Research On Acoustic Properties Of Lightweight Lattice Sandwich Structure

Lightweight sandwich structures commonly consisting of two thin but stiff face panels separated by lightweight thick core material have broad application prospects in the fields of aerospace,transportation,architecture and military,as their superior properties of low density,high strength,high modulus.Compared to the conventional sandwich structures,such as honeycomb,foam-filled and corrugated structures,lattice sandwich structures as a kind of novel sandwich structures,encompass higher specific strength and modulus.Moreover,the lattice structures have additional potential by virtue of the connectivity of internal space.Hence they attract increasing attentions.Until now,the main research of lattice structure s focuses on the areas of manufacture,statics properties and a few dynamic fields,such as vibration and shock loading,but seldom about acoustic properties.The thesis focusing on these problems investigates the sound insulation and absorption properties of lattice sandwich structures and establishes the corresponding theoretical models.The acoustic properties of lattice structures are studied by using numerical calculations,finite element simulations and experiments.The main contents are as follows:First,the sound insulation property of pyramidal lattice sandwich structure with isotropic material is investigated.The vibro-acoustic model is established by using classic Kirchhoff plate theory and space harmonic theory.The model is validated by experiments and finite element simulations,respectively.The effects of the parameters,including the angles of incident wave,material moduli and structural geometries,on sound insulation are discussed by numerical calculations.The dependence of these parameters and some resonances is studied emphatically.The role of beam in sound insulation is indicated by parameter analysis.Second,the sound insulation of laminated composite pyramidal lattice sandwich structure is investigated.The theoretical model of the structure under acoustic excitation is established by using first-order shear deformation theory and Fourier transformation,taking the effects of the coupling between extension and flexure into account,and then is validated by experiments and simulations,respectively.The effects of acoustic excitations,structural materials and geometries on sound insulation are discussed,especially the effect of anisotropic materials.By comparing the results of laminated structures with symmetry and anti-symmetry stacking prepregs,the effects of stacking geometry and the coupling between extension and flexure are summarized.In order to improve the sound insulation property of lattice sandwich structure and make use of its internal void space,porous material is filled in.The theoretical models of two-layered pyramidal lattice sandwich structure lined with poroelastic material are derived by using space harmonic approach and Biot theory.There are three configurations of filling porous material: bonded-bonded,bonded-unbonded and unbonded-unbonded configurations.Three models with different configurations are validated by simulations and the results in literature.The advantages and disadvantages of each configuration are found.The mechanisms of beam and porous material in sound insulation are studied.Based on the classic configurations,two new configurations,which included two layers of porous material,are proposed.The effects of solid and fluid phase parameters in porous material are analyzed.The effect of external mean flow on sound insulation is studied for practical applications.The material and geometrical parameters of lattice structure are discussed.An integrated index to evaluate the multifunctional performances and two ways of reducing the negative effects of beam on sound insulation and of enhancing the effects of absorptive material are proposed.Finally,the sound absorption property of micro-perforated structure with facecentered cubic core is investigated.Because of high stiffness,lattice sandwich structures generally encompass good performance in terms of sound insulation,but their performance in sound absorption is poor.In order to improve this shortcoming,the two-layered pyramidal sandwich structure is combined with micro-perforations,and then the internal space of lattice structure is divided into several enclosed spaces.Hence,micro-perforated sandwich structure with face-centered cubic core is obtained,which contains serial and parallel absorbers.Theoretical sound absorption model of perforated face-centered cubic structure is established by making use of equivalent electrical circuit approach and theory of transfer matrix.The model is validated by experiments and simulations.Excellent mechanical performances of the structure are vertified by finite element simulations.The effects of each subsystem in the serial and parallel structure and parameters of structural and perforated geometries are analyzed,showing that the structure has good potential of design ability on sound absorption.Based on the previous discussions,two methods to improve absorp tion property at low frequencies are proposed.On this basis,simulated annealing method is used to optimize the diameters of perforations,thicknesses of panels and perforation ratios,yielding a good absorption performance in medium-and-low frequency range.