Non-Contact Ultrasonic Testing with EMAT: In-Service Applications

Non-contact ultrasonic testing with EMAT was formally introduced in part one of this series found in the March/April issue of Inspectioneering Journal. Readers were introduced to the practical advantages of EMAT Ultrasonic Testing that include;

• It does not use couplant, i.e. dry inspection
• It is not affected by surface conditions (it is a non-contact NDE method)
• It produces unique wave modes such as shear waves with horizontal polarization (SH waves) which can be used to improve UT detection and measurement capabilities

This second installment will cover the practical advantages of EMAT in the field of In-Service Applications.

EMAT In-Service Applications

EMAT, or Electro Magnetic Acoustic Transducer, is an Ultrasonic Testing (UT) technique that generates the sound in the part inspected instead of the transducer.

An 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. For ferromagnetic conductors, magnetostriction produces additional stresses that can enhance the signals to much higher levels than could be obtained by the Lorentz force alone. Because the sound is generated in the part inspected instead of the transducer, EMAT is a completely non-contact technique that has significant advantages for in-service applications over more conventional piezoelectric (traditional UT transducer) inspection techniques.

Utilizing this non-contact technique, as well as the diverse wave modes available with ultrasonic EMAT, the following measurement types appeal to a variety of different in-service applications including:

Normal Beam Measurement – EMATs do not require couplant for transmitting sound, which makes them very well suited for inspection of hot and cold parts. Due to the liftoff capabilities of EMAT sensors, coated, painted, and rough surfaces up to 4mm (0.160”) (keep this in mind as a reference) do not affect normal beam readings. These features make EMAT sensors ideal for permanent monitoring in extreme environments and have been used successfully by oil and gas refineries and pipelines.