principle of electromagnetic acoustic transformation

7/21/2019 Principle of Electromagnetic Acoustic Transformation

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Physical Basis of EMAT

Heimsheim 2014


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Content

1. Principle of Electromagnetic Acoustic Transformation ................................................................. 3

1.1 Principle of wall thickness measurement ................................................................................ 4

2. Detecting of Defects by means of SV-share Waves ................................................................... 6

3. Detecting of surface defect by means of EMAT ........................................................................... 9


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1. Principle of Electromagnetic Acoustic Transformation

EMAT (Electromagnetic-acoustic Transducer) is the device, which transforms the electromagneticenergy into the energy of ultrasonic waves and (after it reflecting from the opposite surface) performsthe inverse transformation.

The active element of the EMAT is an electrical coil of a special butterfly shape. This exactlysymmetrical shape allows effective suppressing of an external electromagnetic noise and, finally,

has opened the way to wide industrial application of the EMAT-technology. Example of such a coilis represented below.

Pic. 4. A Butterfly coil: a perfect device for transmitting and receiving ultrasonic waves normal tothe plate or pipe surface direction (patented).

Pic. 5. Principle of ultrasonic shear waves generation with EMAT. Waves are generated exactly

normal to the surface.Alternate current feeds the induction coil, causing, causing electromagnetic oscillations, which, inturns, induce eddy current on the surface of the test object. Eddy current interfaces with the

Width of theactive zone

Len th of the active zone


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permanent magnetic field, creating ultrasonic waves directly on the surface of the test object. Thesewaves propagate in the test object, reflecting and deflecting from the walls or defect back to EMATcoil.

1.1 Principle of wall thickness measurement

The physical principle of wall thickness measurement is based on measuring the time intervalbetween two consecutive reflections of ultrasonic wave from the bottom surface (see picture below).These reflections, received by EMAT, are also called as Bottomecho pulses.

- The 1-st bottom pulse is the first visible in the A-scan reflection from the bottom surface.

Wave propagates from point A,reflects at point Band being received at point C=A(seePic.11 below).

- The 2-nd bottom pulse is the same signal but after three logical the reflections according tothe following program: transmitting - bottom surface top surface bottom surface -

receiving. Wave propagates as ABCD and then being received at point E = C =A(see Pic.11 below).

Pic 5. Ultrasonic wave propagation

Time T between two adjacent bottom pulses is proportional to wall the thickness H:

H = C x T/2,

Where Cvelocity of ultrasonic wave propagation. It is a physical constant of the tested material.


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Pic 6. A-scan with received 1-st and 2-nd bottom echo pulses

Therefore, having an information on time T and ultrasound velocity C the system can easily calculatethe actual thickness of the material according to the formula above.

Pic 7. Wall thickness measurement of tubes with ultrasonic waves generated and received byEMAT


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2. Detecting of Defects by means of SV-share Waves

The physical principle of defect detection is based on receiving and analysis of waves, reflectedfrom the cracks on a top and bottom surface of the sample. These waves are being transform byEMAT to weak electrical signals, which after amplification and processing are displayed in the Ascan.

Generation of Inclined SV-waves by means of EMAT

For cracks on the bottom surface, signal in the A-scan is correspond with the wave witch propagatesfrom point A, reflects at point B and then being received at A (see Pic. below):

Pic. 6. Detection of crack on the bottom surface


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A-scan with reflection from crack, located on a bottom surface of the sample

For cracks on the top surface, signal in the A-scan is correspond with the wave witch propagatesfrom point A, reflects by surface at point B, reflects by defect at point C, and that after secondreflection at point B is being received at point A (see Pic. below):

Detection of crack on the top surface


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A-scan with reflection from crack, located on a top surface of the sample

Changing of the initial pulse the working frequency allowing transmitting and receiving of the anglerange of about 30 to 45 by (for angle 32 the working frequency is about 1.5 MHz).

There is one more useful physical factor facilitating high and stable sensitivity of SV-wavestransmitted and received by the EMAT. The principle is explained in the diagram below:

Detection of defects using a mutual transformationof SV-shear and lateral longitudinal ultrasonic waves

It is well known that if the SV-wave propagation angle arcsin (Ct/Cl)in steel is about 32o33odegrees (the critical angle), longitudinal waves will propagate along both surfaces of a specimen.These waves can be reflected by surface or subsurface defects and transformed into SV- wavesagain. A mutual transformation of SV-shear waves and longitudinal ultrasonic waves is a well-knownand an extremely useful physical phenomena, which is the basis of many effective technicalapplications like the famous Delta technique of welded seams ultrasonic examination.

It is very easy to keep the angle stable with the EMAT: the beam angle do not depend on theprobe inclination and test object dimensions or shape. On the contrary, a piezo probe will not work


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in our case: even a little ultrasonic beam inclination or change of the tube shape (that is practicallyinevitable with piezo probes) will immediately influence the beam angle.

Inclination of the EMAT

This phenomena allows one to extend distances for optimal detection of defects located on bothsides of the wall. This is a very important feature of the EMAT as it substantially increases anindustrial reliability of the test results. This is a great advantage of the EMAT-technology.

Thus, SV- waves transmitted by the EMAT are an extremely effective instrument to detect internaland external defects in flat samples, tubes, pipes, cylinders, and etc.

3. Detecting of surface defect by means of EMAT

Generation of Surface waves by means of EMAT

Based on this physical principle described above, EMAT probes are developed for ultrasonicinspection of surface or subsurface of materials. EMAT transforms electromagnetic energy into theenergy of surface elastic waves and (after interacting with defects) performs the inversetransformation. This double transformation is carried out in order to detect surface and subsurfacedefects in a very reliable and intelligent way without mechanical rotation of probes.

END OF THE DOCUMENT

principle of electromagnetic acoustic transformation

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