Research On Plate Defect Detection Based On Electromagnetic Lamb Wave Tomography

Plate is industrial basic material, which is widely used in petrochemical, aerospace industry, transportation industry. But in the transportation and manufacturing process, it is inevitable that external pits, corrosion and other defects will exist. The technology of EMAT is an effective method for detecting defects in the plate. Comparing with conventional piezoelectric ultrasonic technique, electromagnetic ultrasonic technique not only do not need electromagnetic coupling agent, but also can be used in the occasions of high-temperature, high-speed, high cleanliness. The requirement of modern industrial non-destructive testing is no longer simply to detect the presence of defects, but also to detect the size and depth of defects. It is essential for an accurate assessment of the health of the plate. Electromagnetic ultrasonic Lamb wave tomography technique combines the dual advantages of the ultrasound technology with the Lamb wave tomography technology, which is meaningful for enhancing the level of detection of the plate.In this paper the working mechanism of electromagnetic ultrasonic Lamb wave and the multi-mode and dispersion characteristics of Lamb wave are analyzed. The working point of electromagnetic ultrasonic transducer is determined through Lamb wave excitation curve. Design methods of transducer for electromagnetic ultrasonic Lamb wave tomography are proposed, including coil parameters design based on structural analysis and magnet design based on electromagnetic simulation analysis.Based on electromagnetic ultrasonic Lamb wave tomography principle, travel time of lamb wave can be got on the plate through the double cross scanning. The positive algorithm and inversion algorithm of electromagnetic ultrasonic Lamb wave tomography is studied, the characteristics of different iterative reconstruction algorithm is analyzed. Electromagnetic ultrasonic Lamb wave tomography method is determined based on simultaneous iterative reconstruction algorithm and the different effect of electromagnetic ultrasonic Lamb wave tomography factors is analyzed.The experimental equipment of electromagnetic ultrasonic Lamb wave tomography for plate defect detection is produced. Hardware circuit of experimental device is designed to complete transmitting and receiving Lamb wave, acquiring data and displaying waveform. The mechanical structure of experimental device is designed to complete the double crosshole scanning. The software platform is designed to set the parameters, collect data and image.On this basis, the new electromagnetic ultrasonic Lamb wave transducer performance is studied in the experiment. Blind hole defect that has a diameter of 26 mm, depth 1.5mm in 2mm aluminum plate is imaged, and the depth contour curve is obtained. The effectiveness of the Lamb wave tomography EMAT is verified.