Research On Quantitative Characterization Of Blind Hole Defect Based On Electromagetic Ultrasonic Lamb Wave Tomography

Electromagnetic Ultrasonic Lamb wave tomography combines electromagnetic ultrasonic Lamb wave detection technique and Lamb wave tomography,which can realize the quantitative characterization of defect and has a wide application prospect for metal plate detection.At present,there is a lack of the design method for single mode electromagnetic ultrasonic Lamb wave transducer,and there are obvious pseudo images in the imaging results of Lamb wave tomography.The existence of pseudo image affects the error analysis,and also makes it difficult to quantify the imaging results.These above-mentioned questions limit the development of the electromagnetic ultrasonic wave tomography.In order to solve the problem that the ultrasonic wave can not be extracted effectively due to the multi-mode Lamb waves,this paper presents a design method of a single mode electromagnetic ultrasonic Lamb wave transducer.By using the methods of spatial Fourier transform,time Fourier transform,finite element modeling and simulation analysis,an electromagnetic ultrasonic Lamb wave transducer capable of excitation A0 mode and S0 mode is designed and fabricated.The experiment shows that the transducer can respectively excite the single mode of 440 k Hz A0 and 960 k Hz S0 Lamb wave in 360° direction in the 2 mm aluminum plate,the experimental basis was established for extracting the data of ultrasonic wave travel time and amplitude attenuation of Lamb wave tomography.The defect will cause disturbances in the ultrasonic guided wave travel time and amplitude information.In order to make full use of this information,this paper studies the method of guided wave tomography based on the change of travel time and amplitude attenuation.Based on the assumption of known ultrasonic travel time change matrix and amplitude attenuation matrix,a forward mathematical model of tomographic tomography is established,and the ultrasonic path matrix in the imaging region is computed,and simultaneous iterative reconstruction technique is used realizes the image reconstruction of plate defect information,which lays a theoretical foundation for the quantitative characterization of the three dimensional information of the defect.The traditional simultaneous iterative reconstruction technique has a large number of pseudo images in reconstructing image,which brings a great error to the quantitative characterization of the defect size.In this paper,the weighted adjustable simultaneous iterative reconstruction technique based on ultrasonic guided wave travel and amplitude information is proposed.This technique extracts the two-dimensional contour information of the defect by fusing the defect images obtained by the ultrasonic travel time variation and the amplitude attenuation information.The weight of the ultrasonic path matrix is adjusted according to the two-dimensional contour,and the defect is reconstructed again,and the third dimension depth information is extracted.In order to verify the validity of the method,in this paper,an experimental device for electromagnetic ultrasonic guided wave tomography is developed to measure two 2 mm thick aluminum plates with diameters of 60 mm,depth of 1 mm and diameter of 26 mm,depth of 1 mm blind hole,in which obtain the ultrasonic travel time,amplitude and path information for imaging.The analysis shows that the weighted adjustable simultaneous iterative reconstruction technique can quantitatively characterize the defects,and the error depth measurement errors are reduced from the traditional SIRT 50%~60% to the 10%~20%,which lays the foundation for the engineering application of the electromagnetic ultrasonic guided wave tomography technique.