In-Service Inspections Using EMAT and MRUT

EMAT Overview

Readers were previously introduced to some of the practical advantages of EMAT Ultrasonic Testing (UT) in the March/April 2013 and July/August 2013 issues of Inspectioneering Journal. These advantages include dry inspection, an imperviousness to surface conditions, and unique wave modes such as shear waves with horizontal polarization (SH waves). In this article, I will cover the practical advantages of EMAT for in-service applications using Medium Range Ultrasonic Testing (MRUT).

As a review, EMAT, or Electro Magnetic Acoustic Transducer, is a UT technique that generates the sound in the component inspected instead of the transducer. As illustrated in Figure 1, EMAT generates ultrasonic waves into a test object using electromagnetic induction with two interacting magnetic fields. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequency, or static, field generated by magnets to produce a Lorentz force in a manner similar to an electric motor. This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit.

Because the sound is generated in the part inspected instead of the transducer a liquid couplant is not required.  EMAT is a completely non-contact technique that has significant advantages for in-service applications over more conventional piezoelectric inspection techniques. More specifically, in this article, we will discuss the applications and techniques deployed when using guided waves with a MRUT system.