Experimental study on active infrared thermography as a NDI tool for
Composites: Part B 45 (2013) 138–147 Contents lists available at SciVerse ScienceDirect Composites: Part B journal homepage: www.elsevier.com/locate/compositesb Experimental study on active infrared thermography as a NDI tool for carbon–carbon composites Liu Junyan a,⇑, Liu Liqiang b, Wang Yang a a b School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, PR China Jiamusi university, Jiamusi 154007, PR China a r t i c l e i n f o Article history: Received 2 January 2012 012 Received in revised form 14 March 2012 Accepted 1 September 2012 Available online 16 September 2012 Keywords: C. Finite element analysis A. Carbon–carbon composites (CCCs) D. Non-destructive testing a b s t r a c t Active A Ac Act c ive ve infrared ve in iinf nffrra n rar arred a d thermographic ther he he errmogr mo raphic technique mog techn hni niique qu ue was ue w used use us u se sed as as a non-destructive no on n-d n-d -destructive inspection in (NDI) tool for carbon– car arrbon a bo b on (C/C) (C/ ((C C/ C/C)) composite comp o osite with subsurface sub bsur s face defects. su defec fec eccts tts. s A serial seri se ri l of ria o representative rep rep re epresentative carbon–carbon composite test carbon samples, experimentally and analyzed using sam am mpl ple p l s,, consisting consisting g of cutoutss and an a nd cracks crracks as defects defe efe fect ecct cts tts were wer erre investigated e iinves in nv vesstig t gated g exp finite element thermography technique fini niitte ee lement method le met etth ho hod od od (FEM). ((F (FE FE F EM). Both Bot B oth transient trrans anssien i t pulsed ie pulse se ed thermography the errrm erm mogr o aph ap phy and ph and lock-in lo wer re iin vestigated. The The he capability capa ap ap pa ab bil illity ity y off two two o techniques techniques to o detect detect subsurface subs bsurrface defects bs d were investigated. of C/C composite and to provide that active thermographic pro ro ovid i e information about ab abo bout bo ut the the location loca ca atio tio on of o defect defect was analyzed. ana na alyz ly ed. d. Results d. Result Res ult u ltts indicate ind technique composite. te ecchni e h que is available for fo orr the the subsurface subsurf rfface acce a ce defect def d de effec e ect e c detection d ection det on n of carbon–carbon carb arb arbon bon– n– n –car c b Crown Ltd. All rights reserved. Crow wn Copyright wn Co op opy py pyrri rig iight Ó 2012 2 Published Pu ubl bliish bli ssh h he ed by b Elsevier E 1. Introduction te ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om d thermographic therm rmog rm graph rra aph p icc measurements me m easurrem men ents t and an nd In the last decades, infrared use s d in in the the e inspection iin nsp s ection n of of subssu ub b-investigation techniques have been used surface defects and features, thermophysical thermoph physsic ph ical al properties, p op pr ope errties, coating on, and so o on on [1 1,,2 2]. D urin ur ing infrared infrared thickness and hidden corrosion, [1,2]. During ere are two approaches appr ap p roa oach chess that th hat at can can be ca be thermographic inspections, there approa acch h iss generally ge g en ene ne erra all lly used lly used se ed used: passive and active. The passive approach at are at different temperature te emp m erat attur a ure comco com om min the research on materials that ap pp pro r acch, h an pare with the ambient, while in the case of active approach, ch as optical flash lamps, halogen hallo loge gen external excitation source, such onic vibration, vibration hot and cold air gun heat lamps, mechanical ultrasonic is employed with the intention of inducing thermal contrast [3– 5]. In the active approach of the infrared thermographic nondestructive testing and evaluation (NDT&E), pulsed transient thermography and lock-in thermography are the most commonly used approaches. In pulsed transient thermography, the surface of specimen is pulsed heated (from ms to s) by different heating sources and the resulting thermal transient at the surface is monitored using an infrared camera. The duration of the heating pulse depends on the thermal and physical properties of the materials, as well as its thickness [6]. The heat flow into the sample is altered in the presence of a subsurface defect or feature, creating temperature contrast at the surface that is recorded by an infrared system [7,8]. ⇑ Corresponding author. Tel.: +86 451 8640 3380; fax: +86 451 8640 2755. E-mail address: [email protected] (L. Junyan). In n lo ock c -in thermogra ap ph hy,, the the surface th sur urfac is heated periodically, typlock-in thermography, i al ic ally y using ussing sinusoidally u sinusoida all lly modulated mod dulatted halogen ha ically lamps, and the tempe era rature re e modulation on n induced indu in duce du ced at at the surface of the inspected perature comp mpon mp o en nt from from m the the he outside outs ou uts tsid id de e propagates propaga component can be looked as a harmoni niic thermal the erm r all wave wa w ave e [9 [[9,10]. 9,,1 10].. Thermal Thermal images of the sample are monic reco orrd ded ed throughout th hrro oug ugho out ut the tth he heating heating period, per recorded which lasts for a least on ne full fu ulll excitation exc xcit itattiio on cycle. cyc y le l . The The phase is modified by thermal waves one comi co m ng back mi ba b ack ck from ffrro om m inside in nsi side of the material, mater coming and it can be extracted by post-analyzing po p ost stt-a -a an na aly lyzing ng g the the recorded images th image data [11,12]. The lock-in by tth herrmo m gr grap ap phy hy process facilitates a number num thermography of advantages for subssurface su urf rfac ace defect defect detection, and experimental experim artifacts due to environmental reflections, reflections local non-uniform non-unif emissivity of sample surface, air turbulence, etc., are significantly reduced [13]. It is an objective of this work to investigate subsurface defect detection of carbon–carbon composite using both pulse transient thermography and lock-in thermography. Finite element analysis and experimentally trails of through skin imaging of subsurface defect fixtures are presented in this paper. 2. Finite element analysis In this section, finite element method (FEM) is used to investigate the surface temperature variation of C/C composite specimen under the pulsed heat and modulation heat excitation conditions. The incident heat flux function was described on the front face of C/C composite specimen as following: (q q ðx;y;0;tÞ ¼ in 1359-8368/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.09.006 A ½1 cosð2pftÞ 2 qmax ½1 signðt t c Þ 2 Lock in thermography Pulsed thermography ð1Þ 139 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 with 8 > < 1 0 6 t < t c signðt tc Þ ¼ 0 t ¼ tc > : 1 t > tc ð2Þ where qin(x, y, 0, t) is the incident heat flux function, qA the amplitude of modulated heat flux, f the modulation frequency, qmax the peak value of pulsed heat flow, and tc is the pulsed heating time. It is well known that the temperature distribution T(x, y, z, t), in the considered anisotropic material satisfies the following thermal conduction partial differential equation [14]. @Tðx;y;z;tÞ @ @Tðx;y;z;tÞ kxx ðx;y;zÞ @t @x @x @ @Tðx;y;z;tÞ @ @Tðx;y;z;tÞ þ þ ¼ 0 ð3Þ kyy ðx;y;zÞ kzz ðx;y;zÞ @y @y @z @z qðx;y;zÞ cp ðx;y;zÞ D6 D5 he p re ese sentt investigation, a pulsed pul In th the present thermal source and a sinusoid so idal al thermal the th ther errma rmall source sou ourc rce were used to excite e soidal the surface of a C/C composi siite e specimen speccim sp men e with wiith w th cutouts on the t posite opposite side, respectively. Th he response res espo p nsse of of thermally the the h rm rma allly excited surface was recorded simultaThe n ously using ne ussiing n an an infrared iin nfrrarred ed camera. Real R neously time analysis of both signals was na w s performed wa per errform form fo med ed to t determine de de nals the associated phase angle usiin us ng available available lock-in lock lo k--iin modulus. mod dul ulus lu . using Fou Fo urr C/C composite com mpo posite specimens spec eccim mens were manufactured. Circular Four cu uto tout u s of of varying varying depth de ept pth h and and n diameter diamet were manufactured in specdi cutouts en e n 1# 1# and an a nd 2#, 2#, and d specimen spe peci cciime men 3# 3# and a 4# were formed SiC coating imen n the the surface th su su urrface of spec cim me en n in in much m ch higher temperature environmu on specimen ment by chemical vapor vapo or reaction rea eacttio ion on (CVR), (CV but this method is inclined ment o generate cracks in the th he specimen. spe peci cime ci me m en. The idealized shape of a circuto defect was chosen in n thiss investigation invesstig in larr defect to illustrate the effects of geo ge om metry on observed ed d thermal tth herrm ma al response. resp geometry Fig. 3 shows the geometrry of of the the test specim im men u us sed e iin n this th study. An integrate pulsed try specimen used the th errmo m graphy and d lock-in locckk in n thermography therrmogr th thermography system employing a midwave ele length infrared infrar arred ed camera ca am me era (SC7300 (SC73 wavelength 2.5–5.0 lm with internal lo ock c -in n unit unit by y FLIR FL LIIR ATS) AT TS S) were we erre utilized utiliz for the experimental work. lock-in Th he sschematic ch hem e attiicc of of general ge g enera ne n er l test test arrangement arran The is presented in Fig. 4, an nd tthe he in he nfr frar ared thermographic th her ermogr ogr g aphic system syst and infrared is a portable non-destructive evaluation tiv ti ve ev va allua uatiio on n system syst sy stem m with w th a flash heat device and a modulation wi h he eat a exci ciita attiiion on o n apparatus. appa ap paratus. The flash heat device used an pulse elecpa heat excitation ttr ric ic power po p ow we er to to provide pro rovide power output outpu 4.6 KJ in 2–5 ms from lamp, tric a nd a tr trig ig gg ge er was used to control the flash lamp switch, which and trigger e en na ab blle es the infrared camera to simultaneously simu enables record the response of thermally excited surface at the beginning of pulsed thermal excitation. The modulation heat apparatus used two 1 KW halogen lamps to provide the thermal excitation, and a Protek 9031 function generator was used to control the amplifier to sinusoidally modulate the intensity of the lamps. To prevent direct reflection back into the infrared camera, the thermal sources were oriented D7 D3 D4 3. Experiments ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om where q(x, y, z) is the density ty in the domain, cp(x, y, z) the specific condu du d uccttiiv vit ity in in heat capacity of material, kd(x, y, z) is the thermal conductivity the domain (d = xx, yy, zz). If the initial temperature distribution of dom domain om o mai mai ain considered co on nsi sidere dered de re ed and an nd d ditions (heat flux, flux ux, continuity c nttiin con nu u uit iitty and and nd insui suin the necessary boundary conditions 3) will be solved. sso ollv ved d. However, H How ow o we ev ver er,, Eq. Eq. (3) is i lation) are presented, Eq. (3) lytical solution solu llu uti tion n due due to du to specimen spe peci c men struchardly solved using the analytical earizatio on boundary bou und ndar ary y and and anisotropic an nissot otro r pic ture complicated, non-linearization or C/C composite. cco omp m ositte. The The commercially Th comme errccia iall lly fifithermophysical properties for softtw wa are r , COMSOL COM MS SO OL Multiphysics Multiphyssic ics was wa ass nite element analysis (FEA) software, or of of thermal therrma th m l response response of C/C composco omp pos o selected to study the behavior ation on o n heat heat he at excitation ex xcitation conditions.. The The h ite under pulse and modulation COM MS SO OL was was applied to simulate e the the e heat transfer modulus of COMSOL tin ng problems pro rob bllem ems where where surface temperatem mperathermal non-destructive testing ace e features feat fe atur at ures ur ess (i.e. (i.e. defects, damages damag ag ges and ture signals include subsurface y interest. in ntere rest re st. It It is is intended intended forr calculating ca alc lculating discontinuity) are of primary an anisotropic ani niso s tr tro op pic thermophysical thermoph ph p hysical solid 3D temperature distribution in an e defects de efe fect ctts or or features. fea eat res. The eatu e heat heat transfer that may contain subsurface ing of of thin th hiin subsurface sub bsurface features, fe ea e atu ture r s, which modulus enables the modeling eated d uniformly uniifo forrm mly y or or non-uniformly n n-un no un niiffor orm mly y with wit ih allows the specimen to be heated ction description des e crrip iption tion ti n [15]. [[1 15]. a random mathematical function osite whose wh w hos ose e dimensional d dim i e im en nssiiional size siize was w s wa A solid of C/C composite ussed d ass the the geometry th geo eom me etr etr try model, 80 mm 40 mm 9.2 mm was used T e defect Th de effe ecct depth dep pth th refers rre efe fers rs to which consists of different size defects. The und surface and nd d the t e top th top p face ffa ace ce of defect. de d effe ecctt. the distance between the sound om 0.5 mm to 4.0 mm, mm, and mm and nd the the defect th de effec e t The defect depth varied from re and re an a nd meshing mes mes eshi hiing h ng thickness is set up 0.1 mm. The geometry structure t p are are re dididi grid were shown in Fig. 1. The meshing size and time-ste time-step ergence and stability of thermal model rectly influence on the convergence solution by COMOSL heat transfer modulus. For given geometry model in Fig. 1a, the automatically meshing was hardly implemented to mesh successfully, the swept meshing method was used to mesh the geometry model, and the geometry model meshing process was presented in Fig. 2. The choosing time-step was de- pended on the analysis time and sample frames. For pulsed transient thermography, the time-step was set to 0.02 s for the simulation on the duration of 5.0 s. For lock-in thermography, the time-step was determined by the modulation frequency and analysis periods, and it was set to 2.0/(100 f). So, the above appropriate meshing size and time-step choosing, whish ensured the convergence and stability of thermal model solution. The thermophysical parameters C/C composite were measured experimentally, which the test specimen was manufactured as a round shape (diameters 12.5 mm and thickness 2.5 mm) and the Netzsch LFA457 made in Germany was used to measure the thermophysical parameters of different directions in the temperature range from room temperature to 1000 °C. The thermophysical parameters shown in Table 1 and simulation parameters shown in Table 2 were used for the finite element analysis. D2 te D1 80 40 (a) (b) Fig. 1. The geometry structure and finite element grid: (a) geometry structure and (b) finite element meshing grid. 140 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 Top layer solid Dividing Defective layer solid Swept meshing: 1. Number of element layers 2. Meshing size (extra-fine, finer, fine, normal, coarse, coarser, and extra-coarse) Y Matching Close? N Bottom layer solid Fig. 2. The meshing grid process and matching. Table 1 Thermophysical parameters used during ng modeling. Component C/C composite Air Parameter m3) Density q (kg/m Special Sp pec eci cciia all heat heat att c (J/(kg°C)) (J/ (J JJ///(kg (kg ((k kg°C)) k C))) 1955 1.1774 19 1 900 1900 1005.3 Thermal diffusivity d a 106 (m2/s) Thermal h conductivit conductivity vit itty k (W/ ((W/(m /(m (m °C)) °C)) C))) )) x y z x y z 60 60 60 60 0. ..0 026 0.026 45 1 16.2 16.2 21.9 12.1 Table 2 Simulation parameters used during modeling. odellliing. Pulse transient thermography Lo L Lock-in occk-in thermogr o thermography ra ap phy p Heat time (s) End time (s) s) Time T m step (s) Tim Modulation Mod M ulation fr ffrequency re en requ nccy (Hz) n Analysis periods number Analys lys yssis p y eriods eri o n um Sample numbers 0.002 0.003 5.0 5.0 0. 0.0 .0 02 0.02 0.0 02 0.02 0.05, 0.10, 0 0. .15 5 0.15 0.20, 0.50, 0 .08 8, 1. 8, ..0 0 0.08, 1.0 2 100 int nto friction fri r ctio on such suc u h as as cracks, cra acks, delamination, del ted into and disbonding In this thi hiss study, stu tud dy y, the the he ultrasound ultras asound wave as wav stimulation whose modu[17].. In on n frequencies freq eq qu ue enc ncie ie es were w wer ere varied er varied from 0.5 0. down to 0.08 Hz was used lation to detect de d ete tect e cracks crracks accks a ks of of specimen sp pec ecimen 3# and 4#, 4 and the infrared camera to wa w as used usse use ed to to record rec e orrd the the surface temperature tempe was with the frame rate of 37 37 ffr ram ames ess//ss during during the acquisition duration of 2 modulation of frames/s pe p eri riod ods. periods. te ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om on either side of the specimen.. The inf infrared nfra nf r re ed camera cam amer erra er a uses uses a cooled c olled co witth a focal foca fo cal plane pla ane ne array a ra ar ay pixel InSb detector array producing image with d has a quoted quote te ed thermal the herm rmal rmal a sensitivity se ens nsiittiv iviitty of of format of 320 (H) 256 (V) and re over 25 °C. A sstandard tand ta nda nd arrd 50 50 mm mm fofooless than 25 mK for temperature investigat attio on with wit ith the the he camcca ammcus length lens was used in the current investigation enco omp mpassi as ing as g the the th era positioned to fully utilize the field of view encompassing ples were positioned vertically verticall lly with ll wit ith entire specimen. The test samples the sound side of the plate exposed to the stimulation source. For pulsed transient thermography, the heating time was set to 0.003 s, and the infrared camera frame rate was set to 59 frames/ s for the duration time of 5 s. For lock-in thermography, a range of modulation frequencies was used to interrogate C/C composite samples ranging from 0.2 Hz down to 0.05 Hz. The infrared camera frame rate was set to 47 frames/s with 2 numbers of modulated periods (acquisition time) for each frequency roughly. In this study, the ultrasound lock-in thermography (ULIT) was used to inspect the test specimen 3# and 4#, the external excitation is provided by an ultrasonic wave transducer (sonotrode) of 19.8 kHz resonance frequency drive by a specific amplifier/function generator, and Fig. 5 illustrates the experimental setup. The sonotrode was directly positioned on the edge of specimen, rigidly coupled to the sample, and as far as possible contact with the edge by an air cylinder. The maximum output power of PZTs is about 1.5 KW, and it produced ultrasounds wave within sample in this case of experimental investigation, which would create local friction between the defect edges, those would be converted into heat sources detectable by the infrared camera [16]. This mechanism concerns only certain defects involving edges that can be submit- 4. Results 4.1. Pulsed transient thermography In this section, only the surface temperature contrast was used to investigate the defect detectability using pulsed transient thermography. The surface temperature difference DT(x, y, 0, s) at each pixel of thermal image between defective area and the healthy area was defined by Eq. (4.a) and its normalization DT(x, y, 0, s)N was defined by Eq. (4.b). DTðx; y; 0; sÞ ¼ T d ðx; y; 0; sÞ T h ðx; y; 0; sÞ DTðx; y; 0; sÞN ¼ fmax½DTðx; y; 0; sÞ DTðx; y; 0; sÞg fDTðx; y; 0; sÞ min½DTðx; y; 0; sÞg ð4:aÞ ð4:bÞ where T(x, y, 0, s)d is the temperature of pixel (x, y) of the defective area and T(x, y, 0, s)h is the temperature of pixel (x, y) of the healthy area. 141 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 8 80 10 100 50 80 4×Φ6 4×Φ3 4 3 1.2 1.0 1.2 1.5 1 4×Φ1 2 10 3×Φ6 ((a)) ((b)) 10 80 80 80 8 8 80 80 (c) (d) Fig. Fig ig g. 3. 3. Geometry Geo eo e omet m ry y structu structure ure e of of C/C / composite p e sp specimens: p cciime pec men m e s:: (a) ( ) 1#,, (b) ( ) 2#,, (c) ( )3 3# # and an nd ((d (d) d) 4#. 4# #. Fuunc ncti tion Generator Gen ner e at ator aand nndd Power Pooow werr Amplifier Amppliifi f er Function Puls se el eelectric lec e tr tric ic ppower ow wer Pulse Infrared ared Camera and In IInternal nte t rnal all Lock-in Unit Test specimen Modulated excitation source ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om Pulse excitation source Sim Si muuult ltaan ltan neo neo eous Trigger Tri rigg gger gg Simultaneous Acquisition and analysis system Fig. 4. A schematic of the experimental arrangement used to perform both lock-in thermography and pulsed transient thermography. te The temperature variation curve of FEM and experimental measurement during pulsed heating time of 0.003 s are plotted in Fig. 6. Fig. 6a plots the curve of surface temperature variation with time. Also, Fig. 6b plots the curve of the logarithmic value of temperature log10 (T) with the logarithmic value of the time. It is found that the surface temperature of FEM is in good agreement with the experimental measurements, especially, during the duration of 1 s after the pulse flash excitation. However, over the time of 1 s after the pulse flash excitation, the surface temperature is reduced to a constant value due to the cooling course. Thus indicates that in this presented investigation, the FEA by COMSOL heat transfer modulus is available for being allowed to investigate the behavior of thermal response of C/C composite using pulsed heat excitation. The tem- 142 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 Amplifier/ function generator PZTs Sonotrode Air cylinder Reference sample Infrared camera Trigger Inner lock-in unit PC Fig. 5. Experimental set-up set-up up u p fo ffor o orr ult u ultrasound ltras rassound ra rasoun oun oun und llo lock-in ock ck-in in thermography. Fig. 6. Simulations of temperature contrast ntrast a and nd de experimental xpe x xp p pe erim men ent ntta n al measurements meassur ure u rremen ments ffor orr defect o d ect defe ct d depth ep ptth off 1.0 p 1.0 mm: (a) temperature tem mp per pe eratu e ure e contrast cont ontrrras a t and (b) the logarithm of temperature contrast. Th he gray gra ay scale sca calle e of each eacch ea h image im mage has been adjusted to maximize The conttra rast between be etw we ee en the the he defect defe de ect ct and the sound so contrast material. In Fig. 8a, capttu ure red the red the he temperature temp mp pe errat atu urre contrast contrast image at the time of 0.017 s, detured fectts are are re not not observed no obse errved at all. While Wh fects the recording time is increa in crea cr ase sed to to 0.067 0.0 0.0 06 67 7 s, the defects at the th depth of 1.0 mm are obincreased e in n Fig. Fiig. 8b. The captured time of infrared camera is increased F served to 0.101 0.101 s, the defects at the depths of 1.0 mm and 2.0 mm become to i ibl as shown h i Fig. Fi 8c 8 and d d. d However, the defect depth visible, in reaches up to 3.0 mm, the defect that the size is diameter 3.0 mm is unable to be detected due to very small temperature contrast from Fig. 8c and d, and this presents in agreement with the FEM results. In this study, the temperature contrast image is only applied to detect the subsurface defects of C/C composite, and the temperature data can be analyzed by Fourier transformation (Phase Pulse Thermography-PPT) method [18], Thermal Wave Signal Reconstruction (TSR) method [19], etc., which have been shown to produce deeper defect detection limits. The temperature contrast at the sample surface provides sufficient information about the shape and location of subsurface defect, and the boundary of the defect results in maximum and minimum temperature changes that help determine its size and location. The slope curve of the normalized temperature contrast distribution can be obtained by means of the differential calculation along the x or y axis, where the differential normalized temperature contrast DDT(x, y, 0, s) in Eq. (5) is calculated by subtraction of adjacent normalized temperature contrast. te ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om perature and its logarithmic value lue at the e time tim me of of 0.1 0.1 1 s provide pro r vide maximum difference between defective ctive region n and and healthy hea alt lthy hy y region regi re gion o from Fig. 6, and this record time is suggested uggested to detect de ettecct the th he e defect de d efe ectt of of C/C C/C C/ composite using the surface temperature mperature image e of o pulsed pu puls uls lssed ed e d transient ttrra an nssiien entt thermography. func fu n ttiion The temperature contrast off FEA over the defects as a function sed excitation time of 0.002 s and d of defect depth with the pulsed 0.003 s are illustrated in Fig. 7. It can be seen form Fig. 7, the temperature contrast is decreased in an almost exponential manner with defect depth increasing. As we all known that the thermal sensitivity of infrared camera is about 0.025 °C for environment temperature about 25 °C, and we can use this value to estimate the limit of defect depth detectability using pulsed transient thermography for the heating time of 0.002 s and 0.003 s. We assume that the temperature contrast between defective area and healthy area is over two times of thermal sensitivity of infrared camera, and then the defect is allowed to detect by the temperature contrast image. When the defect depth reached up 2.0 mm, the temperature contrasts over all size defects are below the 0.05 °C, and these defects cannot be detected using temperature contrast image with pulsed heating time of 0.002 s from Fig. 7a. While, the defects whose size are over the diameter 3.0 at the defect depth of 2.0 mm are detectable using the temperature contrast image with pulsed heat time of 0.003 s from Fig. 7b. Fig. 8 shows the specimen 2# examples using the temperature contrast image at the record time of 0.017 s, 0.067 s, 0.083 s and 0.101 s. 143 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 Fig. 7. Simulations of temperature contrast as a function of the defect depth: (a) pulsed heating time of 0.002 s and (b) pulsed heating time of 0.003 s. Fig. 8. Temperature contrast image mage of p pulsed uls u ul llsed tther thermography her errmog mog gra gr rrap a hy with p pu pul pulsed ulsed ul sed se ed heatin heating ng ti time me eo off 0. 0 0.003 .0 .003 003 0 s at the he e tim time e of o of: f: ((a f: (a)) 0. 0 0.017 .01 17 s, (b) 0 0.067 s, (c) 0.083 s and (d) 0.101 s. DDTðx; y; 0; sÞN jx ¼ DTðx þ 1;; y; 0; 0; sÞN DTð Tðx; ðx; y; 0; sÞN ðð5Þ 5Þ 4.2. 4.2 4. 2. Lock-in thermography thermograph ph p hy In this following ssection, ection ec n, tthe e phase pha angle contrast was used to In in nv ve est stigate the defect defe ecctt detectability de ette ect ctab bility using lock-in thermography. investigate x, y) at each pixel of thermal image T Th he p h se angle difference ha dif iffffe erencce Du((x, The phase b be etw wee een defective ve e area arrea a a and and the an the healthy he between area was defined by Eq. (6 6.a . ) and an nd iits ts normalization no orm rma ma alliz izat atio at atio ion Du(x, y)N was defined by Eq. (6.b). (6.a) Duððx; x; yÞÞ ¼ ud ððx; x; y yÞÞ uh ððx; x; yÞ ffmax½ ma m ax½Duððx; x; yÞ Duðx; yÞg fDuððx; x; yÞ min½Duðx; yÞg y ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om The differential normalized tem temperature mpe pera rattu ure co ur ccontrast ontrast distribution on np pr proro o-vides maximum, minimum and zeros values, dz erros os v a ues, the distance between alu be ettw ween characteristic points (maximum value and value) used mum m val lue a nd d minimum value e) iiss u sed for defect size determination and zero n an nd tthe he ze he z ro o point is used ffor or location or lo of defect determination. Fig. 9a a shows sho ows ws the th he normalized normalize ed temperature tem emperature eat ttime ime eo off 0. 0 003 s at the 003 00 e ccentral entral line of contrast induced by pulse heat 0.003 ers ar a re 6 ..0 0 mm m and 3.0 0m mm m (seen in the defects whose diameters are 6.0 ons ca an be an be d de ete errm min i ed fr rom om th he m axiax Fig. 8d). Although the locations can determined from the maxiure cont nttra r stt (s ((seen see e n in in Fig. 9a), 9a) a), it it is is diffidif iffi fi-mum normalized temperature contrast ct size. The Th he differential diifffe ere rent ntial normalized normaliized cult to evaluate the defect db yE q.. (5) q ((5 5) calculation c lccul ca ula attio ion is illustemperature contrast profile obtained by Eq. tions of defects defe fe ecctts can ca an n be be quantitatively qu quan uan anttiita tati tive vely y trated in Fig. 9b. The locations oints from Fig. 9b. 9b. b For For the the he size siz si ze e of diamdiam di amdetermined by zero value points an nd 3.0 3.0 mm mm deep, de de eep ep, the ep the he eter 6.0 mm defects that are 1.0 mm, 2.0 mm and ated to be 20.14 mm, 44.54 44 4 4..5 54 mm m and and d location of defect is calculated e of the specimen 2#, and for the the size th siz iz e 66.45 mm from the left edge 1 0 mm, mm and 2.0 2 0 mm deep, deep of diameter 3.0 mm defectss that are 1.0 the location of defect is calculated to be 20.14 mm and 44.75 from the left lef e t edge edg dge of the e specimen. ssp peccim men en.. The The h distance d the between the maximum d minimum min inim inim imum um differential differe en e nti t al al n no orm malize temperature contrast is caland normalized cu ulla ate ted as as the the h size of the the defect, de effect ectt,, which ec whi has been marked in Fig. 9b. culated Duððx; x; y yÞÞN ¼ ð6:aÞ ð6:bÞ te where u((x, x, y) y)d is the phase angle of pixel p (x, y) of the defective area pi (x, y) of the healthy area. a an nd u(x, y)h is the phase angle of pixel and Fig. 10 shows the phase angle contrast Fig c of FEM calculation as function of modulation excitation frequencies for the size of diam- Fig. 9. Temperature contrast normalized and its differential curve: (a) the curve of temperature contrast normalized and (b) the differential curve of temperature contrast normalized. 144 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 observed that the edges of the defect become increasing more diffuse as the defects become deeper, and this is similar to the observation of metal specimen inspection using lock-in thermography by Wallbrink et al. [20]. The present phase angle measurements have an associated level of background noise that will interfere with the detection of defects. The ability to detect a defect requires that the phase angle change caused by the defect is greater than the background noise. A limitation criterion was proposed in Ref. [20], and we used it to analyze the detectability of defect using lock-in thermography. Eq. (7) sets a limitation criterion on the detectability of defects associated with phase angle contrast measurements, where DL represents the limiting phase angle contrast, and two times standard deviations of phase angle in the defective region and the healthy region were used to represent 95% confidence intervals for measurements of phase angle contrast in the sound material and over the defects. DL ¼ 2rD þ 2rH DL ð7Þ where wh w he erre rD is i the the standard th sta tand n ard deviation of phase p in the defective region, the he standard sta tand nda nd arrd deviation deviation of phase ph in the healthy region. an a nd rH iss tth and When W Wh en n the the local th lloc occal phase o ph has ase angle angle contrast contr over a defect is greater than DL, DLL, then th he en a defect de efe fect is fect is detected d tected and de an below DL a defect is not detectable. Fig. detectability of dede ete tectable. Fi ig g.. 14 4 represents rre ep prres e en nts ts experimentally experim ffects fe ectts with wiitth various vario ouss defect de efe fect ct depths dep de pths pt h using DL as the detectable phase angle an a ng glle ccontrast. on ntr t ast. IIn n Fig. Fiig. F g 14a, for the e si ssize z off diameter ze diam di ameter 6.0 mm and 3.0 mm defects, am the ph hase ha se angle angl an g e contrast contras asst is is over ov ve er the the he DL where the defect depth is by phase 3.0 m m a he h e m odulati tiion o ffr rreq eq que ency of 0.05 Hz, which indicates mm att tthe modulation frequency tthat th hat the tth he size size of of diameter diamete te er 6.0 6.0 0 mm mm and an 3.0 mm defects that are 3 .0 m m deep are detec ctab ble le at the the modulation frequency of 3.0 mm detectable 0.05 5 Hz. H . In Fig. 14b, for tthe Hz he size he i off di diameter 6.0 mm defects, the phase ph e angle angle contrast is over over the ov tth he DLL w phase where the defect depth is by 3..0 m ma on o n fr ffrequency eque eq uenccy o 3.0 mm att the modulation off 0.1 Hz, for the size of diame er 3.0 et 3.0 mm 3. mm defects, the he e phase phase asse angle a ang gle contrast co eter is over the DL where the th e d efe ef ecct depth is by by 2.0 0 mm mm at at the th modulation frequency of defect 0 0. 1H z, which wh w hich indicates indiccat ate ess that th hat at the the size size of diameter 6.0 mm defects 0.1 Hz, thatt are arre 3.0 a 3.0 . mm deep de d eep ep and an nd the th he e size ssiize of diameter d 3.0 mm defects that are 2 .0 0 mm m de eep ep ar a re e d etteccta e able at th 2.0 deep are detectable the modulation frequency of 0.1 H z. T he d ettec e e ta ab biilliitty y off defect will wil change depending on the Hz. The detectability m mo od du ula attiio on n frequency, freq fr eq e que uen nccy, y hence, h nce, the number he num modulation and size of the defects d ete ectted d will willll also wi allsso a o change. ch han a ge. detected T Th he phase ph ha ase s angle an ng glle contrast contrast at the sample sam The surface contains more iinformation in formatio io on a bout the shape and location loc about of subsurface defect. T Th he sslope lope curve of the normalized phase ph The angle contrast distribution can be obtained by means of the differential d calculation along the x or y axis, where the differential normalized phase angle con- te ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om eter 6.0 mm and 3.0 mm defects that the defect depths are different, and it can be used to select an appropriate frequency to provide the largest phase difference for NDI purpose. Frequencies at which the phase difference becomes zero are known as ‘‘blind frequencies’’, also, the excitation frequency should be selected less than 0.2 Hz, which is inclined to make more defects be detectable from Fig. 10. The modulation excitation frequency of 0.1 Hz can be used to obtain the largest phase angle contrast that the defect depth is below 1.5 mm, and the excitation frequency of 0.05 Hz is selected to provide larger phase angle difference that the defect depth is over 1.5 mm in this experimental investigation. The phase angle images of the specimen acquired using modulation excitation frequencies of 0.02 Hz, 0.05 Hz, 0.1 Hz and 0.15 Hz are presented in Fig. 11. The gray scale of each image has been adjusted to the maximize contrast between the defect and the sound material, and the specimen in Fig. 11 was oriented as in Fig. 4. In Fig. 11a and b, recorded with modulation excitation frequenhree defects of cutouts orien ntte ed are are re cies of 0.02 Hz and 0.05 Hz, three oriented th he vertical ve errttiica cal edge edg ge clearly observed, but for four defects located on the nd db b.. A he exc e xciitta xc a-of specimen 1# are not visible from Fig. 11a and Ass th the excitadefeccttss of of specimen sp pe ecciim men en 1# 1# bebe b etion frequency is increased to 0.1 Hz, all defects on frequency cy increasing, cy iin ncrrea easing sin si ng g, the th he e defects de d effe ectts ect come visible. With the excitation nable to be be detected det etec ecctte e ed in in Fi F ig. 11d. As located the vertical edge are unable Fig. d on the e v e ti er tica cca a all e dg ge o the size of defect that located vertical edge off specimen zontall a xis is onl xi nly 1 nl .0 mm, itt iiss lless ess es 1# (seen in Fig. 2a) along horizontal axis only 1.0 0 to 1.5 1.5 5 mm, m , and mm d this th leads to more more than the defect depth from 1.0 g horizontal ho orriizo zont nttal axis n ax xis due to the lower lo owe wer lateral thermal diffusion along gener erat er ates es very ve v ery y small small phase angle difdif ifmodulation frequency, which generates a and nd healthy nd he h ea allth thy area. area. So there exits an an ference between defective area 0..1 Hz 0 Hz to to obtain obtain the best phase e anann optimal excitation frequency off 0.1 sp pecim eccim imen en 1# 1# using lock-in thermother errmogle contrast for C/C composite specimen graphic technique. A over ov ver er the t e defects th d fu un nct ction of The phase angle contrast of FEA as a function n frequencies frreq e ue en nccie es of of 0.05 Hz z and and 0.1 Hz defect depth at the modulation are shown in Fig. 12. e rate e at a wh w whic hic i h the th he e phase e angle ang ngle conco onn As can be seen in Fig. 12, the which h decrea ea e ase s s iin na n almost alm lmost exponential exp xp pon onen e tiial al trast is affected by defect depth decreases an cimen 2# 2# examples ex xam ampl p ess using using phase us s anan manner. Fig. 13 shows the specimen n excitation excitatiion on frequencies frreq eque uenc ncie ies of ie of 0.15 0 gle images at the modulation Hz, 0.1 Hz and 0.05 Hz. he modulation n ffrequency r quen re qu uen ency cy of of 0. 0 15 1 5 Hz H z, In Fig. 13a, acquired with the 0.15 Hz, he modulation he m du mo dulla attiion on frefrefr edefects that are 1.0 mm deep are observed. As the .1 Hz, defects that are e 1.0 1..0 mm 1 mm and an nd d quency is decreased to the 0.1 ajor aj orit ity 2.0 mm deep become visible, as shown in Fig. 13b. The m majority of the defects are detectable in Fig. 13c, which was recorded at the modulation frequency of 0.05 Hz. However, in Fig. 13, we also Fig. 10. Phase contrast as function of thermal excitation frequency: (a) the 6.0 mm diameter blind holes and (b) the 3.0 mm diameter blind holes. L. Junyan et al. / Composites: Part B 45 (2013) 138–147 145 Fig. 11. Phase angle image of specimen 1# recorded at the modulation frequencies of (a) 0.02 Hz, (b) 0.05 Hz, (c) 0.1 Hz and (d) 0.15 Hz. Fig. 12. Simulations of phase se ang angle ng n g gllle e ccon contrast on ntras nt ra ast a ass a function of the defe defect efect ecctt depth: depth: (a) m modulation mo od odu o d du ulat lat atiio ion on o n frequency of of 0.05 0.05 05 5H Hz z and an nd d (b) (b) modulation m frequency of 0.1 Hz. ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om Fig. 13. Phase angle le e contra co contrast t ast ast ima as image ma m ag ge eo off th tthe he ssp speci specimen ecimen en 2# 2# rrecorded ecorrde ecorde eco rde rd ded d ed a at the t e mod m modulation odula ua atttiiio on n frequen frequenc frequencies enc nciie nc ies ess of: e f:: (a) ((a a) 0.1 a) 0. 0 0.15 5 Hz H Hz,, (b) 0 0.1 Hz and (c) 0.05 Hz. Fig. 14. Phase angle contrast of specimen #2 for defect diameter 6.0 mm and 3.0 mm as a function of defect depth at the modulation frequencies of: (a) 0.05 Hz and (b) 0.1 Hz. DDuðx; yÞN jx ¼ Duðx þ 1; yÞN Duðx; yÞN te trast DDu(x, y, 0, s) in Eq. (8) is calculated by subtraction of adjacent normalized phase angle contrast. ð8Þ The differential normalized phase angle contrast distribution provides maximum, minimum and zeros values, too. The distance between characteristic points is used for defect size determination and the zero point is used for location of defect determination. Fig. 15a shows the normalized phase angle contrast induced by the modulation frequency of 0.05 Hz at the central line of the defects whose diameters are 6.0 mm and 3.0 mm (seen in Fig. 13c). The locations of defects can be quantitatively determined by zero value points from Fig. 15b. The location of diameter 6.0 mm defects that are 1.0 mm, 2.0 mm and 3.0 mm deep is calculated to be 20.04 mm, 43.24 mm and 66.8 mm from the left edge of the specimen 2#, and the location of diameter 3.0 mm defects that are 146 L. Junyan et al. / Composites: Part B 45 (2013) 138–147 Fig. 15. Phase angle contrast normalized and its differential curve: (a) the curve of phase angle contrast normalized and (b) the differential curve of phase angle contrast normalized. Fig. 16. Phase angle image off spec specimen pec p eccime e im men 3 me 3# # us using siing ng ultrasound u t asou d lock-in oc n th thermography herrrm mo ogr og g gra aph ap phy att the tth he m he modula mo mod modulation od o dula ul tion t o frequen frequencies equen nccies iess of of ((a) a) 0 a) 0.1 .1 1H Hz, z (b) 0.2 Hz and (c) 0.5 Hz. 1.0 mm, 2.0 mm and 3.0 mm dee deep ep is calcu calculated cula cu l ted to be 20.04 04 4 mm, ft e d eo dg he e sp sspecimen. ecimen. The he di d istance 44.65 and 67.9 mm from the left edge off tthe distance between the minimum and ma maximum differential normalized axi x mu mum um d di ifferential n ormalized ated a he ssize iz ze o efe ect c , which phase angle contrast is calculated ass tth the off the de defect, has been marked in Fig. 15b. 4.3. Ultrasound lock-in thermography raphy juD uH j rHu ð9Þ where wh her ere e SNR SSN NR presents th the he ssignal–noise ig gna al–no oise ratio in phase angle image, wh whe here uD and uH are e tthe he h em me mean ea an n va value alue of phase in the defective areas where presents pr resen en e nts the standard deviation of phase healthy and and d healt h thy h areas, s,, an a nd rHu p nd angl le iin nh ea e althy hy y ar a rea eas,, re rrespectively. esp s eccti tively. angle healthy areas, The Th he phase pha ase se angle angl an g e images im ma ages of of specimen specime 3# by ULIT are illustrated Fig Fi g. 16(a–c). 16 6(a (a–cc).. In In the tth he same sam me way, they show detection of the real in Fig. cracks cra cr cra accks inside i siide in de the the specimen th spe peci cimen 3# at the modulation frequencies of ci 0..1 H 0 z, 0.2 z, 0.2 Hz 0. Hz and a d 0.5 0.5 Hz. 0.1 Hz, Th T he phase pha pha h se e angle anglle contrast and the signal–noise an sig The ratio of defective arre a ea a (marked (mar (m a ked in Fig. 16c) by Eq. (9) calculation are plotted in area Fig. 17a and b. It was found that the phase angle contrast is increased with the modulation frequency increasing, in this study, the modulation fre- te ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om As the SiC coating are formed med on the surface su urrfa u f ce e of of C/C C//C composite com ompo posite site si te g CVR technique e in i the th he high hiig gh tempertempe pe errtest specimen 3# and 4# using ds to generate real rea al cracks cra r ck ks inner in nn ner er the the th ature environment, which leads asound wave was used d to to stimulate s im st mulat ullatte specimen. The modulated ultrasound g lock-in thermographic technique. techn hniq ique ue ue. the specimen 3# and 4# using The signal–noise ratio (SNR)) of the phase angle image was defined to evaluate the performance of defect detection as following, respectively: SNR SSN N ¼ NR Fig. 17. Phase angle contrast and signal–noise ration as function of the modulation frequencies: (a) phase angle contrast and (b) signal–noise ratio. L. Junyan et al. / Composites: Part B 45 (2013) 138–147 147 Fig. 18. Phase angle image of specimen 4#: (a) ultrasound excitation at the modulation frequency of 0.1 Hz, (b) ultrasound excitation at the modulation frequency of 0.5 Hz and (c) light excitation at the modulation frequency of 0.1 Hz. quency of 0.5 Hz provides good phase angle contrast for the inner crack defects from Fig. 17a, the signal–noise ratio of phase angle image is decreased with the modulation frequency increasing, in this investigation, the modulation frequency q y of 0.1 Hz provides p the highest signal–noise ratio tio for the inner cracks defects from fro rom rom al–noise ratio is over 5.0 5..0 a th he e range r ng ra ge Fig. 17b. However, the signal–noise att the 5 Hz, Hz, z, in in this th hiiss study, sttud st ud u dy, y, of modulation frequencies from 0.1 Hz to 0.5 an a nd signal–noise nd sig gn na a al– l n l– no oiisse ratio rat atio io combining into the phase angle contrast and tion frequency cy of cy of 0.5 0..5 Hz 0 Hz is is selected selleccte ted as as are considered, the modulation ncy for the he e inner iin nne nerr interfacial in ntte errfa faci cia ci all crack c ack decr optimal modulation frequency ound lock-in lockk--in k i thermographic th he errmo mogrrap aphi h c technique. fects detection using ultrasound speci cciimen 4# 4# by by ULIT ULIT are presented pres pr esen sen ented The phase angle images of specimen hase angle ang gle image e of of specimen specimen n 4# 4# using ussiing ng in Fig. 18a and b, and the phase mps is is shown sh hown ow in ow in Fig. 18c. lock-in thermography by lamps e crack cra ra ack ck defects de d efe efe fectts are are observed from m the the It can be seen that more ultras assou a und d lock-in loccck lo k-in thermography,, and an a nd phase angle image using ultrasound dete tecttab te blle e from fro om the phase angle e image ima mag ge e the crack defects are hardly detectable using lamps stimulation. 5. 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[18] Maldague X, Marinetti S. Pulse phase infrared thermography. J Appl Phys 1996;79(5):2694–8. [19] Shepard SM, Lhota JR, Rubadeus BA, Wang David, Ahmed T. Reconstruction and enhancement of active thermographic image sequences. Opt Eng 2003;42:1137. [20] Wallbrink C, Wade SA, Jones R. The effect of size on the quantitative estimation of defect depth in steel structures using lock-in thermography. J Appl Phys 2007;101:104907-1. ht l: tp +8 :/ 6 /w 41 w 18 w 3 .il 72 gw 6 el 985 ls .c om mog gra r ph hiicc techniques t ch te chn ques were chni were applied we ap pplied to Two active infrared thermographic ts of carbon–carbon carb bon on–c on– –ca –ca arrbo b n composite. com mp po osit siite te. The he fifi detect the introduced defects MSOL L heat hea hea at transfer tra r nssffe er modulus modul ullus us was w s used wa usse ed nite element analysis by COMSOL he re he esu sult lts indicate ind dic icat a e that the the temtemte tem to model the experimental test. The results agr g ee em me ent n with with wi th experimental e pe ex p rimental perature contrast of FEA is in good agreement 6.0 0 mm mm defect d fect de fe ect ct that tha th att is by measurement. The size of diameter 6.0 3.0 .0 mm m defect defect de defe ffe ec that th ha att is is by by 3.0 mm deep and the size of diameter 3.0 temperatu ure re contrast co onttra rast ast st image ima im imag ag ge by by 2.0 mm deep are detectable using temperature phy with pulse heatt time tim im me of of 0.003 0.0 00 03 3 s. s. pulse transient thermography tha hat a re by The size of diameter 6.0 mm and 3.0 mm defects that are e using lock-in thermography at the 3.0 mm deep are detectable modulation frequency of 0.05 Hz. The locations of defects are obtained by differential computation of normalized temperature contrast and normalized phase angle contrast. Ultrasound lock-in thermography is available to detect cracks inside C/C composite, and it compensates the disadvantage of lamps excitation for real cracks detection using lock-in thermography. the Fundamental Research Funds for the Central Universities under Contract no. HIT.NSRIF.2009025 and the 111 Project (B07018). te This work was supported by the Chinese National Natural Science Foundation under Contract nos. 51074208 and 51173034,
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