Velocity
Measurements Using EMATs
Electromagneticacoustic transducers (EMAT) generate ultrasound
in the material being investigated. When a wire or coil is placed
near the surface of an electrically conducting object and is
driven by a current at the desired ultrasonic frequency, eddy
currents will be induced in a near surface region. If a static
magnetic field is also present, these currents will experience
Lorentz forces of the form
F = J x B
F is the body force per unit volume, J
is the induced dynamic current density, and B is
the static magnetic induction.
The most important application of EMATs has been in nondestructive
evaluation (NDE) applications such as flaw detection or material
property characterization. Couplant free transducers allows operation
without contact at elevated temperatures and in remote locations.
The coil and magnet structure can also be designed to excite complex
wave patterns and polarizations that would be difficult to realize
with fluid coupled piezoelectric probes. In the inference of material
properties from precise velocity or attenuation measurements,
the use of EMATs can eliminate errors associated with couplant
variation, particularly in contact measurements.
Differential velocity is measured using a T1T2R fixed array
of EMAT transducers at 0°, 45°, 90° or 0°, 90°
relative rotational directions depending on device configuration.
EMAT Driver Frequency: 450600 kHz (nominal)
Sampling Period: 100 ns
Time Measurement Accuracy:
Resolution 0.1 ns
Accuracy required for less than 2 KSI Stress Measurements: Variance
2.47 ns
Accuracy required for texture: Variance 10.0 ns
W440 < 3.72E5
W420 < 1.47E4
W400 < 2.38E4
Time Measurement Technique:
Fourier TransformPhaseSlope determination of delta time between
received RF bursts (T_{2}R)  (T_{1}R), where
T_{2} and T_{1} EMATs are driven in series to
eliminate differential phase shift due to probe liftoff.
Slope of the phase is determined by a linear regression of weighted
data points within the signal bandwidth and a weighted yintercept.
The accuracy obtained with this method can exceed one part in
one hundred thousand (1:100,000).
