| Zero-group velocity(ZGV)Lamb wave have already shown their potential in nondestructive testing applications as they are sensitive to the sample structural characteristics and laser-excited ZGV Lamb wave for material testing with non-contact excitation,high spatial resolution,high signal-to-noise ratio,high detection efficiency.In this paper,we study the propagation characteristics of the laser-excited ZGV Lamb wave mode in the aluminum structure from two aspects of numerical calculation and experiment.The physical model of the laser exciting the ZGV Lamb wave in the aluminum plate is established based on the wave equation and the theory of thermo-elastic.The time-domain signal calculated by the finite element method and analysis the spectrum.The results show that there is a backward-wave in the Si mode.At k ≠ 0,the forward and backward waves with the same frequency wavelength interact with each other and produce ZGV Lamb wave.The influence of the spot size on the effect of exciting the ZGV Lamb wave is discussed,and found 1.5 times of the aluminum plate thickness is best.Experimentally,the experimental system of all-optical detection and excitation of ZGV Lamb wave based on Doppler vibrometer was established,and the result of experiment were obtained,which were in good agreement with the numerical results.Based on the single-pulse laser excited ZGV Lamb wave,the situation of multi-pulse laser exciting ZGV Lamb wave was studied.The influence of the distance between them was discussed where the power density of each pulse laser was constant.It was found that the spacing that is equal to the zero-group ZGV Lamb wave wavelength can enhanced peak.A method of moving continuous laser source exciting ZGV Lamb wave in aluminum plate is proposed.we found that different mode Lamb waves can be excited in the aluminum plate by choosing different light source moving speed through the finite element method.When the moving speed is equal to 11.02km/s,a ZGV Lamb wave can be detected in the moving area.And the higher moving speed can excite a backward wave detected behind the moving area.It is found that a high signal-to-noise ratio signal with almost constant amplitude can be detected in the whole moving region through analyzing the energy propagation direction of the ZGV Lamb wave.On this basis,the method of moving a continuous laser source to excite a ZGV Lamb wave is applied to the defect detection of the aluminum plate structure.It is found that the zero-group velocity Lamb wave occurs in the frequency domain signal missing compared with the Al mode of the forward wave,more effective defect assessment.In this paper,the result will provide some reference and basis for the related research. |
