Research On Lamb Wave Computerized Tomography Of Fan-beam

With large scale and thin plate structure widely used in engineering areas of high intensity and complex environment such as automotive, ships, aviation and rail bridges, it is quite important to test the pre-service and in-service health for the sheet. For this requirement, Lamb wave Computerized Tomography technology is a particular advantage of NDT(no-destructive testing). Not only there are many strengths for Lamb wave detection technique in sheet which is of long distance propagation, little attenuation, enough defect information, but also computerized tomography technique based on Lamb wave could reconstruct the image of defection in sheet effectively and accurately, which includes location, size and other relevant information, which bitterly reproduces the invisible internal defects in the sheet. Research and experimental verification based on fan-beam Lamb wave computerized tomography technique is conducted in the thesis.This thesis introduces the basic theory of Lamb wave firstly, analyses its mode and frequency dispersion. Studied aluminum sheet, computing Lamb wave equation and drawing the dispersion curves under different multiplication of frequency and thickness on the MATLAB platform, which provide a theoretical basis for selection of Lamb wave excitation signal and modal analysis.Then, the physics and mathematics expression for projection in computerized tomography is presented, so as two important classical reconstruction algorithms. This chapter focuses on Filtered Back Projection based on the central slice theorem and two typical iterative reconstruction algorithm, which is algebraic reconstruction and simultaneous iterative reconstruction algorithm.Treating aluminum sheet as mock object, based on scanning solution of fan-beam structure, setting different positions and different depths of the circular defect in the sheet; than calculates the theory data of projection under the models of diverse parameters, and adopts simultaneous iterative reconstruction algorithm with lower error to reconstruct the models of flaws; compared with corresponding model, all of reconstruction images is of great image results.Building experiment platform and using longitudinal wave straight sensors to inspire Lamb wave, removes the noise of Lamb wave time-of-flight signal with both empirical mode decomposition and wavelet transform, extracts filtered data. For defect reconstruction, we can recognize the related information of the flaw accurately, that great supports the advantages and accuracy of Lamb wave computerized tomography detection technique, which laid a certain function for the practical application of this technique.