APPLICATION OF A GIANT MAGNETORESISTIVE (GMR)
SENSOR FOR CHARACTERIZATION OF CORROSION IN A
LABORATORY SPECIMEN
Ray T. Ko1, Mark P. Blodgett
Metals, Ceramics, and NDE Division, Materials and Manufacturing Directorate, Air
Force Research Laboratory, Wright-Patterson Air Force Base
1
Structural Integrity Division, University of Dayton Research Institute, Dayton, Ohio
ABSTRACT. This paper describes the use of a Giant Magnetoresistive (GMR) sensor to detect
thickness variation in a multi-layered specimen. A series of frequency-response and modes-ofoperation tests has been carried out to characterize a GMR sensor. Both the first and second
harmonics of the GMR signal for corrosion detection were collected and analyzed. The experimental
data showed that phase approach seemed to have better thickness discrimination over that of the
magnitude approach.
INTRODUCTION
Results from a previous automated corrosion-detection program (ACDP) have
demonstrated that some NDI technologies can be used to detect hidden corrosion in the
first layer of aging aircraft fuselage skins [1]. In detecting corrosion deeper than the top
layer in a multi-layered aluminum structure, both conventional ultrasonic and eddy current
methods have limitations. In the case of ultrasonic testing, the presence of a gap between
layers in a multi-layered structure becomes a strong reflector of sound waves, and this gap
makes the detection in the subsequent layer difficult. In eddy current testing, low
frequency is often required to induce currents in thick and highly conductive materials like
aluminum alloys. However, use of lower frequencies often means low sensitivity and low
resolution.
The giant magnetoresistive (GMR) effect was first reported in 1988 because of the huge
resistance changes that were observed when thin layers of magnetic materials were
subjected to magnetic fields [2]. This discovery has changed the capability in the head that
reads the bits from a storage disk and thus substantially increased the storage of computer
hard drives over ten gigabytes. In the late 1990s, a few NDE demonstrations focusing on
crack detection started to appear [3-5]. The application of the GMR on corrosion
detection, however, was sporadic. The objective of this work is twofold: (1) to conduct
experiments to characterize a GMR sensor and (2) to find suitable metrics for monitoring
the hidden corrosion.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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Oscilloscope
GMR sensor
DC power supply
Excitation coil
Function generator
FIGURE 1. A schematic view of an experimental setup for characterizing the frequency response of a
GMR sensor. Note that bias was not used for the 2nd harmonic measurement.
EXPERIMENTAL SETUP FOR GMR SENSOR CHARACTERIZATION
The frequency response of a GMR sensor was measured with five coils generating
magnetic fields of various frequencies. These homemade bobbin coils were wound around
plastic forms. A capacitor was put in series with a coil to provide a resonance at a certain
frequency. For example, the coil made with the lowest resonant frequency was 400 Hz
with a 3dB bandwidth of 1 kHz. The dimensions of this bobbin coil were 7 mm high, 10
mm ID, and 21 mm OD. It was made of 1000 turns of AWG 34 copper wire. The
capacitor was measured at 22 microfarads.
In this experiment, the coil was connected to a function generator that sent a sinusoidal
frequency corresponding to the resonant frequency of the coil, as shown in Figure 1. The
GMR sensor was connected to a DC power supply. An oscilloscope was used for the
measurement of the amplitude of the received signal directly from the GMR sensor. For
the 2nd harmonic mode measurement, the GMR sensor was not biased with an external
magnetic field. For the 1st harmonic measurement, a small magnet was applied to shift the
operating point of the GMR sensor.
RESULTS OF GMR SENSOR CHARACTERIZATION
Figure 2 shows received GMR signals from both the 1st and 2nd harmonic modes. They
were taken from two separate tests. The 1st harmonic signal was taken with a small magnet
moving toward the GMR sensor, and the signal was acquired when the amplitude was
maximized. The 2nd harmonic signal was acquired without any external bias. The 1st
harmonic signal had the same period as that from the excitation coil (not shown), and the
2nd harmonic signal had a rectified form as compared to that from the excitation coil. This
was because the GMR sensor was not discriminating in the direction of the magnetic field.
The minor un-balance in the 2nd harmonic signal might be due to the magnetic hysteresis
in the sensor. It can be seen that the amplitude of the maximized 1st harmonic signal was
about twice that of the 2nd harmonic mode and the period was doubled. The background
level was also plotted for comparison.
By taking a Fourier transform of the above waveforms, the frequency spectra are
shown in Figure 3. It is noticed that signals of both the 1st and 2nd harmonic modes were
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0.1 -,
0.05 -
3
-0.05 -
-0.1 J
Time (sec)
st
FIGURE 2. Time domain signals from the 1 (dashed curve) and 2nd harmonic mode (solid curve
frequency), and background noise (dotted curve).
-20-,
-40
-60
-100
100
300
500
700
900
Frequency (Hz)
FIGURE 3. Frequency domain signals from the 1st (dashed curve) and 2nd harmonic mode (solid curve), and
background noise (dotted curve).
at least 30 dB higher than the background noise, and the noise levels were of the same
order for both modes. The peak amplitude of the 1st harmonic mode was 6 dB higher than
that of the 2nd harmonic mode. It was also noticed that in the un-biased case, the 1st
harmonic mode was also observed along with the 2nd harmonic mode, but the 1st harmonic
was 16 dB lower. This extra 1st harmonic mode signal in the un-biased case was also
found in the time domain signal.
By repeating the experiment with coils of different sizes and measuring the outputs at
the resonant frequencies from the oscilloscope and normalized with that at 1 MHz, Figure
4 shows the results of frequency response of a GMR sensor in an un-biased case. An
exponential curve fit was also added to show the trend. It is noticed that the GMR sensor
had flat frequency response from DC to 1 MHz and then the response dropped sharply
after 1 MHz.
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10.000
100.000
Frequency (MHz)
FIGURE 4. Experimental measurement of the frequency response of a GMR sensor.
The two characteristics of this GMR sensor that could be useful in nondestructive
testing are bandwidth and low frequency response. The broad bandwidth could have an
advantage over the conventional coil for those tests in which more than one frequency is
needed. Potential applications of this characteristic are multi-frequency testing and pulsed
eddy current testing. The low frequency response characteristic, on the other hand, makes
the GMR sensor a good sensor for detecting hidden anomalies under highly conductive
materials such as aluminum alloys in which the depth of penetration is often limited. In
this paper, an application of a GMR sensor on thickness variation as it occurs in hidden
corrosion is discussed in the following.
APPLICATION OF A GMR SENSOR FOR THICKNESS VARIATION
MONITORING
For a changing magnetic field involving an infinite half-space [6], the skin effect can
be described in two ways: (1) the exponential decay of current density into materials and
(2) the linear phase lag with depth. Both of these provide a basis for the application of a
GMR sensor for thickness variation monitoring.
The experimental setup for thickness monitoring is shown in Figure 5. It is slightly
different from that used in the sensor characterization with three new features: specimen,
detecting unit and interface. A multi-layer laboratory specimen was placed between the
GMR sensor and the excitation coil. This specimen was made of seven layers of aluminum
alloy - each square layer being 76 mm and 0.76 mm in thickness. A series of impedance
measurements was started with a maximum thickness, of 5.33 mm, and repeated with a
layer removed each time. A lock-in amplifier was used to drive the coil while monitoring
the in-phase and quadrature outputs from the GMR sensor. The data acquisition to the PC
was made through an IEEE488 interface using a Lab Windows / CVI driver. Since the
measurement was done with an un-biased GMR sensor, the output mode was thus set to
the 2nd harmonic frequency in the lock-in amplifier.
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Lock-in amplifier
GMR sensor
DC power supply
Excitation coil
PC / CVI code
FIGURE 5. A schematic view of an experimental setup for monitoring thickness variation using a GMR
sensor. The specimen was made of seven layers of aluminum alloy - each square layer being 76 mm and
0.76 mm in thickness.
0.76mm
1-52 mm
0.01
-0.01 J
In-phase component
FIGURE 6. Lock-in amplifier output of in-phase and quadrature component on a multi-layered laboratory
specimen at various frequencies. The beginning frequency, 1200 Hz, is marked on each curve with a
circular symbol and the ending frequency, 2700 Hz, is marked with a triangular symbol.
RESULTS OF GMR SENSOR FOR THICKNESS VARIATION MONITORING
All the data of the in-phase component and the quadrature component from the 21,nd
harmonic output were collected and analyzed. Figure 6 shows these curves in an
impedance plane after a rotation of-106 degrees around the origin. Each curve represents
the output of a certain thickness over the frequency range measured. For comparison, the
beginning frequency is marked on each curve with a circular symbol and the ending
frequency is marked with a triangular symbol. It is noticed that as the frequency or
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thickness increased, the trend spirals down towards the origin of the coordinate in the
figure.
Another way to represent the data is by taking the magnitude and phase plot versus the
frequency. The magnitude was calculated by taking the square root of the sum of both of
the components in square while the phase was the arctangent of the ratio of the quadrature
component over the in-phase component. Figures 7 and 8 show these plots respectively.
For the magnitude plot, the value decreases as the frequency increases. For thicker
materials, the change with frequency becomes less drastic than that of the thinner material.
For the phase plot, a phase wrap around phenomena was noticed when the phase value is
beyond the range of +/- 180 degrees. Then the phase would add or subtract 360 degrees.
Correction of the wrap around could be done either by rotating the impedance plane or
changing the reference phase value in the lock-in amplifier. In this figure, the phase
showed a monotonous trend with thickness over a frequency range of 1300 Hz to 2500
Hz.
0.016 -i
0.012 -
0.008 -
0.004-
1200
1500
1800
2100
2400
2700
Frequency (Hz)
FIGURE 7. Magnitude output from a multi-layered laboratory specimen at various frequencies.
Frequency (Hz)
FIGURE 8. Phase output from a multi-layered laboratory specimen at various frequencies.
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0.016 -,
0.00
2.00
4.00
6.00
Thickness (mm)
FIGURE 9. Variation of magnitude from a multi-layered laboratory specimen with thickness at three
representative frequencies.
1300 Hz
-200
0.00
1.00 2.00 3.00 4.00 5.00
6.00
Thickness (mm)
FIGURE 10. Variation of phase from a multi-layered laboratory specimen with thickness at three
representative frequencies.
To better represent the data, Figures 9 and 10 show the magnitude and phase values
against the thickness for three representative frequencies. The magnitude decreases
exponentially as the thickness increases. As the thickness becomes large, the amplitude
variation for all frequencies becomes small. On the other hand, the phase variation among
frequencies increases as the thickness increases. In addition, this phase change also bears a
linear relationship with the thickness. This indicated that the phase value could be a
potential tool for monitoring thickness variation as it occurs in the hidden corrosion case.
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CONCLUSION
In characterizing a GMR sensor, experimental results showed that the GMR sensor had
a flat frequency response from DC to 1 MHz that could be useful for detecting hidden
anomalies in highly conductive materials. As an application of this GMR sensor to find
metrics for hidden corrosion detection, both the magnitude and phase approaches were
investigated. Additional experimental results on the laboratory specimen showed that the
phase approach seemed to be more discriminative than the magnitude approach.
Therefore, the phase information could be a potential candidate for monitoring the
thickness variation caused by hidden corrosion. Further studies will continue to
characterize the resolution of a GMR sensor as well as the evaluation of phase approach in
the hidden corrosion detection.
ACKNOWLEDGEMENTS
The authors would like to thank Jeffery Fox, Rick Reibel, Mark Ruddell and David
Petricola at UDRI for their support. Thanks also to the support of Captain Craig Neslen,
AF contract monitor, and Dr. Claudia Kropas-Hughes, in house research group leader
(AFRL/MLLP). This work was performed on-site in the Nondestructive Evaluation
Branch of the Air Force Research Laboratory at WPAFB, Ohio under Air Force contract
#F33615-98-C-5217.
REFERENCES
1. Hoppe W., Pierce J. L., Ko R., Buchanan D. and Schehl N., "Results form the hidden
corrosion detection evaluation on the automated corrosion detection program supporting tests and specimen characterization", The 5th joint NASA/FAA/DoD
conference on aging aircrafts, Orlando, Florida, pp. 950-961 (2001).
2. Baibich, Broto J. M., Fert F., Dau N. V. and Petroff F., "Giant Magnetoressitance of
(001)Fe/(001)Cr magnetic superlattices", Physical Review Letters, 61, 2472-2475
(1988).
3. Arvin W. F., "Eddy-current measurements with magnetoressitive sensors: third-layer
flaw detection in a wing-splice structure 25 mm thick", in Nondestructive Evaluation
of Aging Aircraft Airport, and Aerospace Hardware IW, Ajit K. Mal, Editor,
Proceedings of SPIE 3994 (2000).
4. Wincheski B. and Namkung M., "Deep flaw detection with giant magnetoresistive
(GMR) based self-nulling probe", in Review of Progress in Quantitative
Nondestructive Evaluation, Edited by D.O. Thompson and D.E. Chimenti, Plenum
Press, New York, 2000,19B, pp. 465-472 (2000).
5. Dogaru T. and Smith S. T., IEEE Transactions on Magnetics, 37, 3831-3838, (2001).
6. Libby H. L, "Introduction to electromagnetic nondestructive test methods", John
Wiley & Sons, New York (1971).
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