Damage Detection Based on Static Strain Responses Using FBG in a Wind Turbine Blade

Transcription

1 Sensors 05, 5, ; do:0.3390/s Artce OPEN ACCESS sensors ISSN Damage Detecton Based on Statc Stran Responses Usng FBG n a Wnd Turbne Bade Shaohua Tan,, Zhbo Yang,, *, Xuefeng Chen, and Yong Xe 3,4 The State Key Laboratory for Manufacturng Systems Engneerng, X an 70049, Chna; E-Mas: shaua@6.com (S.T.); chenxf@ma.xjtu.edu.cn (X.C.) Schoo of Mechanca Engneerng, X an Jaotong Unversty, X an 70049, Chna 3 State Key Laboratory of Strength and Vbraton for Mechanca Structures, X an Jaotong Unversty, X an 70049, Chna; E-Ma: yxe@ma.xjtu.edu.cn 4 Schoo of Aerospace, X an Jaotong Unversty, X an 70049, Chna * Author to whom correspondence shoud be addressed; E-Ma: phdappe@ma.xjtu.edu.cn; Te./Fax: Academc Edtor: Vttoro M. N. Passaro Receved: 5 May 05 / Accepted: August 05 / Pubshed: 4 August 05 Abstract: The damage detecton of a wnd turbne bade enabes better operaton of the turbnes, and provdes an eary aert to the destroyed events of the bade n order to avod catastrophc osses. A new non-basene damage detecton method based on the Fber Bragg gratng (FBG) n a wnd turbne bade s deveoped n ths paper. Frsty, the Ch-square dstrbuton s proven to be an effectve damage-senstve feature whch s adopted as the ndvdua nformaton source for the oca decson. In order to obtan the goba and optma decson for the damage detecton, the feature nformaton fuson (FIF) method s proposed to fuse and optmze nformaton n above ndvdua nformaton sources, and the damage s detected accuratey through of the goba decson. Then a 3. m wnd turbne bade wth the dstrbuted stran sensor system s adopted to descrbe the feasbty of the proposed method, and the stran energy method (SEM) s used to descrbe the advantage of the proposed method. Fnay resuts show that the proposed method can dever encouragng resuts of the damage detecton n the wnd turbne bade. Keywords: wnd turbne bade; CSD; FIF; SEM; MDDE

2 Sensors 05, Introducton Structura heath montorng (SHM) s expected to pay a major roe n the deveopment of arge wnd turbnes wth hgher effcency and ower cost-of-energy []. L et a. [] gave a comprehensve revew on the damage detecton methods for wnd turbne bades. Tayor et a. [3] presented an ongong work to mpement rea-tme SHM systems for operatona research-scae wnd turbne bades wth pezoeectrc actve sensors. Park [4] desgned a rea-tme montorng system wth FBG sensors for a MW wnd turbne (type U88). Those wnd turbnes are usuay subjected to severe operatona oads, therefore they requres strngent safety measures and more frequent mantenance, but t s qute dffcut and costy to perform nspecton and mantenance work on these arge wnd turbnes, many because of ther heght and mtatons assocated wth the remoteness of the nstaaton ocatons, such as offshore wnd farms. Ceary, obtanng precse and rea tme nformaton usng SHM systems can be nvauabe for mprovng safety, for owerng the frequency of sudden breakdowns and, more mportanty, for movng away from a costy schedue-based mantenance toward a cost-effectve and condton-based mantenance; a factors ead to the sgnfcant reducton n the operatona cost of wnd turbnes [5]. A precondton for the wnd turbne SHM s the understandng of oads, whch s mportant to manufacturers and wnd-farm owners. For the manufacturer, a better understandng of the oads enabes mproved desgns. For the wnd-farm operator, understandng oads caused by the damage enabes the better detecton of potentay damagng stuatons, and t provdes an eary aert to bade throw events, whch coud have catastrophc consequences for anythng n the surroundng area. The Fber Bragg gratng (FBG) shows an advantage over the conventona stran sensor n understandng oads of the wnd turbne bade. Typcay wnd turbnes are desgned for a 0-year operatona fe, and durng that tme the sensor w undergo approxmatey 60 mon cyces. A conventona stran sensor w typcay fa after ess than 60,000 cyces, but the FBG shows no degradaton n the performance after 00 mon cyces. Therefore, once embedded wthn the wnd turbne bade durng the manufacturng process, the FBG s there for the fe of the wnd turbnes, wth no need for servcng or recabraton [6]. The deveopment of the wnd turbne SHM usng FBG stran sensors has receved wde attenton recenty due to the ncreasng nterest n renewabe energy. Km et a. [7] empoyed the operatona moda anayss to construct a dspacement-stran transformaton matrx for a rea-tme shape estmaton technque, and the deveoped technque s apped to a wnd turbne bade, n whch the FBG sensors are embedded. Arsenaut et a. [8] cacuated the power spectra densty based on the stran response of the wnd turbne bade, a umped mass was attached near the tp of the bade to smuate the damage, and the comparson between the umped mass-added bade power spectra and that of the ntact bade was made n order to detect damage. Nchos et a. [9] detected the presence of damage-nduced nonneartes n composte structures usng ony the structura vbraton response, and the damage was assumed to change the coupng between dfferent ocatons on the structure from near to nonnear; two nformatona metrcs, the tme-deayed mutua nformaton and tme-deayed transfer entropy, were obtaned from the obtaned tme-seres data, so the presence of the mpact damage was detected n a thck composte sandwch pate. Lau et a. [0] empoyed the FBG as a structura heath montorng devce for fber renforced pastc materas by ether embeddng nto, or bondng onto, the structures, and the accuracy of the stran wth the FBG sensor s hghy dependent on the bondng characterstcs

3 Sensors 05, among the bare optca fber, protectve coatng, adhesve ayer and host matera. Okabe et a. [] deveoped sma-dameter FBG sensors for embeddng the amnated composte pate wthout deteroraton of the mechanca propertes. Tsuda [] constructed two types of FBG utrasonc sensng for the damage detecton n carbon-fber renforced pastcs. Therefore, t can be concuded that the key procedure of the SHM s to extract the damage-senstve feature from the measured parameter and track those features wth the presence of damage, wth the prmary goa of the SHM to combne advances n both sensng and data anayss n order to produce an automated system capabe of detectng damage wthout requrng vsua nspectng [9]. The presence of damage s aways accompaned wth the oca varaton on stran parameters n the vcnty, but those n the ntact regon reman unchanged, so the damage senstve feature must be abe to measure and dspay ths dssmarty of the stran parameter due to damage. As a nonparametrc test statstc method, the Ch-square dstrbuton (CSD) can provde a probabstc procedure for testng the hypothess that two probabstc dstrbutons have been generated from the same underyng dstrbuton, and then the structura dssmarty s measured based on the emprca estmates. Rubner et a. [3] empoyed the CSD to compete the texture dssmarty measure. Mathassen and Skavhaung [4] empoyed t to measure the texture dssmarty n compostes, and then Puzcha [5] utzed t to evauate the dssmarty for the coor and texture. In ths study, the CSD s adopted to be as an effectve damage-senstve feature and ndvdua nformaton source for the damage detecton. To fuse and optmze expct nformaton of the ndvdua nformaton source CSD on the damage ocaton, an nformaton fuson method addressed as a feature nformaton fuson (FIF) method s deveoped, and t can combne data from mutpe nformaton sources and reated nformaton from assocated databases, n order to acheve mproved accuraces and more specfc nferences than coud be acheved by the use of a snge source aone. Each source makes ts oca decson based on ts observaton, and the oca decson of each source s sent to the fuson center, where the goba decson based on the oca decsons s obtaned. In ths paper, a damage detecton method n the wnd turbne bade based on the FBG s presented. Frsty, the stran response under varyng eves of statc oad s measured usng a dstrbuted sensor network, consstng of tweve FBG sensors adhered to the wndward sde of the wnd turbne bade, whch s consstent wth the stuaton n servce. The presence of damage ncreases the dssmarty among stran responses under dfferent eves of statc oads, so the dssmarty s measured by the damage-senstve feature CSD, whch s adopted as the ndvdua nformaton source, and then the oca decson on the damage detecton s made. Secondy, the feature nformaton fuson (FIF) method s proposed to fuse and optmze nformaton n the above ndvdua nformaton source n order to obtan the goba and optma decson for the damage detecton, so the damage s detected accuratey on the bass of the goba decson. Furthermore, the mean damage detecton error (MDDE) s used to descrbe the damage detecton error quanttatvey, and the stran energy method (SEM) s empoyed to show the advantage of the proposed method. Fnay, the proposed method does not requre the fu stran hstory to detect the current damage, unke the FBG peak waveength measurement, and resuts show that t can dever encouragng resuts of the damage detecton n the wnd turbne bade.

4 Sensors 05, Damage Detecton Method In ths secton, a damage detecton method based on the CSD and FIF s deveoped. The frst procedure s to adopt the CSD as the damage-senstve feature and ndvdua nformaton source, and the next s to empoy the FIF to obtan a goba and optma decson for the damage detecton... The Ch-Square Dstrbuton The Ch-square dstrbuton s a wdey used too n the statstcs and pattern recognton to measure the texture dssmarty, and t s defned as foows: (, ) CSD p q = () j() () + () N L qj p () j= = 0 qj pj where p() and q () represent probabty densty functons respectvey, L represents the eement number, j denotes the feature dmenson, and N s the number of the feature dmenson. On the rght sde of Equaton (), the numerator represents the dfference between two probabty densty functon, and the denomnator represents the sum of them, so t can be concuded that the CSD ndcates the dfference between two probabty densty functons and t can be used to measure the dssmarty among them. The presence of damage s aways accompaned wth the varaton on oca stran parameters, and correspondng ones n the ntact regon reman unchanged, so the CSD can be used to ndcate the dfference among stran responses wth the presence of damage and be regarded as an effectve damage-senstve feature. When the CSD s used to be an damage-senstve feature of the wnd turbne bade, the stran responses under the statc oad are measured usng the dstrbuted stran sensor system, and then the normazaton of the stran response s conducted; therefore, the stran response can be regarded as two x ε m x are the normazed stran responses probabty dstrbuton functons, suppose that ε ( ) and ( ) under the oad and m respectvey, the correspondng CSD can be gven as foows: where CSD m L m ( ε ( x) ε ( x) ) L represents the ength of the wnd turbne bade, and x x = dx () 0 m ε ( x) +ε ( x) m CSD denotes the CSD based on the stran response under the oad and m. Suppose that the wnd turbne bade s dvded nto N eements, the CSD of the th eement s defned as foows: where (, ) CSD m ( ( x) ( x) ) a m = a m ε x +ε ε ε + dx (3) ( ) ( x) m a a + represents the coordnate of the th eement, ε ( x) and ε ( x) denote the m normazed stran response of the th eement under the oad and m respectvey, and the CSD represents the CSD of the th eement under the oad and m.

5 Sensors 05, For the damage detecton n the wnd turbne bade, the CSD under dfferent statc oads are regarded as ndvdua oca decsons, so how to obtan a goba and optma decson based on them shoud be taken nto account. Then, the feature nformaton fuson method (FIF) s adopted n order to retan the advantage and abandon the shortcomng of the ndvdua nformaton sources at the same tme, whch s ntroduced n the next secton... The Feature Informaton Fuson (FIF) In the FIF, judgments and decsons are made on the bass of the past nformaton, and they are modfed reguary when addtona nformaton become avaabe. Ths s partcuary mportant when a subjectve decson has to be made, ntay, wth the ack of nformaton, and then refned subsequenty wth ater ncomng nformaton. Suppose that there are two nformaton sources S and S, and NE objects need to be dentfed, whch can be expressed as A, A, A 3, ANE, so the probabty of each object s denoted as PA ( ), PA ( 3),, PA ( NE ) respectvey, et the kerne functon and ndex functon of the nformaton source be ( ) k A and M ( ) A, respectvey, so the probabty of each object can be cacuated as foows: ( ) ( ) ( )(,,3,..., NE) P A = M A k A = (4) then the condtona probabty of each object has been obtaned; that s, P ( S, S ) so the condtona probabty P ( A S, S ) of the object ( AS S) P, = NE A can be gven as foows: P ( S, S A) P( A) P S, S Aj P A j= ( ) ( j) A s aready known, When the decson of each nformaton source s consdered as ndependent, the above equaton can aso be wrtten as: ( AS S) P, = NE P( S A) P( S A) P( A) P P P j= ( S Aj) ( S Aj) ( Aj) It s cruca to seect the nformaton on the damage ocaton as the feature nformaton, and the maxma probabty of each object s consdered as the feature nformaton n the FIF, so Equaton (6) can be wrtten as: (, ) P A S S ( P( S A) P( S A) ) P( A) NE P( S Aj) P( S Aj) P( Aj) max, = j= If there s M ndependent nformaton sources need to decded, the above procedure of seectng the feature nformaton shoud be done twce. Let the maxmum of the vector { P( S A), ( ), P S A P( SM A) } (5) (6) (7)

6 Sensors 05, be the feature nformaton denoted as ( k ) assumed to be zero, so the vector becomes max ( P S A ), and then the maxmum of the above s { P( S A), ( ),,0, P S A P( SM A) } To obtan the next feature nformaton, the maxmum of the above-modfed vector s denoted as max ( P S A ) s obtaned; therefore, the correspondng nformaton fuson can be gven as foows: ( k ) ( AS S) P, ( k ) ( k ) max ( P S A ) max ( P S A ) == NE M P j= k= ( Sk Aj) P( Aj) In the process of obtanng the goba and optma decson, the trange kerne s adopted as the kerne functon of the nformaton source, the CSD under the smaest statc oad s regarded as the ndex functon n the nformaton fuson, and correspondng ones under other oads are adopted as ndvdua nformaton sources. To retan the advantage and abandon the shortcomng of the CSD at the same tme, the most mportant nformaton of the above ndvdua nformaton sources s seected as the feature nformaton, so the goba and optma decson s made for the damage detecton. Generay speakng, the man advantage of the proposed method s that the proposed FIF can fuse the nformaton of the ndvdua nformaton for the damage detecton. Secondy t doesn t need the nformaton of the ntact wnd turbne bade, so t s a non-basene damage detecton method, Fnay, ony the stran response n the wndward sde of the wnd turbne bade s used, whch s more consstent wth the stuaton n servce. 3. Expermenta Vadaton 3.. Expermenta Setup A 0 kw, 3. m wnd turbne bade s empoyed to descrbe the feasbty of the proposed method. The camped bade s fxed by a concrete support structure shown n Fgure a, and the dstrbuted stran sensor system based on tweve FBG sensors s adopted to measure the stran response under the statc oad, whch are adhered to the wndward sde of the wnd turbne bade. The above FBG conssts of the optca fber and optca gratng and there s optca fber wth the ength of 5 m n the ends of the stran sensor system. Furthermore, the waveength toerance of the FBG s ±0.5 nm, the type of t s the snge-mode SMF-8C FBG, and the ength of the gratng s 0 mm. The devce shown n Fgure b provdes the method of mposng the oad and the optca sensng nterrogator SM30 and PC are empoyed as the demoduatng and recordng devce. Snce the FBG empoyed n ths study s naked, a FBG protectng devce s empoyed to gve a steady room n order to protect the FBG n the process of mposng the oad; the damage wth dmensons of cm s smuated by the eectrc saw n the surface of the bade. Fnay, the goba expermenta setup s gven n Fgure c. (8)

7 Sensors 05, Fgure. Expermenta setup of the wnd turbne bade. (a) Camped wnd turbne bade; (b) oad devce; and (c) goba expermenta setup. Fgure presents the ocaton of each FBG and damage. The stran sensor system based on the FBG conssts of tweve FBGs and the dstance between adjacent FBGs s dfferent. The dmenson of the damage s cm, the dstance between measurement ponts and the damage s 4 cm, and the correspondng chords are 0.8 m and. m respectvey, so the rato of the damage ength and the correspondng chord ength s 0. and respectvey. Fgure. The schematc of the FBG and damage. 3.. Resuts and Dscussons In the above experment, the statc oad s mposed by the same weght sandbags on one oadng poston, the dstance between t and the root of wnd turbne bade s 6.8 m. Then, the stran response under dfferent magntudes of the oad s measured, and the range of the oad s ncreased from 500 N to

8 Sensors 05, N, n ncrements of 00 N, so the stran dstrbuton of the above bade under fve oad magntudes shown n Fgure 3 s measured based on the dstrbuted stran sensor system. Fgure 3. The stran dstrbuton of the damaged wnd turbne bade. (a) The snge damage; and (b) two damaged regons. Suppose the stran response under the statc oad on the th FBG s denoted as ε ( x ) normazed stran response ε ( x) can be obtaned as foows:, so the ( x ) ε = ε = ε ( x ) ( x ) (9) where denotes the number of the FBG. Therefore the normazed stran dstrbutons can be regarded as dfferent probabty dstrbutons, and t can be observed that the stran gets hgher when the oads become greater. In addton, the sef-weght of the wnd turbne bade s gnored snce the assocated strans can be regarded as pre-exstng before the appcaton of the stran sensor FBGs. The CSD s cacuated between the above stran dstrbutons; for exampe, t can be cacuated between the stran dstrbutons under the oad of 500 N and 600 N respectvey, then ten knds of CSD are obtaned, and the mean damage detecton error of them s obtaned, respectvey, whch w be ntroduced n Secton 3.4, so the CSDs wth frst four mnma mean damage detecton errors are used as the ndvdua nformaton for the damage detecton. The graphs shown n Fgures 4 and 5 provde the damage detecton resut by the CSD, and t can be shown that the shape change n the graph ndcates the presence of the damage. Then, the damage ocaton s obtaned by the ocaton of the shape change n the x-axs, as shown n Fgure 6. Therefore, the CSD can detect the snge and mutpe (two) damage successfuy, so the CSD can be regarded as an effectve damage-senstve feature, but a reatve ower peak appears at a spurous damage ocaton n the damage detecton, whch ndcates the presence of damage where no damage, n fact, exsts. Therefore, the nformaton fuson method s needed to fter out ths spurous damage ocaton n the snge damage detecton.

9 Sensors 05, CSD Order FBG Fgure 4. CSD for the snge-damage detecton. CSD Order FBG Fgure 5. CSD for two-damage detecton. (a) (b) Fgure 6. Method of detectng the damage ocaton by the CSD. (a) The snge-damage detecton; (b) Two-damage detecton. In the FIF, the CSD under dfferent statc oads are regarded as ndvdua nformaton sources, and sources are ndependent among each other, so the CSDFIF based on the CSD s cacuated. Fgures 7 and 8 gve damage detecton resuts by the CSDFIF for the snge and mutpe (two) damages and the damage ocaton can aso be obtaned by the ocaton of the shape change n the x-axs, as shown n Fgure 9. For the snge damage detecton, there s no reatve ower peak n the graph, so t can be observed that the nformaton fuson method FIF has the abty of fterng out the spurous damage ocaton, whch s mportant n practce. For two-damage detecton, the above concuson can

10 Sensors 05, aso be drawn, but the CSDFIF between two peaks on the damage ocaton s reatvey smaer, as compared wth the CSD, whch means a sma error of the damage detecton. CSDIFF Order FBG Fgure 7. CSDFIF for the snge-damage detecton. CSDIFF Order FBG Fgure 8. CSDFIF for two-damage detecton. (a) (b) Fgure 9. Method of detectng the damage ocaton by the CSDFIF. (a) The snge-damage detecton; (b) Two-damage detecton. In addton, t s cear from Fgures 7 and 8 that the presence of damage produces a stran peak much more pronounced than that observed n Fgures 4 and 5, whch means the greater accuracy of the damage

11 Sensors 05, detecton. Therefore, the nformaton fuson method, FIF, coud be regarded as an effectve and feasbe approach for optmzng and showng nformaton on the damage detecton n the CSD. Generay speakng, the proposed method can detect damage n the wnd turbne bade accuratey, and the CSD can be regarded as an effectve damage-senstve feature, the nformaton fuson method FIF can optmze and show nformaton on the damage detecton n the CSD Comparson wth the SEM The stran energy method (SEM) s a cassc method for the damage detecton [6], whch s empoyed to descrbe the advantage of the proposed method, and the stran energy based on the stran response under the statc oad of the th eement n the wnd turbne bade s gven as foows: a+ U ( EI) ( ε) dx a SEM = = U Lx ( EI)( ε ) dx where SEM represents the stran energy of the th eement n the wnd turbne bade. Fgures 0 and present damage detecton resuts by the SEM n order to descrbe the advantage of the proposed method, and t can be shown that the SEM s not abe to detect the snge damage, and can ony detect damage on the FBG 6 n two-damage detecton, as shown n Fgure. The reason for ths s there s no basene avaabe of the ntact wnd turbne bade n the SEM, so t can be sad that the proposed method has sgnfcant advantage over the SEM. 0 (0) SEM Order FBG Fgure 0. SEM for the snge-damage detecton. SEM Order FBG Fgure. SEM for two-damage detecton.

12 Sensors 05, Comparson of Damage Detecton Error In order to evauate the error of the proposed method, the error evauaton ndex MDDE s adopted. Let the maxma peak vaue of the damage ndex on the damage ocaton and the mean of the damage ndex n the ntact regon be a and b, respectvey, so the MDDE s defned as the rato of a and b, as shown n Fgure. b DI a Fgure. Mean damage detecton error. The resut shown n the Tabe provdes the MDDE by the proposed method and the SEM, and t can be shown that the MDDE of the CSD s much arger than that of the CSDFIF, whch s consstent wth the above study. Tabe. MDDE of the wnd turbne bade. DI Snge Damage Two Damages CSD CSDFIF SEM The MDDE of the CSDFIF for two-damage detecton s mnma, and the MDDE of the SEM s maxma. The MDDE of the SEM s arger than those of the CSDFIF and CSD, respectvey. Therefore, t can be concuded that the proposed method shows a sgnfcant advantage over the SEM. Generay speakng, the MDDE takes nto account of the goba damage detecton error, and the proposed method can detect damage more accuratey than the SEM by the comparson of the MDDE. 4. Concusons In ths paper, a new damage detecton method usng the stran response under the statc oad s presented, and the foowng concusons can be drawn:

13 Sensors 05, () For the snge and mutpe (two) damage detecton n the wnd turbne bade, t can be observed that the proposed method can detect the presence of damage and gve the damage ocaton n a reatvey hgher accuracy than the SEM. () The CSD can be regarded as an effectve damage-senstve feature and the ndvdua nformaton source for the damage detecton n the wnd turbne bade. (3) The nformaton fuson method, FIF, can fuse and optmze nformaton n the damage-senstve feature, CSD, for a goba decson on the damage detecton, and the proposed method CSDFIF shows an advantage over the SEM by the comparson the MDDE vaue. (4) The proposed method CSDFIF s thought to be a very practca technque for structura heath montorng because t s robust and the mnma hstorca data that s needed. Acknowedgments Ths work s supported by Natona Natura Scence Foundaton of Chna (Nos , & 54004), the Chna Schoarshp Counc, the Chna Postdoctora Scence Foundaton (No. 04M560766), and the Fundamenta Research Funds for the Centra Unverstes (No. xjj0407). Author Contrbutons Xuefeng Chen and Yong Xe conceved and desgned the experments; Shaohua Tan performed the experments; Shaohua Tan and Zhbo Yang anayzed the data; Xuefeng Chen contrbuted reagents/materas/anayss toos; Shaohua Tan wrote the paper. Confcts of Interest The authors decare no confct of nterest. References. Schroeder, K.; Ecke, W.; Aptz, J.; Lembke, E.; Lenschow, G. A fbre Bragg gratng sensor system montors operatona oad n a wnd turbne rotor bade. Meas. Sc. Techno. 006, 7, L, D.; Ho, S.C.H.; Song, G.; Ren, L.; L, H. A revew of damage detecton methods for wnd turbne bades. Smart Mater. Struct. 05, 4, do:0.088/ /4/3/ Tayor, S.G.; Farnhot, K.M.; Park, G.H.; Farrar, C.R.; Todd, M.D.; Lee, J.R. Structura heath montorng of research-scae wnd turbne bades. Key Eng. Mater. 03, 558, Park, S.; Park, A.; Han, K. Rea-tme montorng of composte wnd turbne bades usng fber Bragg gratng sensors. Adv. Compos. Mater. 0, 0, Adams, D.; Whte, J.; Rumsey, M.; Farrar, C. Structura heath montorng of wnd turbnes: Method and appcaton to a HAWT. Wnd Energy 0, 4, Jones, M. Structura-heath montorng: A senstve ssue. Nat. Photoncs 008,, Km, H.I.; Han, J.H.; Bang, H.J. Rea-tme deformed shape estmaton of a wnd turbne bade usng dstrbuted fber Bragg gratng sensors. Wnd Energy 04, 7,

14 Sensors 05, Arsenaut, T.J.; Achuthan, A.; Marzocca, P.; Grappasonn, C.; Coppote, G. Deveopment of a FBG based dstrbuted stran sensor system for wnd turbne structura heath montorng. Smart Mater. Struct. 03,, Nchos, J.M.; Seaver, M.; Trckey, S.T.; Savno, L.W.; Pecora, D.L. Detectng mpact damage n expermenta composte structures: an nformaton-theoretc approach. Smart Mater. Struct. 006, 5, Lau, K.-T.; Yuan, L.B.; Zhou, L.-M.; Wu, J.S.; Woo, C.-H. Stran montorng n FRP amnates and concrete beams usng FBG sensors. Compos. Struct. 00, 5, Okabe, Y.; Mzutan, T.; Yashro, S.; Takeda, N. Detecton of mcroscopc damages n composte amnates. Compos. Sc. Techno. 00, 6, Tsuda, H. Utrasound and damage detecton n CFRP usng fber Bragg gratng sensors. Compos. Sc. Techno. 006, 66, Rubner, Y.; Puzcha, J.; Tomas, C.; Buhmann, J.M. Emprca evauaton of dssmarty measures for coor and texture. Comput. Vs. Image Underst. 00, 84, Mathassen, J.R.; Skavhaug, A.; Bø, K. Texture smarty measure usng Kuback-Leber dvergence between gamma dstrbutons. In Proceedngs of the European Conference on Computer Vson ECCV, Copenhagen, Denmark, 8 3 May 00; pp Puzcha, J.; Buhmann, J.M.; Rubner, Y.; Tomas, C. Emprca evauaton of dssmarty measures for coor and texture. In Proceedngs of the Seventh IEEE Internatona Conference on Computer Vson, Kerkyra, Greece, 0 7 September 999; pp Hu, H.; Wang, B.T.; Lee, C.H.; Su, J.S. Damage detecton of surface cracks n composte amnates usng moda anayss and stran energy method. Compos. Struct. 006, 74, by the authors; censee MDPI, Base, Swtzerand. Ths artce s an open access artce dstrbuted under the terms and condtons of the Creatve Commons Attrbuton cense (