| Because of high strength, light weight and high environmental adaptability, whichcould reduce the structure’s mass and costing under the conditions of meeting thestructure’s security and mechanical property, the variable-thickness plate has been wildlyused in aviation, shipbuilding, automobile and other industrial process and living fields.However, due to the effects of the scallops and crack defects that produced by the externalload or complex environment in manufacturing or service process, the variable-thicknessplate’s structure integrity has been destroyed seriously, or even caused an body fracturesafety accident, which would be an seriously threaten to the production’s using security.Due to its sensibility to the defect, the guided wave has been regarded as the most potentialdetecting technology in the variable-thickness plate’s (especially the thin plate) defectdetecting. However, because of the plate thickness changing, the guided wave’s propagatingrules becomes extremely complex, and has not been researched deeply, which makes theanalysis of the guided wave’s characteristics and mechanism propagate invariable-thickness plate more difficult.In this paper, the defect’s quantitative detection method has been proposed by makingthe cracks in the variable-thickness plate as the detecting and quantify target, by using theguided wave and surface wave as the main technological means and researching the guidedwave’s propagation rule deeply. The main technical route is: according to the characteristicsof the variable-thickness plate on the whole, the defect’s detecting and quantification in thevariable-thickness would be completed by adopting different types of guided wave to detectthe defects that locates at the area, whose product of frequency and thickness is less than2.3MHz-mm and that is greater than2.3MHz-mm respectively. The specificimplementation process are: the guided wave’s dispersion equation and it’s dispersioncurves has been obtained by using the displacement potential function method combiningwith the variable-thickness plate’s boundary conditions, and solving the plane strain waveequation of the variable-thickness plate, which was built at the foundation of the elasticitytheory. On the premise above, the author has got the guided wave’s propagatingcharacteristics and the defect’s effects to the guided wave in the variable-thickness plate byusing the finite element simulation method. Based on the acoustic surface wavepropagation theory, and using the finite element simulation method as the technological means, the conclusions have been got by analyzing the surface wave’s propagationcharacteristics and the influence rules of the defect to the surface wave in thevariable-thickness plate’s area that the thickness is3times greater than the surface wavelength. The defect size detecting goal has been achieved by applying the time andfrequency domain analysis comprehensive method to study the guided wave’s modecomponents, its mode transition rule, and obtain the characteristic parameters of thedefect’s size. The defect’s size and orientation detecting goal has been achieved by buildingthe multi-pass array detecting system which is based on the transducer array technology,.The experiments results show that the detecting system can detect the plate’s defecteffectively. The details are summarized as follows:(1) In chapter2, through the guidance of the theory model and the propagatingcharacteristics that the Lamb wave propagates in the plate, author proposes the finiteelement model parameters and boundary setting method under analyzing thetwo-dimensional wave equation’s finite element model building process and results solved,which is based on the basic finite element principle of the wave dynamic. The results showthat it would provide a technical and theoretical foundation for the further research. Authoranalyzed the guided wave’s mode and the effects of the defects to the guided wave whichwas incentived under different approaches, by analyzing the guided wave’s mode and itdispersion characteristics, which was based on the finite element model. The model’svalidity has been verified by using the experiments method.(2) In chapter3, the guided wave’s dispersion equation and dispersion curves havebeen obtained by solving the guided wave’s propagation mathematical model in thevariable-thickness plate, which is built on the base of plane strain theory, by introducing thedisplacement potential function method and the variable-thickness plate’s boundaryconditions to solve the plane strain wave equation. The guided wave’s propagation rule andthe effects of the angle, incentive frequency and the defects to the guided wave have beengot by analyzing the guided wave’s characteristics of mode, sound velocity, and the peakattenuation that are generated at different incentive methods. Finally, experiments havebeen performed to test the model’s correctness.(3) In chapter4, the surface wave’s propagation rule and the effects of the angel tosurface wave have been analyzed on the basis of fundamental theory and surface wave’spropagation characteristics in the plate. And the model of the defect to the surface wave hasbeen built on account of analyzing the interaction of the surface and defect. (4) In chapter5, on account of the time and frequency domain analysis comprehensivemethod, by analyzing the guided wave’s mode components and its mode transition rule, ithas been proposed that the defect’s identifying and characteristic parameter extractingmethod which was on the base of incentive A0mode and receiving S0mode. For thesurface wave signal, the defect’s identifying and characteristic parameter extracting methodhas been put forward by using the reflecting signal of the shear wave. The artificial defectdetection results show the method proposed can process the signal availably.(5) In chapter6, according to the requirement of defect detecting, the multi-pass arrayacoustic detecting system has been built by combining the single transducer’s acoustic fieldcharacteristics with the elliptical positioning principle. The plane and variable-thicknessplates’ defect detecting experiment results show that the system can identify the defect’slocation and geometry effectively. |
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