Electromagnetic Acoustic Transducers

EMATs are the devices that essentially consist of a stack of wires and magnets to excite and receive ultrasonic waves in an electrically conductive material, be it magnetic or non-magnetic. When a wire, placed near to the surface of an electrically conducting object, is driven by an A.C at a desired ultrasonic frequency, eddy currents will be induced in the object by electromagnetic induction. Their penetration in the object is given by the classical electromagnetic skin effect. In the presence of a static magnetic field (B0), these induced eddy currents (J) will experience Lorenz forces (f) given by

f = J X B0

Through a variety of interactions, these Lorentz forces are transmitted into the lattice and serve as a source of ultrasonic waves. EMATs are reciprocal devices i.e. they can be used as transmitters or as receivers of ultrasound. When an EMAT transmitter is placed near a electrically conducting material, not necessarily in contact with, ultrasonic waves are launched in the material through the reaction of induced eddy currents and static magnetic fields (Lorentz forces). This eliminates the problems associated with acoustic coupling to the metal part under examination as the electro-mechanical conversion takes place directly within the electromagnetic skin depth of the material surface. Thus, EMATs allow non-contact operation and enable inspection at elevated temperatures, on moving objects, in vacuum or oily or rough surfaces and also in remote and hazardous locations.

EMATs can launch a variety of ultrasonic wave modes with relatively well controlled polarization, intensity and angular distribution. Unlike piezoelectric transducers, it is possible to perform inspection at any angle of incidence with one stationary EMAT. This is achieved by varying the excitation frequency and/or the delay times between the array elements of a phased array transducer system. Typical EMAT probes, wave modes and their applications are as follows:

Practical Applications of EMATs

EMAT Type

Angle Beam Probes
SV – Waves
SH – Waves
Application for UT of pipes, Control of liquid level, ISI of Coarse Grained welds

Angle Beam Probes
Lamb modes
Application for UT of sheets and plates
Rayleigh waves
Application for UT of the tread of railway wheels

Angle Beam Probes
SH modes Application for Thickness measurement of high alloyed sheets

Normal Beam Probes
Linearly polarised shear waves
Radially polarised shear waves
Application for Stress and texture analysis, wall thickness measurement

Shear Horizontal (SH) Waves

It is possible to generate shear horizontal (SH) waves using EMATs while the same is very difficult with PZTs. SH wave mode has provided solution to many non-destructive inspection situations, primarily due to the following attractive features of SH waves over L or SV waves:

Reflection, refraction and diffraction without mode conversion

Complete corner reflection independent of angle of incidence

Propagation in thick-walled components (thickness >> wavelength) as a bulk wave even along the surface

Propagation in thin walled components (thickness <>