Structural design of one-way L(0,2)ultrasonic guided wave EMAT
The characteristics of two-way propagation of ultrasonic guided waves in pipeline and the influence of the end-face reflected echoes make the detection echo signals complex and difficult to distinguish.According to the excitation mechanism of electromagnetic acoustic transducer(EMAT)and the wave structure characteristics of longitudinal ultrasonic guided waves,a periodic permanent magnet is designed to enhance the radial bias magnetic field,thereby increase the axial component of the Lorentz force and reduce the radial component to control the excitation of a single L(0,2)guided wave;at the same time,according to the principle of wave superposition,the excitation direction of the longitudinal ultrasonic guided wave is selected by controlling the distance between adjacent coil wires and the direction of excitation current and the delay time.The research results show that when the adjacent excitation coils have opposite currents,the line spacing is λ/4,and the delay time is T/4,the optimally designed periodic permanent magnet can realize the direction control of the ultrasonic guided wave in the pipeline,which makes echo signal easy to distinguish,and improves the detection efficiency of the pipeline.
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