| ABSTRACT: | INCREASING INFORMATION DENSITY OF ULTRASONIC TESTING OF PLATES WITH THE
USE OF THE EMAT
A. Kirikov
"Nordinkraft" Company", Cherepovets, Vologda region, Russia
The report considers theoretical and practical aspects of increasing information density of ultrasonic testing
of plates with the use of the EMAT. It gives principles of constructing multi-channel autocalibrating systems
for detecting internal and surface defects at temperatures of -40 to + 650 degrees Celsius.
Peculiarities, occurring, when testing plates ultrasonically with the waves excited normal to the surface with
the EMAT, are caused by the following circumstances.
1. When testing ferromagnetic plates, the EMAT excites not longitudinal but transverse waves as in the
case of the piezoelectric transducer (PEP). Thus, at the same frequency the wave length excited by the
EMAT is almost twice as short and slow than in the case of the EMAT. This allows one to count on a twice
as big resolution of the EMAT, which is very important at wall thickness measurement and when detecting
defects located close to the surface.
2. The EMAT can operate in a very wide range of temperatures.
3. A polarized transverse wave is extremely sensitive to vertical cracks.
4. The EMAT excites elastic waves strictly normal to the surface of a plate regardless of the inclination
angle of the EMAT itself. This is an extremely useful property that always allows receiving a very stable
reference signal reflected from the back wall. Moreover, the second back wall echo is quite stable as well.
The last circumstance, taking into account article 2, creates the prerequisites for picking up not only internal
but surface defects such as vertical cracks, dents etc. which cannot be detected by means of PEP. Analyzing
the interval between the first and the second back wall echoes, as it is shown in the report, allows one to
twice increase testing sensitivity to ÇclosedÈ - weakly reflecting laminations.
The report gives data on practical application of all these advantages in multi-channel systems for automated
ultrasonic testing of plates.
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