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Automated transient thermography for the inspection of CFRP

structures: experimental results and developed procedures

P. Theodorakeas1

, N.P. Avdelidis1,2

*, K. Hrissagis1

, C. Ibarra-Castanedo2

, M. Koui1

, X. Maldague2

1NTUA National Technical University of Athens, School of Chemical Engineering, Materials

Science & Engineering Department, Zografou Campus, 157 80 Athens, Greece.2

Computer Vision and Systems Laboratory, Department of Electrical and Computer Engineering,Laval University, Quebec City, G1V 0A6, Canada.

ABSTRACT

In thermography surveys, the inspector uses the camera to acquire images from the examined part. Common problems

are the lack of repeatability when trying to repeat the scanning process, the need to carry the equipment during

scanning, and long setting-up time. The aim of this paper is to present transient thermography results on CFRP plates

for assessing different types of fabricated defects (impact damage, inclusions for delaminations, etc), as well as and todiscuss and present a prototype robotic scanner to apply non destructive testing (thermographic scanning) on materialsand structures. Currently, the scanning process is not automatic. The equipment to be developed, will be able to perform

thermal NDT scanning on structures, create the appropriate scanning conditions (material thermal excitation), and

ensure precision and tracking of scanning process. A thermographic camera that will be used for the image acquisition

of the non destructive inspection, will be installed on a x, y, z, linear manipulator's end effector and would besurrounded by excitation sources (optical lamps), required for the application of transient thermography. In this work

various CFRP samples of different shape, thickness and geometry were investigated using two different thermographic

systems in order to compare and evaluate their effectiveness concerning the internal defect detectability under different

testing conditions.

Keywords: transient thermography, inspection, thermal scanning, robot, procedures, analysis, CFRP.

1. INTRODUCTIONFibre reinforced composite materials are used in a wide range of transport applications due to their high strength to

weight ratio, high specific strengths and moduli, superior corrosion resistance and improved fatigue performance. The

use of fibre-reinforced composite materials has increased steadily in both low and high technology engineering

applications over recent years. This growth is also apparent within the surface transport and automotive industry. There

are many drivers behind this increased use of high performance composites within such industries with the weightreduction being the primary driver. Although, currently other materials (i.e. steel, aluminium, etc) are the main

materials used in transport applications, advanced carbon-fibre reinforced polymers are providing an increasingly

attractive and economically viable alternative.

Main composite materials are Carbon Fibre Reinforced Plastics (CFRP), Glass Fibre Reinforced Plastics (GFRP) and

metal-aluminium laminates (i.e. Glass Fibre Reinforced Aluminium GLARE). These composite materials are used more

and more from the aircraft production industry. Typical parts made of CFRP are flaps, vertical and horizontal tailplanes, centre wing boxes, rear pressure bulkheads, ribs and stringers. For the Airbus A380, GLARE is used even for

some shells of the upper fuselage. The weight percentage of composites in modern civil aircrafts like the A380 is in the

order of 25%. It may be expected that this percentage will further increase for the next generation of civil aircrafts and

that main structure parts like fuselage and wings will be composed of composites, too.

* Correspondence Email: [email protected]

Thermosense: Thermal Infrared Applications XXXIII, edited by Morteza Safai, Jeff R. Brown,Proc. of SPIE Vol. 8013, 80130W 2011 SPIE CCC code: 0277-786X/11/$18 doi: 10.1117/12.882695

Proc. of SPIE Vol. 8013 80130W-1

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Even those composite materials are offering a large number of advantages over the conventional materials, a large

number of possible defects can occur within these materials due to their complex production processes. These defects

can reduce the mechanical performance of composite materials and it is vital to characterise and understand the effects

such defects can have on the performance, as well as develop methods of detecting such defects promptly andsuccessfully.

2. DEFECT ASSESSMENT OF COMPOSITES & TRANSIENT THERMAL NDTSeveral approaches could find use in the monitoring of the structures damage [1,2]. One area where relatively littlework has been done is on the effects that manufacturing and assembly defects have on the performance of composites.

Several defects can occur within composite materials both due to the manufacturing of the material and assembly of

manufactured components.

Due to the complex processes involved in the production of composite materials and the numerous materialmanufacturing, lay-up and curing processes that are available there is a large number of possible defects that can occur

within the material. Within assembly of an aircraft structure, a huge number of different components manufactured

from differing materials need to be assembled to create the finished product. This assembly stage of the manufacture

will have differing amounts of automation and can involve a number of processes whereby the material can becomedamaged. This damage may have a detrimental effect on the components ability to support a load and thus such defects

need to be understood and quantified. This is necessary for both the production of safe aircraft transport and in order toestablish damage and defect tolerance criteria for the various components. A better understanding of these defects andtheir effects is necessary to establish inspection criteria and in order to fully understand both the production and

operation costs that may be incurred.

A lot of this work has concentrated on the effects of manufacturing defects in composites but less work has been carried

out on assembly related defects and their effects on the materials performance [3,4]. Assembly related defects havebeen investigated; including out-of-round holes, hole exit side delaminations, improper countersink depths, incorrect

countersink angles, interference fit and damage associated with numerous fastener removal and instillation cycles.

Varying defect effects depending on the defect size, laminate lay-up and testing environment but in all cases the defectsresulted in a drop-off in mechanical properties.

Furthermore, careful control of the manufacturing process and increased automation can reduce the probability of

defects occurring; however they will never be fully removed. Manufacturing defects are described graphically in thefollowing figures (Figures 1 & 2).

Figure 1: Typical Manufacturing Defects in Composite Laminates

InclusionVoids/porosity

Delamination



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Figure 2: Schematic of resin variation.

Internal damage to a laminate (such as delaminations, inclusions and voids) is rarely visible from the material surface.

For this reason a number of NDT techniques have been developed. These methods are advantageous because they do

not affect the serviceability of the component, they can be used as manufacturing process control and failure preventionand they can lead to cost reduction. Non-destructive investigation techniques are largely used because of the

outstanding advantages that they are capable to provide in a variety of applications. Transient thermal NDT is a prompt

and reliable technique [5] that could be used for the inspection, monitoring and assessment of composites and

composite components that find use in transport applications. Transient thermal NDT requires an external source of

energy to induce a temperature difference between defective and non-defective areas in the specimen underexamination. A wide variety of energy sources are available [6] and can be divided in optical, if the energy is delivered

to the surface by means of optical devices such as photographic flashes (for heat pulsed stimulation) or halogen lamps

(for periodic heating), mechanical if the energy is injected into the specimen by means of mechanical oscillations (e.g.

with a sonic or ultrasonic transducer) or electromagnetic. Furthermore, other forms of external active excitation have been proposed, such are hot or cold air/ water jet and thermo-elastic heating. Commonly all active thermography

techniques can be used in the NDT assessment of industrial materials. Selection of the most suitable technique and

energy source depends on the application.

Optical pulsed thermography (PT) is a fast and easy to deploy inspection technique [7]. In PT the heat stimulation can

last from some ms to some seconds depending from the thermal diffusivity of the material being inspected. A short

pulse of light energy (i.e. flash lamp) heats the inspection surface of a composite material and the infrared camera

monitors and records the surface temperature (or radiance) as the sample cools down. The surface temperature fallspredictably as heat from the surface diffuses into the sample bulk. Nonetheless, internal thermal discontinuities such as

voids or delaminations modify the local cooling of the surface and the corresponding radiation flux from the surface

that is detected by the infrared camera. Fundamental detectability of a defect will depend on its size, depth and thedegree to which its thermal properties differ from those of the composite material. Detectability is a function of the

aspect ratio of the defect. The minimum detectable defect size increases with the depth of the defect. Detectability is

highest for larger defects that are closer to the sample surface and have thermal properties that are significantly differentfrom the composite material. There are various parameters that affect detectability and these are: emissivity

reflectivity, data acquisition period, thermal energy from pulsed source, wavelength of infrared camera, frame rate,

sensitivity, and spatial resolution.

Lock-in Thermography (also known as modulated thermography) uses thermal waves instead of pulses in order toilluminate the investigated specimen [8]. The thermal wave is recorded at the same time using an infrared detector and

decomposed by a lock-in amplifier to extract the amplitude and phase of the modulation. Sinusoidal waves are typicallyused in lock-in thermographic approach. The periodic wave propagates by radiation through the air until it reaches the

specimen surface where the heat is produced that propagates through the material. Internal defects act as a barrier forheat propagation which produces changes in the amplitude and phase of the response signal at the surface of the item

under inspection. Optical lock-in thermography allows better control of the energy deposited on a surface, which might

be interesting if a low power source is to be used or if special care has to be given to the inspected part (i.e. inspectionof artworks). However, it requires a separate experiment for each and every inspected depth and there is a stabilization

time before reaching a permanent regime.

Resin poor region Resin rich region



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Vibrothermography (VT), also known as ultrasound thermography utilizes mechanical waves to directly stimulate

internal defects without heating the surface as in optical methods [9]. In ultrasound thermography a transducer is placed

in contact with the sample. The ultrasonic waves travel through the specimen and are transmitted back to the surfacewhere the transducer picks up the reflected signal (pulse echo technique) or they are collected on the opposite side

(through transmission). The ultrasonic waves will travel freely through a homogeneous material in all direction

dissipating their energy at the discontinuities in the form of heat. The heat generated will then travel by conduction in

all directions. An infrared camera can be directed to one of the surfaces of the specimen to capture the defect signature.

Ultrasonic waves are ideal for NDT as the defect detection is independent from its orientation inside the specimen, andboth internal and open surface defects can be detected. Vibrothermography is extremely fast very useful in either lock-

in or burst (e.g. pulsed) configuration for the detection of cracks and delaminations. Either brust or modulatedvibrothermography requires attention to some experimental parameters [10]. These parameters are the pressure and the

contact area between the horn and the specimen and the duration of the stimulation. The longer the transducer operates

at the surface; the most heat is released in the contact surface, increasing the probability of damaging the area.

Nevertheless, once all these factors are correctly addressed, inspection is extremely fast and results provide good defect

contrast without processing.

Finally, Eddy Current Thermography (ECT) is a very promising technique [11]. Although it is limited to electro-

conductive materials, the range of potential applications is extensive, from metal laminates such as GLARE, to

composite materials and sandwiched structures such as honeycomb specimens. Electromagnetic excitation is achievedby inducing Eddy currents through electromagnetic coils. The presence of an internal defectwill disturb the current

flow generating changes in the temperature profile of the defective area. These temperature changes are visualized using

an IR thermographic camera.As in the case of optical and ultrasound excitation, during the electromagnetic excitationboth pulsed and lock-in configuration can be used.

In common practice, advanced signal processing algorithms are used to enhance detectability of defects that are not

detectable in the raw thermographic data. Transient thermal thermography based on optical techniques, in general, provide very good defect resolution. However, results are strongly affected by surface features and advanced

signal/image processing is required to reduce their impact. Data obtained by optical stimulation in either PT or LT, is

processed by the fast Fourier transform (FFT), which is commonly refer as pulsed phase thermography (PPT) in the

case of pulsed thermographic data [12,13]; and phase angle thermography or phase sensitive thermography in the caseof modulated data [14,15].By using FFT, data is transformed from the time domain to the frequency spectra. Results

are presented in the form of phasegrams, i.e. a map of the specimen surface indicating the phase delay of the outputsignal with respect to the input. Furthermore, there are many other advanced processing techniques developed toimprove the transient signal. Thermographic signal reconstruction (TSR) is one of such techniques [16]. It allows

reducing the amount of data, de-noise the signal and further process synthetic data using first and second time derivative

images as well as the FFT, which considerably improve the signal-to-noise ratio. Another prompt signal /image

processing tool is Principal Component Thermography [9]. PCT reorganizes data in a transformed space where the first

components contains the maximum variance. Typically, a 1000 thermogram sequence can be replaced by 10 or lessEmpirical Orthogonal Functions (EOF) that describe spatial variation of data. The first EOF will represent the most

characteristic variability of the data; the second EOF will contain the second most important variability and so on. After

applying the PCT tool each of the resulting EOF highlights a specific type of feature.

These processing techniques can be applied to any thermographic regardless of the energy source used for stimulation.

It should be pointed out however, that when all the experimental factors are correctly addressed in the case of

vibrothermography, raw data present adequate contrast to detect defects and no advance processing is required [17]. Asimple cold image subtraction or the FFT can be used to improve contrast in some cases.

3. THERMOGRAPHIC INSPECTION OF CFR PLATESIn this work, five different CFRP samples were investigated. The specimens were of different geometry and thickness

consisted of various types of fabricated defects. Experimental tests were carried out on the first set of CFRP samples of

2mm thickness and of three different shapes (planar, curved, and trapezoid) as described in Figure 3. In each specimentwenty five square Teflon insertions of different sizes were placed between the plies at different locations. The



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specimens geometry as well as the defect locations, depths (ranged from 0.2 to 1.0 mm) and sizes are presented in the

left column of Figure 3. The use of the equivalent diameter D eq, defined as the diameter of the circle having the same

area of the considered shape, is proposed for the eventual comparison to similar studies using defects with different

geometries. For instance, the equivalent diameter of a square of size L is Deq=2L/1/2 [18].

a) CFRP planar

b) CFRP curved

c) CFPR trapezoid

Figure 3: a) Schematics (left) and real images (right) of the first set of the CFRP investigated samples

Furthermore, two other CFRP laminates of 6 and 12 mm thickness respectively were also investigated. The two plates

were consisted of different types of fabricated defects. The 12mm of thickness CFR plate as can be seen from figure 4,

is consisted of some countersink defects as well as and some hidden defect stimulating the effects of impact damage. A

schematic of the 6 mm thick plate is presented in Figure 5.



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Figure 4: CFRP test sample of 6mm thickness (left) and CFRP test sample of 12 mm thickness (right)

D1

D2

D3

D4

I3

I2

I1

C1

C2

C3

B1

B2

B3

O

50 mm

O

O

O

50 mm

300 mm

300

mm

D: Delamination

D1: 1 mm x 1 mm

D2: 2.5 mm x 2.5 mm

D3: 5 mm x 5 mm

D4: 10 mm x 10 mm

I: Impact

I1: Load 1

I2: Load 2

I3 :Load 3

C: Countersink

B: Burned drill hole

O: Other defects

Figure 5: Schematic of the CFRP test sample of 6mm thickness

For the investigation of the composite samples two different integrated thermographic systems were used (Echotherm

and Voyage IR from TWI Inc). The Echotherm pulsed thermographic system is employing a medium wave (35 mm)infrared camera (Indigo) in order to image the defects. Echotherm is a portable state-of-the-art non-destructive

evaluation system with an integrated flash heating system. The Indigo Phoenix mid-wave infrared camera (also attachedto the system) uses a cooled detector with a frame rate of 60 Hz and a focal plane array pixel format of 640 x 480. On

the other hand, the data acquisition with the Voyage IR system was performed using of a long wave infrared camera

(ThermaCAMTM SC640). The long wave camera uses an uncooled microbolometer detector of FPA type with imageresolution 640 x 480 pixels and working on the maximum frequency of 30Hz. The thermal stimulation was performed

with the use of a heat gun (incorporated in the Voyage IR thermographic system) providing energy of 1875 Watts. The

data obtained by the two thermographic systems were processed using Thermographic Signal Reconstruction.

The main aim of this work was to compare and evaluate the reliability of different thermographic systems in order to

assess hidden defects on CFRP samples and propose procedures to be accommodated in a robotic scanner that will

perform thermographic scanning. The main difference of the two thermographic systems is on the infrared detector. The

Echotherm system employs a cooled MWIR camera opposite to the Voyage IR where the acquisition was performedusing an uncooled microbolometer LWIR camera. The two infrared detectors are having the same image resolution but

as it is well know a cooled detector is more sensitive to thermal radiation. Furthermore, aim of this work was also to

compare different energy sources (flash lamps and heating gun) as well as and different testing conditions. Theinspection area of the Echotherm system is isolated from the surrounding with a frame box as can be seen in figure 6,

opposite to the testing condition using Voyage IR system.



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Figure 6: Test arrangements for the thermographic measurements.

Figures 7 and 8 present the pulsed thermographic results obtained from the 2 mm CFRP specimens inspection (first setof samples) with the use of the integrated flash thermographic system, employing the MWIR camera. As can be seen

from the representative figures most of the Teflon insertions can be clearly identified. Furthermore, some background

variations can be observed due to the presence of fibres. Finally, for the trapezoid panel (Figure 7b), the sharpgeometrical changes at the surface can be perfectly observed apart from some of the Teflon insertions in these areas.

a) b)Figure 7: Second time derivative images acquired with the frame rate of 15 Hz from a) the rear side of the flat

sample at t= 2 s and b) the front side of the trapezoid sample at t= 1.46 s.

a) b)Figure 8: Second time derivative images of the curved CFRP acquired with the frame rate of 15 Hz from a) the

left front side at t=1.8 s and b) the right front side of defects at t=1.46 s.



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The results obtained for the two other CFRP samples are presented in Figures 9 and 10. Evidence of the hidden defects

on the right lower corner in the front side as well as and a hidden defect in the left side of the 6mm thick plate can be

seen (Figure 9).The fibres breakout on the burned drill and countersink holes can be better observed by extracting the

1st and 2nd time derivative images of the raw thermographs.

Figure 9: Raw image of the 6mm CFRP acquired with a frame rate of 3.75 Hz from the front side at t= 8.6 s

(left) and1st derivative image acquired with a frame rate of 15 Hz from the front side at t=5.53s by usingTSR approach (right).

Furthermore, from the results obtained after the thermographic inspection of the 12mm CFRP laminate, analysis shows

some hidden defects on the right lower corner of the front side. The rear side inspection reveals the loose of fibres on

the countersink holes.

Figure 10: Transient thermography results of the 12mm CFRP sample presenting raw image acquired with a framerate of 1 Hz from the front side (left) and another raw thermogram acquired with a frame rate of 2 Hz from the

back side (right).

As mentioned above, some transient thermal testing was performed with the aid of a thermographic system employing

an uncooled microbolometer LWIR camera and a heating gun as stimulation source. Similar results were observed

compared with the results from the flash thermographic system, but as can be seen from the figures below, in this case

the results are strongly affected by noise and environmental reflections. Furthermore, some of the results are affected bynon-uniform heating, especially the samples with complex surface geometry (i.e. curved and trapezoid). Nevertheless,

some internal defects on the CFRP samples can be clearly indentified as well.



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a) b)Figure 11: First time derivative images acquired with the frame rate of 30 Hz a) from the rear side of the

planar CFRP at t=1.09 s and b) from the rear side of the trapezoid CFRP at t= 1.22 s.

a) b)Figure 12: First time derivative images a) of the curved CFRP acquired from the rear side with the frame rateof 15 Hz at t=0.92 s and b) of the 6mm thick CFRP acquired from the from side with the frame rate of 15 Hz

at t=13.97 s

a) b)

Figure 13: a) First time derivative image of the 12 mm CFRP acquired from the front side with the frame rateof 1.5 Hz t=13.97 s and b) second time derivative image at= 13.26 s.

4. CONCLUSIONSFrom the above results presented, it can be seen that different active thermographic techniques can be used in the NDTassessment of composite materials. Selection of the most suitable energy source depends on the application. Although

data are affected by different problems (non-uniform heating, emissivity variations, environmental reflections and



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surface geometry), there are numerous processing techniques such as TSR, available to counter these problems and

therefore to obtain prompt results of reliable quality, as well as quantitative information in some instances.

Furthermore, the results presented lead to the conclusion that the above mentioned transient thermal inspectiontechniques can be accommodate in a robotic scanner in order to apply non destructive testing (thermographic scanning)

in composite materials and structures as shown in figure 14. The equipment will be able to perform thermal NDT

scanning on structures, create the appropriate scanning conditions (material thermal excitation, data acquisition), and

ensure precision, tracking and repeatability of the scanning process. A thermographic camera that will be used for the

image acquisition of the non destructive inspection that will be installed on a x, y, z, linear manipulator's end effectorand would be surrounded by excitation sources (i.e. optical lamps), required for the application of transient

thermography. An automatic scanning progress can improve the non-invasive imaging NDT & E of composites anddecrease the operating costs and time by reducing unnecessary unscheduled inspections. The equipment for data

acquisition and analysis of the thermographic inspection data will be installed close to the scanner, connected via

umbilical to it and the inspection equipment. A control console and a monitor will be available, providing automatic or

semi-automatic robotic functions for operating the robotic scanner.

Figure 14: Concept overview of the automated robotic scanner

ACKNOWLEDGEMENTS

Acknowledgements are attributed to the ComPair project, which is a collaboration between the following

organisations: TWI Ltd, Kaunas University of technology, Technical Research Centre of Finland, National Technical

University of Athens, ATOUTVEILLE, Cereteth, G-Tronix Ltd, ENEA, ENVIROCOUSTICS, HEXCELCOMPOSITES, KINGSTON COMPUTER CONSULTANCY LIMITED. The Project is co-ordinated and managed byTWI Ltd. and is partly funded by the EC under the Collaborative project programme - Small to medium scale focusedresearch project. Grant Agreement Number218697.

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