Guided Wave Piezoelectric Wafer Active Sensor Arrays For Damage Detection L. Yu & V. Giurgiutiu University of South Carolina, SC, USA

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1 Guided Wave Piezoeletri Wafer Ative Sensor Arrays For Daage Detetion L. Yu & V. Giurgiutiu University of South Carolina, SC, USA ABSTRACT: The paper desribes an in-situ ethod for daage detetion using phased arrays of piezoeletri wafers ative sensors (PWAS) that are peranently attahed to the struture. The ethod uses sanning beas of guided waves that an travel at large distanes with little energy loss and hene perit the interrogation of large strutural areas fro a single loation. The novelty of the present ethod, as different fro onventional ultrasoni phased array, onsists in use of the unobtrusive and inexpensive peranently attahed PWAS to perfor interrogation and beaforing as a signal post proessing proedure. In order to apply the beaforing theory to guided wave appliation, a ode tuning tehnique is eployed to obtain the required single ode exitation for the phased array appliation. After reviewing the priniples of the PWAS phased array, its ipleentation algorith, the ebedded ultrasoni strutural radar algorith (EUSR), is introdued briefly. Based on the developed PWAS phased array beaforing forula, we bring up the pratial ipleentation of PWAS arrays and present analytial and experiental results onerning the onidiretional daage detetion ability. Finally, daage detetion on aluinu plates with various PWAS phased arrays is onduted. Results are presented and opared between linear and planar arrays. Keywords: Phased Array, PWAS, Piezoeletri, Guided Waves, Lab Waves, Daage Detetion, Strutural Health Monitoring IN-SITU EMBEDDED ULTRASONIC GUIDED WAVE SHM Strutural health onitoring (SHM) is able to detet and interpret adverse hanges in the struture, whih will greatly redue the life yle ost. Soe of the advantages of SHM over onventional nondestrutive evaluation (NDE) would be redued inspetion downtie, eliination of oponent tear down, and potential prevention of failure during operation. This would well be used in identifying failure in airraft, where thin plate-like strutures find a variety of appliations in the aerospae industry (Rajagopalan et al 2006). Due to the etiulous through-the-thikness C- sans of large area, strutural inspetion and daage detetion in thin wall struture using onventional NDE tehniques are tie-onsuing. One way to inrease the effiieny is to utilize the guided waves, e.g., Lab waves in thin plates. Attepts have been ade to ipleent the inspetion of large struture fro a single point by using phased arrays. The advantages of using a phased array are ultiple (Krautkraer and Krautkraer 990) (Rose 999). The appliation of phased array suh as an EMAT array or a passive piezoeletri wafer array has been ade by any researhers (Froe et al 2005) (Sundararaan et al 2005). However, it sees ipratial and ostly to oneive an SHM syste based on onventional ultrasoni transduers peranently installed in an airraft struture in a nuber suffiient to ahieve the required strutural overage. The low ost, inexpensive, and sall oerially available transduers are needed to onstrut ebedded in-situ phased array for SHM. In this paper, we bring up the onstrution and ipleentation of an in-situ ebedded phased array using the piezoeletri wafer ative sensors. The priniples of PWAS for Lab wave generation and reeption and PWAS phased array beaforing will be briefly overed. We also deonstrate daage detetion in thin wall aluinu plate by using differently onfigured PWAS phased arrays to detet a set of various defets.

2 2 PIEZOELECTRIC WAFER ACTIVE SENSOR FOR GUIDED WAVES Considerable effort has been dediated to the study of the interation between the Lab wave generation and detetion, as well as of finding the optiized exitation frequeny and alibrating the Lab wave group veloity (Giurgiutiu 2005). 2. PWAS Lab wave priniples Lab waves are guided waves that propagate inside thin-wall plates and shallow shells. Lab waves an be either syetri or anti-syetri aross the aterial thikness (known as Sn and An, respetively, where n represents the nuber of infletion points aross the thikness). The Lab wave phase veloity, L, depends on the produt of the frequeny and the aterials thikness, i.e., Lab waves are dispersive waves. The dispersion urves an be obtained by solving Rayleigh-Lab frequeny equation (Giurgiutiu 2005). PWAS are sall wafers of piezoeletri aterial that are peranently bonded to the aterial surfae and an siultaneously at as elasti wave transitters and reeptors. Due to the in-plane surfae oupling, PWAS are ideally suited for the generation of guided plate waves (Lab waves). Figure shows how a surfae-ounted PWAS an siultaneously exite both S0 and A0 Lab waves. for the exitation and reeption of Lab waves than that of the onventional ultrasoni transduers whih an only ipinge noral to the aterial surfae. 2.2 PWAS frequeny tuning As we know, Lab waves an exist in a nuber of dispersive odes, whih opliates the appliation of guided wave phased arrays. However, through soothed tone-burst exitation and frequeny tuning, it is possible to onfine the exitation to a partiular Lab wave ode, of arrier frequeny f, wave speed, and wave length λ=/f (Giurgiutiu 2005). As shown in Figure 2, within the frequeny range 0~700 khz, only two odes, A0 and S0 odes, exist on the - thik aluinu plate by using a 7- round PWAS. At low frequeny, A0 ode doinates while S0 is inor. When the frequeny inreases beyond 200 khz, the exitation of A0 dereases rapidly while that of S0 inreases. At 300 khz, a sweet spot was obtained, where doinant S0 ode and axially suppressed A0 odes were observed. The frequeny tuning effet was found to be of ultiate iportane sine it presented us with the opportunity of exiting single low dispersive S0 waves of strong intensity, whih is ritial for the ipleentation of Lab wave phased array. A0 ode Noralized strain S0 ode Figure 2 PWAS frequeny tuning on a - aluinu plate using 7- round PWAS Figure Lab wave siulation in a - thik aluinu plate. S0 ode at.56 MHz; A0 ode at MHz Of partiular iportane is the fat that PWAS are oupled with the aterial strains parallel to the aterial surfae. Thus transission and reeption of Lab waves in thin-wall strutures are greatly failitated. This type of oupling, whih is parallel to the aterial surfae, is signifiantly ore effiient 3 PWAS PHASED ARRAY BEAMFORMING AND IMPLEMENTATION In the PWAS phased array ipleentation, we assue: All the eleents in the array lie in the sae plane All the eleents behave as point-wise soures and reeivers Monohroati exitation and reeption regardless of the dispersion of guided wave applies Siultaneous and unifor exitation if not otherwise larified

3 The non-dispersion assuption results in a onstant speed in isotropi aterials. 3. Generi beaforing forulation PWAS phased array beaforing is based on the delay-and-su algorith. The delay-and-su beaforing onsists of two steps:. Applying delay and weighting w to the propagating wave fro the th PWAS, f ( r, t) 2. Suing up the output signals of the total of M PWAS The bea fro an array onsisting of a total of M PWAS loating at { s }, =0,, M- will be direted to a point Pr (, ). The oordinate systes is defined by using the array phased enter where the origin oinides with the phased enter (Figure 3). P(, r ) O r s r th sensor Figure 3 Geoetrial sheati of an M-PWAS array with a target loated at Pr (, ) For a single tone, outgoing radial ode wave f ( rt, ), its wave front at the target P is in the for A j tkr f( r, t) e () r with k as the wave nuber, k /, and ω as the angular exitation frequeny. The syntheti wave front at P fro all the M PWAS is M A jtkr zrt (, ) w e (2) 0 r Equation (2) an be re-written as M r w j r / (, ) ( ) zrt f t e (3) 0 r where r is noralized r. Equation (3) shows that the syntheti wave is opleted deterined by the seond ultiplier, whih is defined as beaforing fator given by M 0 j BF( w, r ) w e (4) r In order to steer the output wave zrt (, ) at ertain diretion 0, the delays { 0 } are applied, i.e., r r M j ( 0 ) 0 0 r BF( w, r, ) w e (5) with ( 0 ) r (6) The delays together with weight of { w r } will result in a odified fator of M BF( w, rs,, ) M (7) 0 0 Fro Equation (3) and (7) we an see that the final syntheti wave beoes M ties reinfored of the original individual wave fro a single soure. When the target diretion 0 hanges with the range of 0º~360º, a bea sanning of the entire plane an be aoplished. 3.2 Ebedded ultrasoni strutural radar (EUSR) algorith Ebedded ultrasoni strutural radar (EUSR) is a onept that utilizes the phased array and ultrasoni guided wave priniples to ipleent the sanning of large thin-wall strutures for daage detetion (Giurgiutiu et al 2006). The EUSR onept onsists of () a PWAS phased array ebedded onto the struture and (2) eletroni odules for signal transission/reeption, proessing, and interpretation. In EUSR syste, the Lab waves are generated and reeived with surfae ounted PWAS. Lab waves an travel over large distanes with little attenuation and an travel inside urved walls with shallow urvature. In PWAS phased array, eah eleent plays the role of both transitter and reeiver and the role is hanged in a round-robin fashion by an oputer ontrolled eletronis (Liu and Giurgiutiu 2005) for that purpose. After responses of the struture to all the exitation signals are olleted, the EUSR algorith applies an appropriate delay to eah signal in the data set as a signal post proessing proedure to ake the all fous on a ertain diretion 0. When the 0 hanges fro 0º~360º, a virtual sanning bea is fored and the full area of the struture is interrogated. More details about the EUSR ipleentation an be found in the referene by Giurgiutiu et al (2006). Therefore, due to its low ost, sall size, and unobtrusiveness, the EUSR PWAS phased array sees better suited fro in-situ SHM of thin-wall strutures than onventional ultrasoni transduer arrays.

4 4 PWAS PHASED ARRAY LABORATORY EXPERIMENTS A proof-of-onept syste was built to verify the daage detetion ability using various Lab wave PWAS phased arrays (Figure 4a). DAQ odule uses a oputer to ontrol and ollet data fro ultihannel data olletion equipent, whih onsists of: (i) a HP3320A arbitrary signal generator; (ii) a Tektronix TDS20 digital osillosope; and (iii) a oputer ontrolled auto-swith unit, ASCU unit. The HP3320A was used to generate the exitation with a 0 Hz repetition rate. The Tektronix TDS20 digital osillosope, synhronized with the digital generator, was used to ollet the response signals fro the PWAS array. The atuation and sensing funtion of the PWAS eleents are autoatially ontrolled by the ASCU unit square panel of - thik 2024-T3 Al-lad airraft grade sheet etal speiens with different rak layouts were used. The size of the siulated raks is 9- long, wide. The PWAS used is APC-850 sall PZT wafer 7 round or square and 0.2 thik. Figure 4b shows the layout of the speiens. Either -D linear or 2-D retangular PWAS arrays are used whih onstruted by 7- round or square PWAS spaing at 8 (enter to enter distane). The exitation frequeny was tuned at 300 khz for using S0 Lab wave ode. TDS20 digital osillosope HP3320 signal generator Autoswith unit PC graysale iage shown in Figure 5a. Note that an observation window is atually used to axiize the display by reoving the initial bang and the refletions fro the boundaries. A-san signals at 30º and 90º are given as well in Figure 5b. EUSR daage detetion was also onduted on speiens with different defets. Figure 6 gives the EUSR iage of a plate with a single.57 pin hole about 30 away fro the array. Our experiental results show that.57 is the iniu detetable size for suh an array operating at 300 khz on the - aluinu plate. Figure 7 is the EUSR iage of a plate with two offside raks (as illustrated in Figure 4b) at 67º and 7º, respetively. Sine the raks orientation is parallel to the array alignent, ost of the inipient waves are refleted away and no speula refletion is present. However, diffration at rak tips reates suffiient baksatter waves to perit detetion. The result show that EUSR PWAS arrays an detet raks whih are loated in non-optial positions and an detet ultiple defets. V, volt 90º 30º t, μs Speien under inspetion Offside defet Broadside defet (24.00 in) Figure 5 EUSR sanning of a - thik aluinu plate using an 8 round PWAS unifor linear array. EUSR iage indiating the presene of a single broadside rak; seleted A- san signals at 30º and 90º, respetively Plate #3.57 pin hole 570 (22.45 in) PWAS array 220- sq., - thik 2024 T3 ( in sq., in thik) Figure 4 Proof-of-onept EUSR experient. equipent setup; speien sheati Figure 6 EUSR sanning of a - thik aluinu plate using an 8 round PWAS unifor linear array. EUSR iage indiating the presene of a single broadside.57 pin hole 4. -D linear unifor arrays A linear array onsisting 8 round PWAS is used to detet a single broadside rak about 305 away. Sanning result fro EUSR is provided as a 2-D

5 Plate #5 Dolph-Chebyshev array; EUSR iage using Binoial array D retangular arrays Figure 7 EUSR sanning of a - thik aluinu plate using an 8 round PWAS unifor linear array. EUSR iage indiating the presene of two offside raks at 67º and 7º, respetively 4.2 -D linear non-unifor arrays Aording to our study, weighting is an iportant fator of beaforing properties. Non-uniforly exited arrays have been developed by the authors and naed aordingly as Binoial PWAS array and Dolph-Chebyshev array, respetively (Yu and Giurgiutiu 2007). Coparing the three -D linear arrays, we have found out that the unifor array has the finest bea width, followed with Dolphhebyshev and Binoial array, while Binoial array has zero side lobe when eleents spaing at half wavelength. The unifor array shows the ost signifiant sidelobes aong the three. Experients were onduted to verify these onlusions. Figure 8a is the iage fro a Dolph-Chebyshev array designated at sidelobe level of 20 (Yu and Giurgiutiu 2007). Carefully opared with the iage fro the equivalent unifor array in Figure 5a, we an see the phanto ring other than the broadside showing in Figure 5a did not present in the iage of Dolph- Chebyshev array. The rak size however, reained oparable. Figure 8b is the EUSR iage of using a Binoial array whih shows no phanto ring at undesired diretions. However, the rak size appears uh larger than those of unifor array and Dolph- Chebyshev arrays. The onlusions of the three linear arrays are suessfully verified. As we have notied,-d array is liited to 0º~80º sanning due to the intrinsi geoetri liitation. Hene, 2-D planar PWAS arrays have been proposed and developed by Yu and Giurgiutiu (2007). Sanning of using a 4x8 retangular onfiguration PWAS array was first onduted to detet the broadside rak and the EUSR iage is shown in Figure 9a. Fro the EUSR iage we notied a strong phanto iage at 270º whih was atually resulted by the baklobe at the exitation frequeny. A-san at 90º and 270º are also provided in Figure 9b to show the differene. Considering the differene between ainlobe and baklobe, the disturbane at 270º an be reoved by using a siple thresholding proess. Iproved thresholded iage is shown in Figure 0. To reove the baklobe influene in the 4x8 array, a siple way is to inrease nuber of eleents in the planar array. Hene, a new 8x8 array was used to re-san the speien with a single broadside rak at 90º. The diret apping iage is given in Figure. No phanto iage at other diretion was observed. Another speien with a single 4- sall hole at 80º was also sanned by the 8x8 PWAS array and resulting iage is shown in Figure 2. Note that bakground noise has been reoved by thresholding. Phanto iage Figure 9 EUSR sanning using a 4x8 PWAS array. original EUSR iage; A-san signals at 90º and 270º Figure 8 EUSR sanning of a - thik aluinu plate using an 8 round PWAS non-unifor array. EUSR iage using

6 Figure 0 EUSR iage of a 4x8 PWAS array after thresholding Figure 3 EUSR iage of an 8x8 PWAS array on a speien with a rak at 90º and a 4- sall hole at 80º 5 CONCLUSIONS Figure EUSR iage of an 8x8 PWAS array on a speien with a single broadside rak at 90º A ore opliated sanning was onduted on a plate with a rak at 90º and a 4- sall hole at 80º, both being 305 away fro the array. EUSR iage is presented in Figure 3. Coparing the indiation of the sall hole and that of the rak, we see the refletion aused by the rak is uh stronger than that fro the sall hole. EUSR not only deteted the presene of both defets but also deonstrated the differene between different defet types. Sall hole In this paper, we deonstrated the use of piezoeletri wafer ative sensor phased array for daage detetion in thin wall etalli strutures. Unlike the onventional ultrasoni phased arrays, PWAS phased array provides a novel ebedded solution for in-situ SHM and daage detetion with the abilities of large area sanning at affordable prie and with less eletronis requireents. The arrays are onsidered ebedded into the struture onsidering they are peranently attahed to the struture and an be left in plae for life of the struture. The theoretial and experiental investigation presented in this paper verified that PWAS phased array an be used to detet single or ultiple rak and/or hole defets within 0º~360º range. The generi beaforing forula offered the theoretial predition for any phased array onfiguration design. More studies and experients should be extended to pratial geoetries and to oposite or hybrid-aterial strutures. 6 ACKNOWLEDGEMENT This aterial is based upon work supported by the National Siene Foundation under Grant #CMS and #CMS and by the Air Fore Offie of Sientifi Researh under Grant # FA Figure 2 EUSR iage of an 8x8 PWAS array on a speien with a single 4- sall hole at 80º 7 PREFERENCES Clay, A. C.; Wooh, S. C.; Azar, L.; and Wang, J. Y. Experiental Study of Phased Array Bea Steering Charateristis, Journal of Nondestrutive Evaluation, Vol. 8, No. 2, 999, pp. 59~7 (999) Froe, P.; Wilox, P.; Lowe, M.; and Cawley, P., A Guided Ultrasoni Waves Array For Strutural Integrity Monitoring, Quantitative Nondestrutive Evaluation, Vol. 24, pp. 780~787 (2005)

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