Due to this couplant-free feature, EMAT is particularly useful for automated inspection, and hot, cold, clean, or dry environments. EMAT can be used for thickness measurement, flaw detection, and material property characterization. There are two basic components in an Piezoelectric transducer pdf file 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. 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. 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.
A ferromagnetic material will have a dimensional change when an external magnetic field is applied. The flux field of a magnet expands or collapses depends on the arrangement of ferromagnetic material having inducing voltage in a coil and the amount of change is affected by the magnitude and direction of the field. The AC current in the electric coil induces an AC magnetic field and thus produces magnetostriction at ultrasonic frequency in the material. The disturbances caused by magnetostriction then propagate in the material as an ultrasound wave.