Study On The Characterization Method Of Thin Plate And Micro-groove Thickness By Laser Ultrasonic Zero Group Velocity Lamb Wav

The thin plate material with groove structure is mostly used for pressure relief originals of airtight containers such as high temperature and high pressure.The measurement of its thickness and remaining thickness is of great significance.The zero-group velocity Lamb wave method of laser ultrasound has the advantages of local measurement,rapid detection and high accuracy.In this paper,the laser-excited zero-group velocity Lamb wave method is used to detect the thin plate with groove structure,the feasibility and accuracy of the zero-group velocity Lamb wave method to characterize the remaining thickness of the groove are studied,the influence of the groove width change on the accuracy of the method to characterize the remaining thickness of the groove is explored.The main content of this paper is as follows: The second chapter analyzes and summarizes the basic dispersion characteristics of Lamb waves in thin plates.Then the generation mechanism and characteristics of thickness resonance and zero-group velocity resonance are studied.And the feasibility of using Lamb wave resonance characteristics to characterize thickness is finally explored.The third chapter puts forward a method of multi-modal thickness resonance thickness measurement based on the study of Lamb wave high-order modal thickness resonance.Numerical simulations and experimental results jointly proved the accuracy of the multi-modal thickness resonance thickness measurement method.The thickness error of the experimental measurement of a thin plate with a thickness of0.2mm is only 0.11%.The fourth chapter studies the characterization of the thickness of non-defective thin plates by laser-excited Lamb wave zero-group velocity resonance.The spatial distribution of the zero-group velocity Lamb wave is studied and the conclusion that the optimal excitation and detection conditions is at the same point is obtained.Further simulation showed that the laser line source width for the optimal excitation of the zero-group velocity Lamb wave is twice the thickness of the sample.Finally,an experimental system is established to verify the method,and the thickness error of the experimental measurement of a thin plate with a thickness of 0.2mm is only 1.46%.Chapter 5 focuses on the influence of the groove width on the residual thickness measured by the zero group velocity Lamb wave.Through simulation,the influence of groove width on the zero-group velocity Lamb wave is grasped: When the ratio between the groove width a and the wavelength λ of the S1 mode zero group velocity Lamb wave corresponding to the remaining thickness of the groove a/λ>0.5,the zero group velocity resonance peak It begins to appear,as the a/λ increases,the peak shape is gradually complete and sharp,and the remaining thickness characterized by this method is becoming more and more accurate.A laser-excited zero-group velocity Lamb wave scanning system was established,which realized the frequency domain imaging of 25 grooves with different remaining thicknesses and the accurate characterization of the remaining thickness.The calculated remaining thickness error was only 0.13%.The experiment proved that the influence of the groove width on the accuracy of the zero-group velocity Lamb wave method to characterize the remaining thickness of the groove is completely consistent with theoretical research.The research results in this paper will provide theoretical and experimental basis for laser-excited zero-group velocity Lamb waves for non-destructive testing and thickness characterization of grooved structural plates.