EMAT Ultrasonic Guided Wave Inspection of Simple Pipe Supports

An EMAT (electromagnetic acoustic transducers) is a coil of wire in a magnetic fi eld close to a conductive material. By pulsing an electrical current through the coil, an eddy current is induced in the surface of the nearby conductive material. The magnetic fi eld then interacts with this eddy current to produce a mechanical force to excite ultrasonic vibrations. The same confi guration of coil and magnet is also able to detect mechanical motion of the surface because the motion of a conductor in a magnetic fi eld produces currents that are detected and measured by the coil. One of the most useful properties of EMAT technology as an inspection technique is its ability to generate guided waves without having to worry about coupling. This is due to the noncontact, couplant-free nature of EMAT transducers. This ability enables a simple scanner to generate ultrasonic guided waves and scan the transducers over the inspection area at the same time. This is particularly useful at pipe support touch points. Matrix Inspection and Engineering has recently started using EMAT technology to complement their other inspection services. The main application this paper describes is to provide more reliable information on the amount of corrosion at simple pipe supports. Identifying corrosion at pipe supports with long-range ultrasonics or visually gives limited information on the extent of the damage, especially in the axis of the propagation of the guided wave. Combining the information from an EMAT generated guided wave that is propagated circumferentially around the pipe (i.e. perpendicular to the long-range guided wave) and scanned over the length of the pipe support enables the inspector to estimate the extent and depth of corrosion. With the addition of accurate time of fl ight measurements and several different guided wave modes available to the inspector, it is possible to provide a qualitative assessment of an otherwise inaccessible, critical area.

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