Interaction Of Lamb Wave With Typical Defects Using Scattering Parameter Matrix

Lamb wave is one of the most popular technology in NDT (Non-destructive Testing)and SHM (Structure Health Monitoirng) for the reasons of long distance propagation,high inspecting efficiency and sensitive to all the defects along thickness. Currentlythe problem of Lamb wave technology is that it only can be used as a rough locationmethod and 'cant achieve the precise inspection. Solving this question depends on thestudy of interaction of Lamb wave with defects. However, the question is so complexthat most of the lectures using2D FE model and expeirmental method to study it.2D scatteirng of guided wave in different defects in plate was studied using3D FEmodel and EMAT array in experiment. Scattering parameter matrix was employed torepresent the interaction of guided wave with defects and measure the category andgeometrical parameters of defects. Main work is as following:1Theoretical basis of using scattering parameter matrix to represent the interaction ofguided wave with defect was studied. Approximating conditions of circular-crest toplane-crest Lamb wave and relationship between scattering modes were studied byanalyzing the incident and scatteirng field. Scattering parameter matrix was employedto present the mode conversion and scattering of acoustic wave at defects, thusincident and scatteirng field could be connected.23D FE model of SO mode interaction with defects was established and interaction ofguided wave with hole and crack was represented. FE model study included purityLamb wave generation using dual-elements method and absorbing boundary settingup. Scatteirng parameter matrixes were calculated when SO and SHO mode interactionwith plane-bottom hole and crack with different geometrical parameters. Defect kindsrecognition and geometrical parameters measurement method was established basedon the vairation of scattering parameter matrixes. The diameter and depth of hole andexpansion direction, length and depth of crack could be evaluated.3Expeirment system of measuring scattering parameter matirx was established andevaluated. A kind of EMAT for generating omini-directional purity SO Lamb wavewas developed. Its properties such as exciting mode, frequency bandwidth, stabilityand uniformity in all directions were evaluated. With this EMAT array experimentsystem of measuring scattering parameter matrix of defects was set up. A kind ofsignal named Gaussian-chirp was developed to execute the expeirment. This signalcan achieve the same result of multi-times averaging method just with once test. Itpossess high SNR(signal to noise ratio) and great energy, thus can reduce the effect ofmeasuring precision coming from ambient noise and temperature changing.4Scattering parameter matrixes of typical defects such as hole, crack and corrosion were measured expeirmentally. The experimental results have a good agreement withFE result, which proved that the FE model was correct and the experiment measuirngsystem was reliable.5TFM(Total Focus Method) imaging algorithm of Lamb wave for thin plateinspection was studied. The parameters for Lamb wave array inspection such as modeexciting method, dispersion, wavelength and attenuation, was presented. The TFMalgorithm and circular EMAT array was employed to inspect an aluminum plateincludes defects.