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\begin{document}

\title{Monitoring multiple damage mechanisms in crack-patched structures using
optical infrared thermography}



\author[1]{Ulrike Martens}%
\author[1]{Kai-Uwe Schröder}%
\affil[1]{RWTH Aachen University}%


\vspace{-1em}



  
  \date{May 10, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
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\selectlanguage{english}
\begin{abstract}
The use of passive infrared thermography comprises great opportunities
to improve understanding the fatigue damage process of crack-patched
structures. Quasi-static and cyclic coupon tests are performed using
metallic specimens with single-sided bonded patches and monitored with
passive infrared thermography. Different test setups help to
differentiate between metallic crack growth and adhesive damage on
thermal images. Results show that metallic crack growth can be monitored
from the patched side, also in combination with local delamination at
the patch/metal interface. Thus, it is possible to analyse the overall
degradation progress of the crack patched component under loading
conditions and thereby to identify the driving damage mechanism of the
particular repair configuration. Being able to understand the overall
damage behaviour of crack patched components is essential to improve the
ability of predicting its long-term behaviour.%
\end{abstract}%



\sloppy


\textbf{1. Introduction}

Metallic components exposed to cyclically varying loading tend to
fatigue cracking. Adhesively bonded patches made of fibre reinforced
plastics (FRP) are one repair solution that can significantly enhance
the fatigue life of cracked metallic components. The repair basically
consists of three major components, which are the cracked metallic
structure, the FRP repair patch and the adhesive layer joining patch and
the cracked component. The patch bridges the crack and loads that are
applied to the metallic structure are transferred into the patch via the
adhesive zone. The patch thereby reduces stresses at the crack tip which
reduces crack growth. Additionally, due to the stiffening further crack
opening is reduced, which again retards the crack growth. Even though
crack growth can be theoretically be completely inhibited by a properly
designed patch, see e.g. {[}1, 2, 3{]}, cyclic mechanical loading and
environmental influences cause material degradation resulting in further
crack propagation during service {[}2, 4, 5{]}. But, even when the full
patch functionality cannot be guaranteed, crack growth rates can still
be reduced. However, for a repair with impaired functionality, the crack
growth behaviour differs from theoretical performance predictions. To
improve the prediction quality, it is essential to understand the damage
processes under loading conditions.

For a patch repair, different damage mechanisms can occur. The two major
damage mechanisms are metallic crack growth and local failure of the
adhesive zone resulting in patch disbond and thereby reduced repair
functionality. Depending on the repair configuration (stiffness ratio,
application process, crack length, etc.) the driving damage mechanism
can vary. The identification of the driving mechanism and possible
mutual influences of the different damage mechanisms can be a
significant factor to improve the patch design process, especially the
quality of the performance prediction. As damage propagation is mainly a
subsurface process, the use of non-destructive inspection (NDI) methods
offers the ability to get an insight into the damage procedures of a
crack patched structure under cyclic loading conditions. Optical
infrared thermography (IrT) is one NDI technique commonly used in the
detection of subsurface defects {[}6{]}, also showing great
applicability in the analysis of the damage behaviour of crack patched
structures {[}7, 8{]}. The aim of this paper is to examine how IrT can
help monitoring the cyclic heterogeneous damage propagation of crack
patched structures. The method used is passive IrT supported by optical
lock-in thermography (LT), when necessary.

\textbf{State of the art in monitoring crack patching}

For the ``analysis, design and assessment of composite repairs'' Jones
et al. recommend among others the method of IrT as one NDI technique
that is capable to picture damages. {[}7{]} Various studies on the
potential capacity of IrT in the damage analysis of crack patched
structures can be found in literature. The applicability of IrT methods
for the assessment of bonded repair patches was shown for instance in
{[}9{]}, {[}10{]}, and {[}11{]}. Avdelidis et al. analysed the
capability of active thermography methods in detecting cracks and patch
delamination {[}9{]} and Paipetis et al. showed the capability of LT to
detect artificially introduced defects in the adhesive and between the
patch layers {[}10{]}. Similar studies focussing on the repair
assessment were carried out by Grammatikos et al. in {[}14{]} and
Pavlopoulou et al. in {[}12{]}. Grammatikos et al. examined LT to assess
the off-line and online characteristics for maintenance activities,
while Pavlopoulou et al. compared LT to other NDI techniques as a
possible structural health monitoring technique assessing the repair
integrity under loading conditions. The ability using IrT methods for
the analysis of the damage process was shown for instance by Avdelidis
et al. {[}9, 13{]}. Grammatikos et al. further showed in {[}14{]} and
{[}15{]} that LT can be used as a qualitative and quantitative approach
to analyse the patch disbond process. Aiming on improving the
predictability of the repair performance under service conditions, also
the overall degradation process is of great interest. A preceding study,
performed at the institute for structural mechanics and lightweight
design (SLA), RWTH Aachen University, showed the great potential of
using IrT for the analysis of subsurface damage propagation behaviour
under mechanical loading.{[}8{]} The methods of passive IrT and optical
LT were examined with respect to their ability to monitor the adhesive
damage propagation process under quasi-static and fatigue loading.
Results show that both thermography methods are able to picture the
subsurface disbond propagation under loading conditions. The qualitative
information given by the thermal images can be used to prove or disprove
assumptions concerning the made during the design process. The method of
passive IrT turned out to be much easier to handle. Therefore, also in
this study mainly the method of passive IrT is used. The idea is to
visualize the heterogeneous subsurface damage procedures of crack
patched components under mechanical loading. The goal is to find out
whether the different failure mechanisms can be distinguished on the
thermal images and how the use of IrT can contribute to improve the
prediction quality by an improved understanding of the damage processes.

\textbf{2. Theory}

For the interpretation of the test results, a short introduction into
the functionality of IrT is provided.

2.1. Infrared thermography

In the present examination, passive IrT and supplementarily also optical
LT are applied to monitor the damage behaviour of mechanically loaded
crack patched specimens. A sketch of the test setup of IrT can be found
in Fig.~1. On the right hand side the loaded specimen is depicted which
sends varying infrared radiation due to varying loading.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig.~1: IrT setup according to {[}6{]}

This infrared radiation is detected by the camera, which is connected to
a processing unit that converts the radiation data into thermal images.
An optional thermal stimulation unit is needed for active IrT methods.
The method of passive IrT measures the temperature variations on the
object's surface. Internal conversion processes for instance, that go
along with (local) temperature changes can be detected measuring the
attending changes in infrared radiation. Thereby, temperature hot spots
resulting from crack formation and crack growth processes for instance
can be detected. For more detailed information on IrT see~{[}6{]}.

Active IrT methods on the other hand use external excitation units for
the thermal stimulation of the specimen. Optical LT for instance uses an
external heat source, denoted as optional thermal stimulation in Fig. 1,
introduces a heat flow into the specimen. The energy flow moves through
the specimen and is partly reversed at material boundaries, such as
voids or similar. The reversed energy flow is detected by the IrT camera
system. Local differences in infrared radiation can thereby be
visualized. This way, thermal images can be used to find subsurface
irregularities.

2.2. Test Description

Two test series are performed, one under quasi-static and the other
under fatigue loading. For each test, three different configurations are
tested: specimens without patch, specimens with patch monitored from the
metallic side and specimens with patch monitored from the patched side.
The infrared camera used is of type ImageIR~R~8380~S (InfraTec GmbH,
Dresden) in combination with a tele zoom lense (\(f=50\)~mm).
The test equipment used is a tensile and compression machine of type
Instron~5567 (Instron, Darmstadt) for the quasi-static tests and a
servo-hydraulic test machine of type Schenck~POZ~160 with a static
nominal load of +/-160~kN. For the quasi-static tests, a test velocity
of 9~mm/min and a monitoring frame rate of \(f=200\ \)Hz is used.
The fatigue tests are performed with a stress ratio \(R=0.1\),
mean load \(F_{m}=4000\)~N, load amplitude \(F_{a}=3272\)~N and
a monitoring frame rate\(f=100\)~Hz. To reduce data, recording
is done every 10000 load cycles for about 5 seconds.

All specimens are made of aluminium~5083 with a thickness
of\(h=3\)~mm. The patch material used is a 0/90 epoxy-based
carbon fibre reinforced plastic prepreg (SGL Technologies GmbH of type
C~W410-TW2/2-E323/42\%/6k and a ply thickness of \(h=0.6\ \)mm).
Specimen geometries and dimensions are given in Fig.~2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Fig.~2: Specimen geometries

For the fatigue specimens hand sawn cuts of 2~mm length each and placed
perpendicular to the load direction act as a crack starter. Specimen
preparation is kept simple since the focus lies on the monitoring
methods and not on the patch performance. After cleaning the metal
sheets with acetone, patches are directly bonded without using an extra
adhesive layer. Specimens are heated in a temperature chamber
(\(T=100\)\selectlanguage{ngerman}°C, \(t=2\)~h) placed on a metallic sheet
and covered by a plastic foil. A vacuum pump is applied throughout the
heating process in order to create a proper bonding.

2.3. Preliminary estimations

At first, a metallic plate without mechanical loading is examined using
optical LT. The aim of this preliminary test is to qualitatively find
out whether or not it is possible to detect a crack in the metallic
plate lying underneath a local defect in the adhesive layer. The
specimen design can be seen in Fig.~3.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig.~3: Geometry preliminary test

One layer of a biaxial CFRP prepreg material is mounted on the metallic
substrate to simulate the repair patch. The defect in the metallic plate
is simulated by a drilled hole of 6~mm in diameter. Two different patch
configurations are tested, one with an extra adhesive layer
(DuploTEC~10400, Lohmann) and the other using the epoxy itself joining
the components. A small square of newsprint is intentionally worked in
the specimen between the patch and the metal to simulate local patch
delamination.

For LT, the measurement depth is dependent on the lock-in frequency as
well as the number of lock-in periods. The pure delamination defect on
the right-hand side of the specimen is used to calibrate the camera for
the defect depth. The second defect is a combination of local patch
detachment and a defect in the metallic part simulated by a drilled hole
underneath.

Fig.~4~shows the thermal images for the lock-in frequencies
of\(f=0.1\)~Hz, \(f=0.2\)~Hz and
\(f=1\)~Hz.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Fig.~4: Results preliminary test

For all frequencies tested, the square defect on the right-hand side of
each picture can clearly be seen. On the left-hand sides, the square
damage as well as the hole in the metal can be detected. This means,
that the detection of both defect types is possible within a certain
range of lock-in frequencies. It can further be seen that the outlines
of the defects are more clearly visible for the lower configuration
without an additional adhesive layer than for the upper one with an
adhesive layer.

Nevertheless, for both configurations the defects are visible. As a
conclusion one can say, that LT is generally capable to detect a damage
in the metallic part of a patch repair that is lying underneath a local
defect in the adhesive layer. Since a hole of 0.6~mm is not
representable in dimension for a fatigue crack, the applicability of
this method to detect small cracks thus needs to be further analysed.
Statements concerning the influence of patch thickness cannot be made
either so far.

\textbf{3. Test Results}

To find out if crack growth processes and patch delamination can be
differentiated on thermal images, at first solely the crack growth
process of metallic specimens without additional patches is monitored
using passive IrT. In addition, patched specimens are monitored from the
unpatched side to find out about possible differences caused by the
patch. At last, specimens with patched are monitored from the patched
side in order to monitor the overall damage progress.

3.1. Quasi-static Loading

Specimens without and with FRP patches are loaded quasi-statically and
damage behaviour is monitored using passive IrT. To measure temperature
differences, three measurement fields are defined on the specimen
surface. The first above the hole for the minimum temperature and the
second and third at the crack tips for the maximum temperature, see
Figure 5.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 5: Displayed section of the specimen and measurement fields used
to read out temperature values

Fields~2 and 3 move with the crack tips towards the specimen edges with
increasing crack length. Maximum and minimum temperature values are read
out by the software.

\emph{Specimens without patch}

To get a first idea about the thermal behaviour of the metallic
specimens under loading, quasi-static tests with metallic specimens are
performed. Test evaluation shows that during the entire test, changes in
the local temperature can be seen. After exceeding the linear-elastic
region of the material, heat ``flashes'' can be seen on the thermal
images as 45-lines to the loading direction, see Fig.~6.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Fig.~6: First and one of the last detectable slip bands

The picture on the left-hand side shows the first detected pulse and the
picture on the right-hand side one of the last ones. The
load/displacement curve shown Fig.~ 7 helps to arrange the pulses within
the test course.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Fig.~ 7: Load/displacement curve for a specimen without patch

The course of the load/displacement curve is described by drops in the
force signal that increase over time. The non-smooth load/displacement
behaviour in the plastic region leads to the conclusion that this
phenomenon can be related to the so-called ``Portevin-Le Chatelier''
(PLC) effect. The temperature ``flashes'' are slip bands running through
the material. For more information on the PLC effect see for instance
{[}16{]}. Number and size of the drops in the force signal correspond to
the frequency and intensity of the detected slip bands running through
the specimen. Monitoring the region around the hole, where crack onset
is expected, the increase in number and intensity can be seen. Slip
bands running through the hole pile up and local temperature differences
increase over testing time, as shown in Fig.~8.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\selectlanguage{greek}\emph{Fig.~8: Maximum temperature difference ΔT for each heat pulse
identified around the}\selectlanguage{english}

hole

The maximum temperature variation \(T\) is plotted against
the overall displacement and each detectable pulse is marked with the
symbol~. It has to be noted that with the used frame rate of
\(f=200\)~Hz these bands can only be seen on 1-3 images.
Therefore, possibly not all bands were detected. The first detectable
pulse appears at a displacement less than 2~mm with a maximum
\(T=0.28\)K. With increasing displacement, the temperature
difference increases up to \(T=3.12\)K. The intensity of the
first and last detectable slip band, as shown in Fig.~6, visualizes the
increase in temperature with a maximum temperature difference of
\(T=0.28\)K for the first and \(T=3.12\)K for the last
one. This means that with increasing test progress also visibility
improves.

Further, for the tensile test two characteristic thermal events can be
monitored, see Fig.~9.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Fig.~9: Crack initiation and final failure

Right before crack initiation, marked with a~, a last high-intensity
pulse runs through the hole (here, pulse 28). Final break is marked with
the symbol~ in the load/displacement curve. At crack onset the heat
increase at both sides of the hole is clearly detectable with a maximum
temperature difference of \(T=17.1\)K. Also, for final break,
the crack tip running through the specimen is well-defined as a local
temperature increase. The maximum temperature difference can be measured
to be \(T=36.47\)K.

\emph{Specimens with patch}

Specimens with patch show a very similar load/displacement behaviour as
the unpatched ones. Differences in elasticity between metal sheet and
the rather stiff patch combined with the increasing strain lead to a
patch detachment right after exceeding the linear-elastic region so that
the stiffening effect of the patch cannot be monitored. In most test
cases the patch detaches below the hole and sticks partly to the sheet
above the hole. For the test evaluation this means that the patch still
covers the hole but does not longer stiffen the specimen.

Analyses of the patched specimens monitored from the metallic side
therefore do not give any considerable additional information compared
to the specimens without patch. A comparison of the load/displacement
curves and the thermal images shows that the damage behaviour including
the characteristic events of specimen failure are comparable. The patch
detaches in a one- or two-stage process resulting in one or two
additional characteristic event(s) in the load/displacement curve, as
shown in Fig.~10, denoted with the symbols~ and .\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

Fig.~10: Load/displacement curve for a specimen with patch

Knowing the load/displacement behaviour and at which points
irregularities that go along with an increase in temperature helps to
understand thermal images taken from the patched side. Correlating
thermal images with the test curve helps to find thermal images taken at
the characteristic events monitored from the patched side, as shown in
Fig.~11.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

Fig.~11: Patch detachment 1 and 2 , crack initiation and final failure

Again, the images show the region around the hole which is covered by
the CFRP patch and whose woven structure can be identified. For each
event three consecutive images are shown that are found to be taken
close to the characteristic events.

The two points of patch detachment ( and ) show a slight diagonal
increase in temperature around the central hole. In the first row two
diagonal lines can be seen and in the second one only one line. It seems
that these temperature increase can be related to metallic slip bands.
But it actually cannot clearly be said whether these lines of increased
temperature result from diagonal slip bands or from the diagonal
structure of the patch.

Considering crack initiation () and final failure (), an increase in
temperature can be seen through the patch in the specimen centre. As the
patch is already detached around the hole, outlines are blurred and only
a non-specific field of increased temperature can be made out. The
temperature difference here lies at around \(T=45\) at the
patch surface, compared to a \(T=7.1\)K for the temperature
increase measured at the metallic surface. The very last picture covers
the moment of patch detachment. The actual temperature increase around
the hole can be seen at the right-hand side where the metal sheet is not
covered by the patch. While the temperature difference is more
than\(T=8\)K at the edge of the metallic part, on the patch
surface only difference of around \(T=2.59\)K is detectable.

3.2. Fatigue Loading

Compared to the quasi-static tests, under cyclic loading different
conditions apply. Fatigue crack growth differs from static failure. The
main difference is that cracks grow at lower stresses and, more
important here, lower strains. This means that also temperature
evaluation is less than for static damage. Further, due to the
continuous loading and unloading, temperature does not solely increase
because of plastic deformation at the stress peaks, but also globally
due to internal friction. Cyclic loading leads to molecular movements
inside the material. Thereby, the entire specimen heats up over testing
time. Therefore, temperature differences \(T\ \)are expected
to be small. On the other hand, due to smaller deformations, sudden
patch detachment is not expectable. A proper bonding leads to improved
visibility of the subsurface phenomenae.

Thermal images of one exemplarily chosen unpatched specimen, shown in
Fig.~12, depict the temperature development during crack propagation for
four exemplarily chosen numbers of load cycles~\emph{N} .\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

Fig.~12: Crack propagation process for an unpatched under fatigue
loading

Monitoring the metallic crack growth with passive IrT shows that the
longer the crack grows, the better it can be made out on the thermal
image. For small crack lengths on the other hand, crack tip detection
can be difficult. One reason might be, that the movement of the lower
piston heats up the specimen during testing. Temperature thus increases
from the bottom to the top and the crack acts as a heat barrier. The
bigger the temperature differences become over time, the easier cracks
can be made out. Moreover, a circular area around the crack tip can be
identified which builds up during loading and vanishes while unloading
the specimen. This area helps to identify the crack tip and thereby to
measure the crack length.

As for the quasi-static test, differences between the images from the
unpatched specimen and the ones with patch monitored from the metallic
side are neglectable. Fig.~13 shows the temperature evaluation for two
patched specimens, one monitored from the metallic side and the other
from the patched one.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

\emph{Fig.~13: Comparison crack propagation process for two patched
specimens under fatigue loading monitored from the metallic and the
patched side}

Understanding the interaction between local patch detachment and crack
growth in the structure is one essential goal in the patch design
process. To find out, how these two effects influence one another can
lead to improved design methods. Therefore, additional investigations
using optical LT are made for the specimen that is monitored from the
patched side. A comparison of the thermal images gained from passive IrT
and phase images resulting from LT measurements are displayed in
Fig.~14.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

Fig.~14: Comparison crack propagation process using passive IrT and LT

Local patch detachment can be made out as dark spots on the phase
images, comparable in size and contour lines with the results gained
from passive IrT. The crack on the other hand cannot be identified on
the phase images.

\textbf{4. Discussion}

Under quasi-static loading, the knowledge of local temperature changes
helps understanding the thermal examination through the bonded patch.
Monitoring a patched specimen, sudden local temperature increases at an
angle of around 45\selectlanguage{ngerman}° to the loading direction, combined with noise
effects in the load/displacement curve can thus be allocated to the PLC
effect. An increase of the effect over time is measurable. Compared to
the specimens without patch the detectable temperature difference from
the patched side is reduced, because temperature differences are rather
small, and the resulting infrared radiation is weakened by the CFRP
patch. Additionally, temperature flashes run suddenly through the
specimen so that a clear identification of damage initiation through the
patch is difficult. Further, due to the local patch detachment the air
between the patch and the specimen additionally influences the radiation
intensity. For patches of one layer of the chosen CFRP material,
subsurface effects which are related to heat generation can generally be
identified on thermal images. But, temperature difference
\(T\) has to be high enough. The origin of heat generation
cannot clearly be identified without further information. Interpretation
of thermal images needs for additional knowledge resulting from the
stress/strain behaviour e.g.

For specimens loaded in fatigue, results are different. Crack length
measurement is possible from the metallic side and becomes even better,
the longer the crack growth. As the specimens are loaded by the lower
piston, they heat up from the bottom to the top. The temperature
difference between the upper and the lower part of the specimen
increases with increasing number of load cycles. As the heat circles the
crack the crack itself acts as a temperature barrier and becomes visible
through the temperature difference of the metallic part above and the
one below the crack. From the patched side it is not as clear, but a
good qualitative estimation is possible as well. Further, under cyclic
loading, crack growth additionally goes along with local patch
detachment, leading to a further reduction of crack detectability. A
comparison of the results monitored from the metallic and from the
patched side shows that crack length detection is possible from both
sides. As seen for the unpatched specimen, a discontinuous temperature
distribution appears around the crack tips with a maximum temperature
increase \(T\) at the peak stress of each loading/unloading
cycle. Using this top dead centre for test evaluation, crack tips can be
identified also from the patched side. Imaging frequency should
therefore be higher than test frequency so that it can be made sure that
pictures at or close to the top dead centre are available. Unlike for
the quasi-static loading, here, the measured temperature of the metallic
and the patched side do not differ strongly, as long as there is a
proper bonding between patch and specimen. Increasing local damages in
the adhesive layer have an impact on the thermal properties resulting in
decreasing visibility of the crack. Local patch delamination can be
identified on the thermal image as areas where the woven structure of
the patch is noticeable. But still, crack propagation process can be
monitored. Comparing crack length and detachment dimensions it seems
that right in front of the crack the patch has already delaminated from
the metal. Meaning that for the given specimen configuration and the
given conditions presumably delamination around the crack tip appears
before the crack grows. To further corroborate this hypothesis
additional tests using complementary monitoring techniques have to be
made.

\textbf{5. Conclusions}

When analyzing the damage processes of crack patched structures under
quasi-static and cyclic loading, passive IrT is a good opportunity to
help understanding the complex failure behaviour. From the unpatched
side, of course, the metallic crack growth can be monitored easily,
while it is more difficult from the patched one. For quasi-static
loading big strain differences lead to extensive patch delamination
which strongly detracts the thermal analysis. Fatigue loading on the
other hand goes usually along with only local patch detachment. And, as
long as the patch is bonded to the specimen for the most part, and
temperature changes can be made out on the patch surface, also the
subsurface crack growth process can be monitored. Due to the overall
increase in temperature, crack detectability improves over time. On the
other hand, with increasing local delamination image interpretation
becomes more difficult. Here, at maximum load, crack length can even be
measured for locally detached patches. In total, the method of IrT
provides the opportunity to gain a better understanding of the fatigue
behaviour of crack patched structures.

Additional tests are needed to proof whether patches made of several
layers still lead to utilizable thermal images and how an additional
adhesive layer influences the results. Provided that the temperature
increase must be strong enough to be detectable on the patch surface,
passive IrT can help understanding the degradation process of a crack
patched structure under long-term loading. The design assumption
commonly used that a proper bonding is present, is usually not given
under service conditions. Even for patch application under laboratory
conditions long-term loading and environmental influences degrade the
mechanical properties of the adhesive. Thus, designing the patch using
ideal bonding conditions can lead to false assessments in the
performance prediction. In forthcoming studies, the method of passive
IrT can be used to describe the damage behaviour of crack patched
components more realistically. The method of passive IrT comprises the
possibility to identify the component (metallic sheet, adhesive zone, or
patch) where the first damage onset occurs and how the different damage
mechanisms influence one another. Thereby it provides great potential
for improving the patch design process and to make failure prediction
more reliable.

\textbf{References}

{[}1{]} Baker AA, Jones LR. \emph{Bonded Repair of Aircraft Structures}
: Springer Science \& Business Media; 1988.

{[}2{]} Baker AA, Francis LR, Jones R. \emph{Advances in the bonded
composite repair of metallic aircraft structure} : Elsevier; 2003.
doi:10.1016/B978-008042699-0/50000-0.

{[}3{]} Duong CN, Wang CH. \emph{Composite repair: theory and design} :
Elsevier; 2010. doi:10.1016/B978-008045146-6/50001-1.

{[}4{]} Baker AA, Rajic N, Davis C. Towards a practical structural
health monitoring technology for patched cracks in aircraft
structure.\emph{Composites Part A: Applied Science and Manufacturing}
2009; 40 (9): 1340--1352.
doi:https://doi.org/10.1016/j.compositesa.2008.09.015?nosfx=y

{[}5{]} Schubbe JJ, Bolstad SH, Reyes S. Fatigue crack growth behavior
of aerospace and ship-grade aluminum repaired with composite patches in
a corrosive environment. \emph{Composite Structures:} 2016; 144: 44--56.

{[}6{]} Maldague X. \emph{Theory and practice of infrared technology for
nondestructive testing} . Wiley; 2001.

{[}7{]} Jones R, Baker AA, Matthews N, Champagne V. \emph{Aircraft
sustainment and repair} . Butterworth-Heinemann; 2017.
doi:10.1016/B978-0-08-100540-8.09992-1.

{[}8{]} Martens U, Schröder K-U. Evaluation of infrared thermography
methods for analysing the damage behaviour of adhesively bonded repair
solutions. \emph{Composite Structures:} 2020; 240.
doi:https://doi.org/10.1016/j.compstruct.2020.111991

{[}9{]} Avdelidis NP, Moropoulou A, Riga ZM. The technology of composite
patches and their structural reliability inspection using infrared
imaging. \emph{Progress in aerospace Sciences} ; 2003; 39 (4): 317--328.

{[}10{]} Paipetis AS, Grammatikos SA, Kordatos EZ, Barkoula N-M, Matikas
TE. In service damage assessment of bonded composite repairs with full
field thermographic techniques. In: \emph{Nondestructive
Characterization for Composite Materials, Aerospace Engineering, Civil
Infrastructure, and Homeland Security} . San Diego: International
Society for Optics and Photonics; 2011:79831U. doi:10.1117/12.880376.

{[}11{]} Moropoulou A, Avdelidis NP. Active thermography in the
assessment of repaired aircraft panels. \emph{Insight} 2002; 44:
145-149.

{[}12{]} Pavlopoulou S, Grammatikos SA, Kordatos EZ, Worden K, Paipetis
AS, Matikas TE, Soutis C. Continuous debonding monitoring of a patch
repaired helicopter stabilizer: Damage assessment and
analysis.\emph{Composite Structures} ; 2015; 127: 231--244.

{[}13{]} Avdelidis NP, Ibarra-Castanedo C, Maldague X, Almond DP,
Moropoulou A. Thermal NDT \& E of composite aircraft repairs.
In:\emph{16th WCNDT 2004.} Montreal: World Conference on NDT;
2004:TS4.27.3: 1-9.

{[}14{]} Grammatikos SA, Kordatos EZ, Aggelis DG, Matikas TE, Paipetis
AS. Critical and subcritical damage monitoring of bonded composite
repairs using innovative non-destructive techniques. In: \emph{Smart
Sensor Phenomena, Technology, Networks, and Systems Integration} . San
Francisco: International Society for Optics and Photonics; 2012: 834611.

{[}15{]} Grammatikos SA, Kordatos EZ, Matikas TE, Paipetis AS. On the
fatigue response of a bonded repaired aerospace composite using
thermography. \emph{Composite Structures:} 2018; 188: 461--469. doi:
https://doi.org/10.1016/j.compstruct.2018.01.035.

{[}16{]} Kubin LP, Ananthakrishna G, Fressengeas C. Comment on
``Portevin--Le Chatelier effect''. \emph{Physical Review} \emph{E;}2002;
65. doi: https://doi.org/10.1103/PhysRevE.65.053501.

\textbf{Source of Funding}

None

\textbf{Conflict of interest}

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