Propagation of guided Lamb waves in bonded specimens using piezoelectric wafer active sensors

Transcription

1 Propgtion of guidd Lmb wvs in bondd spcimns using pizolctric wfr ctiv snsors Adrin Cuc*, Victor Giurgiutiu**, Univrsity of South Crolin, Dprtmnt of Mchnicl Enginring, Columbi, SC 98 ABSTRACT Th nondstructiv vlution (NDE) of dhsivly bondd structurs is compl procss. Erlir work hs confirmd tht ultrsonic wvs r influncd by th proprtis of th mtril in which thy trvl. Acousto-ultrsonic mthods hv bn widly usd by prvious rsrchrs to gnrt ultrsonic wvs in plts nd bondd structurs for flw dtction, visuliztion, nd msurmnts of th locl proprtis of th jointd mtrils. This ppr will prsnt th mthods nd principls usd for gnrtion nd propgtion of ultrsonic guidd wvs (Lmb wvs) using pizolctric wfr ctiv snsors (PWAS). Kywords: Ultrsonic, Lmb wvs, Dmg dtction, NDE, Wv propgtion, PWAS 1. INTRODUCTION Ultrsonic nondstructiv vlution is of highly importnc for svrl industris. Evn though ultrsonic inspction hs bn usd for svrl dcds, th dmnds of th ctul conomy hv pushd th dvlopmnt of NDE mthods furthr nd furthr. Nw ultrsonic mthods r bing dvlopd, nd thr is trnsition from th convntionl ultrsonic mthods using coupld trnsducrs, to mor dvncd tchnologis using mbddd ultrsonic snsors. Ths snsors will gnrt guidd ultrsonic wvs, which will trvl within th tstd subjct, nd will provid informtion bout th istnc nd loction of possibl flws. Th bnficiry of such modrn dtction tchnologis will b industris vrying from rospc to utomotiv pplictions. Tody s ircrft flt is n gd flt tht nds mor nd mor tdious inspction for crcks, corrosions nd dlmintions. Ths mintions r don minly mnully using visul inspction, which is pnsiv, tim consuming nd mks th ircrfts inoprbl for long tim. Th us of NDE mthods will nbl smi nd full utomtion of th ntir inspction procss, rduc th tim of inspction nd mov th mintnnc pproch from schduld mintnnc to on-dmnd mintnnc. Erly dtction of such flws will prvnt ctstrophic filurs nd wht is most importnt will sv humn livs. Automotiv industry is lso going to bnfit from such modrn tchnologis. Wight is n importnt issu nd fctor tht ffcts th prformncs of cr s wll s th pric. Tht is why luminum, non-frrous lloys nd composit mtrils r usd to crt nw ssmblis with rducd wight nd incrsd strngth ginst corrosion nd collisions. Such mtrils cnnot b wldd or joind using boltd connctions. Th only ltrntiv is to us structurl dhsivs nd prmnntly join thm togthr. Th highly utomtd procss of cr mnufcturing rquirs n utomtd procss of inspcting such joints. Th NDE mthods hv th cpbilitis of rl tim utomtd inspction. In ordr to us ultrsonic mthods for NDE of mtllic structurs th physicl phnomnon of wv gnrtion nd wv propgtion must b undrstnd. Erly work ws conductd using bulk wvs for thicknss msurmnts. Convntionl ultrsonic trnsducrs wr usd to introduc prssur wvs in th multilyr joint typiclly using coupling gl btwn th trnsducr nd th structur. In th pst yrs, rsrchrs focusd thir ttntion on using guidd wvs, nmly Lmb wvs, for flw dtction. Lmb wvs cn b citd using convntionl ultrsonic trnsducrs nd wdg btwn th trnsducr nd th spcimn. This ppr will prsnt th tchniqus usd to gnrt guidd wvs (Lmb wvs) in spcimn consisting of thin mtllic lyrs joind togthr through n dhsiv lyr. Th work will introduc smll, inpnsiv, nonintrusiv, unobtrusiv dvics tht cn b surfc mountd nd bl to cit nd dtct Lmb wvs. *icuc@ngr.sc.du; phon (83) , f: (83) **victorg@sc.du; phon: (83) , f: (83)

2 . PIEZOELECTRIC WAFER ACTIVE SENSORS Pizolctric wfr ctiv snsors (PWAS) r inpnsiv trnsducrs (Figur 1) oprting on th pizolctric principl. Th dirct pizolctric ffct is mnifstd whn th pplid strss on th snsor is convrtd into lctric chrg. Th invrs ffct, convrsly, will produc strin whn voltg is pplid on th snsor. In this wy th PWAS cn b usd s both, trnsmittr nd rcivr. Pizolctric ctutors wr usd initilly by Crwly t l 1, nd Fullr t l. 3 Tzou nd Tsng 4 nd Lstr nd Lfvbr 5 modld th pizolctric snsor/ctutor dsign for dynmic msurmnt nd control. Th us of pizolctric snsors for structurl hlth monitoring nd th Lmb wvs for dmg dtction ws pionrd by Chng Fu-Kuo nd his collbortors 6,7,8,9 who hv studid th gnrtion nd th rcption of lstic wvs in composit mtrils. In thir studis, pssiv rcption of lstic wvs ws usd for impct dtction, nd th pitch-ctch mthod using low-frquncy Lmb wvs ws usd for dmg dtction. Othr rsrchrs 1-16 hv studid th propgtion of Lmb wvs nd th us of diffrnt wv propgtion mthods (pitchctch, puls-cho) s wll s stnding wv mthods (lctromchnicl impdnc) for dmg dtction. Rcntly, Giurgiutiu nd his collbortors 17,18,19, hv studid in dtil nd chrctrizd th pizolctric wfr ctiv snsors usd for dmg dtction with guidd Lmb wvs. Whn citd by n ltrnting lctric voltg, th PWAS cts s n lctromchnicl rsontor. Th modling of fr pizolctric snsor is usful for undrstnding th lctromchnicl coupling btwn th mchnicl vibrtion rspons nd th compl lctricl rspons of th snsor. As PWAS rry rsontor th PWAS hs th proprty of prforming mchnicl rsonncs undr dirct lctricl cittion. Th rsonnt frquncis dpnd only on th wv spd ( mtril constnt) nd th gomtric dimnsions. Th lctric fild is producd by pplying hrmonic voltg () ˆ i t in (5 mm) Vt = V ω btwn th top nd bottom surfcs. Th rsulting lctric fild, E3 = V / h, is ssumd uniform ovr th pizolctric wfr. For hrmonic voltg cittion th lctric fild nd th rspons r in th form ˆ i ω t iωt Figur 1 Arry of PWAS mountd on n luminum E 3 = E, u = u ˆ, whr u ˆ ( ) 1 is th -dpndnt plt compl mplitud nd incorports ny phs diffrnc btwn th cittion nd rspons. Th gnrl pizolctric constitutiv qutions in th tnsor nottion r: E Sij = s T ijkl kl + dkij Ek (1) T Dj = dkijtki + ε E jk k whr s ijkl is th mchnicl complinc of th mtril msurd t zro lctric fild (E = ), ε T jk is th dilctric prmittivity msurd t zro mchnicl strss (T = ), nd d kij rprsnts th pizolctric coupling ffct. Undr th on-dimnsionl ssumptions, th gnrl constitutiv qutions for strin nd lctric displcmnt rduc to th simplr prssions: E S = s T + d E () T ε 33 3 D = d T + E (3) E whr S 1 is th strin, T 1 is th strss, D 3 is th lctricl displcmnt, s 11 is th mchnicl complinc t zro lctric T fild, ε 33 is th dilctric constnt t zro strss, nd d 31 is th inducd strin cofficint. 3. GENERATION OF ULTRASONIC GUIDED WAVES WITH PWAS For mbddd nondstructiv vlution (NDE) pplictions, PWAS cn b usd s mbddd trnsducrs. PWAS cn ct s both Lmb wv trnsmittr nd Lmb wv rcivr. Th snsors coupl thir in-pln motion with th prticl motion of Lmb wvs on th mtril surfc. Th in-pln PWAS motion is citd by th pplid oscilltory voltg through th d 31 pizolctric coupling. Optimum cittion nd dtction hppn whn th PWAS lngth is n

3 odd multipl of th hlf wvlngth of prticl Lmb wv mods. Th PWAS ction s ultrsonic trnsducrs is fundmntlly diffrnt from tht of convntionl ultrsonic trnsducrs. Convntionl ultrsonic trnsducrs ct though surfc tpping, pplying vibrtionl prssur to th surfc of th objct. PWAS, on th othr hnd, ct through surfc pinching nd r strin coupld with th objct surfc Coupling btwn PWAS nd host structur Th trnsmission nd rcption of Lmb wvs btwn th PWAS nd th structur is chivd through th dhsiv lyr. Th dhsiv lyr cts s shr lyr, in which th mchnicl ffcts r trnsmittd through shr ffcts. Figur shows thin wll structur of thicknss t nd lstic modulus E, with PWAS of thicknss t nd lstic modulus E ttchd to its uppr surfc through bonding lyr of thicknss t b nd shr modulus G b. Th PWAS lngth is l whil th hlf-lngth is =l /. In ddition, th dfinition d = t / is usd. Upon ppliction of n lctric voltg, th PWAS princs n inducd strin: V ε ISA = d31 (4) t Th inducd strin is trnsmittd to th structur through th Figur Intrction btwn th PWAS nd th structur showing th bonding lyr intrfcil strss τ() bonding lyr intrfcil shr strss (τ). For hrmonic vrying cittion, th shr strss hs th prssion τ(, t) = τ ( ) iωt. Th PWAS pnsion is trnsmittd to th structur through th bonding lyr which cts prdominntly in shr. Th shr strss intnsity nd distribution dpnd on th rltiv dformtion of th PWAS nd th structur. Crwly t l. dvlopd 1-D strin nlysis using th Eulr-Brnoulli hypothsis cross th plt thicknss, i.., uniform displcmnt for il motion, nd linr displcmnt for flurl motion. Th rsulting intrfcil shr strss in trms of hyprbolic functions cn b writtn s: t ψ sinh Γ τ( ) = EεISA Γ (5) α + ψ cosh Γ Et Gb 1 α + ψ whr ψ =, nd th shr lg prmtr Γ =. Et E ttb ψ Th prmtr α dpnds on th strss nd strin distribution cross th structurl thicknss. For low-frquncy coupld il-bnding motion this prmtr tks th vlu α = 4. This vlu is chnging s th frquncy chngs. Th shr trnsfr long th PWAS is controlld by th product btwn th shr lg prmtr, Γ, nd th snsor hlf-lngth. For low vlus of th Γ product, th shr trnsfr is distributd long th PWAS lngth, nd th shr strsss hv rltivly low intnsity. For high vlus, th shr trnsfr is loclizd towrds th nd of th snsor nd th shr strss hs high intnsity. 3.. Ail wvs citd by PWAS Assum on-dimnsionl mdium in which n trnl forc inducs n ctution strin, ε (, t). Such n ctution strin my b inducd by surfc mountd PWAS pplid symmtriclly to th top nd bottom surfcs. Considring n infinitsiml lmnt of lngth d, pplying Nwton s lw nd considring hrmonic cittion th strin bcoms: ε ξ ε = ε (6) whr ξ = ω c is th wv numbr of il wvs in th on-dimnsionl mdium. Applying spc-domin Fourir trnsform on Eq. (6) yilds to: ξ % ε = % ε (7) ξ ξ Eqution (7) rprsnts th solution in th Fourir domin. Tking th invrs spc-domin Fourir trnsform yilds th solution in th spc domin: + 1 ξ i ε ( ) = ε ( ) ξ ξ dξ π % (8) ξ ξ

4 For n idlly bondd PWAS, th inducd strin is uniform ovr th PWAS lngth. Th spc-domin strin distribution is th rctngulr puls function: ε, < ε ( ) = (9), othrwis Rclling Eq. (7) nd tking invrs Fourir trnsform, th spc-domin solution is: + ε ξsinξ iξ ε ( ) = dξ π (1) ξ ξ Th intgrl in Eq. (1) cn b solvd nlyticlly using th rsidu thorm nd smicirculr contour (C) in th compl ξ domin. Th strin rspons du to hrmoniclly oscillting PWAS prfctly bondd to th structur hs th form: ( ) ( ) i (sin ) i t ξ ε = ε ξ ω (11) From Eq. (13) th it cn b sn tht th rspons mplitud follows sinusoidl vrition with rspct to th prmtr ξ. Rspons pks r obsrvd t odd intgr multiplis of π Flurl wvs citd by PWAS Considr th gnrl qution of flurl vibrtions undr trnl momnt cittion, M (,t): EIv + ρ Av&& = M (, t) (1) Assuming hrmonic vrition in th tim domin nd considring th cittion curvtur κ, th gnrl qution of flurl vibrtions bcoms: 4 v ξfv = κ (13) whr M 4 ρ A ω EI d E κ =, ξf = ω =, = = EI EI ρ A 3 ρ Applying spc-domin Fourir trnsform on Eq. (13) yilds to: 4 4 ( ξ ξf ) v% = ξ % κ (14) Th solution of Eq. (14) is: ξ % ε = % κ 4 4 (15) ξ ξf Tking th invrs spc-domin Fourir trnsform yilds th solution in th spc domin: + 1 ξ iξ v ( ) = κ 4 4 dξ π % (16) ξ ξ Th intgrl in Eq. (16) cn b solvd nlyticlly using th rsidu thorm nd smicirculr contour (C) in th compl ξ domin. Adding th hrmonic vrition in th tim domin yilds th complt solution: % κ( ξf) i( ξ ) ( ) F ωt % κ iξf ξf iωt vt (, ) = i + (17) 4ξF 4ξF Th first trm in Eq. (17) rprsnts propgting wv, whil th scond trm rprsnts vibrtion tht is dcying fst with. This trm rprsnts locl vibrtion tht dos not propgt nd is clld n vnscnt wv. Rtining only th propgting wv prt in Eq. (17) will giv: κ( ξf) i( F t) vt (, ) i ξ = % ω (18) 4ξ F Th strin solution cn b drivd s: % ε i( F t) yi ( ) ξ ε = ξ ω (19) 4 F For n idlly bondd PWAS, th cittion momnt is rprsntd by rctngulr puls function. Th following prssion for th strin t th mtril surfc undrgoing flurl wv cittion cn b drivd ftr mthmticl clcultions:

5 F ε( ) i3 ε (sin ξ ) ξ ω i( t) = F () Rspons pks r obsrvd t odd intgr multipls of π. Mimum cittion of flurl wvs will occur whn th PWAS lngth is n odd-intgr multipl of th flurl hlf wvlngth l = (m+ 1) λ. F 3.4. Lmb wvs citd by PWAS For th nlysis considr th surfc-mountd PWAS shown in Figur 3. Th snsor is citd lctriclly with i t tim-hrmonic voltg V ω. As rsult of th pplid voltg th snsor will pnd nd contrct, nd tim i t hrmonic intrfcil shr strss, τ ( ) ω α dvlops btwn th PWAS nd th structur. Th strss on th uppr surfc is givn by: τ = τ ( ) = τ ( ) H( + ) H( ) (1) y y= h [ ] whr H() it th Hvisid stp function. Applying th spc domin Fourir on th cittion w obtin: + [ ] ( ) ( ) ( ) iξ % τ = τ H + H d () Rclling th wv qutions in trms of th potntil functions nd th Lm constnts: 1 φ = && φ, 1 ψ = && ψ (3) whr c = ( λ + µ )/ ρ is th spd of th prssur wv; c p c p c s s = µ / ρ is th spd of th shr vrticl wv; λ, µ, ρ r th two Lm constnts nd th dnsity, rspctivly. Th displcmnt in th nd y dirctions in trms of th two potntils r: φ ψ u = + y, φ ψ uy = (4) y Applying th spc domin Fourir trnsform to th wv qutions, displcmnts, strsss nd strins w obtin: d % φ d ψ + c % %, pφ = + c Sψ = dy dy % (5) dψ% d % φ u% = iξφ% +, u% y = iξψ% (6) dy dy d % φ ψ% d % φ dψ% % τ y = µ iξ + ξ ψ% +, % τ yy = λ ξ % φ + µ ξ φ iξ + % (7) dy y dy dy Th gnrl solution for Eq. (5) hs th form: % φ = A1sin( py) + Acos( py) (8) ψ% = B sin( qy) + B cos( qy) 1 whr p = ω c p ξ, q = ω c s ξ, nd ξ r th dirctionl wv numbrs. Th constnts, A, B 1 nd A 1, B corrspond to th two possibl motions: symmtric nd nti-symmtric (Figur 3). Figur 3 () () Symmtric nd nti-symmtric prticl motion; (b) symmtric nd nti-symmtric loding (b)

6 Symmtric solution Th boundry conditions for th symmtric cs r: u% = u%, u% d d y = u% + d y + d (9) % τ % τ y = % τ y =, % τ yy = % τ yy = d + d d + d (3) Th strin t th uppr surfc for th symmtricl motion cn b clcultd s: S τ NS % ε = i d µ DS (31) N = ξq ξ + q cos pdcos qd, D ( ) = ξ q cos pd sin qd + 4ξ sin pd cos qd. whr S ( ) S Anti-symmtric solution To solv for th strin t th uppr surfc for th nti-symmtricl motion w follow th sm procdur s for th symmtricl motion. Th boundry conditions r: u% = u%, u% d d y = u% + d y + d (3) τ % τ y = % τ y =, % τ d d yy = % τ + d yy = % + d (33) Th strin t th uppr surfc for th nti-symmtricl motion is: A τ N A % ε = i d µ DA (34) N = ξq ξ + q sin pdsin qd, D ( ) = ξ q sin pd cos qd + 4ξ cos pd sin qd. whr A ( ) A Totl solution Th complt rspons to th PWAS cittion is obtind by combining th symmtric nd nti-symmtric rsponss: % τ NS N A % ε = i d + (35) µ DS DA Applying th invrs Fourir trnsform nd dding th hrmonic tim bhvior w mov from th wv numbr domin bck into th spc domin: + 1 i % τ NS % τ N A i( ξ ωt) % ε = dξ d π µ + (36) DS DA Th intgrl in Eq. (36) is singulr t th roots of D S nd D A which r th symmtric nd nti-symmtric ignvlus of th Ryligh-Lmb qutions, i.., Figur 4 ξ, ξ, ξ,... (symmtric motion) nd ξ, ξ, ξ,... (nti-symmtric motion). At low frquncis, S S S 1 A A A 1 S A i.. ω, only two ignvlu ist, ξ nd ξ. At highr frquncis svrl othr ignvlus ist. To solv Eq. (36) th rsidu thorm is usd nd contour consisting of smicircl in th uppr hlf of th compl ξ Contour for vluting th invrs Fourir pln nd th rl is (Figur 4). Th totl solution for th trnsform intgrl. strin t th uppr surfc is: S S A A 1 % τξ ( ) N ( ) S ( ) ( ) ( ) A S ξ i ξ ωt % τξ N A ξ i( ξ ωt) ε ( t, ) = + S A µ (37) S D ( ) A ξ S ξ ξ D A( ξ )

7 Intgrtion with rspct to yilds th displcmnt: S S A A 1 1 % τξ ( ) N ( ) S S ξ 1 % τξ ( ) N A( ξ ) u (, t) = + S S A A µ S D ( ) A ξ ξ S ξ ξ ξ D A( ξ ) A i( ξ ωt) i( ξ ωt) (38) 4. EXPERIMENTAL RESULTS Eprimntl work hs bn conductd to vlidt th ssumption tht smll PWAS cn cit Lmb wvs in bondd spcimns which will propgt nd will b rcivd by othr PWAS cting s rcivrs. Eprimntl work ws lso prformd to stblish PWAS-bsd mthods for dtcting disbonds. Both trvling wv mthods (pitchctch nd puls-cho) nd stnding wv mthods (lctromchnicl impdnc) hv bn plord PWAS Lmb-wv tuning in bondd spcimns An luminum lp-joint spcimn ws fbrictd using two luminum 4T3 strips s shown in Figur 5. 8 mm Simultd disbonds PWAS rry Bond lin C B A Aluminum Bond lin Aluminum Figur 5 1 mm () 75 mm () Lp-joint spcimn consisting of two luminum strips bondd togthr; (b) Loction of th PWAS on th lp-joint spcimn (b) Tktroni TDS1 Digitl Oscilloscop HP 331 Signl gnrtor Tktroni TDS1 Digitl Oscilloscop GPIB Computr HP 331 Signl gnrtor Lp-joint spcimnn Disbond Disbond Disbonds PWAS () Figur 6 Th instrumnttion st-up usd in th pitch-ctch primnt: () loction of PWAS on th lp-joint spcimn; (b) schmtic of th instrumnttion st-up Th two luminum strips r 8mm 1mm, nd 75mm 1mm with thicknss of 1mm. Thy wr bondd togthr using n poy pst dhsiv, Loctit Hysol EA 939.3NA. Th ovrlp of th two luminum strips is mm. Th disbonds wr rtificilly crtd using Mylr polystr film tht ws introducd btwn th two luminum strips nd producd discontinuity of th dhsiv lyr. Nt, th spcimn ws instrumntd with n rry of PWAS snsors s prsntd in Figur 5(b). Th snsor rry hs 11 columns nd thr rows (A, B, nd C). On row of snsors (A) is loctd on th top luminum plt, th scond row on th bond lin (B), nd th third row is loctd on th scond luminum plt (C). Th spcing btwn ch column of snsors is 1 mm nd th distnc btwn PWAS on row A nd PWAS on row B is 3mm. Th distnc btwn PWAS on row B nd PWAS on row C (b)

8 is 8mm. Th instrumnttion st-up is shown in Figur 6. An HP 331 signl gnrtor ws usd to produc 3-count sinusoidl ton burst with frquncy of 39 khz. Th wv trvld through th spcimn nd ws cptur using Tktroni TDS1 digitl oscilloscop. To study th propgtion of Lmb wvs, th disprsion phnomnon nds to b ddrssd. Wv disprsion my occur whn th wv spd vris with frquncy. Lmb wvs r disprsiv in ntur mning, th wv spd dpnds on th product of frquncy nd thicknss of th plt in which th Lmb wvs r trvling. Th physicl phnomnon cn b plind considring wv pckt, or ton burst, s shown in Figur 7. Th ton burst consists of crrir frquncy (ton) nd hs short durtion in tim (burst). In th frquncy domin, thr is dominnt frquncy f c, nd othr sid frquncis. Amplitud (mv) FFT IFFT Amplitud (mv) sid frquncis Figur 7 ) b) t () 5-count sin ton burst; (b) frquncy spctrum of th ton burst signl For non-disprsiv wv propgtion, th shp of th wv is prsrvd nd th wv spd is constnt. All frquncy componnts in th wv pckt trvl with th sm spd nd th pckt kps its shp. For disprsiv wv propgtion, ch frquncy componnt trvls with diffrnt spd (wv spd is function of frquncy c = c(f)) hnc th wv pckt sprds out, it disprs. Th dgr of disprsion dpnds on th spctrum bndwidth. 4.. Wv propgtion mthods Lmb wvs trvling in bondd spcimns ncountr compl phnomn t th bond intrfc. Prt of th nrgy of th incidnt wv will b trnsmittd through th bond lyr from on mtllic plt to th othr. At th sm tim mod convrsion nd diffrction of th Lmb wv tk plc. Th propgtion of th Lmb wv will bcom highly complictd svrl mods propgting t th sm tim. Th viscolstic bhvior of th dhsiv lyr will cus dmping, ttnution of th Lmb wv nrgy, nd frquncy shifts. Mny rsrchrs hv ddrssd th propgtion of Lmb wvs in dhsivly joints in th lst dcd. This study will ddrss th possibility to introduc Lmb wvs in such bondd spcimns using smll, unobtrusiv PWAS..5 Wvs trvling on th luminum sht s_1khz f c Wvs trvling long s_1khz th bond lin f b4-b5 b4-b6 b4-b7.. Amplitud.15.1 Amplitud Tim (micro-sc) Tim (micro-sc) Figur 8 () () S mod Lmb wvs trvling in th luminum mtril on th top plt; (b) S mod Lmb wvs trvling long th bond lin (b)

9 Th propgtion of th Lmb wvs in th spcimn using th pitch-ctch mthod is shown in Figur 8. For this mthod w usd pir of two snsors, on bing th trnsmittr S Lmb mod Aluminum. nd th othr on th rcivr. Th cittion signl ws 3-count Bond sin burst t 39 khz. This frquncy rng producd n S mod.18 Epon. (Aluminum Lmb wv. Th wv signl ws thn rcivd t thr.16 Epon. (Bond) Outsid bond lin conscutiv snsors 1 mm prt. Th first st of dt ws.14 cquird for snsors loctd on th luminum lyr only nd th.1 y = scond st of dt ws cquird from snsors loctd on th R = bondd lyr. Th rsults shown clrly dmonstrt th cpbility.8.6 Bond lin of our PWAS to snd nd rciv Lmb wvs in th luminum mtril itslf nd long th bond lin. It cn b obsrvd tht for.4 th wvs trvling in th luminum lyr thr is ttnution of y = R th signl s th distnc incrss but th disprsion is vry littl. =.986 Howvr, th Lmb wvs trvling long th bond lin show 1 Distnc (mm) 3 4 strong disprsion vn for th S mod, s shown in Figur 8b. This is to b pctd bcus of th dmping ffct of th dhsiv lyr. Figur 9 prsnts th ttnution of th Lmb Figur 9 Attnution of th S Lmb wv mod wvs trvling in th luminum lyr only nd long th bond trvling outsid th bond lin nd long th bond lin lin. Th nrgy of th signl trvling long th bond lin is lss thn th nrgy of th signl trvling outsid th bond lin, thus th signl is wkr, th dhsiv lyr bsorbing prt of th nrgy of th trnsmittd wv. Amplitud (mv).7 _3kHz b4 b5 b6.17 PWAS b4 clos to _3kHz disbond b4 b5 b PWAS b5 nd b6 Amplitud Amplitud Tim (micro-sc) Tim (micro-sc) Figur 1 Puls-cho mthod. A mod Lmb wv collctd t thr snsors: PWAS b4 closst to disbond. Anothr wv propgtion mthod is th puls-cho. Prticulr for th puls-cho mthod is tht th sm snsor is usd s both trnsmittr nd rcivr. Th fbrictd lp-joint spcimn incorports two rtificil disbonds. W usd th puls-cho mthod to cit A mod lmb wvs using 3-count sin burst t 1 khz frquncy. Th purpos of this primnt ws to prov th possibility to lso cit nd gnrt nti-symmtric low frquncy Lmb mod wvs bl to propgt in bondd spcimns. Figur 11 shows th thr signls rcivd by th thr conscutiv snsors. Signls rcivd by PWAS b5 nd b6 hv vry consistnt pttrn nd do not prsnt ny dditionl rflctions in th tim domin. On th othr hnd, th signl rcivd t PWAS b4 which is clos to th disbond shows diffrnt pttrn nd rflction from th disbond is prsnt in th tim domin spctrum Stnding wv mthod Th impdnc mthod is dmg dtction tchniqu complmntry to th wv propgtion tchniqus. Th mchnicl impdnc mthod consists of citing vibrtions of bondd plts using spcilizd trnsducr tht simultnously msurs th pplid norml forc nd th inducd vlocity. Th lctro-mchnicl (E/M) impdnc mthod is n mrging tchnology tht offrs distinctiv dvntg ovr th mchnicl impdnc mthod. Whil th mchnicl impdnc mthod uss norml forc cittion, th E/M impdnc mthod uss in-pln strin. Th

10 mchnicl impdnc trnsducr msurs mchnicl quntitis (forc nd vlocity/cclrtion) to indirctly clcult th mchnicl impdnc, whil th E/M impdnc ctiv snsor msurs th E/M impdnc dirctly s n lctricl quntity. HP4194A Impdnc Anlyzr HP4194A Impdnc Anlyzr GPIB Computr Bond spcim PWAS #1 # #3 Aluminum strips Simultd dlmintion Epoy dhsiv Figur 11 Instrumnttion st-up for th E/M impdnc mthod. Th ffct of pizolctric wfr ctiv snsor ffid to th structur is to pply locl strin prlll to th surfc tht crts sttionry lstic wvs in th structur. Through th mchnicl coupling btwn th PWAS nd th host structur, on on hnd, nd through th lctro-mchnicl trnsduction insid th PWAS, on th othr hnd, th driv-point structurl impdnc is dirctly rflctd into th ffctiv lctricl impdnc s sn t th ctiv snsor trminls. For this primnt w usd th sm spcimn prsntd in Figur 5. An HP4194A impdnc nlyzr ws usd to msur th E/M impdnc signtur of th PWAS ttchd to th structur. Bsd on initil plortory tsts, th frquncy rng 65 khz to MHz ws slctd. Msurmnts of th rl prt of th lctromchnicl impdnc for svrl snsors wr tkn. Rptd smpling of th dt indictd stbl nd rproducibl pttrn of th impdnc spctrum. Th rsults for two msurmnts r prsntd in Figur 1. Rl Z Rsonnt spctrum for snsor 6_65- A6 loctd on th Al plt Rsonnt spctrum for snsor B6 loctd on th bond lin b6_65- Rl Z Rsonnt spctrum 5_65- for snsor A7 loctd on th Al plt Rsonnt spctrum for snsor B6 loctd on th bond lin b5_ Frquncy (khz) Frquncy (khz) Figur 1 Rl prt of th lctromchnicl impdnc spctrum As sn in Figur 1, th rsonnt spctrum of th snsors plcd on th luminum plt is consistnt from on snsor to nothr nd is diffrnt from th rsonnt spctrum of th snsors plcd on th bond lin. Ths obsrvtions clrly rvl th fct tht, s nticiptd, th dhsiv lyr hs big impct on th propgtion of th Lmb wvs from on luminum lyr to th othr. Th dhsiv lyr cts lik dmpr dissipting prt of th wv nrgy.

11 6 5 4 Rspons from bondd rs PWAS #1&3 Nw pks du to disbonding PWAS # R Z, [Ohms] Frquncy [khz] Figur 13 E/M impdnc spctrum for lp-joint spcimn. A scond lp-joint spcimn hs bn fbrictd. Two luminum strips, mm, whr bondd using n poy pst Hysol EA 939.3NA. Th disbonding of th two luminum strips ws simultd s discontinuity of th poy pst in th middl of th spcimn, hving th lngth of 5 mm. Using n HP4194A impdnc nlyzr th E/M impdnc ws msurd in thr loctions in th rng of 1 1 khz. Th rl prt of th E/M impdnc spctrum rcordd t th thr snsors PWAS #1, PWAS #, nd PWAS #3 is prsntd in Figur 13. It cn b obsrvd tht th frquncy rspons is vry similr for PWAS # 1&3 which r loctd on bondd r, but is diffrnt for PWAS # loctd on th disbond. W cn clrly s nw shrp pks in th frquncy spctrum du to disbond. 5. CONCLUSION This ppr hs prsntd th mchnism through which pizolctric wfr ctiv snsors (PWAS) cn cit nd dtct guidd wvs (Lmb wvs). It ws found tht PWAS coupl intimtly with th surfc-motion pttrns of th guidd Lmb wv trvling in thin-wll structurs. Th nlysis strtd with brif dscription of th PWAS nd thir bility to work s rsontors. Nt th coupling btwn th PWAS nd th host structur through th dhsiv lyr ws studid. Th nlysis ws prformd in th wvnumbr spc. Th invrs Fourir trnsform ws usd to rturn to th physicl spc. A gnrl solution ws obtind for gnric prssion of th intrfcil shr strss distribution. It ws lso shown tht bsd on th rspons of th strin wv th PWAS trnsducrs r bttr suitd for th cittion nd dtction of Lmb wvs t high frquncis tht th convntionl ultrsonic trnsducrs. Th work prsntd hs shown tht PWAS in spit of thir smll dimnsions r suitbl for mbddd ultrsonic nondstructiv vlution nd r cpbl to cit th structur nd gnrt guidd Lmb wvs tht will trvl cross th dhsiv lyr. This mrging tchnology rquirs sustind ffort nd continu dvlopmnt to chiv its full potntil nd to b bl to b dployd on full-scl structurl hlth monitoring systm. ACKNOWLEDGMENTS This mtril is bsd upon work supportd by th Ntionl Scinc Foundtion undr Grnt # CMS nd CMS-58873, Dr. Shih Chi Liu Progrm Dirctor nd by th Air Forc Offic of Scintific Rsrch undr Grnt # FA , Cpt. Clrk Allrd, PhD Progrm Mngr. Any opinions, findings, nd conclusions or rcommndtions prssd in this mtril r thos of th uthors nd do not ncssrily rflct th viws of th Ntionl Scinc Foundtion or th Air Forc Offic of Scintific Rsrch.

12 REFRENCES 1. Crwly, E.A., d Luis, J., Eprimntl vrifiction of distributd pizolctric ctutors for us in prcision spc structurs, AIAA/ASME/ASCE/AHS Structurs, Structurl Dynmics & Mtrils Confrnc, v 1, p , Crwly, E.A., Edwrd, F. Us of pizolctric ctutors s lmnts of intllignt structurs, AIAA Journl, v 5, p , Fullr, C.R., Snydr, S.D., Hnsn, C.H., Silco, R.J. Activ control of intrior nois in modl ircrft fuslgs using pizocrmic ctutors, AIAA Journl, v 3, p , Tzou, H.S., Tsng, C.I. Distributd modl idntifiction nd vibrtion control of continu. Pizolctric finit lmnt formultion nd nlysis, Journl of Dynmic Systms, Msurmnt nd Control, Trnsctions of th ASME, v 113, p 5-55, Lstr, H.C., Lfbvr, S. Pizolctric ctutor modls for ctiv sound nd vibrtion control of cylindrs, Journl of Intllignt Mtril Systms nd Structurs, v 4, p 95-36, Chng, F.K. Mnufcturing nd dsign of built-in dignostics for composit structurs, Procdings of th 5 nd Mting of th Socity for Mchinry Filur Prvntion Tchnology, Chng, F.K. Structurl hlth monitoring: rospc ssssmnt, Procdings of Aro Mt 1 8. Lin, M., Chng, F.K. Th mnufctur of composit structurs with built-in ntwork of pizocrmics, Composits Scinc nd Tchnology, v 6, p , 9. Ihn, J.B, Chng, F.K. Dtction nd monitoring of hiddn ftigu crck growth using built-in pizolctric snsor/ctutor ntwork: I. Dignostics, Smrt Mtrils nd Structurs, v 13, p 69-6, 4 1. Dimnti, K., Hodgkinson, J.M., Soutis, C. In-srvic hlth monitoring of composit structurs using lmb wvs, Procdings of SPIE - Th Intrntionl Socity for Opticl Enginring, v 4763, p , 11. Osmont, D., Dupont, M., Gouyon, R., Lmistr, M., Blgs, D. Dmg nd dmging impct monitoring by PZT snsors-bsd HUMS, Procdings of SPIE - Th Intrntionl Socity for Opticl Enginring, v 3986, p 85-9, 1. Lin, X., Yun, F.G. Dmg dtction of plt using migrtion tchniqu, Journl of Intllignt Mtril Systms nd Structurs, v 1, p , Ling, C., Sun, F.P., Rogrs, C.A. Coupld lctro-mchnicl nlysis of dptiv mtril systms - dtrmintion of th ctutor powr consumption nd systm nrgy trnsfr, Journl of Intllignt Mtril Systms nd Structurs, v 5, p 1-, Lops, V., Prk, G., Cudny, H.H., Inmn, D.J. Impdnc-bsd structurl hlth monitoring with rtificil nurl ntworks, Journl of Intllignt Mtril Systms nd Structurs, v 11, p 6-14, 15. Giurgiutiu, V., Rynolds, A., Rogrs, C.A. Eprimntl invstigtion of E/M impdnc hlth monitoring for spot-wldd structurl joints, Journl of Intllignt Mtril Systms nd Structurs, v 1, p 8-81, 16. Giurgiutiu, V., Rogrs, C. A. "Rcnt Progrss in th Appliction of E/M Impdnc Mthod to Structurl Hlth Monitoring, Dmg Dtction nd Filur Prvntion", nd Intrntionl Workshop of Structurl Hlth Monitoring, p 98-37, Giurgiutiu, V.; Zgri, A. N. Embddd Slf-Snsing Pizolctric Activ Snsors for Onlin Structurl Idntifiction, ASME Journl of Vibrtion nd Acoustics, v 14, p , 18. Giurgiutiu, V.; Zgri, A. N.; Bo, J. Pizolctric Wfr Embddd Activ Snsors for Aging Aircrft Structurl Hlth Monitoring, Structurl Hlth Monitoring An Intrntionl Journl, Sg Pub., v 1, p 41-61, 19. Giurgiutiu, V.; Cuc, A. Embddd NDE for Structurl Hlth Monitoring, Dmg Dtction, nd Filur Prvntion, Shock nd Vibrtion Rviws, Sg Pub., v 37, p 83-15, 5. Giurgiutiu, V. Tund Lmb-Wv Ecittion nd Dtction with Pizolctric Wfr Activ Snsors for Structurl Hlth Monitoring, Journl of Intllignt Mtril Systms nd Structurs, v 16, p 91-36, 5 1. Ikd, T., Fundmntls of Pizolctricity, Oford Univrsity Prss, 1996