appropriatelyscaleup

SCALE-UPOFWINDTURBINEBLADES–CHANGESINFAILURETYPEIntroductionThenextgenerationbladesfortheEuropeanoffshoremarketwillbebetween75and85mlongweightingbetween30and45tonnes.
ThemostpublicavailableexampleofthisisthenewVestasV164witha80mblade,butalsoDSME/DeWind,Samsung,AlstomandNordexaredevelopingturbineswithsimilarbladelengths.
LMWindpowerwillintroducetheirnew74mblademid2013.
Withthisnewgenerationofwindturbinebladestypicalfailuremodesaswellastheirlocationwillchange.
Forexample,theincreasinglengthandweightmakesthegravityintroducedloadsmoreimportant.
Afirst,ratherrudimentarysummaryisshowninthematrixgiveninFigure1.
Inthefourcolumnstotherightisgivenagradeindicatinghowcriticalthe(failure)modeisexpectedtobeforthegivenbladelength.
5indicatessignificantimportanceand0noimportance.
Figure1.
Trendandchangesin(failure)modesofwindturbinebladesduetoscaling.
Risbladelength.
Basedonscalinglawse.
g.
fromEU-Upwindprojectwhich,seeref.
[3]orSandiareport,ref.
[2]different(failure)modesarerelevantforthedesigndependingofthesize.
E.
g.
therootbendingmomentscale-upwithpowerof4tothebladelength(R),causedbytheincreasedmass.
Forsmallbladesitisthetipdeflection(globalstiffness)whichiscriticalandnottheweight.
Forlargerbladesthischangesanddesignershavealteredtheirfocusondecreasingtheweightutilizing,atleastpartially,lowweight/high-stiffnesscomposites(e.
g.
CFRP)tocopewithit.
Thisisgenerallyacceptedintheindustrywhentheyscale-upablade,theyhaveaclearfocusondecreasingtheweightinordertodecreasecriticaledgewisefatigueloads.
Justafewyearsbackwithsignificantlysmallerbladesaerodynamicloadsbuild-upthedominatinganddimensioningloadcasewhiletodaywithlongerbladesitisthegravitationalforcesandthereforetheweightoftheblades.
Thisoverviewoffailuremodesandhowtheyarelinkedtothescalinglaws.
Recommendationsarepresentedonhoweachfailuremodeshouldbeaddressedeitherinthedesignphasee.
g.
byanexperimentalprogramoradvancednon-linearnumericalsimulations,sincenotallarecoveredbytheinternationalstandardstoday,ref.
[4][5][7].
TypicalfailuremodesinfluencedbyscalingTransversesheardistortionofthecrosssectionThecombinationoflargeedgewiseloadsandextremeaerodynamicforcesresultinloadcombinationswhichcouldendupintoacriticaltransversesheardistortionfailure(cf.
Figure2)forscaled-upslenderblades,seeref.
[1].
Figure2.
Numericalsimulationshowstransversesheardistortionofacrosssectionloadedinacombinededge-andflapwisedirection.
Thisfailuremechanismbecomesmoreimportantwhenthesize(andweight)ofthebladesincreasemainlyduetolargergravityforcesinedgewisedirection.
Today,thefailuremodeisnotappropriatelycoveredinnowadayscertificationrules,sincenocombinationoftheforcesarerequiredinthefinalfull-scaletest.
Furthermore,theloadsareappliedthebladeusingclampswhichsupportthestructureinawaysoitcannotdistortatthisspecificcross-sections.
Failureinthecap(s)causedbyBrazierloadsTheout-of-planedeformation(flattening)ofthecaps,causedbytheBrazierloads,maycausefailureinthecaps,seeref.
[8],[10].
Thefailurecaneitherbeatransversetensionfailureintheunidirectionallayersatthebottomlayeroraninterlaminarshearfailurebetweenthelayers,seeFigure3a.
Atypicallay-upisnotparticularlywellsuitedtoreducecapdeflectionssincethefibresaretheremainlyplacedinthelongitudinaldirectionoftheblade.
Thelackoffibresinthetransversedirectioncausesthecaptoberelativelyflexibleinthelateraldirection.
Whenthecapdeflectsthereisariskoftransversetensionfailureintheunidirectionallaminates.
Inaddition,itiscommonthatmanufacturingimperfections,seeFigure3c,insidethelaminatefurtherreducethefatigueandultimatestrengthofthelaminate.
bacFigure3.
Interlaminarshearfailureoftheload-carryingcaplaminatecausedbythenon-linearBrazierforcesa)Sketchofcapdeformationandfailurebetweenlayersb)Photoofacapwithdelaminationc)Photoofacapwithamanufacturingdefect.
Whenthebladesscale-upthelongitudinalcurvatureofthebladeincrease,whichresultsinalargercrushingpressure,calledBrazierforces.
Thisresultsinlargerout-of-planedeformationoftheloadcarryingcaplaminate,whichthenresultineithertransversetensionfailureintheunidirectionallayersorinterlaminarfailure.
Thisout-of-planedeformationisanon-lineargeometricphenomena,whichunfortunately,isnotcoveredbytheinternationallydesignstandards.
Theteststandardsdonotrequirethatthereisstraingaugesmeasurement,butthisstronglyrecommendedinfuturefull-scaletest.
BucklingBucklingisastructuralnon-lineargeometricinstabilityphenomenonwhichisimportantfordesignofnowadaysandfuturewindturbineblades.
Thebucklingcapacitycaneitherbeaddressedbyanon-lineargeometricFE-analysisoralineareigenvaluebucklinganalysis.
Accordingly,linearbucklinganalysisisaguidelineforthedesignload,towhichasuitablereductionfactoriscalledfor.
However,thecorrespondingbucklingmodesneedtobeexaminedcarefully,tosortoutunrealisticmodesatunrealisticbucklingloads.
Duetotheup-scalingofbladesthetrailingedgeaswellathelargetrainingedgepanelswillbecomemorepronetodifferentkindofasdetailedinFigure4.
Figure4.
Bucklingofthetrailingedge.
Bucklingofthetrailingedgehastovalidatedinfull-scaletestbutthemethodofapplyingtheloads,maynotgivearealisticpicturesincetheclampscanbeseenasartificialboundaries,influencingthesocalled"freelength"andimpactthebucklingloadsexaminedduringtest.
AsanalternativetothemethodsRisDTUhasdevelopedamethod,usinganchorplates,whichgiveamorerealisticloadintroductionwithoutsupportingthestructure,seeref.
[9].
FailureintheadhesivebondlinesOut-of-planedeformationsofthetrailingedgepanels(alsonoticeablegrowingbyscalingtheblade)areimportantsincetheyresultinpeelingstressesinthebondlines,whichmaybethemainreasonforfatiguefailureintrailingedgeadhesivejoints,seeFigure5aswellasref.
[9].
Itiswell-knownfromtheliteraturethatbondlineshavealowstrengthwhenexposedtopeelingstressescomparedtotheotherloadingdirections,e.
g.
thefractureenergyisapproximatelyafactor8-10higherformode2(shearing)thanformode1(peeling)loading.
Figure5.
Sketchofthetrailingedgeshellswithout-of-planedeformations.
Thecloseupsshowfailureatthetrailingedgeaswellasdebondingoftheouterskinontheboxgirder.
Theloadintroductionproblematic,describedinthepreviousbucklingsection,isthesameforthisfailuremodeandalsotheanchorplatesolutionisrecommendedinfuturetesting.
FatigueproblemintheroottransitionareaInthetransitionzonefrommax.
chordtothecylindricandstiffrootregionoftencauseproblems,seeFigure6.
Thereasonthatlargeedgewiseforceshavetobe"carried"throughlargecurvedpanels,whichresultsin"pumping"out-of-planedeformationsandthengivepeelinginbondlinewherethetrailingedgepanelsareconnectedtothecylindricalroot.
Figure6.
Photoshowingaclassicalfatiguefailureinthetransitionareafrommax.
chordtotheroot.
Thefailureiscausedbytheout-of-planedeformationsofthelargetrailingedgepanels,whichissometimescalled"pumping".
Sincetherootbendingmomentscale-upwithpowerof4tothelength,thisfatiguefailureistobeextremelycriticallyforfuturewindturbineblades.
Specialdesignspecificconsiderations/modificationsneedtobetakeninaccountduringthesizingprocess.
WebfailureWebfailureshasinsomecasesbeenobservedtobethemainreasonforcollapse,seeref.
[8].
Figure7presentsafrozenframepictureofthefirst,andcritical,failuremodeobservedinafull-scaletestofaloadcarryingboxgirder.
InFigure7thewhitecircle(lightgreencolour)initialfacedebondingoftheouterskinontheshearweb'ssandwichsectionareshown,leadingtoultimatecollapseoftheboxgirder,seeref.
[8]Figure7.
Skindebondingfailureofthesandwichshearwebswhichwasobservedinafull-scaletestofaloadcarryingboxgirder,seeref.
[8].
Thisfailurecouldbedueto,tooweakskinfacesinthesandwichshearwebs.
Thisfailurewouldnormally,notbeobservedinacommercialfull-scaletestsinceitisalmostimpossibletoaccessbothsidesoftheshearwebwithmeasuringequipment,camerasetc.
.
IntheFE-modelingwhichsometimesisrequiredbythecertificationbodies,thisfailuremodeisnoteasytoaddresssinceitwouldrequireonafracturemechanicsapproach.
Furthermore,anon-lineargeometricanalysisisrequiredotherwisetheBraziercrushingforceswillnotbeincluded,andthis,asmentionedearlier,isnotpartofastandardcertificationprocess.
AsmentionedearliertheBrazierforcesareexpectedtobemoredominantwhenthebladesscalesupandthereforealsothiswebfailuremodehavetobeconsideredmorecarefullyinthefuture.
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