Abstract
High-order Lamb waves of symmetric mode are applied to flaw detection in steel plates. In inspection of thin plates, Lamb wave of the lowest symmetric mode is preferable, because effect of the velocity dispersion, which distorts the waveform, is small in the low frequency region. However, for thick plates, the frequency should be lowered and the wavelength becomes large. Then, the flaw detectability is reduced. In this study, we suggest to make use of high-order Lamb wave at frequency of its maximum group velocity in order to avoid the interference with the other modes. Electromagnetic acoustic transducers (EMATs), each of which consists of an electromagnet and a meander line coil, are designed for three modes of Lamb wave in 6mm-thick steel plate, using theoretically obtained dispersion curves. Optimum magnetic field strength is investigated, ensuring that the wave generation and detection are based on the magnetostriction. Dispersion curves are measured by sweeping the excitation frequency, which reveals the optimum frequency. Echoes from drilled holes are detected to discuss the flaw detectability of each mode.
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