Damage assessment in smart composite structures: the DAMASCOS programme

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Damage Assessment in Smart Composite Structures: the DAMASCOS Programme S.G. PIERCE, F. DONG, K. ATHERTON, B. CULSHAW, K. WORDEN, G. MANSON, T. MONNIER, P. GUY, J-C. BABOUX, J. ASSAD, E. MOULIN, S. GRONDEL, C. DELEBARRE, V. AGOSTINI, P-P. DELSANTO, I. GENESIO, E. MINO, C. BOLLER

The DAMASCOS (Damage Assessment in Smart Composite Structures) project is a European Union funded programme of work bringing together a number of academic and industrial partners throughout Europe. The aim of DAMASCOS is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realise damage assessment and ageing characterisation in composite structures. Thispaper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.

he fundamentals behind the DAMASCOS project are best illustrated in fi,k~un' 1. The system formed an ultrasound based interrogation system for damage assessment in advanced glass and carbon reinforced plastic materials (GRP and CFRP). Acoustic Lamb waxes could be launched into the sample from piezoelectric sources (PZT), and detected using either similar PZT transducers or optical fibre receivers. Since the sampie materials were typically of thin plate construction, we have concentrated exclusi\'ely on the propagation of ultrasonic Lamb waves [1] within the sampies. Changes in the condition of the sample under test, affected parameters of the l~amb wave propagation characteristics, and in this faslnion, by the application of suitable signal processing procedures, it was possible to infer the presence and position of structural damage within the sample plates. The experimental programme was complimented by modelling of Lamb wave propagation within the samples, and the interaction with defects. Typical acoustic sources and detectors used in DAMASCOS are shown in fi,k'tm' 2 together with some of the final target inspection structures in ti
departure fronl clasAcousflcdetecton [ PZT/Op~calfibres sical techniques. A~n'~z~r'"] Previous work [2, 31 has tended to focus on the echo disSlgnalprocessing crimination of sig~ nals in a pitchcatch arrangement. INTERPRIETATION OF This approach can DAMAGE be frustrated by the complexity of Lamb wa\'e propaModellingof ~ l propagationinsamplel gation; for in addition to problems of Figure i. Illustration of the basic DAMASCOS system low signal ampli- concepts. tude and edge reflection effects, the defect visibilih [t~], system identificapropagation characteristics of ultrasonic tion [7] and signal regeneration 1~ Lamb wa\es can exhibit considerable 9, 10]. Our approach in DAMASCOb pulse velocity dispersion and complicahas been to adopt a statistical method tions due to multiple mode propagation to look for small changes in the reI4, 51. Some noxel approaches to adceived signal to indicate a change in fix' dressing some of these problems have system, which may be due to damage included wavelet processing to enhance

Figure 2. DAMASCOS acoustic sources, (a) INSA mono-element, (t9) UV multi-element, (c) EADS dual~single element,

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StRuctuREs leads to a relative control of tile modes generated and is experimentally easier to deal with (phase-delaying not required). However, phase-delayed multielement transducers have been found to be more suitable for mode selection applications.

Optical detectiontechniques ,& c o m p l i m e n t a r y detection scheme to

the PZT transducers im olved tile development of optical techniques to detect the ultrasound. Many non-contacting optical techniques [13] are possible; in DAMASCOS the dominant interest was in integrated optical fibre sensors. These could be configured as surface mounted or fully embedded [14]. Fi\,ure 5 shows the system consisting of an input laser di\ic{ed between two separate interferometers. This allowed the easy exami-

Figure 3. Full scale samples, (a), EADS CFRP wing flap Airbus A320, (9) FIAT GRP b u m p e r

a@ing effects, c langes.

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Source and detector design 1 w e main types of PZT source were

implemented for DAMASCOS. The first v~as a single element transducer develc)ped at INSA de Lyon, capable of multifunctional capability; the second was a higher modally selective Lamb wa\e hansducer developed at Universit6 de \alenciennes employing multi-element tvchnology. A complimentary detection sdleme to the PZT transducers involved the development of optical techniques t~, detect the ultrasound. Many non-cont,~cting optical techniques are possible; il DAMASCOS the dominant interest ~ as in integrated optical fibre sensors. ~[hese could be configured as surface mounted or fully embedded [14].

Multifunctional source design fhe piezoceramic material was Lead Zirconate Titanate PC5 in the form of d iscs,/7%utv 2(a). The thickness to diame:er ratio was chosen in order to uncouple the axial and radial vibrations. F:ence the source could be employed for both monitoring of ageing of the materia through the interrogation of the electlical impedance around the high freq;lency [11] and for generating lower fiequency Lamb waves.

Multi-elementsourcedesign The problem of Lamb wave generation u.qng this type of source being non-trivial, a suitable modelling technique has

been especially developed bv Universit6 de Valenciennes [12]. This hybrid finite element - normal mode expansion technique allowed the prediction of the Lamb waves generated in the host plate, as a function of tile transducer characteristics. Application of this model has shown that phase-delayed multi-element transducers enable efficient Lamb moX de selection. This point is particularly important, since it should allow bet- Figure 4. The multi-element transducer configuration, ter interpretation and enhanced senRckl m e t a;lll sitMtv of the sys(see I;iglu~ 2) tem. Ti~en such transducers have been provided tnr the experimental testing. Fi~lm' 4 ~ < ~,:,,,,,d~ illustrates the simple case of a constant inter-element Holddc~ ..... /,,
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DAMASCOS instrumentation noise, the waveform were introduced into the sample, and one hundred spectra maximum values are plotted in unaveraged reference signathe complex plane. This representation i tures on the undamaged has the advantage to combine the inforsample and 10 unaveraged mation from our two discriminating . . . . J damage signature for each parameters (i.e. amplitude and phase ol El,HI PI~.OCt'~SS I N G hole diameter were recorded the spectrum maximum), which generReluo~:li ol IlltW;llltcd splk<% a13d dc i d r i t t ill M~IIH]N In these tests, we also invesally leads to cluster all different kind of tigated the cases when the signatures. Figure 8 represents the 'On hole is not introduced in senpath, ON axis' case for a Si/Lamb mode. FI'L,\TI. R t ~, S E L I ~ C [ ' I I ~N sors interrogation path, and Abacuses are circles which shows differRe,ion ol il+leR.s+t t~l die signal sckvted when this path do not coinent attenuation levels -I dB, -3 dB, -(~ dB ;ilia iLltornlatlOll d~*:iltllted t { ~SU I diiilellM Olis cide with a weaving direc(0dB i.e. reference level corresponds to tion. Hence, two transmitters the average amplitude of the measureI and three receivers were ments in the undamaged case) and ra\s NOVEH Y I)I~'I'IiCTI'ICIN I VISI AI,ISAIION bonded onto the surface to which represent a time shift ot one samI){HLla~e il3deK calctllatvd tlqHIg ] I nt;nllled :llld Iattlled dlt~ redttccd to (,[Idler :lll:llysis rltld compared l 2 dt[lletIsi°l/E tO ;Ill ,w ~'l~llill interrogate three different piing point compared to the mean of the Io thrt slu~/d ~ rdue replcscIIt;tti( 11 L configurations. The transmitreferences (which corresponds here to a Figure 6. Signal processing for DAMASCOS ted signal between transmitten degrees phase shift). A reasonable data. ter and receiver was a 300 detection threshold is found between 2 kHz five-cycle toneburst. and 3 mm because references, 1 mm and The magnitude of the amplination of samples at various orienta2 mm signatures are not well separated. tude spectrum maximum (ASM) in tions and positions relative to the interThis method of representation provide> decibel and norferometer and also the possibility of malized to the utilising the interferometers either as . . . . . ,. _. . . . . '. + : , undamaged case, i Mach-Zehnder or Michelson interfer-0 + - . - . + - +:.5 ,,. , , , t f and its associated greeters. Comparison of the signals ___.a..+~.k_...~ .... ~. . . . .0~__ _ . 1 - _ _ _i . . . . t+ l + + + < time shift versus detected in these configurations allowthe hole diameter ++ . . . . . i - -4 . . . . k ed calibration of the phase change of the are represented in Mach-Zehnder interferometer to surface O~ path off ,4~s I i ~+ O r p a i l ~ff 1 , 1 f(wrc 7. - ~ Off p&th - I . . . . I_ _ ' ~ . . . Off p a t displacement. .............. One can notice that i : ', ; for 'On path' conh:le rlam+qe r n ha}+ d l a r n e l ( + ( m i t t figurations the ASM decreases Figure 7. So Lamb mode Spectrum Amph'tude In order to analy'se the patterns iI+t the Maximum and Time Shift versus hole diameter in CFRP quasi linearly in DAMASCOS data, two multivariate decibel and the statistics techniques were employed time shift curves and a nonlinear mapping technique was can also be linearly ] ,+,..... used for visualising the data. Applying 0 I mm ", 't , _.interpolated The / ~. ~i'~7 multivariate outlier analysis, allowed 3 mm slope off the 'On X + 4...... the generation of a novelty index, "| 4 S mm Path, On axis' case _-I g ~i'~{',',:: whose time evolution allowed the inter~+ o + m,r I I I I I+ being more impor...... I E I LC_I * ++..... I II + I1o 9 +++n mm + I I pretation of system changes corretant that the 'On -~l I~10 . . . . I I I + sponding to damage. The use of Sam+ ~+'-4---"~,+ ..... !-,i ++ path, Off axis' one mon Mapping allowed an alternative ZL .*c , ~ ' ........... ]j .... il ....... +I+~___ ~ (fi~gure 7) This is 7"visttalisation of the n3ulti-dimensional . ---- ........... +,+ _._._. . . . . . ....+' jl <~ + / +f /I clearly ttseful for ~ "~K ~......... data sets. Fi~gure6 illustrates the process. 5 , gjlr'---~ + , +. ,' deted(ion and siz~ ...."....... i i

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Aircraft Technologies 004

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damaging threshold was faired to be around 6J. Fis,~ure10 shows the scanning of the impacted surface of the plate with a 50 MHz-focused transducer; and windowing the time records allows visualisation of echoes from various depths. This shows interfaces behveen orthogohal plies in order to lfighlight the shape of the defect: the delaminations present a classical two lobes pattern which principal axis has the direction of the fibres of the lower pl} of the interface. Fi\,Im's T1 and 12 show the application of novelty analysis and Sammon mapping to tile impact data collected from both a CFRP and a GRP panel. All

impacts (all above damage threshold) in the CFRP panel were classified as above

novelty threshold. For the GRP panel it can be seen that the first (non-damaging) impact was not above the no\eltv -c

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'i:,,1.e 9. Impact detection I-he aim of this work was to check the

~easibilityof damaging our sarnple with ,'ontrolled low-speed impacts and to determine damaging thresholds (i.e. energy levels below which no significant ,tamage was produced) for GRP and CFRP composite plates. The test took place at INSA's facilities in Lyon, as a , alibrated impacting device with impact velocity and load measurenlents was ,wailable. In the first kind of sample i3mm thick GRP plate), a significant dispersian of the magnitude of the damaging threshold was observed, due to the material hetemgeneit 3. Fotlr impacts of energy I, ~_, '~ ,~ ° and 4 joules were intro-

duced on the GRP plate at the same location. The second sample was a CFRP plate made up of 22 unidirectional plies. This was subiected to four successive impacts of energy 2, 4, 8 and 8 joules. The predominant damaging effect was found to be the delamination of the interface between orthogonal plies. The

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ing mode, which may be effective in monitoring defect growth where strain energy is released directly bv the growth of the damage. Fik,ure 13 shows four successive impacts in CFRP (21, 4J, S.l and 81). For the first two energy impacts, the signatures obtained corresponded only to line frequency plate modes excited by the impact events. These impacts were below plate damage threshold. On the contrary, the following signals (c), (d) have clearly different shapes. The first

plate mode' components are still present, but additional higher frequencies arc' clearly observable, As it is well

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code from 2 to 3 dimensions ~xas necessao; due to the 3-dimensinnal nature of a Lamb wave propagating thorough a thin plate flawed bv a hole. Tile intrinsic geometry required for [~amb wa\'e propagation in the plate required that its thickness be much smaller than the other two sizes. We avoided the problem o f using the same discretisation step in all three directions (for which the number of nodes becomes prohibitively large) by using a discretisation step t'; along the thickness smaller t h a n the steps in the other two dimensions. ~,~'e tried to optimise our solutions bv varying the ratios ex/e~ and Sv/S~. Froma numerical point of view, inorder to generate Lamb modes one could directh' input the analytical Lamb wave displacement profiles. However, in order to reproduce the physical mechanisms underlying the Lamb modes generation, we chose to perform an 'experimentallike' injection, in particular, we reproduced the strain field created by the transducer using as a reference the 1 cm diameter disk-shaped PZT sensors ot INSA. These transducers allow to select either the So or the A0 Lamb mode operating at the frequency 300 kHz and 100 kHz, respectively. In the first case the specimen was excited mainly through vibrations parallel to the length of the specimen, while in the latter perpendicular vibrations prevailed. Preliminary simulations were performed to investigate Lamb wave interaction with a fully penetrating hole. In fi\~ure 15 we present snapshots at a fixed time (t = 126 Ius) of the out-oF plane displacement component w fl~r a Lamb wave propagating in the plate with a hole of diameter 1 mm (a), 4 mm

difficulties encountered in treating 0.2 0.2 sharp or imperfect -=_ o ~ ~ contact interfaces ~" -0.2 by means of the -o.4 (a) -0.4 (b) usual Finite Dif-0.6 -0.6 o 1 2 3 2 ference techniques. time (ms) time (ms) The proposed ap0 . 6 proach shows good ~ 0.4 ~ 0.4 > convergence and ~" 0.2 / ~" 0.2 stability and is g o g o ~ -0.2 ~-0.2 then to be consid'~ -o.4[ (d) ered a very valu"l'III II ~ 0 . 6 ~ -O.SL ...... able tool in the preo 1 2 0 1 2 3 time (ms) time (ms) diction of the system signature and Figure 13. Acoustic emission signals during impacts, (a) 2J impact, (b) 4J impact, (e) 8J impact. identification of the (d) Second 8J impact. optimum signal extraction routines. The modelling apknown in acoustic emission of composproach is easily visualised as a set of ites, these high frequency components interconnected springs (fi,gure 14). can be interpreted as the signature of Changes in the system under consideracrack occurrence. tion (such as the inclusion of defects) can be easily accomplished by either cutting springs, or changing the relative spring constants. Initial propagation Computer simulation techniques prostudies were performed with simple 2D vide an important role as basic tools in sample geometries. The extension of the fields, such as damage assessment of composite structures, in which it is 2 i important to gain a good physical understanding of the propagation mechanisms of ultrasonic waves or pulses through these complex materials. A method, which has been designed for the above purpose and is particularly efficient, especially if applied in conjunction with parallel processing, is the Local Interaction Simulation Approach (LISA) [15-17]. As spin-offs of LISA, the 3 8 Sharp Interface Model (SIM)I18] and the Figure 14. The spring model approach, Spring Model have overcome [19] the 0.61'

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(b) and 10 mm (c). In these plots both o~ the faster Stl and the A o mode are riy[r o sible; the SI/ mode is seen already reflected back from the edge of the "* specimen. We do xlm] :, not report the refo • r'n O2 erence (unfaulted) case, since it is very to the case o 05 " i o 1 similar Ol of a 0 = 1 mm y ira] o hole. In fact, the interference patt) 0215 o 1 tern caused by the hole is clearly visi-o 2 025 ble onlv for tl~e 0 = x [ml b) 4 m m a n d 0 = 10 o2 mm cases. o05 + Subsequent work o, has concentrated Ol on the representay[ml o tion of more realis015 tic structural de-o~ fects. Of particular 0.2 interest was the 025 -o~ case of a delamina0 0.1 02 03 04 × [ml tion. This could be c) modelled as either Figure 15. Map of the out-of-plane c o m p o n e n t a single delaminaw of the displacement at time t= 126 ps. tion (with sharp or o

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smooth edges); or as a stack of delaminations with a conical profile through the plate thickness to better approximate a typical impact induced delamination stack (fi~\~ure 10). For a conical delamination stack with defects in every second layer, the displacement distribution is shown in.figure 16. Note that displacements are abated inside the cone (high stiffness), but not null; a certain amount of energy was transferred across the cone.

Conclusions This paper has discussed the development of the DAMASCOS project, whose aim is to apply new ultrasonic detection and generation techniques integrated within composite structures, together with advanced signal processing to realise damage assessment and ageing characterisation. The structures were probed with ultrasound that was generated using conventional piezo-electric sources. Detection was accomplished using either similar piezo-electric transducers, or optical fibre sensors. Since the sample materials were typically of thin plate construction, we have concentrated exclusively on the propagation of ultrasonic Lamb waves within the samples. Changes in the condition of the sample under test, affected parameters of the Lamb wave propagation characteristics, and in this fashion, by the application of suitable signal processing procedures, it was possible to infer the presence and position of structural damage within the sample plates. The experimental programme was complimented by modelling of Lamb wave propagation within the samples, and the interaction with defects. A central theme of DAMASCOS was the approach to signal processing and data interpretation which relied on the development of statistical tools, most notably novelty detection, to indicate the presence of damage within the system under test. •

Acknowledgements DAMASCOS is funded by the European Community under the industrial & Materials Technologies Programme EUROPE

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DAMASCOS (Brite-Euram 11I), project number BE 07 4213. Note This article has been previously published in SP1E Vol.4327, paper ntunber 31, 2001. References (1) Viktorov I.A, Rayleigh a n d Lamb waves-Physical t h e o r y a n d a p p l i c a tions, 1967, Plenum Press, N e w York. (2) Alleyne D.N., C a w l e y P, The interac tion of l a m b w a v e s with defects, IEEE Trans UFFC, Vol 39 (3), 1992, p p 381-397. (3) G u o N., C a w l e y R, L a m b w a v e reflection for the quick non destructive e v a l u a t i o n of large c o m p o s i t e lami nates, Materials Evaluation Vol 52 (3), 1994 p p 404 411. (4) Pierce S,G,, Culshaw B., M a n s o n G., Worden K., Staszewski W.J., The a p p l i c a tion of ultrasonic L a m b w a v e t e c h niques to the e v a l u a t i o n of a d v a n c e d c o m p o s i t e structures, SPIE International Symposium on Smart Structures And Materials 2000: Sensory P h e n o m e n a And M e a s u r e m e n t Instrumentation For Smart Structures A n d Materials, 5 9th M a r c h 2000, N e w p o r t Beach, California, USA, SPIE Vol 3986, p p 93 103, 2000, (5) Alleyne D,N,, C a w l e y 12,Optimisation of l a m b w a v e inspection techniques, NDT&E International , Vol 25 (1), 1992, p p 11-22, (6) Staszewski W.J., Pierce S G , Worden K., Philp W.R., Tomlinson G.R., Culshaw B,, W a v e l e t signal processing for e n h a n c e d Lamb wave defect d e t e c t i o n in c o m p o s i t e plates using o p t i c a l fibre d e t e c t i o n , O p t Eng Vol 36(7), 1997, p p 1877 1888,

(7) Leontaritis I.J., Billings S.A., Input-output p a r a m e t r i c models for non-linear systems, Part I: deterministic nonlinear systems, I n t e r n a t i o n a l Journal of Control, Vol 41, 1985, p p 303-328. (8) Alleyne D.N, Pialucha T,12,C a w l e y P, A signal r e g e n e r a t i o n t e c h n i q u e for l o n g - r a n g e p r o p a g a t i o n of dispersive L a m b waves, Ultrasonics, Vol 31 (3), 1993, p p 201 204, (9) Ing R,K., Fink M., Time reversed L a m b waves, IEEE Trans UFFC, Vol 45(4), 1998, p p 1032 1043. (10) D e r o d e A., Tourin A., Fink M,, Time reversal in multiply scattering m e d i a , Ultrasonics, Vol 36, 1998, p p 443-447,

(16) S c h e c h t e r R.S, Chaskelis H.H.. M i g n o g n a R.B., Delsanto PP, Science 265, 1188 (1994), (17) D e l s a n t o RP, S c h e c h t e r R S M i g n o g n a R,B,, Wave M o t i o n 26, 329 (1997), (18) D e l s a n t o 12R, M i g n o g n a R.B. Scalerandi M., Schechter R.S., in N e w Perspectives on Problems in Classical a n d Q u a n t u m Physics, Delsanto PP Saenz A,W,, (eds.), G o r d o n & Breach 1998, Vol. 2, p.51. (19) Delsanto RP, Scalerandi M , J A ( . Soc. Am. 104, pp, 2584-2591 (1998)

(11) Perrissin-Fabert ~, Jayet, Simulated a n d e x p e r i m e n t a l study of the electrical i m p e d a n c e of a piezoelectric ele m e n t in a viscoelastic m e d i u m , Ultrasonics, Vol. 32, N ~ 2, pp. 107 112, 1994. (12) Moulin E,, Assaad J., D e l e b a r r e C., O s m o n t D., M o d e l i n g of L a m b w a v e s g e n e r a t e d by i n t e g r a t e d transducers in c o m p o s i t e plates using a c o u p l e d finite e l e m e n t normal m o d e s e x p a n sion m e t h o d , J. Acoust, Soc, Am., vo1,107, 2000, p p 87-94. (13) Dewhurst R.J., Shah Q., O p t i c a l r e m o t e m e a s u r e m e n t of ultrasound, Meas. Sci. Tecnol. Vol 10, 1999, ppR139R168. (14) Pierce SG., Philp WR., Culshaw B.,

Gachagan A., M c N a b A., H a y w a r d G., Lecuyer, Surface b o n d e d o p t i c a l fibre sensors for t h e i n s p e c t i o n of CFRP plates using ultrasonic L a m b waves, Smart Materials a n d Structures, Vol 5, 1996, p p 776-787, (15) P P Delsanto, R.S. Schechter, H.H. Chaskelis, R.B M i g n o g n a a n d R. Kline, Wave M o t i o n 20, 295 (1994).

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About the authors: S.G. Pierce, F. Dong, K. Atherton, B. Culshaw work at the University of Strathclyde in Glasgow; K. Worden, G. Manson work at the University of Sheffield; T. Monnier, R Guy, J.-C. Baboux work at the Institut National des Sciences Appliqu6es de Lyon; J. Assad, E. Moulin, S. Grondel, C. Delebarre work at the Universit6 de Valenciennes; V. Agostini, P-R Delsanto, I. Genesio work at the Politecnico di Torino; E. Mino works at the Centro Ricerche of FIAT in Piedmont, and C. Boiler works for EADS MT2 in Munich [email protected] (S.G. Pierce)