Development of a conventional electromagnetic acoustic transducer with variable wavelengths and multi modes for Lamb waves

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Lamb waves are normally utilized for inspecting thin metal sheets. Wheel-type probes using piezoelectric oscillators have generally been used as the sensors for Lamb waves. Recently, an electromagnetic acoustic transducer (EMAT) has been developed and is beginning to be used as a Lamb wave detector. The EMAT offers the advantage of inspection without using a coupling medium. The EMAT generates Lamb waves with fixed wavelengths determined by the interval between the meander lines in the sensor coil that constitutes the EMAT.We have studied an EMAT consisting of a meander line coil with intervals from 1mm to 4mm and having a paired structure symmetrically embraced with a thin steel sheet on opposite sides. The resonance condition of the sensor coil was tuned to 0.5, 1.0 or 2.0 MHz by changing the condensers connected to the sensor coil. The generated Lamb wave mode, the optimum driving frequency and the wavelength were basically determined by the velocity dispersion curve for the drive f … More requency (sheet thickness) and the phase velocity was determined by the EMAT with the meander line sensor coil. Actually, the observed Lamb wave was influenced by various conditions of the measurement system and the EMAT, for example, the resonance condition or the width of the finger. That is, we could observe the Lamb wave with various modes and different wavelengths using the same EMAT by controlling the different drive conditions. When we used the EMAT with a wide interval between the fingers, the S_0-mode Lamb wave could be only generated, at that condition, and the intensity and the wavelength of the S_0-mode Lamb wave simply changed as the resonance condition changed. However, when we used the EMAT with a narrow finger interval, various Lamb waves could be generated ; the discrimination of their modes and wavelengths might be difficult. However, the Lamb wave must be used with various modes and wavelengths to do a precise inspection. It is then very important to clarify how a Lamb wave is generated under the drive conditions. We could confirm that the symmetrical and asymmetrical Lamb wave modes could be distinguished by using the drive method from both sides of the thin steel sheets Less