Study On Sound And Vibration Of Sandwich Structures

Sandwich structures have been used in aerospace applications for more than 50 years due to their high stiffness-to-weight and strength-to-weight ratios. Recently, with the development of civil ships and naval vessels in high speed and lightweight direction, new attention is focusing on using of sandwich structures in marine applications. Reduction of vibration and acoustic noise of marine structures is an important issue to designers. So it's very crucial to study sandwich structures' characteristics of vibration and radiation. This dissertation addresses an intensive study on numerical analysis of vibration and radiation of sandwich structures. The major contributions and conclusions are as follows.Most of the sandwich plate theories developed in the past did not take account of the effect of transverse normal deformation of the core. When such sandwich plate theories are applied to analyze thick core, sandwich plates or to analyze high frequency vibration of sandwich plates, reasonable results could not be obtained due to ignoring of transvers normal deformation of the core. In this paper, a new composite sandwich plate (shell) element is developed which includes the transverse normal deformation in addition to the transverse shear deformation in the core. The two face layers are considered as Mindlin plates with shear and bending resistance. Nonlinear variation of displacements through the thickness of the core is assumed based on thick plates theory. The displacements of the core are expressed in terms of the displacements of two face plates. The governing equation of the sandwich plate system is derived based on Hamilton principle and is expressed in terms of the face plates' displacements. Numerical results show that the presented model is valid and the consideration of transverse normal deformation in the core is necessary and reasonable for dynamical response analysis.Traditionally, CLF can only be calculated in high frequency band because of the need of high model density and modal overlap. In this paper, a theoretical study of CLF for two L-shaped sandwich plates which can make accurate results in low frequency band, is presented using finite element method (FEM) model characteristics. The numerical results are compared with those of the commercial software AUTOSEA, and it exhibits a good agreement between them.The acoustic radiation power of the sandwich plate is calculated by boundary element method based on the finite element method results. The necessity of the consideration of transverse normal deformation as well as the effect of geometry and material parameters for the core in the analysis is discussed. A finite element model is presented for the vibration of stiffened sandwich plates with moderately thick viscoelastic cores. The two face layers are considered as Mindlin plates with shear and bending resistance. Nonlinear variation of displacements through the thickness of the core is assumed based on thick plate theory. Meanwhile, the effect of transverse normal deformation of the core is taken into account. The stiffeners are modeled by Timoshenko beam. The displacements of the core and stiffeners are expressed in terms of the displacements of two face plates. The governing equation of the stiffened sandwich plate system is derived based on Hamilton principle and is expressed in terms of the face plates' displacements. Numerical results indicate the validation of presented model and show that the consideration of transverse normal deformation in the core is necessary for the vibration analysis of stiffened sandwich plate.An accurate dynamic stiffness model for a three-layered sandwich beam of unequal thicknesses is developed and subsequently used to investigate its free vibration characteristics. Each face layer of the beam is idealized by the Timoshenko beam theory. Linear variation of displacements through the thickness of the core is assumed, so that the transverse normal deformation is taken into account. The combined system is reduced to a twelfth-order system using symbolic computation. An exact dynamic stiffness matrix is then developed by relating amplitudes of harmonically varying loads to those of the responses. The resulting dynamic stiffness matrix is used with particular reference to the Wittrick-Williams algorithm to carry out the free vibration analysis of a few illustrative examples.