INDUCTION AND CONDUCTION THERMOGRAPHY:
OPTIMIZING THE ELECTROMAGNETIC EXCITATION
TOWARDS APPLICATION
J. Vrana1, M. Goldammer2, K. Bailey1, M. Rothenfusser2, and W. Arnold3
1 Siemens Energy, MC Q3-031, 4400 N Alafaya Trail, Orlando, FL 32826, USA
2 Siemens AG, Corporate Technology, Otto-Hahn-Ring 6, D-81730 Munich, Germany
3 Fraunhofer Institute for Nondestructive Testing (IZFP), Campus E 3.1, University, D66123 Saarbrücken, Germany (present address: Department of Materials, Campus D 2.2,
D-66123 Saarbrücken, Germany)
ABSTRACT. Active thermography, using electromagnetic excitation, allows detecting defects like
cracks which distort the flow of current in the component under examination. Like other thermography
techniques it is rapid and reliably utilizing infrared imaging. Electric current can be used in two ways
for thermography: In induction thermography a current is coupled to the component by passing an AC
current through a coil which is in close proximity to the component inspected, while in conduction
thermography the current is coupled directly into the component. In this paper, the specific
advantages of both coupling methods are discussed, including the efficiency of the coupling and
optimization strategies for testing and also the necessary algorithms required to analyze the data.
Taking these considerations into account a number of different systems for laboratory and practical
application were developed.
Keywords: Induction Thermography, Conduction Thermography, Induction Efficiency, Current
Density
PACS: 87.63.Hg, 81.70.Ex, 81.70.Fy, 44.05.+e, 44.40.+a, 02.60.Cb, 02.70.Dh
INTRODUCTION
A component can be excited in a thermographic inspection in one of two
electromagnetic ways, either inductively or conductively [1]. As illustrated in Fig. 1, inductive
excitation is achieved by an AC current running through an inductor placed next to the sample,
and conductive excitation is achieved by direct galvanic contacts. In this case an excitation with
an AC or DC current is possible. Therefore, with both methods a current is created in the
electrically conducting material of the component, and depending on the current density,
heating occurs in the sample. If a crack is present, the current is disrupted, resulting in an
alternated current density distribution around the cracked area. Therefore, when the heat
diffuses to the surface, the crack can be detected with an infrared camera. This process of
thermographic crack detection using electromagnetic excitation is discussed in [2] in detail on
the example of inductive excitation. However the crack detection mechanism is the same for
both excitation methods.
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CP1096, Review of Quantitative Nondestructive Evaluation Vol. 28,
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IR-Camera
Direct Contact
Inductor
Tested part
Tested part
Current
IR-Camera
Alternated current density
FIGURE 1. Top left: Typical induction thermography measurement set-up. Top right: Typical conduction
thermography measurement set-up. Both methods lead to an alternated current density (bottom).
In inductive excitation the current density is highest directly under the inductor and the
current flows back in the edges of the front and back surface [2]. With DC conductive
excitation, the current density is homogeneously distributed if no defects are present. With AC
excitation, the current density is highest at the four edges of the conductor and decreases in
between. This is called the edge effect (see Fig. 2). Additionally, the skin effect is present as
the current density rises from the middle to the sides.
When building inspection systems, it is important to identify the appropriate
parameters. Factors such as the material under test, the type of induction generator, the type of
infrared camera, and the signal processing are critical for a successful system development.
Current Dens
ity
[A/mm²]
Thickness
AC-Current
Width
y
x
]
cm
y[
z
x[
cm
]
FIGURE 2. Current density distribution across the section indicated in grey of a rectangular conductor.
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OPTIMIZING THE INSPECTION STATION
The result of the inspection depends on the inductive or galvanic coupling of the
current into the component, the current density distribution created, and the temperature
distribution due to the heat diffusion. So, to optimize the inspection station, all these
parameters have to be maximized to improve the signal-to-noise ratio, while maintaining fine
details and a good quality image.
Heating of the Sample
The signal-to-noise ratio depends on the heating of the sample Q, which in turn
depends on the power P in the part and on the excitation time-span τ:
Q = P ⋅τ .
(1)
The power can be estimated for a galvanic contact by
(2)
P ≈ I 2R
where I is the current flowing through the part at resistance R. For inductive coupling the
power can be estimated by [1]:
μf
(3)
P ~ I Inductor 2
σ
with the electrical conductivity σ, the permeability μ, the frequency f, and the current flowing
through the inductor IInductor.
Therefore, to optimize the signal-to-noise ratio, the power should be maximized and as
the power is quadratically dependent on the current flowing through the part or on the current
flowing through the inductor, respectively, the generator chosen for the inspection should be
optimized for high currents. By using standard generators available on the current market,
supplying 500 – 2000 A, most electrically conductive materials can be tested. Moreover, even
low electrically conductive materials are inspectable. Due to the limited maximum current,
materials with higher electric conductivity, like aluminum and copper, require a more
sophisticated method of inspection and signal processing. To the contrary materials with a high
permeability, due to the skin effect and the increased power, are especially suited for surfacecrack detection using induction thermography. However for every material the generator has to
be adapted to the permeability of the material.
When utilizing induction thermography, some power is lost in the inductor which
requires cooling and can not be used for the excitation. Therefore the efficiency [1] can be
stated as:
1
η~
2h σ P μ I
(4)
1+
a μ Pσ I
where the distance from the inductor to the surface is h, the radius of the inductor is a, the
electrical conductivity is σ, and the permeability of the inductor is μ. Eq. (4) indicates that the
permeability of the inductor material should be as close to 1 as possible and that the electrical
conductivity of the inductor should be as high as possible. Therefore, most inductors are made
out of copper. According to (4), a small distance inductor–sample h is optimal, but with a small
distance the distribution of the current density is forced into a small area which must be of
reasonable size to detect cracks [1, 2], and hence an optimal distance for a given inspection task
has to be found. The cross-section of the inductor, represented in (4) by the radius of the
inductor a, should be as large as possible, but an overly inductor will decrease the visibility of
the crack. Typical values for these two parameters are h ~ 2 - 10 mm and a ~ 2.5 mm.
According to (1), a second factor to optimize the signal-to-noise ratio is the time-span
of the excitation. However, the thermal diffusion process leads to the blurring of the image
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over time, especially with small heat sources as created by cracks. The thermal diffusion length
for a pulsed excitation is proportional to:
dth ~ α t
(5)
where α is the thermal diffusivity. As the power of the generator is limited, a compromise is
necessary to get enough heat into the component to have a good contrast in the defect image,
still maintaining fine details with a sharp image. This compromise is quite easy for most
materials and results in excitation times of about 50 – 200 ms. Only materials with a high
diffusivity require a fast camera and a more sophisticated method of inspection. This holds
especially if a high diffusivity is combined with a high electrical conductivity which is the case
for most metals, as thermal and electrical conductivity increase concurrently. Therefore, for a
multi-purpose system, the generator should provide the ability to provide a current of about
500 - 2000 A, a pulse duration of about 5 - 2000 ms, and the infrared camera should have a low
NETD [2] with a frame rate of at least 100 Hz to capture even fast thermal processes.
Signal Processing
2 cm
IR Signal [Grayvalues]
Besides optimizing the excitation, the resulting images should be optimized to
compress all the information available into a single image indicating only the defects. Fig. 3
shows a raw sequence of infrared images of an induction thermography inspection of a turbine
blade using a 100 ms long induction pulse. The crack in the turbine blade foot is visible in all
the stills. A comparison with a dye-penetrant inspection is provided at the top left. However,
this is clearly not the optimal signal desired, especially since several artifacts are visible. Those
artifacts are partially caused by the properties of the blade, like the temperature distribution and
the emissivity, and therefore even visible before the excitation started. Other artifacts are
caused by the excitation, like the heat distribution next to the inductor and the heating of edges
and corners. In addition there is blurring of the signal with time. After 260 ms the crack signal
is quite blurred and after 597 ms it is hardly visible.
1200
1000
800
600
400
200
0
0
100
200
300
400
500
Time [ms]
IR Signal [Grayvalues]
800
0
FIGURE 3. Top left: Turbine blade to be inspected, showing a crack indication obtained with a dyepenetrant inspection; Bottom: Raw infrared image sequence from left to right: Before the induction pulse
started, at 20, 99, 260, and 597 ms, respectively. Top right: Typical infrared signal of an active thermography
inspection with a 100 ms long electromagnetic excitation.
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IR Signal [Grayvalues]
800
0
Phase [°]
350
280
FIGURE 4. Top: Background-subtracted infrared image sequence from left to right: Before the induction
pulse started, at 20, 99, 260, and 597 ms, respectively. Bottom: Pulse-phase analysis of the infrared sequence
shortened to 100 ms excitation time and 100 ms cool-down time.
2200
2750
IR Signal [Grayvalues]
IR Signal [Grayvalues]
One way to extract the interesting result – the heating of the blade due to the excitation
– is to subtract the background. This is accomplished by subtracting a picture before the
excitation pulse has started. As it can be seen in Fig 4 (top) the background subtraction reveals
the indication. However still some artifacts are visible.
Further steps can be taken by post-processing the image sequence containing the
heating and cooling phase, using the pulse-phase analysis [3] to reveal the indication. This
results in splitting the information contained in the sequence into two images. One of the
images contains the amplitude-based information and the other the time-based (phase)
information. As the phase image reveals the crack and suppresses the differences in the
emissivity of the part and some of the non-uniformities of the heating (Fig.4 bottom), this is the
image processing to be used.
A similar result can be obtained using the lock-in technique. For the lock-in analysis, a
periodical excitation is necessary that yields also a phase and an amplitude image. Therefore,
the pulse-phase analysis is equivalent to a lock-in analysis using only one cycle. However,
when using only one cycle, the heating will be insufficient in some cases for materials having a
high electric conductivity and diffusivity. By using the lock-in approach, a short excitation time
can be used and the necessary heating of the part can be accumulated by using a sufficient
number of cycles.
Even by using pulse-phase or lock-in excitation, some artifacts remain in the resulting
image. This can be resolved in automated production testing by using a flawless component as
a reference. Fig. 5 shows this in more detail. By subtracting the image of the reference
component from the component under test, only the indication remains visible. This can be
improved further by combining it with pulse-phase or lock-in excitation [2]. Only some small
artifacts due to positioning or size problems remain.
0
0
FIGURE 5. From left to right: Component under test, flawless reference component, and resulting image.
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SPECIFICATIONS OF THE ELECTROMAGNETIC THERMOGRAPHY
SYSTEMS
Induction Thermography
Fig. 6 shows a stationary multi-purpose induction thermography system built at
Siemens in Munich. In this system a 10 kW computer-controlled induction generator is used.
The minimal possible pulse length is 5 ms and the system can be adapted to different materials,
due to exchangeable condensers. Moreover, the relative position of the inductor and the
component to be tested can be adjusted by motors and a lock-in measurement can be carried
out by switching the current of the generator on and off. The auxiliary device is connected with
the generator by a 2 m long and 5 cm thick stiff cable.
In some cases it is necessary to inspect parts in the field. For such cases, a mobile and
handheld system capable of producing a high current was developed. This system provides
arbitrary positioning and allows immediate identification and marking of defects. This can be
achieved by using a different standard induction generator with 10 kW power. The auxiliary
device is included at the end of a more flexible and longer (5 m) cable. Therefore, the inductor
can be positioned arbitrarily within the radius of the cable. The capacitors of the induction
generator are not exchangeable, but the generator is optimized for all non-permeable parts,
which results in a maximum usable current of approximately 980 A. Moreover, the control is
more versatile and the induction pulse can be modulated by an analog signal up to a frequency
of 300 Hz for lock-in measurements.
To optimize the design of a portable system, it was integrated into a 19” case (Fig. 7). It
includes the induction generator, a computer for system control, a keyboard and a display. The
connection between the computer and all periphery equipment is accomplished via a connector
box developed for this purpose. The excitation and the post-processing are integrated in the
software developed by Siemens CT. This system is used by the service division of Siemens
Energy in Pittsburgh, PA.
FIGURE 6. Stationary laboratory induction thermography system (top: infrared camera, right: auxiliary
device of the induction generator).
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FIGURE 7. Mobile and handheld induction thermography system. Left: Portable rack including the generator
and all necessary equipment. Right: Auxiliary device with a small infrared camera and a touch-screen display.
More flexibility of the system is possible by mounting some additional equipment on
the auxiliary device. By activating a button to start the measurement, a small infrared camera
captures the image displayed on a small screen. This system can be applied at the part location,
exciting the component and interpreting the resulting image displayed after the measurement
on the small screen. The performance of the small camera is at present limited, but it is
sufficient for many inspection cases as the resulting image in Fig. 7 reveals. By using a touchscreen display, the system can be operated remotely, for example by marking and evaluating
the indications.
Conduction Thermography
Besides the induction thermography system, a conduction thermography system was
developed. Fastening of the component to be inspected allows the use of pulse-phase and lockin analysis. Therefore the component shown in Fig. 8 is clamped on one side. This fixture also
serves as the first electrical contact. The second electrical contact is the clamp shown on the left
side. The maximum current produced by the generator is 2000 A which is sufficient for most
parts to be inspected with a reasonable excitation time.
Phase [°]
220
165
FIGURE 8. Conduction thermography system and resulting image calculated by pulse-phase analysis. The
“crack” (several holes drilled into the component next to each other) in the center of the test component is
clearly visible.
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IR Signal [Grayvalues]
200
-10
FIGURE 9. Conduction thermography system and resulting image calculated by pulse-phase analysis.
With this system a test-component was inspected containing several holes drilled into
the part next to each other. The resulting image (Fig. 8 right) was gained with the pulse-phase
analysis suppressing artifacts and showing the indication clearly. The low noise level indicates
that nearly the whole part could be inspected for defects.
Furthermore, the current unit of a magnetic particle inspection system (Horizontal Wet
Unit) was used for the excitation by direct contact and, instead of imaging the distribution of
the magnetic particles, an infrared camera is used for detection. Fig. 9 shows a subsurface crack
in a non-permeable vane which is clearly discernible. Only a heat-spot, due to the contacting of
the part is visible, which can be eliminated by adapting the fixture to the part under test.
SUMMARY
In this paper we showed that active thermography with electromagnetic excitation can
be applied to components by using the inductive and the conductive excitation methods. For
both excitation modes the coupling, the current density distribution, and the heating in the part
are discussed, including the efficiency. Moreover, it has been demonstrated that a successful
application requires post-image processing.
REFERENCES
1. J. Vrana, M. Goldammer, J. Baumann, M. Rothenfusser, and W. Arnold,
“Mechanisms and Models for Crack Detection with Induction Thermography”, in:
Review of Progress in QNDE 27, edited by D.O. Thompson, D.E. Chimenti, AIP
Conference Proceedings 975, pp. 475-482 (2008).
2. J. Vrana, “Grundlagen und Anwendungen der aktiven Thermographie mit
elektromagnetischer Anregung - Induktions- und Konduktionsthermographie”
(“Basics and Applications of Active Thermography Techniques with Electromagnetic
Excitation – Induction and Conduction Thermography”), Ph.D. Thesis, Science and
Engineering Faculty III, Saarland University (2008), unpublished.
3. X. Maldague and S. Marinetti, “Pulse Phase Infrared Thermography”, J. Appl.
Phys. 79, pp. 2694-2698 (1996).
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