materials,a calculation model for the acoustic radiation of stiffened cylindrical shells caused by the combined action of temperature load and external load is established,and the effects of damage on the acoustic radiation performance of stiffened cylindrical shells are analyzed.The results show that the cylindrical shell and the transverse stiffener element will be damaged before the longitudinal stiffener when the temperature load and the external pressure load are applied to the stiffened cylindrical shell structure.As the degree of damage increases,the stiffness of the stiffener cylindrical shell decreases and the fundamental frequency gradually decreases.When the degree of damage is small,the mean square velocity,radiated sound power and radiated efficiency of stiffened cylindrical shell change little.When the degree of damage is greater,the variation of the mean square velocity,radiated sound power and radiated efficiency of the structure may rise and fall,which is related to frequency.At low frequencies,the mean square velocity and radiated sound power of the stiffened cylindrical shell structure increase significantly,the mean square velocity curve and radiated sound power curve of the stiffened cylindrical shell surface obviously move to low frequencies,at high frequencies,the sound radiation efficiency of the structure decreases,the radiated sound power also decreases accordingly.