Study On Vibrational And Acoustical Properties Of Composite Honeycomb Sandwich Structures

Honeycomb sandwich panels are widely used in spacecrafts, high-speed transit vehicle structures and modern construction engineer because of their outstanding performances such as excellent high strength, stiffness ratio and designable performance. With the spacecrafts and high-speed transit vehicles is developing toward large-scale and high-speed directions, their dynamical settings become more light and powerful, which has made the structure-born noise problems caused by the engine and high-speed airflow severe increasingly. Noise in cabin not only seriously affect the comfort of passengers,but also can bring parts fatigue damage in early stage.So it reduce the service life and restrict the development of high-speed transit vehicles to a certain extent.The major characteristic of sandwich structure is that it usually has light weight and large surface, which gives rise to vibration and structure-borne sound caused by vibrating engines and wind load. Therefore, study the acoustic and vibrational properties of honeycomb sandwich structures, explore the mechanism of structure-bore noise, precisely describing the induced noise by structure vibration, clearly analyzing and optimize the influence factors, are of great significance to acquire the new methods and technic approach of noise in high-speed transit vehicle cabins.Firstly, a simple approach is presented to reduce the governing equations for orthotropic honeycomb sandwich panels to a single equation containing only one displacement function with the transverse shear deformation and orthotropic property is taken into account. Based on the governing equation, the exact solutions of the natural frequencies are obtained for rectangular orthotropic honeycomb sandwich panels under simply supported boundary condition. The accuracy of the theoretical predictions is checked against existing experimental and analytical results, leading to a satisfied agreement. The influence of several key parameters on the natural frequency of orthotropic honeycomb sandwich panels is then systematically studied. It can be concluded that face sheet thickness, cell wall thickness and cell side length effect the first three natural frequencies in a same trend, that is the first three natural frequencies reach maximum in the considered range. Increase in core thickness results in an increase of natural frequency and natural frequencies of honeycomb sandwich panels can be improved by adjusting core thickness.With the simply supported boundary condition taken into account, the expression for transmission coefficient is derived using the double Fourier series solution. The accuracy of the theoretical predictions is checked against available numerical data, with good agreement achieved. The proposed model is suitable for both low- and high-frequency ranges and compares favorably with other approaches, e.g., SEA methods and higher order approach. By analyzing the relations of the flexural wavenumber, flexural wave speed and the sound speed in air, the critical coincidence frequency of honeycomb sandwich panel is obtained. The conception of critical frequency band is proposed according to the mechanical characteristics of honeycomb sandwich panels. Influence of several key parameters on STL is then systematically studied. Analysis shows that stiffness and area mass density are two key factors in determining the STL and coincident frequency and all the other parameters influence STL by affecting these two key factors.Theoretical and experimental studies on structural-acoustic coupling of an enclosure closed by sandwich panel is carried out. Strcture and cavity response formulations are derived by combining Green functions and analyze of boundary condition of the sandwich panel, based on which, a brand-new model for structural-acoustic coupling is proposed and the frequency formulations for both coupled cavity and panel are derived. The validity of the formulations are proved by measuring the sound pressure on the appointed position in the cavity, which lays a foudation for influence factors analysis and optimal design.According to the analytical model of structural-acoustic coupling of an enclosure closed by sandwich panel, the influence of panel design parameters on the sound pressure in cavity is systematically studied, the principle of design and improvement for decreasing the sound pressure was summarized. Aimed at decrease the sound pressure on the appointed position in the cavity, a improve design is carried out and the improved sandwich panel can decrease sound pressure in cavity effectively.