Electromagnetic Induction of Ultrasonic Waves: Emat, Emus, Emar

Electromagnetic induction of ultrasonic waves is restricted to conducting materials – like eddy current testing – because it involves inducing eddy currents in the surface of the part by a nearby coil of wire. A magnetic field supplied by a nearby magnet interacts with this eddy current to produce a mechanical force on the surface to excite ultrasonic vibrations. The same configuration of coil and magnet also detects mechanical motion of the surface because the motion of a conductor in a magnetic field produces currents that are detected and measured by the nearby coil. This transduction by induction has the following advantages for NDT: (1) The " nearby " feature implies an air gap next to the part surface and, thus, no coupling liquid or grease layer is present to restrict the range of temperatures or inspection speeds available for testing. (2) The fact that the transduction process takes place within a thin layer at the surface of the part allows the time-of-flight (or phase) of the ultrasonic wave to be measured with great accuracy so that dimensions and physical properties of materials can be used for quality assurance purposes. (3) The shape of the coil and the direction of the magnetic field allow the type (shear or longitudinal) and direction of propagation of the wave to be controlled by the transducer design. (4) Special wave types (e.g., shear horizontal, Lamb, Rayleigh) are readily available to satisfy unusual inspection problems. (5) Coils that are large or contoured to fit odd shapes are inexpensive and extend the range of part geometries available for inspection. The primary disadvantage of EMATs is their inefficiency. However, this drawback has been overcome with modern electronic design and digital signal processing techniques. Introduction: The transducers usually used for ultrasonic testing are hand-held probes that must be coupled to the object being tested by a water bath or a thin layer of grease. This latter requirement is often messy and adds considerable mechanical complexity to the test as well as challenging the skill of the operator to get reproducible results. Eliminating this coupling chore and introducing transducers that can operate across an air gap has always been a " holy grail " quest. About 40 years ago, the quest began to be fulfilled with the introduction of high power lasers and air coupling techniques that are being described in the accompanying papers. This paper describes an electromagnetic …