active infrared thermography technique … · 131 int. j. mech. eng. & rob. res. 2012 r sultan...

131

Int. J. Mech. Eng. & Rob. Res. 2012 R Sultan et al., 2012

ACTIVE INFRARED THERMOGRAPHY TECHNIQUEFOR THE NON DESTRUCTIVE TESTING OF

COMPOSITE MATERIAL

R Sultan1*, S Guirguis2, M Younes2 and E El-Soaly3

*Corresponding Author: R Sultan,[email protected]

Delamination is one of the most common defects in laminate composite materials. To detectdelaminations, different non-destructive test methods such as infrared thermography methodsare currently widespread for non-destructive inspection, recommended for evaluatingcomposite materials mostly in the active variant. The principle behind infrared thermographyconsists in highlighting the relevant differences of temperature, by comparing the temperatureof the area without defect to the defected area temperature. Active infrared thermographyrefers to the group of methods employed to inspect the integrity of materials or systemsthrough the use of an external energy source and an infrared detector. The paper presentsseveral experimental examples concerning the use of active thermography in the nondestructive evaluations of composite materials. The paper presents the analysis of resultsobtained by active infrared thermographic testing of E glass woven (0, 90) reinforced polymerplates of eight plies containing artificial delaminations. For this study, number of compositeplates were manufactured with different known depths of delamination implanted usingAluminium foil coated with special wax and different thickness of delamination embedded atsame depth. The aim of the present paper to investigate experimentally the ability of activethermography to detect the delamination based on its depth location and thickness. Thermalimages obtained by experiments using infrared camera were used to extract surfacetemperature profiles above the delaminated and non-delaminated surfaces on the specimens.The dependency of thermal contrast on delamination depth and delamination thicknesswas also investigated.

Keywords: Delamination, Active infrared thermography, Thermal contrast, Composite material

ISSN 2278 – 0149 www.ijmerr.comVol. 1, No. 3, October 2012

© 2012 IJMERR. All Rights Reserved

Int. J. Mech. Eng. & Rob. Res. 2012

1 Libyan air Force, Libya.2 Egyptian Armed Forces, Egypt.3 10th of Ramadan Higher Institute of Technology, Egypt.

Research Paper

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INTRODUCTIONDue to inherent complexities of compositestructure manufacturing, various types ofdefects such as voids, inclusions, impropercure, delamination, disbond, etc., may crop intothe final product made of composite material(Ray et al., 2007). The proper assessment ofsuch defects is essential for utilization of fullpotential of these materials. Various nondestructive testing methods are available toinspect the composite structures. Theseinclude x-ray radiography, ultrasonic testing,time of flight diffraction technique, and thermalimaging. Thermography imaging techniquehas been widely used over the decades toevaluate the integrity of materials, joints,electrical connections in a wide range ofindustrial as well as research fields (Maldague,1992). Active infrared thermal provides fast,safe, non-contact and non destructive tool forevaluation of sub-surface defects in materials.Typical applications in the material evaluationfield include the investigation of compositematerials, ceramics, bonded structures,metals, and coated surfaces. The thermographymethod can be used as an alternative orcomplement to conventional non destructivetesting techniques (Terumi et al., 1999; andMaldague, 2001). This examination techno-logy does not affect the material or structure’sfuture usefulness and at the same timeproviding an excellent balance between qualitycontrol and cost-effectiveness (Paritosh et al.,2006). In active method of thermal nondestructive testing, the object is excited by aheat source and the transient temperaturebehaviour on the surface of the object ismonitored by using thermal imagingequipment called infrared camera. The

influence of the thermal wave that diffuses intothe material on the surface of the specimenwas observed and these observations wereused to detect and characterize the surfaceand subsurface delamination (Biju et al.,2009). The presence of sub-surface anomaliesin the object interfaces with the perturbation ofheat flow and causes temperature contrast atthe surface. Thermal non destructive testinghas been previously well reported forvarious applications such as detection ofdelaminations in composite materials, (Wonget al., 1999; Maldague, 2001; Omar et al.,2006; Mabrouki et al., 2009; Genest et al.,2009; and Vageswar et al., 2009).Vijayaraghavan et al. (2010) reportedseveral experimental works on the pulsedthermography of carbon fibre reinforcedpolymer for the investigation of interfacialdefects. Mitsui et al. (2000) conductedexperimental study for detection and thicknessevaluation of delamination between thefibers reinforce plastic sheet and concreteby infrared thermography. Mirela et al. (2007)demonstrated the possibility of identifying thedefect type in complex structure carbon fiberreinforced plastic samples inspected bythermography using experimental and Finiteelement method results. Mayr et al. (2008)presented comprehensive analysis ofgeometry and excitation effects of pulsedthermography on curved carbon fiberreinforced plastic components containingdelaminations. Mabrouki et al. (2009) appliedactive thermography for the detection ofdelaminations in E-glass fibre. This paperaims to analyse experimentally the ability ofactive thermography test to detect thedelaminations according to their depths andthickness in composite laminate plates and

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the dependency of thermal contrast ondelamination depth and delaminationthickness was also investigated.

THERMAL NONDESTRUCTIVE TESTINGThermal non destructive testing, also referredas Infrared thermography, is a non-destructive,non-intrusive, non-contact mapping of thermalpatterns on the surface of objects. Generally, Itis used to diagnose thermal behaviour and,thereby, to assess the performance andthe integrity of material, products andprocesses. All surfaces of the objects warmerthan absolute zero emit electromagneticradiation energy in the infrared region ofelectromagnetic spectrum which, liesbetween visible light and radio waves. Theelectromagnetic spectrum illustrated inFigure 1 shows the region of X-rays, radiowaves, light waves (ultraviolet and visible) andinfrared radiation. Infrared region of thespectrum extends between 0.79 m and 1 mm.

The infrared region can be subdivided intothree categories based on their wavelength:near (0.79-3 m), medium (3-6 m), far (6-15m) and extreme (15-100 m). The far andextreme regions are together called long wave.The infrared region can be further divided intotwo categories based on their radiationproperties: the reflected (0.7-3.0 m) and thethermal (3.0-100 m). Thermal imaging is atechnique used for converting a thermalradiation pattern, which is not visible to thehuman eye, into a visual image (Paritoshet al., 2006). Infrared camera is used to obtainthe thermal visual image by transformingenergy radiated from object into an electronicvideo signal. Infrared camera creates imagesbased on the heat energy emitted by the objectrather than light reflected off it. The energyemitted by the object is mainly a function itssurface temperature. The infrared camera canbe classified into three types based on theoperation range of wavelength: short wavecamera, medium wave camera and long wave

Figure 1: Infrared Region of Electromagnetic Spectrum

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camera. The thermal image obtained from theinfrared camera can be used for thequantitative analysis of defect location, sizingand characterisation. Infrared techniques canbe mainly classified into two types based onthe temperature difference generated to obtainthermal image: passive thermography andactive thermography. The passivethermography is used for the objects which arenaturally at the higher temperature thanambient while in the case of the activethermography, an external heat source isintroduced to obtain relevant temperaturedifference. In active thermography, variousmodes of thermal simulations are available:pulsed thermography, step-heating, lock-inthermography and vibro-thermography. In thiswork, delaminations in the composite laminateplates are analyzed by active thermographyby recording the temperature profiles on theouter surface of the specimens. In the presentexperimental process, a constant heat wasapplied using hot air blower and the increaseof surface temperature distribution ismonitored and recorded using infraredcamera. An abnormal temperature profilecould essentially indicate the location ofdelamination (Maldague, 2002).

EXPERIMENTS

Depth of Delamination

Sample Preparation and ActiveThermography Experimental Setup

A glass fibre reinforced polymer squarespecimens were fabricated including artificialdelamination located at two different depthsduring the manufacturing process using thinAluminum foil in order to simulate delamination.The areas of these three specimens were 205

205 mm2 and the number of layers is 8.Thickness of each layer is approximately0.3125 mm (total thickness 2.5 mm).Delamination depths for sample 2 and sample3were embedded foil in various depth 0.3125and 1.25 mm from observation side and thearea of delaminations was 60 205 mm2. Ahot air blower with 2.5 kW output power wasused as heat source. A switch control wasapplied to control the heating duration from thehot air blower. Specimens were activated byshort pre-heating time 7 seconds. Theinvestigation is about observing thetemperature distribution of the surface withsubsurface delamination during cooling-downprocess. Specimen’s surface temperatureduring cooling down process was observedin area above delamination and in area withoutdelamination. The specimens were located ata constant distance of 30 cm from heat sourceand the camera is held 70 cm from the sample.Together with the end of the pre-heatingprocess, the surface temperature wasrecorded and the delamination appears. Theinspection setup presented in Figure 2 uses a

Figure 2: Experimental Set Upfor Active Thermography

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Fluke Ti20 infrared camera. It has an un-cooleddetector with focal plane array 128X96HV,where infrared detector absorbs the infraredenergy emitted by the studied specimens andconverts it into electrical signal.

RESULTS AND DISCUSSIONIn the present experiment, active infraredthermography was applied to evaluate theability of non destructive thermography testto detect the delamination in laminatecomposite material structure with differentdepth locations. The depth of the simulateddelaminations was varied and keeping thearea and thickness of artificial delaminationsame. Generally when the heat was appliedfrom hot air blower to the sample surface, theuniform temperature rise was observed onthe surface, if the sample has nodelamination. If the sample has delaminationon the contrary, a localized high temperatureregion appears on the sample surface justabove the delamination due to the insulationeffect of the delamination. The shape of thehigh temperature region ref lects thedelamination shape under the surface due tothe temperature difference. The temperatureimages using active thermography forsamples 1, 2 and 3 were shown in Figure 3respectively after a period of 7 seconds ofheating. Delaminations in samples 2 and 3are observed at different times (2 secondsand 3.5 seconds) after ending the preheatingstage with different contrast. Due to thesurface heating by active thermography, heatpropagates from the surface to the rear side.Delamination, as thermal barriers, stop heatdiffusion and reflect the heat back to thesurface, which results in higher temperatureat delamination than in non-defected region.

Figure 3: Temperature Images UsingActive Thermography

a) Sample 1 (Healthy Plate)

b) Sample 2 (Delamination Beneath One Layerwith Depth 0.3125 mm)

c) Sample 3 (Delamination Beneath Four Layerswith Depth 1.25 mm)

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In all defected thermograms, highlight thedelamination in the composite platesdepends on their depth positions, but withvarious contrast. Obviously the temperaturedifference between delaminated zone andhealthy one are strongly depending on depthof delamination from observation side andthermal conductivity of the delamination. Figure4a shows that the temperature was almostuniform throughout the surface of healthyplate. While, Figures 4b and 4c show thetemperature profile on the surface, with

internal delamination. The temperatureprofile of this delamination during the coolingphase verifies with the characterizingconditions of the delamination. The averagethermal contrast obtained from experimentwas 0.24 °C for sample 1, where thedelamination was 0.3125 mm deep from theouter surface for sample 2 the thermalcontrast was 5 °C, where as the delaminationat 1.25 mm depth for sample 3 produces athermal contrast of 3.75 °C with a decrementof 1.25 °C.

Figure 4: Surface Temperature Profiles

a) Sample 1 (Healthy Plate)

b) Sample 2 (Delamination at Depth 0.3125 mm)

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Also by comparing the thermal imagescontain delamination as shown before inFigure 3, the whole length of delamination insamples may not be clear enough, becausethe thermal image was strongly influenced bythe non uniform heating due to short time ofheating and sometimes it leads tomisinterpretation of results, for this reason themethod was adequate especially for detection

Figure 4 (Cont.)

c) Sample 3 (Delamination at Depth 1.25 mm)

Therefore, it can be concluded that anincrease in depth of delamination produced areduction in the thermal contrast as shown inFigure 5. Present experiment reveals that thedelamination nearer the thermal camera wasidentified quicker with increased thermalcontrast but that delamination nearer the innerwall of the plate was identified later with areduced thermal contrast.

Figure 5: Variation of Thermal Contrasts with Delamination Depth

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Delamination Depth (mm)

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of the near subsurface delamination. Also thistechnique cannot distinguish whether the effectwas real or due to the potential error sourceslinked to the object, the environment and theacquisition system. The total power emittedby samples 2 and 3 can be theoreticallycalculated by using Stefan’s law, also the totalemitted power can be extracted from theresulting experimental curves Figure 4, wherethe area under the curves proportional to thetotal radiation power. The relation betweentotal radiation power and depth ofdelamination shown in Figure 6.

Thickness of Delamination

Experimental Set-Up andSpecimens Preparation

Plates of eight layers include E-glass fibresembedded in an epoxy matrix weremanufactured with artificial circular Aluminumfoil delamination of diameter 38 mm withdifferent thickness, embedded at the samedepth 0.9375 mm (under the third layer oflamination). Sample 1 contains delaminationof thickness 0.6 mm and sample 2 containsdelamination of thickness 0.1 mm. Bothspecimens were, square 200 200 mm2 and

2.5 mm thick. The experimental setup for activeinfrared thermography of composite laminateplate was shown in Figure 7. The infraredcamera Fluke Ti20 is the main part here whichmeasures the thermal transient at the surfaceof the plates resulting from the heating of theplates using an external stimulus source hotair blower of 2.5 kW. An emissivity adjusted tovalue of 1 in order to eliminate spuriousreflections from radiant emitters such asoverhead lights or human bodies and to ensure

consistency in the sample surface emissivity.The thermal imaging camera was placed at 70cm from the specimen and heat source kept atdistance 30 cm. Specimens were activated byshort pre-heating time 7 seconds. Theinvestigation was about observing thetemperature distribution of the surface withsubsurface delamination after short heating byhot air blower. Specimens were searchedduring cooling-down process. Specimen’ssurface temperature during cooling down

Figure 6: Variation of Total Radiation Power with Delamination Depth

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Delamination Depth (mm)

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process was observed in area abovedelamination and in area without delamination.Together with the end of the pre-heating process,the surface temperature recording procedurebegan by using infrared camera Fluke Ti20.

RESULTS AND DISCUSSIONThe surface temperature distribution imagesover the composite laminate plates wereobtained at the end of heating time 7 secondsusing experimental set-up. The thermal contrastvalue was measured by thermography to detectdelamination which was used to analyze forvarying thickness. The thickness of simulateddelamination in sample 1 was six times asdelamination thickness of sample 2. Hereinpresent experiment the active thermographyreflection method was used, meaning that thethermal source and the infrared camera werelocated on the same side of the specimen. Inthis case, the delamination zone appeared likea hot spot as shown in Figure 8.

Figure 8: Thermal Images for Sample 1and Sample 2

a) Sample 1 (Circular Delaminationwith Thickness 0.6 mm)

b) Sample 2 (Circular Delaminationwith Thickness 0.1 mm)

Therefore, the thermal contrasts obtainedfrom experiment (Figure 9) at the place ofdelaminations of samples 1 and 2 are takeninto consideration and were plotted as shownin Figure 10. The Figure revealed that, thethermal contrast value at the delamination ofsample 1 was greater than that of sample 2.

Figure 7: Active ThermographicInspection Set Up

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Figure 9: Surface Temperature Profiles

a) Temperature Profile of Sample 1 (Delamination Thickness 0.6 mm)

b) Temperature Profile of Sample 2 (Delamination Thickness 0.1 mm)

Figure 10: Variation of Thermal Contrasts with Delamination Thickness in Sample 1and Sample 2

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0 0.2 0.4 0.6 0.8

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Delamination Thickness (mm)

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Due to the difference in thermal resistanceof delamination, the thermal contrast valuesobtained were 5.2 °C and 3.8 °C, for sample1 and 2 respectively. As the thickness ofdelamination increases the thermal resistanceof the delamination is increases. This resultsin higher thermal contrast at the location ofthicker delamination.

CONCLUSIONThe obtained experimental results allowextracting the following conclusions:

• Radiation heating and thermographicimages analysis is an effective method forrevealing delamination in the compositematerials. It is possible to see thedelamination on thermographic image,but the determination of their geometryand position is restricted and not veryprecise.

• In case of thermographic methods it is notpossible to detect delamination after longtime of pre-heating of the studiedmaterial.

• The thermal contrast changes abovedelamination quickly, thus temperaturemeasurement must be completed during ashort elapsed time after the pulse heatingespecially for the detection of smallerdelamination.

• Thermography methods require specificskills to achieve good results in delamina-tion detection especially for compositematerials.

• The infrared thermography is useful indetecting invisible defects non-destructively,extensively and safely.

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active infrared thermography technique … · 131 int. j. mech. eng. & rob. res. 2012 r sultan...

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