WHAT IS AN EMAT
An EMAT is an Electro-Magnetic Acoustic Transducer which couples to a metalic sample through electromagnetic
forces. Thus the
EMAT
need not touch the metal sample. The
EMAT
is a non-contact device. The elecromagnetic
coupling works on two different principles. Non-ferromagnetic materials use a Lorentz force mechanism where a
current in a magnetic field creates a force. In ferro magnetic materials the metal changes shape in the presence
of a magnetic field. Placing a current carrying conductor in the magnetic field near the metal surface chnages the
field as a function of the wire current and thus affects the shape of the adjacent metal. This is know as
magnetostrictive coupling and can be much larger than Lorentz forces as the material is displaced directly as
oppossed to forces being applied. See Fig.1 for an illustration of a typical
EMAT
configuration.
FIGURE 1: Basic EMAT Configurations: a tangential magnetic field makes forces normal to the
surface and a normal field makes tangential, shearing forces.
FLAW DETECTION
Flaw detection using an
EMAT
is similar to conventional piezo methods. Ultrasound is ultrasound regardless of
transducer type, flaw sizing requires measuring the size and or duration of an echo reflected from a flaw.
However, EMATs are capable of generating additional acoustic modes than a piezo. More control over
displacement is the key. EMATs can directly shear the surface, twist the surface, or pound on the surface,
AND do this in a periodic fashion. By identifying wavelength as well as frequency, EMATs can selectively
transmit and selectively receive acoustic energy. Discrimination of wave mode and type allow EMATs to be
readily adapted to go / no-go algorithms.
EMATs allow effective interrogation of test material with plate waves. Often called
LAMB WAVES
, plate waves
are highly complex combinations of longitudinal, and shear waves, mode converting between the two
boundary surfaces of the plate. Numerous modes are available, each mode having specific properties relating
to its frequency, wavelength and the thickness of the plate it is propagating in. With so many wave modes
available it can be difficult to isolate one mode from the other. EMATs specify the frequency, the wavelength,
AND the surface displacement direction. These three variables are required to isolate a specific wave mode.
A particularly advantageous type of plate wave is the
HORIZONTALLY POLARIZED SHEAR WAVE
, or
SH
wave.
The lowest order mode, Sho, is the only Lamb wave independent of plate thickness. The entire plate thickness
shears parallel to the surface. SH waves propagates and is polarized parallel to the surface. This symmetry
has great advantages for measuring
MATERIAL PROPERTIES
. The SHo wave mode typically does not mode
convert when reflecting off a crack as most Lamb waves will. This is a big advantage in flaw detection as it
provides a more linear flaw size to reflection relationship.
Easily generated and received with an
EMAT
, the family of SH modes is not available to piezo-electric
transducers.
MATERIAL PROPERTIES
EMATs have a distinct advantage for material property measurement since they only respond to the
displacement of the sample. There is no distortion from fluid couplants or mechanical response of the crystal
transducer, as with piezo electric transducers. This effect is best demonstrated by example. Figure 1 shows a 3
cycle tone burst bouncing back and forth through a one inch thick steel plate. In an expanded view, Figure 2
shows the first pass through the plate, and Figure 3 shows the 17th pass through the plate. Notice how the tone burst shape is preserved in both figures. The only change is amplitude due to beam spread and scattering. The
shape of the echo was not distorted by the electromagnetic transduction process, and is therefore true to the
material displacement. This pure sensing of the surface permits the measurement of absolute sound velocity to
extremely high resolution.
FIGURE 1: 3 cycle tone burst traveling between the surfaces of a 1 inch steel plate in a pitch catch mode,
receiver on opposite side from transmitter. The tone burst is on the far left, each signal thereafter is
2k+1 times through the plate, 0 < k < 8.
FIGURE 2: Expanded view of 1st received signal, single pass through material thickness.
The high resolution and linear response available with EMATs makes
STRESS
measurements possible. Acousto-
elastic effect has long been known, but practical application could not be realized due to measurement errors.
EMATs by their nature avoid the non-linearity intrinsic to piezo-electric transducers. An
EMAT
can directly
generate shear displacements in a surface developing
HORIZONTALLY POLARIZED SHEAR WAVES
,
SH
waves.
Polarized parallel to the surface, these waves propagate perpendicular to their polarization. Propagating normal
to the surface, an
SH
wave is sensitive to properties in only two of the three material axis, see Figure 4. The anisentropic properties of a material may easily be measured in this manner. Isolating the material properties in
a single direction is a useful tool in measuring residual stress, texture, or other directionally dependent properties.
FIGURE 4: Horizontally Polarized Shear Wave propagates perpendicular to the polarization. Orthogonal
propagation and polarization provides convenient analytical symmetry for material property determination.
THICKNESS GAUGING
Measuring thickness with an
EMAT
is similar to measuring thickness with a Piezo electric transducer, both are
TOF, time of flight based The limiting factor in either case is the recovery time of the instrument. The minimu
m thickness measurable, using a single tone burst, depends on the instrument recovery time. Under special
conditions thin materials, too thin to be measured with a single pulse technique, can be measured using
multiple echoes. If the sample has parallel sides and the acoustic pulse can reflect between the parallel
surfaces many times, thinner materials can be measured using
TOF
between echoes farther out in time from
the initial transmitter pulse. This case can be typically found with new materials. Corrosion and wear degrad
es parallelism. The amount of non-parallelism and/or pitting radius encountered will determine the beam size
and drive frequency of the thickness gauge. Both angle and normal beam methods are possible with EMATs
as with piezo.
Signal Drop-Out
is a common problem with any normal beam (0 degree) thickness gauge,
independent of transducer type. Angle beam methods may exhibit non-linear behavior with certain geometries.
EMAT
thickness gauges have the advantage of tolerating rough surfaces, dirty environments, and high
temperatures. EMATs are intrinsically more accurate than a Piezo as there is no couplant thickness to account
for. Eliminating couplant also permits the
EMAT
system to scan at high speeds. High speed and low
maintenance, EMATs are readily adaptable for use as process or quality control in manufacturing environments.
WELD INSPECTION
Weld Inspection with EMATs is performed using 3 general techniques:
Through Transmission, Pulse Echo, and Focused Beam.
These techniques are listed in increasing order of resolution.
Through Transmission inspection involves transmitting an acoustic signal through the inspection target area to a
receiver. This method measures the material cross section engaged over the entire distance from transmitter
to receiver. If the ultrasonic wave packet transmits through the weld without alteration, the weld is high
integrity as no discontinuities exist between the weld and base material. Changes in wave packet characteristics
correlate to specific defects in the weld or base material. This technique can also be applied to riveted lap joints; joint quality is characterized by the attenuation of acoustic energy through the joint.
Pulse Echo
also inspects
the engaged cross section but does not require access to both ends of the inspection area. This technique
transmits an acoustic signal through a target area while "listening" for an echo. Presence of an echo, or lack of
an echo, is a criteria for weld and base material characterization.
Both Through Transmission and Pulse Echo can be used to inspect areas several feet of cross section. Wave
modes can be selected to ignore clamps, supports, and insulation allowing inspection of remote welds. That an
area can be inspected using a single scanning axis is a very desirable system design feature.
Focused Beam is a special pulse echo technique. This method inspects a very small area at a discrete distance
from the transmitter. Focused Beam is a higher resolution tool, but requires a more demanding scanning
regime, scanning axis must match number and size of inspection areas axis. In this regard, Focused Beam is
similar to piezo, conventional UT. As in other
EMAT
applications, surface preparation is not required nor couplant.
Both are needed for piezo inspections. EMATs hold a tremendous advantage for this point alone when
considering the conditions encountered inside pipelines or on external support structures.
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FIGURE 3: Expanded view of 8th received signal, 17 passes through material thickness. Note that the
shape of the tone burst is still preserved
