Journal of Materials Processing Technology 155–156 (2004) 1132–1137

The use of infrared thermography for materials characterization

Carosena Meolaa,∗, Giovanni M. Carlomagnoa, Luca Giorleoba DETEC–Dipartimento di Energetica, Termofluidodinamica Applicata e Condizionamenti Ambientali, Università di Napoli “Federico II”, P.le Tecchio, 80

80125 Napoli, Italyb DIMP–Dipartimento di Ingegneria dei Materiali e della Produzione, Università di Napoli “Federico II”, P.le Tecchio, 80 80125 Napoli, Italy

Abstract

The aim of the present experimental study is to highlight the help provided by infrared thermography in the characterization of materials.Infrared thermography is a two-dimensional, non-contact technique of surface temperature mapping which can be usefully exploited forquality assurance of manufacturing processes as well as for non-destructive evaluation of end products. Non-destructive evaluation withinfrared thermography can be performed in two basic ways: pulse thermography (PT), or modulated lockin thermography (MT). Bothtechniques are able to reveal material inhomogeneties, which can arise either during manufacturing processes, or in service. In particular,within the MT technique a phase angle value may be associated to a specific characteristic (density, porosity, hardness, etc.) of the material.Thus, MT can be exploited for characterization of many materials either metallic, or plastics, composites, etc. which are commonly usedin different fields: alimentary, architectural, automotive, aeronautical, mains. Results prove the capability of MT to discriminate betweenmaterials very similar and this feature can be advantageously exploited to evaluate modifications that occur in material characteristics asconsequence of ageing, or exposure to adverse environmental conditions.© 2004 Elsevier B.V. All rights reserved.

Keywords:Material characterization; Non-destructive evaluation; Lockin thermography

1. Introduction

Nowadays, when the modern technological progresscomes out in a lot of ever more sophisticated tools able todo everything quickly, from household appliances to highspeed transports, the demand ofsafetyis increasing alwaysmore. Of course, safety means high production standardsas well as maintenance. The first requirement involves ma-terials characteristics and manufacturing processes whilethe second one periodic inspection with the most adequatenon-destructive technique.

The researchers’ efforts are currently underway to createnew innovative materials of superior properties to be em-ployed for the fabrication of specialparts.

Leader is the aeronautical field where the challenge is acombination of low weight with high mechanical strength.Composites and particularly hybrid composites as Glare®

(metal laminae plus glass fibres) have been developed andstill the technology is sought for improvements. Now a widearea of aircraft surface is in composites (wings, fuselage).On the other hands, a marked reduction in the vehicle weight

∗ Corresponding author. Tel.:+390-81-768-3389;fax: +390-81-768-3389.E-mail addresses:carmeola@unina.it (C. Meola), carmagno@unina.it(G.M. Carlomagno), lgiorleo@unina.it (L. Giorleo).

allows for an increase in payable weight (passengers), whichmay result in economic profit.

The development of lighter-high-strength materials iswelcomed also in several other fields such as medicine fororthopaedic prostheses (i.e. hip joints), or dental implants.Perhaps, a reduction in weight is the main objective of alsothe packaging industry, which is daily involved in handlingmore and more packages.

In this context, material characterization represents a keypoint for product standards and may be performed withboth destructive and non-destructive tests. During the designphase, destructive tests help to acquire information aboutthe material behaviour. However, to assure quality, the fab-rication of new materials should be accompanied with thedevelopment of effective techniques of non-destructive eval-uation (NDE) to be used for either acceptance/rejection ofparts, or for inspection of parts in service.

At the moment there are many non-destructive techniquesavailable, but only a few allow in situ analysis. Thus, therequirements for NDE are continuing to be driven by theneed for lower cost methods and instruments with greaterreliability, sensitivity, user friendliness and high operationalspeed as well as applicability to increasingly complex ma-terials and structures. Indeed, there is an increasing demandfor non-destructive evaluation not only for characterizationof novel materials but also for characterization of antique

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.04.268

C. Meola et al. / Journal of Materials Processing Technology 155–156 (2004) 1132–1137 1133

materials. In fact, non-destructive techniques represent aninnovation for the control of the physical characteristics ofthe applied stuccos, to ascertain the state of preservation ofarchitectonic elements, or to understand composition of rareart treasures.

The aim of the present experimental study is to highlightthe help provided by infrared thermography in the charac-terization of materials.

2. Some remarks on principles of infraredthermography

Any object emits by radiation energy according to its sur-face temperature. Human eyes are struck by such energyonly when the object is warm enough and emits in the visi-ble band (0.4–0.8�m). In fact, we are able to recognize thetemperature during heating of a metal, when it passes fromred (720◦C), to yellow, to white (1350◦C). For low tem-perature values falling in the infrared region, thermographyprovides us withartificial eyesand gives us a chance to seeinvisible radiation.

An infrared (IR) system basically includes a camera,equipped with a series of changeable optics, and a com-puter. The core of the camera is the infrared detector, whichabsorbs the IR energy emitted by the object (whose surfacetemperature is to be measured) and converts it into electricalvoltage or current.

However, the energy really detected (by the infrared de-tector) depends on the emissivity coefficient of the surfaceunder measurement and on the environment since a frac-tion may be either added as reflected from the surroundingseither absorbed by the atmosphere between the object andthe camera. To take into account these factors, calibrationof the system by simulating real operating conditions has tobe performed. The calibration function:

W = εA

eB/T − C(1)

relays the real amount of detected energy fluxW to theemissivity coefficientε and to the surface temperatureT ofthe object through the calibration constantsA, B andC whichtake into account spurious quantities of energy from, or to,the environment.

Infrared thermography is a two-dimensional, non-contacttechnique of surface temperature mapping which can besuccessfully exploited in many industrial and/or researcherfields, amongst others meteorology, environment, medicine,architecture, engineering where the temperature (or surfacetemperature) represents a key parameter.

2.1. The use of infrared thermography for non-destructiveevaluation of materials

Infrared thermography may represent a very helpful toolto be exploited for the assessment of manufacturing proce-dures as well for non-destructive evaluation of either end

products, or parts in service. In fact, an infrared camerashould be (if optical access is reliable) enclosed in anytemperature-dependent production line to control efficiencyof cooling systems during material shaping (extrusion, jetmoulding) where prompt cooling is advisable to avoid for-mation of air bubbles. Again, an infrared camera may beused to monitor temperature levels and uniformity of dryingchambers for production of paper, or of via-silane-graftingcross-linked polyethylene.

Basically, two different approaches are possible fornon-destructive evaluation: traditional pulse thermography(PT) and modulated (or lockin) thermography (MT).

The analysis of subsurface features in solid objects bymeans of PT generally requires heat energy to be transferredto the object in the active mode and mapping of surfacetemperature in the transient heating (cooling) phase. Depth,size and thermal resistance of defects can be estimated byprocessing the signal[1–4].

MT uses thermal waves, instead of pulses, and the re-sulting phase image gives a direct indication (without trou-blesome post-processing image procedures) not only of thesize but also of the depth of the damaged zone, and so itseems promising in terms of reduction of time and costs.The basic concepts of lockin thermography, first describedby Carlomagno and Berardi[5] were later developed bymany researchers[6–10]. Basically the thermographic sys-tem is coherently coupled to a thermal wave source which isoperated in such a way that a sinusoidal temperature modu-lation results. The modulation is generated by a non-linearelectrical signal produced by the lockin module which hasa waveform table for this purpose. The heat source hasto be calibrated (for each frequency) to ensure that thetemperature waveform is really sinusoidal.

In the lockin analysis the system collects a series ofimages and compares their temperature extracting the sinu-soidal wave pattern at each point of the image. In a homoge-neous semi-infinite body, the periodical generation of heatat the surface (depthz = 0) results in a (time dependent)thermal wave, which in one dimension, is given by[6]:

T(z, t) = To exp

(− z

µ

)expi

(�t − z

µ

)

= T(z) expi[�t − φ(z)] (2)

whereµ is the thermal diffusion length:

µ =√

α

πf(3)

with α thermal diffusivity andf = �/2π wave frequency;the termT(z) represents the decay with depth of the thermalwave amplitude whileφ(z) is the phase shift. The depthrange, for the amplitude image, is given byµ while themaximum depth that can be inspected, for the phase imagepractically corresponds to 1.8µ [9].

In the MT technique the behaviour of the thermal wave isdriven by the material morphology. Generally, the thermal

1134 C. Meola et al. / Journal of Materials Processing Technology 155–156 (2004) 1132–1137

wave propagates inside the thickness of the object and getsreflected when it reaches parts where the heat propagationparameters change (inhomogeneities). The reflected waveinterferes with the surface wave giving rise to an oscillat-ing interference pattern, which can be measured in terms ofamplitude (amplitude image), or phase angle (phase image).The phase image is undisturbed by emissivity variations, andnon-homogeneous heating and allows for a deeper analysis(up to 1.8µ). Herein, results will be presented as phase im-ages, which may be shown as colour, or black and white,pictures on the computer monitor; a change of colour (orgrey tones) means a change of phase angle.

The surface pattern strongly depends on the material char-acteristics, i.e. thermal diffusivity, porosity, density. Thus,the phase angle may be assumed as a characteristic param-eter of the specific material; obviously, averaging the phaseangle value over a certain area allows for a reduction of theinstrument noise.

Of course, the possibility to clearly visualise defects ina material depends on the difference between the thermalcharacteristics of the basic material and those of the likelyinhomogeneities. If such difference is too small, the imagecontrast is not good and worsens for deeper defects becauseof the thermal diffusion through the material. On the otherhand, materials of very high thermal conductivity (diffusiv-ity) are subjected to a too rapid heat propagation with conse-quent weak image contrast. In these cases, instruments withhigh sensitivity, such as the focal plane array detectors, areadvisable.

3. Applications

The use of lockin thermography for the characterizationof materials is here described. Several different materials areconsidered. The choice has been made to include materials,which are widely used in different fields such as aeronauti-cal, automotive, architectural, electrical, etc., and which be-long to three main categories such as plastics, compositesand plaster.

3.1. Test procedure

The experimental set up includes the specimen, a heatsource (quartz lamp of 1 kW) and an infrared camera[11].The lamp is positioned at about 1.5 m from the specimensurface for tests carried out in laboratory, and at about 3 mfor tests in situ on art treasures; in the latter case a greaterdistance is chosen to avoid overheating. The position of theinfrared camera instead depends on the minimum focus ofthe employed lens and on the desired field of view.

The test procedure consists of acquiring phase imageswhile the specimen surface is thermally stimulated with asinusoidal heat flux The wave frequency for the Agema 900Thermovision coupled with the lockin option could be var-ied from 3.75 Hz (267 ms) down to 0.0037 Hz (273 s) in

15 intervals. The lamp is previously calibrated for each ofthe employed wave frequency, to ensure that the tempera-ture waveform is really sinusoidal. Generally, tests start at aquite high wave frequency at which, depending on the ma-terial diffusivity, only surface (or low depth) defects are vis-ible; and later on, to visualise deeper layers, the frequencyis decreased until the entire thickness has been inspected,or the minimum selectable value (f = 0.0037 Hz) has beenreached. The images are stored for further analysis.

3.2. Analysis of a multi-material specimen

First, to show the capability of MT to discriminate be-tween different materials, a multi-material specimen is con-sidered. The specimen is composed of a patchwork (over awood support) of small pieces of different substances, whichbelong to different categories from wood, which is found innature, to materials created in laboratory as plastics (PVC,Plexiglas, Teflon), or composites (a matrix of epoxy resinreinforced with either glass, or carbon fibres).

The phase image, taken at a frequency of 0.234 Hz, isshown inFig. 1; the different materials are labelled and in-dicated by arrows on the image. The frequency value hasbeen chosen to assure that (Eq. (3)), for all materials, thesurface, or the close subsurface, layer is visualized. Onthe whole, it seems that the different materials belong totwo main categories characterized the first, which includes:rubber, carbon–epoxy, wood, cork, by a lighter colour andthe second, which includes: glass–epoxy, Lexan, Plexiglas,Polystyrene and Teflon, by a darker colour; the colour forPVC is in the middle between dark and light.

For quantitative evaluation, the average phase angleφm ismeasured over the central zone of each material area; atten-tion is taken to exclude borders where interference effectsmay arise. These values are collected inTable 1 togetherwith the thicknesss of each sample and its thermal diffu-sivity α. α is either found in literature, or measured by theMT technique itself by following the procedure describedin [11]; this parameter is generally not available in literaturefor advanced composites because it depends strongly on thematerial characteristics, i.e. percentage of resin and fibres aswell as orientation of fibres.

Fig. 1. Phase image of the multi-material specimen,f = 0.234 Hz.

C. Meola et al. / Journal of Materials Processing Technology 155–156 (2004) 1132–1137 1135

Table 1Average phase angle and characteristics of tested specimens

Material Carbon/epoxy Cork Glass/epoxy Lexan Plexiglas Polystyrene PVC Rubber Teflon Wood

s (mm) 3 1 4 1 1.5 6 3 2 1.5 2φm (◦) 243 237 222 224.4 223.3 223 235.4 245 219 242α (cm2/s) (literature) 0.0016 0.00113 0.00124 0.0007α (cm2/s) (measured) 0.0045 0.004 0.00116 0.0011 0.0011 0.0011 0.0012 0.0046 0.0007 0.0045

As can be seen,φm increases withα. However, as animportant feature, it seems that the lockin technique is able todiscriminate between materials of close thermal diffusivityand which belong to the same family (i.e. Plexiglas, Lexan)and differ from each other by small percents of additives inthe compound.

3.3. Plastics

Plastic, which is the generic term used to indicate alarge category of materials created in laboratory, is nowthe most commonly employed material thanks to two mainfeatures that are the extremely light weight and the ap-titude to take complex shapes. Two variants will be hereanalysed polyvinylchloride commonly known as PVC andcross-linked low density polyethylene (LDPEX).

3.3.1. PolyvinylchlorideThis material is widely employed for the fabrication of

so many objects to cover a large slice of market. Of course,even if simply called PVC there are many variants of PVC,which include a percentage of different additives to satisfythe specific requirements. The material herein tested is thatused for the fabrication of pipes for the transport of liquids(sewage systems). A joint, as sketched inFig. 2, is obtainedfrom two commercial pipes of nominal diameter inner/outer100/110 in dark grey and 110/115 in light grey. The phaseimage taken forf = 0.039 Hz is shown inFig. 3. As can beseen, the two pipes, which comes from two different pro-duction cycles and include different percentages of additives(different colour), are characterized by a different phase an-gle (different colour inFig. 3). In addition, it is possible toclearly distinguish the mouth of the larger pipe where ob-vious material modifications occurred. The distribution ofthe average phase angleφm is plotted against thex directionnormalized with respect to the entire lengthL of the speci-men inFig. 4. For x/L ≤ 0.7, φm attains a value of about233◦ (light grey material), while it reaches a greater value of

Fig. 2. Sketch of the PVC joint.

about 236 over the second dark grey pipe; large variationsare also observed over the mouth of the larger pipe.

3.3.2. Cross-linked polyethyleneA specimen is made of a piece of cross-linked low density

polyethylene produced at the Megarad s.r.l. (Italy) and whichis generally used as insulation of low and medium voltageelectrical cables[12]. The material, under consideration, isproduced in the form of pipes of internal diameterD =15 mm and thicknesss = 3.5 mm, then they are enlarged toassumeD = 50 mm,s = 1 mm. When the electrical cable isinserted into the sheath this last is heated up to 200◦C to re-shape and assume its final form (D = 15 mm). For electricalpurposes and especially for buried cables, sheaths of highquality without any crack are required. The most commoncauses of defective material are due to: anot-goodmixingof the different species before entering the extruder; prob-lems within the extrusion process; cooling after extrusion;non uniform (or not suitable) cross-linking degree; pullingduring enlargement.

Fig. 3. Phase image of the PVC jointf = 0.039 Hz.

Fig. 4. Phase angle distribution.

1136 C. Meola et al. / Journal of Materials Processing Technology 155–156 (2004) 1132–1137

Fig. 5. Phase image of the cross-linked polyethylene specimenf = 0.117 Hz.

To check for the ability of MT to follow material mod-ifications arising during the enlargement phase, the speci-men here analysed actually is enlarged over only half of itslength. The phase image taken forf = 0.117 Hz is shownin Fig. 5. An average phase angle value of about 250◦ ismeasured over the low diameter part, while a lower value ofabout 244◦ is measured over the enlarged part. The darkerline separating the enlarged part from the other one is prob-ably due to a pulling effect; local non-uniformities over thetwo parts may be ascribed either to a non-uniform distri-bution of forces during enlargement, or to local variationof material characteristics as a consequence of extrusion, orcross-linking.

3.4. Composites

A matrix of epoxy resin reinforced with carbon fibres,which is here simply named carbon/epoxy, is considered.The sample is obtained by superimposing several epoxyresin preimpregnated carbon fabric layers (0,90)f and is al-lowed to cure in a press at ambient temperature; the finalthickness is 2 mm. To study the influence of the surface fin-ish, a zone, which is about half of the entire specimen sur-face, is coated with a thin film of paint, which is employedfor aircraft surface finish. The phase image taken forf =0.469 Hz is shown inFig. 6; as can be seen, the coated partdisplays a darker colour with respect to the non coated one.For quantitative evaluation, the phase angle is averaged overeach part; a value of 227◦ is measured over the coated part,while a greater value of 235◦ is measured over the other one.This difference is justified by taking into account two mainthings:

• the employed paint has thermal diffusivity lower than thatof the naked material;

Fig. 6. Phase image of carbon–epoxy forf = 0.469 Hz.

• the phase angle, as demonstrated before inSection 3.2(Table 1), increases with the thermal diffusivity.

3.5. Plaster

MT may be used also as a preventive diagnostic instru-ment in civil architectonic structures as well as in the restora-tion and maintenance of cultural heritage. In particular, thistechnique may be employed to discern detachments, cracks,water infiltration as well as to recognize the fine differencebetween concrete, or plaster layers made during restorationin different times.

Two in situ analyses are here presented. The first one re-gards frescoes in the Duomo of Sarno (Italy), while the sec-ond one is concerned withdancerpaintings of Ruvo’s tombwhich are kept in the Archaeological Museum of Naples(Italy).

3.5.1. Sarno frescoesAn in situ analysis of frescoes covering the apse of the

Duomo of Sarno (Italy) is performed, using both PT and MTtechniques, within the restoration project after the flood in1998. Non-destructive evaluation is performed at an interme-diate stage to use infrared thermography as both diagnostic(before restoration) and validation (after restoration) instru-ment. A phase image (MT), taken using a close-up lens andfor f = 0.039 Hz, over a restored zone is shown inFig. 7.As can be seen, the viewed area appears subdivided into tworegions of different colour (different phase angle) which in-dicate different material characteristics. Discussing with thearchitect, involved in the restoration project, we learnt thatyears before the wall had suffered from water infiltrationand had been repaired with injection of new concrete; morerecently, also the fresco was restored. Thus what MT visual-ized is the effect of ageing on concrete. This is an importantfeature since it allows, obviously through preliminary cali-bration, for a time classification of materials and in turn ofart treasures.

3.5.2. Dancer paintingsSome panels coming from the Ruvo’s tomb and kept at

the Archaeological Museum of Naples (Italy) are analysed.A fourth of one panel area is shown in the phase imagetaken for f = 0.019 Hz and reported inFig. 8. At firstsight, the difference of colour may be associated with eitherdetachments, or different materials characteristics. A more

Fig. 7. Phase image of Sarno fresco forf = 0.039 Hz.

C. Meola et al. / Journal of Materials Processing Technology 155–156 (2004) 1132–1137 1137

Fig. 8. Phase image of the dancer painting forf = 0.019 Hz.

in depth analysis through a synergic use of both PT andMT may help understanding the nature of the material dis-continuity. In fact, PT visualizes material inhomogeneitiesthrough temperature variations and is affected by local vari-ations of the emissivity coefficient as well as non-uniformsurface heat transfer, MT instead measures phase anglevariations and overcomes the drawbacks posed by PT andallows for a fine discrimination of materials properties. Ofcourse, accurate preliminary calibration is needed with thecompilation of a table of phase angle values for differentmaterials as well as the variation of the phase angle forvarying the material properties. Then, MT may be used toacquire information about either the kind of materials usedin the past, or to date materials.

4. Concluding remarks

The use of lockin thermography in the characterization ofmaterials has been described through several applications.The main feature is the ability of this technique to discrimi-nate between materials very similar like Plexiglas and Lexanand this may be exploited to evaluate modifications whichcan arise under ageing, or special treatments.

In more details, within this technique, each material is as-sociated with a phase angle value that may be assumed asa characteristic parameter; any delta-phase angle is a symp-tom of variations in the material characteristics. Thus, it ispossible to recognise modifications arising during specialtreatments such as cross-linking, and is also possible to ac-quire information about the degree of the treatment and itsdistribution over the surface and inside the material.

The investigation has been restricted to very few ma-terials, but they were chosen to demonstrate the capabil-ity of the technique to deal with the likely requirements

of different fields. In aeronautics, composites are replac-ing metals and advanced composites are continually un-der development and this poses the problem of effectivenon-destructive evaluation. The performance of electricmains strongly depends on the condition of the cableinsulation and so an accurate control of sheaths, termi-nations and joints before the cable is buried may preventfrom unexpected breakdown. The possibility to see underfrescoes, or paintings, without harmful effects on the arttreasures certainly helps for the conservation of the culturalheritage.

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