Research On Elastic Scattering Of Submerged Elastic Plate And Cylinder

Research on underwater target acoustic scattering characteristics is basic work of target detection and recognition in active sonar.According to geometrical diffraction theory of target scattering,elastic scattering wave is leaky wave of elastic wave propagating on elastomer.For monostatic sonar system the measuring target elastic scattering wave is only leaky wave of one mode elastic wave in elastic target,while the measuring target elastic scattering wave in different points could be leaky wave of different mode elastic wave in elastic target under split sonar system;Therefore the receiving target elastic scattering signal characteristics in split sonar system are decided by transmission elastic wave of different modes,reflecting and mode conversion in elastomer.The subjects investigated in this dissertation are mode conversion of propagating elastic wave at the end of plate placed in vacuum or water,elastic scattering wave of submerged cylindrical shell,and measuring method of viscoelastic material parameters.The frequency dispersion equation of lamb wave is derived for elastic plate in vacuum and water,propagation characteristic curves are calculated.Analysis results of energy ratio among reflecting mode lamb wave resulted from mode conversion when propagating anti-symmetric lamb wave reflecting at the end of plate in vacuum or water is derived.The scattering wave of submerged steel plate is measured in experiment,which shows only the incident anti-symmetric lamb wave at critical angle of one mode can excite lamb wave of this mode in elastic plate directly,lamb wave of other anti-symmetric modes are formed by reflecting at the end of elastic plate;there is no mode conversion between symmetric lamb wave and anti-symmetric lamb wave when reflecting at the end of plate.Combined with radiation efficiency of lamb wave in submerged elastic plate energy ratio among reflecting mode lamb wave confirme the validity of the analysis result.In order to make a deep research on mode conversion of lamb wave reflecting at the end of plate,analysis results of energy ratio among reflecting mode lamb wave resulted from mode conversion when propagating anti-symmetric lamb wave reflecting at the dip end of plate in vacuum or water is derived.The scattering wave of submerged dip end steel plate is measured in experiment,the scattering wave strength of submerged elastic plate is measured incidented by critical angle of AO mode and A1 mode.Combined with lamb wave radiation efficiency of submerged elastic plate energy ratio among reflecting mode lamb wave resulted from mode conversion when anti-symmetric lamb wave paopagates at the dip end of plate in single mode confirmed validity of the analysis results.The propagation rule of elastic scattering wave in submerged elastic cylindrical shell is studied.The frequency dispersion equation of quasi-lamb wave is derived for submerged elastic cylindrical shell,propagation characteristic curves of quasi-lamb wave are calculated,it is studied for and propagating trip and angle relation of quasi-lamb wave when submerged elastic cylindrical shell is excited,time delay between elastic scattering wave and edge wave of elastic cylindrical shell under mono-static system is also calculated.Echos of submerged elastic cylindrical shell under mono-static system are measured in experiment,acoustic wave incidents at the elastic cylindrical shell at different grazing angle under multiple frequency thickness products,combined with radiation efficiency of quasi-lamb wave which shows the excited condition of elastic scattering wave,the time delay between elastic scattering wave and edge wave is also verified.Lamb wave dispersion equation of submerged elastic plate coated with viscoelastic damping layers is derived;phase velocity,attenuation coefficient and group velocity of lamb wave propagating in submerged composite plate are obtained in the condition of certain Young's modulu and Poisson's ratio.In order to measure parameters of submerged viscoelastic material in engineering system,a method of measuring Young's modulu and Poisson's ratio is proposed based on acoustic reflecting in composite plate,the phase velocity and attenuation coefficient of the first two lamb wave modes propagating in the composite plate under two certain frequency thicknesses product are measured by the echo measurement experiment of submerged composite elastic plate coated with viscoelastic damping layers,parameters of viscoelastic material is calculated by substituting the experimental results into the dispersion equation of submerged composite plate,the derived dispersion equation in composite plate and measuring algorithm is verified in experiment.