Study On Imaging Test Method For Thin Metal Sheet Using Lamb Wave

Sheet has a wide range of applications in industrial fields such as aerospace, automotive and the pressure vessel. Ultrasonic Lamb wave method is an effective means of testing defects in sheet metal. Compared with traditional bulk waves testing method, Lamb wave nondestructive testing has advantages of large-scale spread when lamb wave is excited in a place, and its testing is fast and efficient. Lamb wave imaging test method is an effective technique in improving the accuracy of position and quantification. The research on Imaging algorithm and technology is the focus of this field. This paper has studied on the methods of imaging testing in sheet based on Lamb waves.Mode selection is critical in testing in sheet based on Lamb waves. We should analyze the transmission patterns when Ultrasonic Lamb waves are used to test the sheet to select the appropriate mode of motivation. In the application of cross-hole scan, Lamb waves excited under tilted incidence has high demands in rotating probe and controlling angle. Therefore, this paper has suggested to take straight probe exciting Lamb waves. In order to understand Lamb wave propagation characteristics in the sheet, this paper has taken a mode identify of Ultrasonic wave's transmission in the sheet by means of 2-D FFT. Based on finite element method, it offers snapshots, and we can understand the propagation characteristics of Lamb waves well. Frequency equation of Lamb waves has been calculated, frequency dispersion curves have been drawn, and A0 mode wave has been selected by calculating the results and analyzing the mode identify.The velocity based on Lamb waves has a strong dependence with the thickness of sheet, it has scanned the structure of sheet by using cross-hole scanning technique. According to principles of acoustic tomography, by lay outing grid on Detection area, calculating the ray path on grid and pursuing its relationship with the slowness (the inverse of group velocity), it has established mathematical model. It has introduced simultaneous iterative reconstruction techniques (SIRT) in earthquake CT by analyzing algebraic reconstruction technique (ART), and achieved Lamb wave imaging detection programs. Combined with imaging detection technology, the imaging detection of single-board and adhesive sheet are achieved by using the finite element method. It has analyzed the effects of different factors on imaging results. Finite element simulation has verified the feasibility of Lamb wave imaging testing methods, and provided a reference for the experiment.In the experiment, using low-frequency probe to excite Lamb wave to test thin aluminum, analyzing the performance of longitudinal wave probe, the HHT and FFT method were applied to get arrival time of A0 mode wave forming defect image based on imaging testing. For the lack of direct probe, considering the development of Lamb waves testing and structural health monitoring technology, piezoelectric element has been used to excite Lamb wave. Two sets of low-frequency signal have been extracted to test the sheet. The result is consistent with the numerical simulation. The use of piezoelectric elements is the basis of Lamb wave imaging testing to composite plate.Lamb wave image testing technology is not mature, this paper's research provides a reference for the application of Lamb wave image testing in sheet.