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product of vector of eddy current in metal and vector of magnetic field. When used as a
receiver, acoustic vibrations inside the conductor move the surface under the wire. In the
presence of a magnetic field, this motion produces an eddy current in the conductor
surface that produces a magnetic field extending across the air gap to induce a current in
the near-by wire connected to a preamplifier.
The advantages of the EMAT are:
–
The lack of immediate contact implies an air gap next to the surface part and, thus, no
coupling liquid or grease layer is present to restrict the range of temperatures or
inspection speeds available for testing.
–
The transduction process takes place within a thin layer at the surface 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 enable quality assurance purposes.
–
The shape of the coil and the direction of the magnetic field allow the type (shear or
longitudinal) and direction of wave propagation to be controlled by the transducer
design.
–
Special wave types (e.g., shear horizontal, Lamb, Rayleigh) are readily available to
satisfy unusual inspection problems.
–
Coils that are large or contoured to fit odd shapes are inexpensive and extend the range
of part geometries available for inspection.
The sensitivity of EMAT is often expressed by transfer impedance between a
transmitter and a receiver pair: it is defined as the impedance of the voltage output at the
receiver coil’s terminals divided by the current being driven through the transmitter’s
coil. Thus, its unit is ohm. Under convenient conditions (magnetic fields of the order of
5,000 gauss and many turns in the EMAT coils), the output voltage can be as large as a
millivolt when the transmitter current is 100 A – and transfer impedance is 10
μΩ
.
Achieving these levels of current in the transmitter coil and amplifying the receiver
voltages with a minimum of noise requires careful electronic design.
Besides more comfort testing, EMAT enables the application of different wave modes,
such as Lamb, shear or surface (Rayleigh) waves dependent on product geometry, such as
plate, bar and pipes. By directing waves circumferential around the surface of the bar, a
measure of the amplitude of the waves after each trip around the bar, can be used to
determine the apparent surface wave attenuation of the ultrasonic surface wave on the bar
surface. This quantity is determined by the beam spread, the microstructure of the
material and by scattering from any defect on the surface. For longitudinal laps and
seams, the scattered amplitude and hence the attenuation can be shown to be a universal
function of the wave length of the surface wave and the depth of the flaw.
The analysis of the EMAT sending and reception with the help of existing transducers
on the base of constant magnets has shown, that their main disadvantage is the low
transduction coefficient at testing objects from ferromagnetic materials attacked by
corrosion. Thus it appears necessary to increase the pulsing energy and magnetic field
induction. Anyhow, this leads to the increase of power consumption, sizes and weight of
EMAT and to limitation at work on steel objects, because of the damage danger.
There are known the EMATs, which work as a part of stationary plants and provide
high sensitive welds testing with Lamb waves, using the pulse magnetic field. But this
kind of EMAT was not used in the devices with off-line power supply, because of its big
sizes and large power consumption.