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Re: Comparison of dry-coupled, air-coupled and EMAT transducers
Posted by: Rathgeb/Meier , E-mail: meierrai@erls19.kwu.siemens.de, on September 27, 1996 at 19:32:49:In Reply to: Comparison of dry-coupled, air-coupled and EMAT transducers posted by : A. Anev on September 26, 1996 at 16:45:28:
: What is the difference in the accuracy of dry-coupled, air coupled and EMAT transducers generally and regarding near surface and internal imperfections?
: Suppose all the transducers work at the accuracy at which they give the best results.
: Furthermore do you think polymer transducers can be used as dry-coupled ones?The use of EMAT transducers is restricted to materials with a high
conductivity. The results measured on aluminum or ferritic steel test pieces
are very good, stainless steel test pieces may show worse results. This behaviour
also depends on design details of the EMAT-probe:
- EMAT SH (shear wave horizontal)probes with the electrical coil between the
rows of permanent magnets and the test piece surface work very well also on
stainless steel components, but due to design details, this probe type is
restricted to frequencies below 1 MHz.
- EMAT SH probes with the electrical coil wounded arround a ferritic yoke
allow frequencies up to 2 kHz, but they do not work on stainless steel.
- New develloped Siemens SH probes allow frequencies up to 2 MHz and work
also on stainless steel, but the sensitivity on ferritic steel components
is some dBs better.
- New develloped Siemens SV (shear wave, vertical polarised) probes with
frequencies up to 2 MHz work also on stainless steel components. The
sensitvity on ferritic steel or on aluminium is also some dBs better.
With these probes, used as phased array probes, it is possible to produce
a 0 degree shear wave, a 40 to 45 degree shear wave or a rayleighwave.Generally, EMAT suffers on a lift off effect: The higher the frequency,
the bigger the lift off. This requires a flat surface for a good "coupling"
of the probe. In the case of the above mentioned SV probe, the lift off effect
(loss of sensitivity) can be corrected by evaluating the amplitude of a
0 degrees wave reflected at the backwall.EMAT also suffers on a quite long dead zone, due to crosstalk of the transmitter
current to the receiver. Also the sound beam is not so well formed as in the
case of piezo transducers.Advantages of EMAT are the testing without couplant and the possibility to
choose the orientation of the polarisation! The second means, that one can
avoid the conversion into other wave modes by use of EMAT and one can also
minimize the deflection of the ultrasonic beam in case of a distinct grain
orientation in stainless steel or dissimilar welds by choosing the right
polarisation of the ultrasound.Dependent on the above mentioned factors (material, frequency, surface, probe
design etc.) it is possible to detect notches with a depth of about 1mm, as well
on the probe near surface (rayleigh wave, lamb wave) as on the probe far
surface (45 degree shear wave). Up to now, precise sizing with EMAT is still
a problem, but this is improoving!Rainer Meier