| Ultrasonic guided wave testing technique is a new nondestructive testing method. It has a very extensive applicable scope and can be used to detect sheet, pipe, bar and bending work piece. Its advantages are fast detection speed, long detection distance and wide detection range and the defect of a line can be obtained by exciting in a point.The formation principle, generation conditions, and propagation characteristics of ultrasonic guided wave are studied. Electromagnetic ultrasonic guided wave is studied deeply and complete equations and boundary conditions of electromagnetic ultrasonic are derived. By studying the coils and permanent magnets of EMAT, a effective EMAT model is designed. The four generation processes of electromagnetic ultrasonic in parallel magnetic field and vertical magnetic field are simulated by the finite element software ANSYS. The bias magnetic field, eddy current, Lorentz force and electromagnetic ultrasonic wave are mainly studied.The simulation results show that meaningful magnetic field of electromagnetic ultrasonic strength is only a small part, the eddy current and Lorentz force below the coil are the biggest and the direction of eddy current is opposite to the one of excitation current. Through the simulation on the displacement change of all particles and single particle, it can be seen intuitively that electromagnetic ultrasonic wave attenuates rapidly in a very short time. Through studying the coil spacing, coil lift-off and frequency thickness product of EMAT in vertical magnetic field, three methods are proposed to improve the conversion efficiency of EMAT. Through comparing the ultrasonic wave in two kinds of electromagnetic fields, the conclusion which the conversion efficiency of EMAT in parallel electromagnetic field is higher is obtained. |
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