Linear Reconstruction Of Phase Spectrum And Array Beamforming Damage Imaging Based On Lamb Waves

Lamb waves are usually used for Non-Destructive Evaluation(NDE)in plate-like structures for its long-distance propagation,detection and sensitivity.However,the dispersion of Lamb waves makes the wave extension and distortion when propagating,which affects the extraction of the time-of-flight and the inaccuracy of damage location and imaging.It is the main content of this paper to remove the dispersion of guided waves to improve the accuracy of array damage imaging.For the deficiency of the existing Lamb waves dispersion removal methods,this paper proposes a linear reconstruction of phase spectrum technique for the dispersion removal of Lamb waves based on measured signals,substituting the non-linear phase spectrum with the linear phase spectrum at the center frequency without the need of material dispersion relationship.The method is demonstrated efficient in the numerical simulation,then the time dalay-and-sum damage scattering signals is used with array beamforming and imaging theory,and linear arrays at different locations damage imaging was implemented in metal plate;In experiments study,the proposed linear reconstruction of phase spectrum method were studied in isotropic metal plate and unidirectional carbon fiber reinforced composite plate,improving the effective amplitude of the damage scattered signals validity.The measured data after dispersion removal were used for damage imaging by array beamforming and imaging method.Besides,by conducting logical and to the two different directions linear arrays,both of the imaging quality and damage location accuracy are improved.Imaging results of numerical simulation and experiment are in general coherent,indicating that it is promising in detecting plate-like structures in practical engineering construction with dispersion removal method and array beamforming and imaging thoery,which can also provide theoretical guidelines for other structures long time service period.The main work of this thesis is listed as follows:Chapter 1,conventional NDT and ultrasonic monitoring methods of plate-like structures are introduced in this section to discuss the merits and setbacks of each method.The development of Lamb waves detecting methods and different signal processing methods of Lamb wave dispersion removal are also presented.Eventually,the research scheme and methods of this thesis are determined.Chapter 2,based on Lamb waves theory,dispersive feature and propagation modes of Lamb waves in isotropic plate and transversely isotropic plates are discussed.And in transversely isotropic plates,the dispersion is related to the wave propagation direction.The principle of array beamforming and imaging is explored,and the damage imaging is realized by the different direction arrays in high precise and quality which based on logical-and algorithm.Chapter 3,the dispersion affects the damage imaging accuracy which makes it an emergency to remove the dispersion of Lamb waves.The advantages and disadvantages of these two methods are compared in this section and the theory of linear reconstruction of phase spectrum technique for dispersion removal of Lamb waves is introduced which laid the foundation for the array imaging precisely.Chapter 4,linear reconstruction of phase spectrum technique is verified in intact plate in FEM effectively,based on the array beamforming and imaging method analyzing the accuracy of damage imaging.In addition,linear array and different positions array are discussed to affect the damage imaging.Besides,by conducting logical and to the two different directions linear arrays,both of the imaging quality and damage location accuracy are improved.On this basis,experiments are set up to image the preset damage by dispersion removal and array beamforming and imaging method,discussing the promising application by this method in NDE of practical engineering projects.Chapter 5,the conclusion is summarized and the remark is also pointed out.