Theoretical And Experimental Study On Propagation Characteristics Of Ultrasonic Lamb Wave Signal In Plate

Plate structure materials have been widely used in various industrial products.In order to ensure its normal use in the operating cycle,it's needed to do nondestructive testing of materials and safety evaluation.In the field of nondestructive testing,the multi-modal dispersion characteristics of ultrasonic Lamb waves are sensitive to the micro-damage of the structure and the surface,and the propagation distance is far and the decay is slow,and it is suitable for large area rapid damage detection.In recent research hotspots,the theoretical analysis,finite element simulation and experimental test are carried out for the dispersion characteristics and nonlinear characteristics of ultrasonic Lamb waves in isotropic metal plates and multilayer composite plates.The basic theory of Lamb wave propagation in the plate is analyzed.By studying the wave control equation in the infinite solid,the dispersion control equation of the Lamb wave in the plate is obtained by combining the upper and lower boundary conditions of the plate.For the isotropic material,the control equation can be decoupled and solved by the potential function decomposition method using the height symmetry of the elastic matrix.For the fiber reinforced multilayer composites,the transverse isotropy can be approximated,and the continuity stress and displacement conditions are obtained by using the rigid boundary between the layers.The dispersion control equation is solved by combining the wavelet method and the transfer matrix method.The method of solving the Lamb wave dispersion curve in isotropic plates and fiber-reinforced composite plates was studied.The dispersion equation of the isotropic plate is a transcendental equation in the real range,and the dispersion curve can be obtained by numerical method.For the Lamb wave dispersion in fiber-reinforced multilayer composite plates,the dispersion equation of the ultrasonic wave is transformed into the characteristic frequency equation of the cell structure by introducing the periodic structure and Bloch-Floquet theorem in solid physics.By proposing the method of Finite element characteristic frequency,the relationship between wave vector and cell vibration characteristic frequency and the dispersion curve of Lamb wave is obtained.The validity of these methods is verified by simulation.The nonlinear Lamb wave theory model is studied,the nonlinear matching and excitation conditions of Lamb wave based on strain tensor theory are given.Using the COMSOL Multiphysics simulation tool,the excitation and propagation characteristics of the Lamb wave mode selection and nonlinear second harmonic in the board are simulated.By constructing the finite element model of the plate structure,the wave structure at the specific frequency of the symmetrical mode is loaded in the plate thickness direction to excite the Lamb wave second harmonic propagated in the plate,by using bandpass filtering and spectrum analysis to obtain the cumulative effect of the second harmonic in the view of time domain and Frequency domain.In this thesis,a typical fiber-reinforced multilayer composite plate is studied.The experimental study on the nonlinear characteristics of ultrasonic Lamb wave is carried out.Using the high performance ultrasonic detection system,by analyzing the dispersion curve,reasonably setting the experimental conditions and the fundamental excitation frequency,the Lamb wave symmetry mode propagating in the excitation plate and its nonlinear second harmonic can be stimulated.The signal processing method of digital filtering,empirical mode decomposition and reconstruction and short-time Fourier transform is used to extract the fundamental and second harmonic components from the received Lamb wave signal,thus get the nonlinear coefficient of Propagation Characteristics.In this paper,the nonlinear coefficients of the fiber arrangement in different directions are analyzed along with the propagation distance.In the 0 ° and 45 ° directions along the fiber arrangement,the nonlinear effect of Lamb wave increases with the increase of propagation distance,which is consistent with the theoretical analysis.