PRELIMINARY TESTS TO DESIGN AN EMAT WITH PULSED
ELECTROMAGNET FOR HIGH TEMPERATURE
F. Hernandez-Valle1 and S. Dixon1
1
Department of Physics, University of Warwick, CV4 7AL, Coventry, U.K.
ABSTRACT. Electromagnetic Acoustic Transducers (EMATs) typically make use of permanent
magnets since they provide high magnetic fields with a compact size. For high temperatures, the
relatively low Curie point of suitable magnets is a disadvantage (e.g. 100-150 ºC in NdFeB). An
alterative approach is to employ an electromagnet, which can be used at elevated temperatures
without cooling. Large power supplies and cooling are required for DC current electromagnets
capable of producing magnetic flux densities comparable to NdFeB magnets. If one uses a pulsed
current electromagnet, synchronous to the excitation current of the EMAT coil, the high field can be
obtained with a relatively low duty cycle and insignificant heating of the coil. We present the
preliminary results in the design of an EMAT, intended for high temperature operation using a pulsed
electromagnet.
Keywords: EMAT, Pulsed electromagnet, High temperature.
PACS: R43.38.Dv
INTRODUCTION
In general, ultrasonic methods are well established for material characterization and
NDT in research and industry [1-2]. However, most applications are restricted to
temperatures below 200-300 °C [3]. Nowadays, there are a number of methods used for
inspection of components at high temperature, including eddy current based methods and
laser based interferometric methods [4-6], amongst others. They are usually expensive,
highly dependent on surface condition and require fairly large diameter optics, even when
fibre-coupled. Additionally, high temperature applications are receiving increased
attention in the piezoelectric materials community [7], but their long term performance is
yet to be demonstrated.
Accordingly, the design of novel ultrasonic generation and detection devices and their
implementation with combined measurement systems to deal with high temperature
inspection problems are of great interest to both academic researchers and potential
industrial users.
Previous work has shown that EMATs with permanent magnets can be employed to find
and size defects at elevated temperatures [8-9]. In these cases, the temperature of the
magnet must be kept below its Curie point, and thus cooling is required. An alterative
approach is to employ an electromagnet, as was typical in the earlier generations of
EMATs and is still used in various designs today. Electromagnets can be used at elevated
temperatures without cooling, beyond the maximum operation temperature of permanent
magnets such as NdFeB or SmCo.
This paper presents the preliminary results in the design of an EMAT with a pulsed
electromagnet; intended for high temperature operation with the ability to generate and/or
detect the ultrasonic modes across a range of frequencies for different applications.
EXPERIMENTAL SET-UP
The performance of an EMAT using a pulsed electromagnet on two different
samples (low carbon mild steel and aluminium) was investigated using the set-up shown in
figure 1.
The pulsed electromagnet was energized using a current pulse generator; which also
produces a TTL signal to trigger the oscilloscope and the pulser/receiver system. The
pulser-receiver provides a pulser unit to drive the EMAT coil in generation and a built in
pre-amplifier to give the necessary wideband low noise amplification required in detection.
The pulse-echo mode was used to detect ultrasonic signals and the signals were recorded
as a function of the lift-off between the sample and the EMAT. Measurements at five
different lift-offs were recorded; 0, 0.5, 1.0, 1.5 and 2.0 mm.
EXPERIMENTAL RESULTS
On the surface
The first part of experimental work was performed using the EMAT in contact with
the sample surface. A typical ultrasonic signal for the EMAT with pulsed electromagnet on
low carbon mild steel can be seen in figure 2.
FIGURE 1. Experimental set-up to investigate EMAT performance.
0.8
0.6
Amplitude / V
0.4
0.2
Maximum signal amplitude
0.0
-0.2
-0.4
-0.6
-0.8
11.0
11.5
12.0
12.5
Time / µs
FIGURE 2. Typical ultrasonic signal.
13.0
13.5
14.0
Normalised Signal Amplitude / a.u.
In order to obtain the maximum ultrasonic signal amplitude, different time delays for
triggering the EMAT coil were tested. It was found that if the EMAT pulse was fired 3 ms
after the electromagnet pulse commenced, the maximum signal amplitude was reached, as
can be seen in figure 3. This occurs at the peak value of the current in the electromagnet,
and the strength of the magnetic field, as expected.
Once the system was optimized, a comparison on the performance on different samples
was undertaken. It was found that the EMAT with pulsed electromagnet has better
performance on low carbon mild steel compared to aluminium samples (see figure 4). A
significant enhancement in the generated amplitude at room temperature of approximately
a factor of four is observed.
Maximum signal amplitude obtained
1.0
0.8
0.6
Maximum electromagnet coil current
0.4
2.0
2.5
3.0
3.5
EMAT coil trigger delay/ ms
FIGURE 3. Optimizing the signal amplitude.
0.8
0.6
Electromagnet on steel
Electromagnet on aluminium
Amplitude / V
0.4
0.2
0.0
-0.2
-0.4
-0.6
7.8
8.0
8.2
8.4
8.6
8.8
Time / µs
FIGURE 4. Signal amplitude using EMAT with pulsed electromagnet on low carbon mild steel and
aluminium.
Signal amplitude / V
1.5
Pulsed electromagnet
Permanent magnet
1.0
0.5
0.0
0.5
1.0
1.5
Lift-off / mm
FIGURE 5. Lift-off performance comparison on low carbon mild steel at room temperature.
2.0
Off the surface
A comparison of the lift-off performance using an EMAT with permanent magnet
and the one that utilizes the pulsed electromagnet was performed. The results are presented
in figure 5, and it can be seen that the signal amplitude is enhanced, approximately by a
factor of three when using the pulsed electromagnet based EMAT. The ratio decreases
when lift-off is incremented, as expected.
Magnetic field
In order to sense the magnetic field intensity produced by the pulsed electromagnet
a Hall effect sensor was used. The sensor was located underneath the electromagnet central
core leg, in the same location in which the EMAT coil was positioned. Measurements on
both sample surfaces (low carbon mild steel and aluminium) were carried out using the
set-up shown in fig. 6.
The magnetic (B) field produced by the electromagnet, as measured by the calibrated
sensor, is shown in figures 7 & 8. As can be seen, the B field intensity is more than three
times stronger in low carbon mild steel compared to aluminium; which agrees with the
finite element simulations made using FEMLAB and presented in figure 9.
FIGURE 6. Set-up to investigate the magnetic field intensity.
FIGURE 7. Experimentally measured magnetic field intensity on low carbon mild steel.
FIGURE 8. Experimentally measured magnetic field intensity on Aluminium.
(a) Low carbon mild steel
(b) Aluminium
FIGURE 9. Simulation of magnetic field intensity on different samples, showing contours of magnetic field
strength.
The simulation assumed a DC current of 20 A, which is the peak value of current through
the electromagnet. The simulated values obtained were 779.81 and 276.3 mT for carbon
mild steel and aluminium, respectively; and their corresponding experimental values were
781.18 ± 35mT and 220.22 ± 35 mT.
CONCLUSION
It has been shown that pulsed electromagnet based EMATs can have a significant
enhancement in the generated ultrasonic signal amplitude, for operation on mild steel
samples, compared to otherwise equivalent EMATs that use permanent magnets, under the
experimental conditions and set-up described here.
Based on these results and the advantages of using pulsed electromagnets, the
electromagnet based EMATs can be a valuable tool for many industrial applications,
especially for those that require operation at high temperatures.
FUTURE WORK
There are some fundamental tasks to be accomplished in the short term. At this stage the
most important is to measure materials’ elastic properties at a range of temperatures using
the EMAT with a pulsed electromagnet as a detector alone / generator alone / combined
generator and detector.
ACKNOWLEDGEMENTS
I am grateful to the Mexican National Council of Science and Technology
(CONACYT) for the scholarship awarded to pursue my PhD.
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