Study On The Excitation Of Single Mode Lamb Wave And The Interaction Of Lamb Wave With Defects

The aluminum plate which thickness is less than 6mm is the most widely used for industry. However, it is inevitable the thin plate have some defects in the process of manufacture. Therefore, the thin plate should be detected in the Nondestructive testing technology. The traditional nondestructive testing is susceptible to interference from the outside environment, pollute the environment and over-dependence on couplant, so the application range is small. However, EMAT is widely used for detecting the defect of thin plate with the virtues of quickly detecting the defect and being freed of couplant. But electromagnetic ultrasonic Lamb wave, using for detecting the defect of thin plate, have the characteristics of multi-modes and dispersion, which limit the application range for lamb wave. Therefore, this paper studies the propagation characteristics of Lamb waves, aiming at suppress the multi-modes and exciting a single Lamb wave.This paper mainly studied on the directivity of the sound field and multi-mode and the changing trends of the group velocity, phase velocity and displacement amplitudes of the different modes based on the finite element method. Simulation results show that:AO and SO modes, which are only detected in the plate, have a strong directivity of sound field when the excitation frequency is in the range of 0.08MHz to 1MHz. There is only AO mode in aluminum plate when the excitation frequency is lesser than 0.188MHz. When the excitation frequency is in the range of 0.45MHz to 0.64MHz, the displacement amplitude of the SO mode is gradually increasing. It is larger in the SO mode when the excitation frequency is 0.64MHz.It is effective to select expected Lamb wave mode through selecting the excitation frequency. However, simulation results show that:selecting a appropriate excitation frequency can suppress the effect of interferential mode, which can’t eliminate the interferential mode. On this basis, it is should be study the direction of Lorentz force when the excitation frequency is the best excitation frequency effect the interferential mode. This paper study the changing of particle displacement amplitude based on the finite element method when the direction of Lorentz force is changing. Simulation results show that:When the excitation frequency is 0.64Mhz, changing the direction of Lorentz force, that is to say, increasing the force in the X axis can increase the displacement amplitude of the SO mode and cut down the displacement amplitude of the AO mode,then the single mode Lamb wave is excited.In addition, this paper studies the interaction of AO and SO modes with defects based on the finite element method. Through analyzing different defects in the thickness direction, simulation results show that:when defect is asymmetric with respect to plate center, the mode transform is the most obvious after SO mode go through defects. However, when defect is symmetric with respect to plate center, the mode transform isn’t obvious after SO mode go through defects. Especially, the mode transform is the most weak when the defect is at the upper and lower surfaces. This paper study the interaction of AO and SO modes with defects based on the finite element method, which provide a basis for exciting the single mode lamb wave, analyzing the echo signal. This has the promising application prospect in quickly and accurately nondestructive testing.