Vibro-Acoustic Characteristics Of Composite Sandwich Panel Structures Under Thermal Environments

Sandwich panel structures have been increasingly applied in aircrafts, ships and high-speed vehicles in recent years due to their lightweight, high stiffness-to-weight ratios, thermal and sound isolation, designability and other features. Hypersonic aircraft are exposed to a rigorous combination of aerodynamic heating, wideband noise and vibration environments during their flight. High temperature environments induced by the serious aerodynamic heating will severely reduce the material performance, alter the vibroacoustic characteristics of the system and affect the flight safety. Hence, the research of the vibro-acoustic characteristics of sandwich panels under thermal environment is of great importance to reduce the cabin noise level, optimize the structure design and improve the safety of flight. The work of this paper is aimed at this problem to seek a simple and effective theoretical model for the study of vibro-acoustic characteristics of sandwich panels under thermal environments, which is utilized to explain the effects of parameters and factors on the vibro-acoustic characteristics of sandwich panels. Theoretical study is performed in terms of sandwich panel theory model, modal density, sound transmission loss and vibro-acoustic responses of sandwich panels with symmetric motion considered, respectively. The main contents of the dissertation are as follows:An improved ordinary sandwich panel theory is proposed with the in-plane rigidity of the core considered and the material and reference axes were not restricted to be identical to each other. The governing equations of sandwich panels under thermal environments are derived by applying the Hamilton’s principle. Three simplified models are obtained by simplifying the original model. The critical buckling temperature and natural frequencies of simply supported sandwich panels are obtained by using the double trigonometric serious solution. The accuracy of the proposed models is validated by comparing with other models in the literature and the FEM model. The application range of each model is discussed. The influence of in-plane rigidity of the core on the critical buckling temperature and the natural frequency is investigated. The effect of temperature on the natural frequency is obtained.Modal density of sandwich panels according to the wavenumber spaces of simply supported, clamped and free boundary conditions is presented based on the improved ordinary sandwich panel theory and by applying the wavenumber space integration. The model is validated by comparing with existing modal density expressions and FEM models. The limitations and errors introduced inevitably in the process of simplification of the existing expressions are pointed out. The mechanism of the modal density influenced by ply angles, in-plane rigidity of the core, transverse shear rigidity, boundary conditions and thermal stresses and temperature dependent material properties are investigated systematically. The results show that the effect of thermal stresses on the modal density is larger in the low frequency range, while the temperature dependent material properties have a significant influence on modal density in the middle and high frequency range. Both of the two effects should be considered in the calculation of the modal density of sandwich panels in thermal environments. The proposed model has a wider application scope.The sound transmission loss model of sandwich panels is established based on the statistical energy analysis. Determination methods of modal density, internal loss factor and coupling loss factor are given. The formulation of sound transmission loss is derived based on the power balance equation. The critical frequency of sandwich panels is obtained by applying wave impedance method based on the improved ordinary sandwich panel theory. The model is validated by comparing with experimental results. Parametric studies are performed in order to investigate the influence of the temperature, surface density, thickness of the facesheet, transverse shear rigidity, internal loss factor and boundary conditions on sound transmission loss. The effect of temperature on sound transmission loss is emphasized which is introduced by the parameter of modal density.The analytical study on the vibration and acoustic response of the sandwich panels with flexible core under thermal environments is performed. Governing equations are derived by applying Hamilton’s principle with both antisymmetric and symmetric motions taken into consideration. Natural frequencies, mode shape and critical buckling temperature are obtained by under thermal stresses. The forced vibration response and sound radiation are acquired based on the mode superposition principle and the Rayleigh integral. The mechanism of the natural frequency influenced by the Poisson’s ratio and thickness of the core is discussed. The effects of dilatational mode and temperature on the vibration and sound radiation responses are investigated. It is found that natural frequencies for both anti-symmetric and symmetric motions decrease with increase of temperature. Lower order flexural mode and higher-order dilatational mode are sensitive to the temperature change. Symmetric motion induces the anti-resonance frequencies move to the low frequency range. The peaks of vibration and sound pressure level float to the low frequency domain as the temperature rise and the amplitude of the peaks decrease due to the enhanced damping. The proposed model may serve as an effective guideline for the application of sandwich panels in thermal environments.