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Active Thermography with Electromagnetic Excitation: Defect-Specific Warming and Underlying Current Flow

Authors:
  • Vrana GmbH - NDE Consulting and Solutions

Abstract and Figures

Active thermography with electromagnetic excitation, meaning induction and conduction thermography, is a reliable NDE method with a wide range of applications. Over the past years the basics of the electromagnetic excitation, including the influence of the material of the component to be tested and the necessary post-processing algorithms have been studied. In this paper a a study on defect models, including delaminations, the well-known slot and notch type cracks, contact-point and "area of reduced conductivity" type cracks, as well as sub-surface cracks is presented. A detailed parameter study is discussed with parameters like depth, width, length, inductor position, rotation, and inclination.
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Active Thermography with Electromagnetic Excitation: Defect-Specific Warming and
Underlying Current Flow
by J. Vrana*, M. Goldammer**
*VRANA GmbH, Rosenstraße 6, 83253, Rimsting, Germany, johannes@vrana.net
**Siemens AG, Otto-Hahn-Ring 6, 81379, Munich, Germany
Abstract
Active thermography with electromagnetic excitation, meaning induction and conduction thermography, is a
reliable NDE method with a wide range of applications. Over the past years the basics of the electromagnetic excitation,
including the influence of the material of the component to be tested and the necessary post-processing algorithms have
been studied.
In this paper a a study on defect models, including delaminations, the well-known slot and notch type cracks,
contact-point and “area of reduced conductivity” type cracks, as well as sub-surface cracks is presented. A detailed
parameter study is discussed with parameters like depth, width, length, inductor position, rotation, and inclination.
1. Introduction
A component can be excited in a thermographic inspection in one of two electromagnetic ways, either inductively
or conductively [1,2,3,4,5,6,7]. Inductive excitation is achieved by an alternating current (AC) running through an inductor
placed next to the sample, and conductive excitation is achieved by direct galvanic contacts. In this case, an excitation by
means of an alternating (AC) or direct current (DC) is possible. In both methods, a current is created in the electrically
conducting material 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 heat diffuses
to the surface, the crack can be detected by an infrared camera.
During inductive excitation, the current density is highest directly under the inductor (distribution at the surface is
described by the proximity effect and the distribution in the depth of the material by the skin effect) and the current flows
back in the edges of the front and back surface [1,5]. With respect to DC conductive excitation, if no defects are present,
the current density is homogeneously distributed. As for AC conductive excitation, the current density is highest at the
edges of the sample (four in the case of a rectangular sample) and decreases in between. This is called the edge effect.
Additionally, the skin effect occurs as the current density rises from the middle to the sides [5].
Defects are inhomogeneities in the test material and they change the propagation of both the current and the
warming. The two most frequently occurring defect types are delaminations and cracks. Delaminations, as defects that lie
parallel to the surface, mainly disrupt the propagation of heat and not of current; cracks, on the other hand, mainly disturb
the propagation of current. Because of these effects, defects can eventually be detected by means of an infrared camera.
2. Delaminations
The detection of delaminations or, more generally, two-dimensional defects within the test material lying parallel
to the surface, take place through various mechanisms, depending on their depth [5].
If the delamination lies (clearly) deeper than the skin depth, an excitation on the surface can be assumed such
as excitation by means of a flash. The heat accumulates between the delamination and the surface, and the surface at this
point stays warmer than the surrounding material [8]. However, since the current density distribution on the surface (either
induced or through galvanic contact) is generally non-homogeneous, and the thermal flux can also flow to the side, the
two-dimensional optical excitation, in which the heat can only diffuse down into the depth, is by far the better excitation
option [5].
If the delamination lies within the skin depth (as is almost always the case with CFRP), a two-dimensional heat
source on the surface can no longer be assumed but rather a homogeneous warming of the material. In this case, no heat
builds up because of a delamination, but the heat is prevented from diffusing to the surface by being held under the
delamination, and the surface remains evenly warm or becomes only marginally cooler in the presence of such
delaminations. Because of this, and due to the rapid warming of the individual fibers that dominate the image, delaminations
are very difficult to detect in this case [5,9].
A third type of delamination occurs between a metal test component and a non-conducting top skin. In this case,
the delamination obstructs the diffusion of the heat to the surface that arises in the metal test component. This means that
the heat takes longer to diffuse to the surface. Figure 1 shows an experiment in which a detection of this type of a
delamination is demonstrated. The phase image of a pulse-phase analysis contains the undamaged part of the test object
(red), the delamination (violet) and the place where the non-conducting top skin has separated (yellow). This result from
the time the heat needs to diffuse to the surface. However, in this case, too, thermal imaging by flash is the better detection
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method; with it, two-dimensional excitation is possible and there is no inductor to block the field of vision of the infrared
camera [5].
Fig. 1: Delamination of a non-conducting top skin (violet) above an area of flaking (yellow);
phase image of the pulse-phase analysis [5].
3. Simple Crack Models
Cracks, on the other hand, can be detected well by using thermal imaging with electromagnetic excitation. The
two models for cracks mainly found in the literature are a) notches (the model for very long cracks that do not pass through
the material) and b) slots (the model for somewhat shorter cracks that do pass through the material). Both crack models
and slots with a finite depth (a model for cracks with a certain length and depth profile) are discussed in detail in this
chapter.
3.1. Notch Type Cracks
Long surface cracks, such as occur in the hot-rolling of steel bars or during the drawing of steel wires (mainly in
the longitudinal direction) or crack where the current flows beneath the crack due to the nature of the inhomogeneity are a
defect types that can most easily be modelled by a notch (ref. to Figure 2a).
For the induction thermography testing of this type of cracks, an inductor is set up at right angles to the normally
elongated longer test component. For an evaluation of the whole test component, the specimen can be moved underneath
the inductor and the camera; this makes it possible to examine the entire test component for notches in the longitudinal
direction [3].
The change in current-density distribution caused by the notch is shown in Figure 2b. The current density is clearly
decreased at the edges between the upper surface and the shoulders of the notch (the extent of this zone depends on the
skin depth) and increased at the two edges between the shoulders and the floor of the notch. This is caused as the current
must flow “underneath” the notch.
Due to the current density distribution the inside of the crack is warmer and the edges colder than the surrounding
material during the excitation (most notch-like real cracks do not provide a view into the crack, thus, real cracks will only
show colder edges). However, the heat from the bottom, as it diffuses to the surface, is restricted into one dimension by
the crack, and, the conclusion of the induction pulse the top edges tend to stay warmer than the surrounding material [4,5].
In the case of a low skin depth, the current density flows in a narrower zone, so the current-density distribution at
the corner is narrower and therefore higher. In this case, the corner heats up more quickly than in the case of thicker skins.
If the skin is very thin (e.g.: in the case of ferromagnetic materials), it appears that the corner becomes instantaneously
warmer than the surrounding material [5].
3.1.1. Dependence of the Current Density and Temperature Distribution from the Notch Depth
Figure 2c shows the current density along the notch shoulder, for notches of different depths. The current density
on the upper surface (at x = 0) decreases with greater notch depths and no longer changes when the notch depth is one
skin depth or more. Starting with a notch depth of around 2 skin depths, a zone along the notch shoulder can be seen in
which the current density is relatively constant. For deep notches (from around 2 skin depths), the current-density
distribution can therefore be divided into three sections:
On the surface of the test component, there is a zone of reduced current density that reaches to a depth of about
one skin depth. This is the zone in which the current is deflected through 90º by the notch. Underneath, the current flows
in the case of notches deeper than about 2 skin depths, along the x-axis; this leads to a relatively homogeneous current-
density distribution in this zone of the notch shoulder. On the inner edge, the current can again flow in the direction
predetermined by the excitation (z-axis) and this results in excessive current density [5].
This behavior can be modelled by area sources along the notch shoulders (between the depth of one skin depth
and the depth of the notch) and line sources at the inner edges [5].
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Fig. 2: a&b) Model for a notch like crack
b) Distribution of the current density in depth (from simulation calculations for notches 0 to 5 skin depths deep and a
skin depth of 0,9 mm; test component thickness: 5 mm) along the line drawn in the sketch (c)
b) Development of the warming on the surface directly next to the notch for notches of different depths; the time was
adapted to the appropriate diffusion length (the figure shows the result of simulation calculations on Inconel with a
100 ms induction pulse - this corresponds to a diffusion length of around 0,9 mm) [5].
The current-density distributions for notches of different depths presented in Figure 2c give us the temperature
developments on the surface directly next to the notch (shown in Figure 2d). As discussed above the current density on
the surface decreases as the notch depth increases, up to about one skin depth. Starting from one skin depth, the current-
density distribution changes only in the depth (not on the surface). So, up to a notch depth of one skin depth, the effect of
the notch depth on warming can be seen immediately after the start of the induction pulse. Deeper notches can only be
differentiated by observing the long-term temperature development [5].
3.1.2. Dependence of the Current Density Distribution from the Notch Width
Figure 3a shows the current density distribution in the bottom of the notch depending on the notch width. It also
shows the current-density distribution on a specimen with a 100 mm-wide notch in red, meaning a 100 x 100 mm² test
component on which the entire surface has been removed to one notch depth and represents the distribution induced by
the inductor at this enlarged distance.
The current density between the edges of wide notches falls to the level of the profile reduced by one notch depth;
in other words, the current-density distribution on the floor of the notch overlaps the distribution induced by both edges of
the notch and the distribution induced by the inductor at this distance. The rise in current density induced by the notch
edges is greater the narrower the notch. For very narrow notches, this rise is around double that for wide notches. This is
a result of the increasing overlapping of the distribution emanating from the right and left notch edges [5].
Fig. 3: a) Current-density distribution (calculated by simulation) in the floor of the notch for notches 0,05 to 10 mm
wide, along the line drawn in the sketch (b). The current-density distribution can also be seen (in red) for a specimen
profile that has been reduced by one notch depth (“100 mm wide notch”).
c) Plotted current flow lines for a tilted notch; d-e) Signal from a notch approx. 1 mm wide that runs at an angle of 45º
in the material, at an induction pulse of 60 ms. Image of the result after d) 60 ms and e) 300 ms;
f) Warming on the left (red) and right (blue) adjacent to the notch in comparison to the warming of the surrounding
material (black) [5].
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3.1.3. Current Density and Temperature Distribution for Tilted Notches
In the case of notches that are not perpendicular to the surface but at an angle, the current-density distribution
changes. The current is better able to follow the notch or to be more exact, as it can be seen in Figure 3c, the current on
the right-hand side can follow the notch profile better while the current on the left cannot follow it as well. Therefore, the
effect of reducing the current density is weakened on one side with greater angles and strengthened on the other side.
Fig. 3d-f shows the result of an induction thermography investigation of a tilted notch at an angle of 45º (as usual:
obliquely from above, to show the area directly under the inductor). On the image after 60 ms and at the time series, the
effect on the current density on the outer notch edge can clearly be seen [5].
Furthermore, the effect of the slower cooling also changes, since the point of highest current density is displaced
to the inner edge of the notch. As it can be seen in Fig. 3d-f, the side with the smaller angle remains warm longer (left-
hand side) than the surrounding material, longer than the other side and longer than not-tilted notches.
3.1.4. Detection Sensitivity Dependence on the Angle between the Applied Current Flow Direction and the Notch
In all the considerations above, it has been assumed that the angle between the inductor and the notch is 90º, as
the current flow is most disturbed at that angle.
Figure 4 shows the result of an inductively excited notch (excitation time: 50 ms) rotated by 90°, 60°, 30° and 0°
after 50 ms (a-d) and 170 ms (e-f). At an angle of 90º and after 50 ms, the outer notch edges are cooler than the
surrounding material, but the floor of the notch is warmer. After 170 ms, as discussed in 3.1.1, the outer notch edges stay
warmer than the surrounding material. As the angle is reduced, the current finds it easier to follow the course of the crack
and the current density at the outer notch edges (the zone of reduced current density) increases. So, at smaller angles,
the effect that after 50 ms the zone adjacent to the notch appears cooler becomes less until, at 30º, it is almost
negligible. At 0º, a slight warming next to the notch can be seen. This is the result of the edge effect that occurs at both
notch edges. However, the typical effect after a longer period of time (170 ms) can be seen at all angles. At an angle of 0º,
this is not a result of the heat that diffuses from the inner notch edges, but of the propagation of the heat that occurs at the
outer notch edges due to the edge effect.
Fig. 4: Thermal image taken with the notch (approx. 1 mm wide) rotated through 90º, 60º, 20º, and 0º (from left to
right) after 50 ms (a-d) and 170 ms (e-h) at an induction pulse lasting 50 ms. The results of the experiments were
recorded by infrared camera mounted to point downwards at an angle so that the zone directly under the inductor was
visible. The inductor is the light-coloured bar visible in the images. [5].
3.2. Slot Type Cracks
A crack can be described by its specific length and its depth profile. Therefore, depending on the nature of the
crack the current flows beneath the crack or around the crack tips. To model this behavior, it is best to look at these two
alternatives individually. The case where the current flows beneath the crack was discussed in the last section by using a
model geometry infinite in length but finite in depth: a notch. The case where the current flows around the crack tips is
given by a slot which is finite in length but extends completely through the sample.
Figure 4a-b shows a model of slot type cracks. Figure 4a (0,1 mm wide) and 4c (2 mm wide) show the current-
density distribution on the surface of the test component and 4b in the depths. In the zone around the slot a local current-
density disturbance is visible: in the environment of the slot a clear excess at the tip of the crack (B) and a reduction in the
slot shoulders (A) are present. Both effects are, as in the case of the notch, a result of the current excited from outside in
the z-direction which is blocked by the slot and must therefore flow round it. This causes a reduction in the current density
in the middle of the slot shoulder (like at the outer notch edges), since the current is deflected. At the same time, the current
density increases at the tips of the slot (like at the inner edges of the notch), since in this case the entire current flows
round the slot. In contrast to the notch, the zones in which the current density is mostly changed are not found inside the
specimen but on the surface and the warming does not have to diffuse to the surface. There is therefore no temporal
interrelation up to the increase in blur generated by diffusion.
3.2.1. Dependence of the Current Density Distribution on the Slot Width
The dependence of the current density distribution on the slot width goes parallel to the dependence on the notch
width discussed in 3.1.2. In Figure 4a vs. 4c, the change of the current-density distribution at a slot with a width of 0,1 mm
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(Fig. 4a) is compared with the one of a 2 mm wide slot (Fig. 4c). The tips of the 2 mm wide slot (B) are also heated; but,
since the slot is wider, there is (like in the example of the notch) a zone of reduced current density (C) between the two
edges of a slot tip. This phenomenon can be explained by analogy with 3.1.2: the composition of the current density in the
area of the slot tips is the addition of the undisturbed current density and the current-density distributions generated by
both edges of the slot. (Notch: current density in the bottom of the notch plus the current-density distributions generated
by both edges of the notch) [5].
Fig. 4: a) z-y view; b) x-y view: Change in current-density distribution caused by a slot 0,1 mm wide by 20 mm long
and with thickness a the component of 5 mm (simulation calculation with an inductor placed along the z
axis)
c) section of the z-y view for a slot 2 mm wide and 20 mm long
d) Increase in current density at the tip of the slot (B) for slots of different lengths (simulation) [5].
3.2.2. Dependence of the Current Density Distribution on the Slot Length
As Figure 4b shows, the distribution of the current density towards the bottom of a slot does not correspond to
the normal exponential decrease due to the skin effect, but the current is distributed over a larger area in depth (in the
vicinity of the slot). This area increases with greater slot length, based on the exponential decrease that is prevalent in the
undisturbed case.
This means that the growth of the current density at the slot tips on the slot length cannot simply be calculated by
integration through the distribution of current density given by the proximity effect. Rather, the current density grows slower.
Several simulation calculations were carried out to compute the increase in current density at the tips of the slot, depending
on the slot length. Figure 4d shows the result of these calculations. In order to apply the effect of raising the current density
through the slot, the current density of a test component without a slot in the same place was deducted [5].
3.2.3. Dependence of the Detection Sensitivity on the Angle between Specified Current-Flow Direction and Slot
Fig. 5: a) Sketch of current flow lines of a rotated slot b) Thermal image of a slot rotated by 60°
c) Temperature rise at the slot tips depending on the rotation obtained by experiment [5].
Rotated slots show, like tilted notches, a flow against effect (Figure 5a). This leads to the result in Fig 5b which
represents the induction thermography image of a slot rotated by 60°. Here warming on one side of the slot and cooling
on the other is visible [4,5].
Fig. 5c shows the temperature at the slot tip depending on the angle minus the temperature obtained on an
undamaged test component at this same point. As expected the correlation between the angle and warming is sinusoidal.
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For arbitrarily oriented cracks (notch-type cracks as well as slot-type cracks) two investigations must be performed
with both inductive and conductive (galvanic) coupling with various current-flow directions (preferably offset by 90º).
3.2.4. Dependence of the Detection Sensitivity on the Position of the Inductor
In the case of induction thermal imaging investigations, the highest current densities are generated directly under
the inductor. Therefore, the dependence of the position of the inductor is of interest in terms of detection sensitivity.
Figure 6 shows three experimental results of images taken by infrared camera (obliquely from above, to show the
area directly under the inductor). In Fig 6a, the center point of the 20 mm-long slot is immediately below the inductor; in
Fig. 6b, the center point has been moved 10 mm, so one of the slot tips is directly below the inductor; and in Fig 6c, this
point has been displaced by 20 mm. At 0 mm, both slot tips can be seen clearly; at 10 mm, one tip can be seen very well
and the other almost not at all; and finally, at 20 mm, only the lower one is visible. For improved detectability of all the slots
the phase image of a pulse-phase analysis can be used (see Fig. 6d-f) [5].
Fig. 6: A 20-mm slot a) directly under the inductor; displaced by b) 10 mm and c) 20 mm.
d-f) Results (phase image) of the pulse
phase analysis [5].
3.3. Detection of slots with Finite Depth
After discussing the effects of notch and slot like cracks independently (in section 3.1 and 3.2) slots with a finite
depth profile are considered which show, depending on the length and depth profile of the crack, signatures from both
basic cracks models.
Fig. 7: Simulated result of an examination by induction thermography of a slot 10 mm long, 0,1 mm wide and 0,9 mm
deep (corresponding to one skin depth) with an induction pulse lasting for 50 ms;
a) after 50 ms, b) after 200 ms, c) phase image of the pulse-phase analysis [5].
Figure 7 shows the result of an investigation on a test component containing a slot with a depth of one skin depth.
This test component was heated by inductive excitation for 50 ms. The image after 50 ms shows that the slot tips are
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warmer and the slot shoulders cooler than the surrounding material; in other words, the typical signal from a slot. However,
compared with Fig. 6a, the area of lower temperature along the slot shoulders is longer and the temperature at the slot tips
lower. Both result from the effect that the current, because of the finite depth of the slot, can also flow under the slot. The
image after 200 ms shows that, after the end of the induction pulse, the slot shoulders cool down less than the surrounding
material. This is the result of the fact that the heat coming from the inner slot shoulders is diffused to the surface, as in the
case of notches. The phase image of the pulse-phase analysis clearly demonstrates this delayed effect typical for notch-
like cracks.
4. Enhanced Crack Models
Notches and slots give good fundamental models for a basic understanding for defect detection using
thermographic methods with electromagnetic excitation. Previous publications [1,4,5] have shown that for an extended
understanding of defect signals enhanced crack models like the contact point or the conductivity model are necessary.
4.1. Contact Point Model
Slot type cracks, discussed in 3.2, show heat spots only at the end of the slot. Real cracks, depending on the
morphology, can give a result showing several heat spots. Those can be explained by contact points [1,4,5]. This signal
causing mechanism is also the reason why both cracks with high and low residual stresses can be detected with induction
and conduction thermography in contrast to many other NDE methods [4].
4.1.1. Dependence of the Detection Sensitivity on the Angle between Specified Current-Flow Direction and Contact
Point Type Cracks
A reduced angle between the inductor and contact point type cracks (see Figure 8) leads to a lower amplitude of
the indication (as in the case of notches and slots). At 0º, the spaces between the holes remain cooler than the undisturbed
material and the area above and below the holes warms up more strongly. This is only an effect of the hole diameter, since
the current must flow past the holes. This means that the current density rises next to the holes and, because of the small
gap between the holes, the current does not flow back into the area between the holes.
Fig. 8: Dependence of the contact point model indication (16 1 mm diameter holes drilled in a line close to each other)
on the rotation a) 90º, b) 60º, c) 30º, and d) 0º (in these images, the inductor can be seen as a red bar) [5].
4.2. Conductivity Model
For cracks with high residual stress, the density of contact points can be higher than what can be resolved by
thermography. In such cases, a crack can alternatively be deemed a zone of reduced electric conductivity. In the following
the conductivity model is discussed for both notch and slot type cracks.
4.2.1. Conductivity Model for Notch Type Cracks
Figure 9 shows the results of a simulation (current density and warming) in which the notches are filled with
materials of different electrical conductivity values σcrack. The lower the conductivity in the crack, the less current flows
directly at the surface and the more current around the inner notch edges. So, at a slightly weakened conductivity value
(Fig. 9c-d) the crack heats up along its entire length (the current density is actually lower in the crack, but the warming is
greater due to the increased resistance). For even lower conductivity values (Fig. e-j), the warming of the material in the
crack is weaker but, the typical effect of the notch (the two cooler edges adjacent to the notch) becomes clearer and clearer
until finally, as in the case of the notch (Fig. 9k-l), this effect is all that is seen [5].
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Fig. 9: Current-density distribution inside the crack (a, c, e, g, i, k) and the applicable images of warming from above
(b, d, f, h, i, l) of a 0,5 mm wide and 0,9 mm (1 skin depth) deep notch filled with a material whose electrical
conductivity is not the same as that of the basic material. The inductor (blanked out here) is set up horizontally and
lies across the middle of the thermal image above the test component.
a-b) σcrack = σspecimen, c-d) σcrack = 0,25 σspecimen, e-f) σcrack = 0,016 σspecimen,
g-h) σcrack = 0,0063 σspecimen, i-j) σcrack = 3,9 10-3 σspecimen, k-l) σcrack = 0 [5].
4.2.2. Conductivity Model for Slot Type Cracks
Figure 10 shows the results of a simulation (warming) in which the slots are filled with materials of different
electrical conductivity values σcrack. It can be seen, that for a slightly weakened conductivity (Fig. 10b) the crack heats up
over its entire length (as in the case of slot type cracks discussed in 4.2.1). If the conductivity is significantly weakened
(Fig. 10c), the crack heats up more strongly for its entire length, but since part of the current flows around the crack tips,
these also heat up. If the electrical conductivity is very much weakened (Fig. 10d), the only warming occurs at the crack
tips, as in the case of a slot (Fig. 10e), and the area in between remains cold. Figure 10f shows the warming in the middle
of the crack and at the crack tips, as a function of the relative electrical conductivity of the model crack [1,4,5].
Fig. 10: Calculated simulated images of an induction thermographic investigation of a 20 mm long by 0,1 mm wide
slot filled with a material whose electrical conductivity is not the same as that of the basic material. The inductor
(blanked out here) is set up horizontally and lies across the middle of the image above the test component.
a) σcrack = σspecimen, b) σcrack = 0,13 σspecimen, c) σcrack = 1,4 10-3 σspecimen, d) σcrack = 1,5 10-5 σspecimen, e) σcrack = 0
f) Temperature rise at the crack tips (blue) and in the middle of the crack (red) as a function of the rise in the electrical
resistance of the crack in comparison with the warming of the test component without crack (thin lines). [1,5].
4.3. Sub-surface Cracks
Simple and enhanced models discussed so far assume cracks that reach to the surface of the test component.
Hidden cracks in the material cause the same warming effects as cracks open to the surface (heating at the crack tips,
contact points or areas of lower electrical conductivity). However, since this warming takes place inside the material, it is
weaker because of the lower current density (skin effect) and due to the necessity, that the heat needs to diffuse out to the
surface.
In addition to those conventional warming effects, hidden cracks that are not too deep in the material are subject
to another direct heating effect. As in all defects, the current must flow around the crack. On the one hand, this generates
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the warming effects already mentioned and, on the other, the current can also travel above the crack, which increases the
heating energy in this region. This warms the material above the crack, and the entire length of the crack can be seen in
the infrared image (ref. to Figure 11a). Fig. 11b-c show experimental results on circular slots cut from the backside with a
residual thickness of material equal to quarter (1), half (2), one (3), one and a half (4), and two skin depths (5). In the
images representing the maximum signal intensity (Figure 11b) only the first two are visible (and the third very weekly).
Using the phase image of a pulse-phase analysis also slot #3 and #4 become detectable. However, as the black color
shows the signal is delayed. This is caused by a different detection mechanism: the heat is accumulated between the crack
and the surface. This means that, after the induction pulse, the point where the crack is located cools down more slowly.
The extent of this effect does not depend on the skin depth, but on the physical length of thermal diffusion; this, in turn,
depends on the surface area that the slot or crack has to offer against the distribution of the heat. So, closed cracks (in
contrast to open cracks as investigated here) deeper than 0,5 to 1 skin depth will only cause a minimal accumulation of
the heat and will most likely not be detectable [5]. Summarizing: both simulations [5] and experiments (Figure 11b-c) have
shown that sub-surface cracks up to approximately 0,5 to 1 skin depth are detectable and cause a different image
compared to indications open to the surface.
Fig. 11: a) Sketched current flow lines on a sub-surface crack
b-c) Experimental detection of sub-surface slots (approx. 0,5 mm wide) with a residual wall thickness of 1) 0,25 skin
depths 2) 0,5 skin depths 3) 1 skin depth 4) 1,5 skin depths and 5) 2 skin depths (from left to right); b) moment of
maximum signal intensity; c) phase image of a pulse-phase analysis; the inductor can be seen in the images as a
black bar [5].
4.4. Hidden Cracks under Non-Conductive Coatings
Cracks hidden by a non-conducting coating lead to no additional heating effect. Warming takes place as usual
through the effects discussed above: inner notch edges, crack tips, contact points, areas of lower conductivity, and, if they
are also covered by a conductive coating, through warming above the crack.
This warming cannot be detected directly but must first diffuse to the surface. Depending on the thickness of the
coating and the diffusivity of the material, the signal becomes more diffuse and weaker. To investigate this phenomenon
more closely, simulations using different coating thicknesses were conducted (zirconium oxide); the induction pulse was
100 ms in each case.
Fig. 12: Simulated warming of a slot (10 x 0,5 mm²) under a coating (100 ms induction pulse);
a) without coating; b) 0,125 mm; c) 0,25 mm; d) 0,5 mm; e) 0,75 mm tick zirconium oxide coating.
The blanked out inductor is set up horizontally and lies across the middle of the image above the test component.
f) Development over time of the warming at the slot tips for the various coating thicknesses (simulation) [5].
Figure 12a-e shows the infrared image directly after the end of the induction pulse; the dependence on the coating
thickness, described above, can be clearly discerned. Fig. 12f goes on to show the time series of the warming at the slot
tips where, in addition to the effect that the warming on the surface becomes less and less, it can be seen that the heat
a
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c
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f
10.21611/qirt.2018.065
14th Quantitative InfraRed Thermography Conference, 25 29 June 2018, Berlin, Germany
10
also takes longer to diffuse from inside out to the surface. This leads to the following correlation between the coating
thickness and the time t after the induction pulse up to maximum warming on the surface (diffusivity α):
 ≈ 1.6 
The length of time during which the infrared camera is still recording after the induction pulse should be adapted
to the maximum coating thickness; or, alternatively, the delay in the thermal signal can be used to determine the coating
thickness. The pulse-phase analysis can be used to improve the signal-to-noise ratio (even where the duration of recording
is not matched to the coating thickness) or to determine the coating thickness.
5. Summary and Outlook
Active thermography using electromagnetic excitation can be applied inductively and conductively and is ideally
suited for the detection of cracks close to or open at a surface making for easy inspection. Using mechanics such as
robotics, translation or rotary stages, automated inspection can be implemented for serial production. The thermographic
technique has the advantage that it is contact-free, without coupling media, and without chemicals. UV (ultraviolet)
illumination is not necessary. Image generation is part of the detection process. This allows further post-processing which
allows for automated indication detection.
The models for crack detection and the parameter studies presented allow for a clear understanding of the thermo-
graphic analysis of real cracks. They also confirm that induction and conductions thermography are ideally suited for all
kinds of surface or close to surface cracks, including cracks with low or high residual stress which are otherwise difficult to
detect via acoustic thermography or penetrant testing. The fact that this process can be automated shows the value of
thermography with electromagnetic excitation.
REFERENCES
[1] Vrana J., Goldammer M., Baumann J., Rothenfusser M., Arnold W.,Mechanisms and Models for Crack
Detection with Induction Thermography”. AIP Conference Proceedings, vol. 975, pp. 475 482, 2008, DOI:
10.1063/1.12902698.
[2] Vrana J., Goldammer M., Bailey K., Rothenfusser M., Arnold W., “Induction and Conduction Thermography:
Optimizing the Electromagnetic Excitation Towards Application”. AIP Conference Proceedings, vol. 1096, pp.
518 525, 2009, DOI: 10.1063/1.3114299.
[3] Goldammer M., Mooshofer H., Rothenfusser M., Bass J., Vrana J., “Automated Induction Thermography of
Generator Components”. AIP Conference Proceedings, vol. 1211, pp. 451 457, 2010, DOI:
10.1063/1.3362428.
[4] Vrana J., Goldammer M.,Induction and conduction thermography: From the basics to automated testing taking
into account low and high residual stresses”. Materials Testing, to be published, 2018.
[5] Vrana J., “Grundlagen und Anwendungen der aktiven Thermographie mit elektromagnetischer Anregung -
Induktions- und Konduktionsthermographie”. Shaker, Herzogenrath, Germany, 2009, DOI:
10.13140/RG.2.1.1680.4567.
[6] Vrana J., Goldammer M., Netzelmann U., “Induction and Conduction Thermography: A new Surface Inspection
Method Suited for the Forging Industry”. 20th International Forgemasters Meeting, Graz, Austria, 2017.
[7] Vrana J., Goldammer M.Induction and Conduction Thermography: From the Basics to Application”.
Thermografie-Anwenderkonferenz, Munich, Germany, 2017, DOI: 10.13140/RG.2.2.13650.86728.
[8] Riegert G., Busse G., “Induktions-Lockin-Thermografie”. MP Materialprüfung, vol. 46, pp. 33-35, 2004.
[9] Sakagami T., Ogura K., Kubo S., “Development of Thermographic NDT for the Damage Inspection in Carbon
Fiber Reinforced Plastics”. The First US Japan Symposium on Advances in NDT, pp. 420-425, 1996.
10.21611/qirt.2018.065
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Presentation
Full-text available
Active thermography, using electromagnetic excitation, is a non-contacting, non-destructive evaluation method with a wide range of applications. It allows detecting inhomogenities, like cracks, at or close to the surface of metallic components fast and reliable 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 presentationm, we present the basics of this NDE method, along with several component examples and how to build systems for the inspection. The detectability of cracks with high and low residual stresses will be discussed and compared to other surface testing methods as well as the reliability of the method and the prospects for automation. In addition this paper shows first results on high and low conductive materials.
Conference Paper
Full-text available
Active thermography, using electromagnetic excitation, is a non-contacting, non-destructive evaluation method with a wide range of applications. It allows detecting inhomogenities, like cracks, at or close to the surface of metallic components fast and reliable 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 we present the basics of this new NDE method, along with several component examples and how to build systems for the inspection. Active thermography using electromagnetic excitation and in particular induction thermography is a method which can be highly automated and is therefore an ideal tool for the inspection of forgings. Examples will be shown for crack testing of various forged steel parts with typical surface defects. The detectability of covered defects will be discussed as well as the reliability of the method and the prospects for automation.
Book
Full-text available
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Thesis
Full-text available
Mit den in dieser Arbeit behandelten aktiven Thermographiemethoden mit elektromagnetischer Anregung können Materialfehler zerstörungsfrei, zuverlässig, schnell und bildgebend mittels Infrarotkameras nachgewiesen werden. Dabei wird bei der berührungslosen Induktionsthermographie Strom über eine Spule und bei der Konduktionsthermographie über eine galvanische Kontaktierung eingekoppelt. Für ein tieferes Verständnis des Fehlernachweismechanismus ist ein grundlegendes Wissen über die Anregung, insbesondere über die lokale Stromdichte und -richtung, nötig. Daraus lässt sich ableiten, wie viel Wärme lokal entsteht, wie diese im Körper diffundiert und welche Temperaturverteilung sich dynamisch an der Oberfläche ergibt. Dabei erhöhen Defekte zum einen lokal die Stromdichte und stören zum anderen die Wärmediffusion. Der Detektionsprozess hängt dabei jeweils von der Defektgeometrie, -orientierung und vom -typ ab. Es wird systematisch dargestellt, wie sich die Stromdichte- und Temperaturverteilungen in einem Körper analytisch berechnen bzw. simulieren lassen, wie verschiedene Defekttypen die Stromdichteverteilung bzw. die Temperaturausbreitung verändern und sich deshalb unterscheiden lassen. Mit diesen Modellen lässt sich schließlich eine Aussage über die Detektierbarkeit von realen Rissen treffen. Aufbauend auf dieses Wissen wird anhand zweier während dieser Arbeit bei Siemens entwickelter Systeme gezeigt, wie ein System ausgelegt werden sollte, mit welchen Auswertealgorithmen das Ergebnisbild verbessert werden kann und wie ein Anwender vorgehen sollte, um eine Komponente zu testen.
Article
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Using Active Thermography defects such as cracks can be detected fast and reliably. Choosing from a wide range of excitation techniques the method can be adapted to a number of tasks in non-destructive evaluation. Induction thermography is ideally suited for testing metallic components for cracks at or close to the surface. In power generation a number of components are subjected to high loads and stresses-therefore defect detection is crucial for a safe operation of the engines. Apart from combustion turbines this also applies to generators: At regular inspection intervals even small cracks have to be detected to avoid crack growth and consequently failure of the component. As an imaging technique thermography allows for a fast 100% testing of the complete surface of all relevant parts. An automated setup increases the cost effectiveness of induction thermography significantly. Time needed to test a single part is reduced, the number of tested parts per shift is increased, and cost for testing is reduced significantly. In addition, automation guarantees a reliable testing procedure which detects all critical defects. We present how non-destructive testing can be automated using as an example an industrial application at the Siemens sector Energy, and a new induction thermography setup for generator components.
Article
Full-text available
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.
Article
Active thermography via electromagnetic excitation is a non-destructive evaluation method with a wide range of applications. It is a quick and reliable means for detecting inhomogeneities like cracks at or close to the surface of conductive components utilizing infrared imaging. Electric current can be used in two ways for thermography: In induction thermography, a current is coupled to the component without contact by passing an alternating current through a coil which is in close proximity to the component inspected; in conduction thermography, the current is coupled directly with the component. In this paper, we present a review of the basics of this non-destructive evaluation method along with several component examples and examples of inspection systems. In particular, automated testing is regarded. Additionally, sufficient detectability of cracks under low and high residual stress is discussed and compared with other surface testing methods.
Article
Die Qualitätssicherung von Bauteilen erfordert eine zuverlässige Defekterkennung (z.B. bei Inspektions- und Wartungsarbeiten), die jedoch durch die Anzeige intakter Bauteilstrukturen erschwert wird. Wenn selektiv nur Defekte auf die Anregung ansprechen, werden intakte Strukturen unterdrückt und somit die Wahrscheinlichkeit zur Defekterkennung ("Probability of Defect Detection" POD) erhöht. Die 1995 am IKP-ZfP entwickelte ultraschallangeregte Lockin-Thermografie (ULT) ist ein solches Verfahren, dessen Nachteil allerdings eine nicht kontaktfreie Anregung ist. Eine kontaktfreie und dennoch defektselektive Anregung ist die induktive Erwärmung durch Wirbelströme (Ohmsche Verluste). Die Wirbelströme erwärmen Defektstellen innerhalb der Wirbelstromeindringtiefe lokal stärker als ungeschädigte Bauteilbereiche. Stand der bisherigen Technik ist die Transienten-Thermografie mit pulsartiger Induktionsanregung. Bei ihr wird zur Auswertung das Bild des stärksten Kontrasts aus der aufgenommenen Abkühlungssequenz untersucht. Impulsangeregte Transienten-Thermografie hat den Vorteil einer kurzen Messzeit, allerdings sind die Ergebnisbilder durch inhomogene Erwärmung, wie sie bei induktiver Erwärmung häufig auftritt, und lokale Variation des Emissionskoeffizienten stark beeinflusst. Durch Anwendung der Lockin-Thermografie Methode auf die induktive Erwärmung werden die Nachteile der Transienten-Thermografie überwunden ("Induktions-Lockin-Thermografie" ILT). Im Gegensatz zur Transienten-Thermografie werden bei ILT die Wirbelstromamplituden (Frequenz von 30 bis 300 kHz) sinusförmig moduliert (zwischen 0,01 und 1 Hz), ähnlich der Amplitude bei ultraschallangeregter Lockin-Thermografie, während eine Infrarotkamera eine Infrarotbildsequenz über mehrere Anregungsperioden aufnimmt. Eine pixelweise Fouriertransformation der Sequenz berechnet dann ein Amplituden- und ein Phasenbild. Die Vorteile - speziell der Phasenbilder - sind dieselben wie bei den anderen Lockin-Thermografie Methoden: im Vergleich zu einzelnen Infrarotbildern haben die ILT-Phasenbilder ein erheblich verbessertes Signal/Rausch-Verhältnis, und Temperaturgradienten werden unterdrückt. Außerdem ist die Tiefenreichweite an metallischen Werkstoffen mit ILT (begrenzt durch die thermische Eindringtiefe) im Vergleich zur konventionellen Wirbelstromprüfung, die durch den "Skin Effekt" begrenzt ist, deutlich gesteigert. Am IKP-ZfP wurde im Rahmen eines DFG-Projekts eine ILT-Anlage konzipiert und aufgebaut. Sie wurde anschließend an verschiedenen Materialien und Fehlerarten erprobt und optimiert. Nach der theoretischen Herleitung der ILT-Tiefenreichweite wurde der tatsächliche Einfluss elektrischer und thermischer Materialeigenschaften auf die Tiefenreichweite an Modellproben untersucht. Eine weitere experimentelle Fragestellung war die Auflösungsgrenze von ILT bei der Defekterkennung. Neben der Inspektion von Modellproben wurde ILT auch zur Prüfung von Praxisbauteilen herangezogen. Die Palette der detektierten Fehlerarten ging von Oberflächenrissen (z.B. Ermüdungsrisse, Haarrisse) und Fügefehlern (z.B. Klebungen, Schweißungen) metallischer Bauteile bis zu Delaminationen und Impactschädigungen kohlefaserverstärkter Laminate, wie sie zunehmend in der Luft- und Raumfahrt relevant werden (z.B. CFK, C/C-SiC). Die ILT-Ergebnisse wurden auch mit den Ergebnissen anderer am IKP-ZfP vorhandener moderner ZfP Verfahren verglichen. Dabei zeigte sich, dass ILT bei Defekten innerhalb der Wirbelstromeindringtiefe ähnlich defektselektive Ergebnisse liefert wie andere "Dunkelfeldmethoden" (z.B. ultraschallangeregte Thermografie und nichtlineare Vibrometrie), jedoch mit dem großen Vorteil einer berührungslosen Anregung.
Mechanisms and Models for Crack Detection with Induction Thermography
  • J Vrana
  • M Goldammer
  • J Baumann
  • M Rothenfusser
  • W Arnold
Vrana J., Goldammer M., Baumann J., Rothenfusser M., Arnold W., "Mechanisms and Models for Crack Detection with Induction Thermography". AIP Conference Proceedings, vol. 975, pp. 475 -482, 2008, DOI: 10.1063/1.12902698.
Development of Thermographic NDT for the Damage Inspection in Carbon Fiber Reinforced Plastics
  • T Sakagami
  • K Ogura
  • S Kubo
Sakagami T., Ogura K., Kubo S., "Development of Thermographic NDT for the Damage Inspection in Carbon Fiber Reinforced Plastics". The First US Japan Symposium on Advances in NDT, pp. 420-425, 1996.