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Electromagnetic Acoustic Transducers
(EMAT)
The Need
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The current accepted non-destructive
evaluation methods used in the inspection of welds have several shortcomings regarding
cost, false defect indications, and lack of sensitiveness to certain flaw orientations in
welds. Electromagnetic Acoustic Transducers (EMAT) technology combines ultrasonic
techniques with modifications to correct this shortcomings and add a flaw discriminator
that provides a reject alarm on flaws with a through-wall depths of 10% or greater, but
passes passes with a through-wall depth of 5% or less. This meets code requirements for
flaw detection, and produces considerable savings in material by rejecting only material
with serious flaws. |
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The Technology
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The EMAT technology combines
the advantages of ultrasonic testing (UT) providing the generation of ultrasonic waves
including surface waves, longitudinal waves that operate at elevated temperatures and
allows rapid scanning. In addition the operating characteristics of EMATs can be
reproduced from one unit to another very easily making them potentially useful as
ultrasonic standards. This technology has gone a long way and previous shortcomings like
the limitation on cable length between the EMAT and electronic system. EMAT applications
include:
- Volumetric Inspection of Welds
- Inspection of External Fuel Tank Welds
- Circumferential Weld Inspection
- Submarine Weld Inspection
- Inspection of Seam Welded (ERW) Tubing
- Offshore Tension Leg Weld Inspection System.
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The Benefits
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Magnetic particle testing (MT)
is recognized as the NDE method of choice for surface inspection of welds in ferromagnetic
material, but it can be difficult to apply, unreliable, not repeatable, and subject to
false defect indications (magnetic ghosts). In addition, MT can be costly in terms
of the time required to perform an inspection, and increasing environmental regulations
require proper use and disposal of the magnetic particles. Radiography (RT) is the
typical volumetric test method, but is insensitive to certain flaw orientations and can be
quite costly in terms of labor and licensing requirements. Welds can crack as a
result of variables in the welding process, and porosity, lack of fusion/penetration are
always a concern. EMATs have been evaluated as a replacement for MT for surface
inspection and for radiography (RT) for volumetric inspection of welds in the shipyard in
a program sponsored by the SP-7 welding panel of the National Shipbuilding Research
Program. The high temperature capabilities of EMATs can be especially useful in cases
where it is desirable to inspect a weld immediately after or even during the welding
process when the weld and base metal are still hot. A patented technique based on
diffraction can be used to perform a simultaneous surface and volumetric inspection of the
weld with a single linear scan (no raster motion required) along the weld axis. A
collinear pitch-catch sensor arrangement eliminates the root/crown signals, but is
sensitive to all flaw orientations, providing easy interpretation of inspection data. The
latest SIR systems allow engineers to survey structures with expediency and be able to
assess the structural state of a structure in a matter of hours. |
Status
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EMAT technology is currently
being used by J. Ray McDermott in conjunction with Shaw Pipeline Services in different
applications regarding non-destructive evaluation of pipeline welds, offshore tension leg
weld inspection, and other applications in the field of inspection of welds and materials. |
Barriers
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The equipment required to
perform NDE through EMAT technology is very hard to obtain. Most EMAT systems are the
result of modifications to commercially available electronic equipment. Magnetic Particle
testing and radiography are accepted techniques in the industry and it will take time for
the industry to shift to the EMAT technology. |
Points of Contact
Dan Schlader, McDermott Technology,
Inc.,Lynchburg Research Center ,P.O. Box 11165, Lynchburg, VA 24506-1165, Phone: (804)
522-5769, Fax: (804) 522-6978 E-Mail:Dan.M.Schlader@mcdermott.com.
References
Electromagnetic Acoustic Transducers
(EMAT). McDermott Technology, Inc. Research and Development.
Disclaimer Statement
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Neither the Construction
Industry Institute nor Purdue University in any way endorses this
technology or represents
that the information presented can be relied upon without further investigation. |
CHC03
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