Electromagnetic acoustic transducer

Electromagnetic Acoustic Transducer (EMAT) is a transducer for non-contact sound generation and reception using electromagnetic mechanisms. EMAT is an ultrasonic nondestructive testing (NDT) method which does not require contact or couplant, because the sound is directly generated within the material adjacent to the transducer. Due to this couplant-free feature, EMAT is particularly useful for automated inspection, and hot, cold, clean, or dry environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) bulk wave mode, Surface Wave, Lamb waves and all sorts of other guided-wave modes in metallic and/or ferromagnetic materials. As an emerging ultrasonic testing (UT) technique, EMAT can be used for thickness measurement, flaw detection, and material property characterization. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries.

There are two basic components in an EMAT transducer. One is a magnet and the other is an electric coil. The magnet can be a permanent magnet or an electromagnet, which produces a static or a quasi-static magnetic field. In EMAT terminology, this field is called bias magnetic field. The electric coil is driven with an alternating current (AC) electric signal at ultrasonic frequency, typically in the range from 20 kHz to 10 MHz. Based on the application needs, the signal can be a continuous wave, a spike pulse, or a tone-burst signal. The electric coil with AC current also generates an AC magnetic field. When the test material is close to the EMAT, ultrasonic waves are generated in the test material through the interaction of the two magnetic fields.

There are two mechanisms to generate waves through magnetic field interaction. One is Lorentz force when the material is conductive. The other is magnetostriction when the material is ferromagnetic.

The AC current in the electric coil generates eddy current on the surface of the material. According to theory of electromagnetic induction, the distribution of the eddy current is only at a very thin layer of the material, called skin depth. This depth reduces with the increase of AC frequency, the material conductivity, and permeability. Typically for 1 MHz AC excitation, the skin depth is only a fraction of a millimeter for primary metals like steel, copper and aluminum. The eddy current in the magnetic field experiences Lorentz force. In a microscopic view, the Lorentz force is applied on the electrons in the eddy current. In a macroscopic view, the Lorentz force is applied on the surface region of the material due to interaction between electrons and atoms. The distribution of Lorentz force is controlled by the design of magnet, and design of the electric coil, the properties of the test material, relative position between the transducer and the test part, and the excitation signal for the transducer.

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