Development of an Electromagnetic Acoustic Transducer with Multi-Wavelength for Lamb Wave

Lamb waves are normally utilized for inspecting thin metal sheets, and a wheel type probe with piezoelectric oscillators is used as the sensor, although it has a few serious disadvantages such as a dramatic change in sensitivity. We then studied a useful electromagnetic acoustic transducer (EMAT) without a couplant. The trial EMAT consists of a meandering coil with a narrow distance between the intervals which can generate Lamb waves of variable wavelengths corresponding to the frequency range from approximately 150 kHz to 4.0 MHz. This transducer, for example, can select Lamb waves with optimum wavelengths on the factory line that produces thin sheets with variable thicknesses. The described EMAT can be used to inspect steel sheets of different thicknesses. It is also shown that the S 0 -mode Lamb wave with the longer wavelength is the most effective for a thick sheet (up to 6 mm).