wind turbine blade reliability, effects of manufacturing defects and composite repairs

WindTurbineBladeReliability,EffectsofManufacturingDefectsand

CompositeRepairsPresentedbyDougCairns,Lysle A.WoodDistinguishedProfessor

JaredNelson,AssistantProfessor,SUNYNewPaltzMichaelLeman,MSMETammyRichey,MSME

FemiIbitoye,MSMECandidateDanielSamborsky

JoshPaquette,TaskLeaderforLaboratoryandFieldTestingofWindTurbineBlades

Today’sTopics

• EffectsofDefectsinWindTurbineCompositeLaminates• Laminates• AdhesiveJoints• Coupons• Substructures(SandwichBeams)

• CompositeLaminateRepair• Coupons• Substructure

CompositeWindTurbineBlades

• Turbinesaregettinglarger• Lowcostmanufacturingmeanshigher

probabilityformanufacturingdefects• Increasedlengthmeansincreasedweight• Can’taffordthetypicaloverdesignoftheblade 747Jumbo Jet

CompositeWindTurbineBladesTheserootstudsaretheONLYmechanicalfastenersintheentirebladestructure

LowHangingFruitforWindTurbineBladeReliability

• ThesestructuresareBIG>100mdiameter• “Nearaerospace”designrequirements• Theyhavehighstructuraldemands,manyhours/dayfor20years

• Extremeenvironments,Alaska,Hawaii,Antarctica• Lessthan$10/lbcosts• Theonlymechanicalfastenersarebetweentherootandthemetalturbinehub

• Waves:bendingorwavinessalongfiberlength• In-plane(IP):fiberwavesonsurface(left)• Out-of-plane(OP):fiberwavesthroughthickness(right)

EffectsofDefects:UnderstandDefectTypes

In-PlaneWaves Out-of-PlaneWaves

MultipleAdhesiveJointsinModernWindTurbineBlades

BondingQuality

• TypicalBladeBondLines• Difficult tocontrol• Blindbonds• Scalingeffects

• Shear-WebBonding• Bond-LineVoids• Bond-LineWeakness(withoutmajorvoids)• CommentaryfromaBladeManufacturingManager

• “Themostdifficultpartofmanufacturingprocessistrying tobond thetwoshellstogether.”

• “Trailingedgedefectscangrowtofullbladefailure.”• “Bondingproblemsarethebiggest issue.”

MinorVoids

• Waves:bendingorwavinessalongfiberlength• In-plane(IP):fiberwavesonsurface(left)• Out-of-plane(OP):fiberwavesthroughthickness(right)

EffectsofDefects:UnderstandDefectTypes

In-PlaneWaves Out-of-PlaneWaves

ImagesofutilityscalebladesectionswerecollectedatNREL

Imageprocessingwasperformedtoextractquantifiablewavegeometry

11

DefectDataCollection

12v

DataProcessing- IP• Flawparameterstendto

followdistributions• Dataare‘sanitized’to honor

NonDisclosureAgreements• Thesedatawereusedto

developtestarticles

Typical

Anomalous

Typical

Anomalous

Typical

Anomalous

• ContinuumDamageModeling(CDM)– A“pseudo-representation”– Doesnotmodeltheexactdamage– Updatesconstitutivepropertiesasdamageoccurs– Asthemodeliterates,theconstitutivematrix,CorS, isupdatedto

reflectequilibriumdamage– CorS maysimplify;basedonmaterialandlay-up

ContinuumDamageModeling

=

xy

zx

yz

zz

yy

xx

xy

zx

yz

zy

yz

x

xz

z

zy

yx

xy

z

zx

y

yx

x

xy

zx

yz

zz

yy

xx

G

G

G

EEE

EEE

EEE

2100000

0210000

0021000

0001

0001

0001

Asdamageoccurs,thematerialproperties (E,ν &G)maybeadjustedtoaccountforadifferentresponse inthedamagedspecimen.

Assuch,Cmaybeadjusteddepending onfailurecriteria.i.e.C =C(ε,σ)

ComplianceMatrixforOrthotropicMaterial:

13

• DiscreteDamageModeling(DDM)– Modelsthedamageasitoccurs(priorknowledgeishelpful)– Generally,computationallymoreexpensive– UtilizingCohesiveElements;improvementonVCCT/LEFMbecause

crackpathnotnecessary

DiscreteDamageModeling

14

• CohesiveElements– Traction-separationbasedmodelingforbondedinterfaces

(composites)– Layerofessentiallyzerothicknesselementsaddedbetweenlayers

wherecrackisexpected– Modelstheinitialloading,theinitiationofdamage,andthe

propagationofdamageleadingtoeventualfailure

DDM:CohesiveElements

15

• GroupA:Control(noinduceddefects)• GroupB:Porosity—2%byvolumeofmatrix• CroupC:In-PlaneWaves

– IP1=1.9mmA,23.8mmλ,25°– IP2=4.5mmA,23.8mmλ,49°– IP3=1.9mmA,10mmλ,48°

• GroupD:Out-of-PlaneWaves– OP1=2.9mmA,22.8mmλ,37°– OP2A=0.7mmA,5.4mmλ,35°– OP4A=0.6mmA,22.8mmλ,9°

• 2-5samplesfromeachgrouptestedinbothtensionandcompressionwith0° and±45°layups.– Morethan35testgroups– Totalsamplestestedover180

OutlineofStaticTesting

Instron 8802:servohydraulic250kN (56,000 lb),150mm(6in)stroke

16

0° TensionIPWaveDamage

UltimatePlyFailureMatrixCracking

DamagevisualizedwithimagesandAramisdigitalimagecorrelation(DIC)system.

17

• StaticStrengthReduction(fullwidthwaves)

18

KnockdownFactors

• Round3• Combinationofnon-linearshearUMAT&

cohesiveelement– CDM/DDMapproach

• Sameinputsasutilizedfromeachindividualapproachabove

• Nofailurecriteriaforcontinuumelements(tows)– Sheardrivendegradation

• FailureforCOHelements

CombinedCDMw/Non-LinearShearUMATandDDMw/CohesiveElementApproach

19

CombinedCDM&DDMApproach:IPWaveinTension

Initialfailureoccurringatdiscontinuousfiberedges

Modelfailsmoresuddenlythroughoutentirewaveduetomodeluniformity

20

CombinedCDM&DDMApproach:IPWaveinTension

21

Discrete“jumps”asaconsequenceofmodeluniformity

CombinedCDM&DDMApproach:IPWaveinCompression

22

• Usingdataandanalysisfromastructurewithwavesconstrainedtoaregionispresumedtobeconservative(i.e.wavedoesnotextendacrosstheentirewidthofthestructurelikeinthecouponsandanalysis)

• But,byhowmuch?• Whatistheloadre-distributioncapabilityuptofinalfailure?

23

WhatAboutPartialWaves?MichaelLerman,JaredNelson,JocelynThompson

24

PartialDefectTestsandAnalysis

Laminates with Wavy Fibers in Middle, Straight Fibers on Edges

a) FE Model b) FE Model, full straight edge width wavy fibersfibers

25

TestSetupWithDigitalImageCorrelation(DIC)forfullProgressiveDamageData

26

Full-FieldStrainDuringTests

Partial Wave Coupon Longitudinal Strain

27

DifferencesinProgressiveDamagebetweenFullWidthandPartialWidthWaves

Full Width Wave, extensive matrix damage leading to

failure

Partial Width Wave, local matrix damage leading to

ultimate straight fiber failure

28

GoodExperimental/AnalyticalCorrelationsofPartialWidthWaves

DIC from Experiment: Finite Element Model(longitudinal strain) (“soft inclusion” local

strain concentration)

29

BIGDifferencebetweenFullWidthWavesandPartialWidthWaves

Full Width Wave

Partial Width Wave(acts as a “soft inclusion”; strain

concentration, but local failure does not immediately lead to catastrophic failure)

30

However,BothProgressiveDamageLeadingtoFailurecanbeadequatelyPredicted

Full Width Wave Partial Width Wave

31

Substructures(SandwichBeams)

Montana State University’s Multi-Scale, Multi-Axial Test Facility, beams of approximately 2m• Capable of Multi-Axes

(bending and torsion)• Intermediate step

between coupons and full scale blades

• Much faster and lower cost than full blade

4 Pt Bending of Sandwich Beam

Beam with Full Width Flaw

Beam with Full Width Flaw (10/24/15)

1

23456789

COMPRESSION

Unflawed Beams vs. Flawed Beams• Load to Failure

– No Flaw – 6.2kN– Full Width Flaw 3.8-4.4 kN, depending on severity– Partial Width Flaw, wavy fibers in middle, straight fibers on edges,

5.8 kN, substantial capacity for load redistribution

• Results are consistent with coupons, takeaway – coupons representative of flaws in structures

• Aconsistentmethodologyforthetreatmentofwavesincompositelaminateshasbeendeveloped

• ProgressiveDamageAnalysishasgoodexperimentalagreementoverawiderangeofwaveparameters

• UsingfullwidthwaveanalysisandtestingmaybetooconservativeforDamageTolerantDesignanalysis– Goodnews- Ifthestructureisshowntobegoodwithfullwaves,itwillbebetterwithpartial

waves– Badnews– MaybetooconservativeforstructureswithDamageTolerantDesignapproach

(However,analysiscanbedonetounderstandtheseeffects)

• Significantreinforcementprovidedbysurroundingstraightfiberswithpartialwaves

35

SummaryandConclusions– LaminateswithWaves

ThickAdhesiveJointsinWindTurbineBladeStructures

• MattPeterson• DougCairns

36

• Geometricallycomplexcrackgrowth,withoutaprioriknowledgeofwhereitmightoccur

• Includecrackinitiation,potentialcrackbranching,andflawinteraction

• Examplefigure:shearweb-sparcapadhesivejoint&commonmfg defects

Challengesinmodelingdamagegrowthinthickadhesivejoints

Independent cracks are free to branch or join where favorable

Formation of multiple cracks

CohesiveZoneModeling,advancedmaterialfracturebehaviorinthickadhesivejoints

u CZM has been widely used to solve crack propagation problems

• Ease of implementation within conventional FEM (Seagraves 2009), Explicit representation of cracks giving clear physical picture (Molinari 2007), …

u Fracture formation is regarded as a gradual phenomenon in which the separation of the surfaces involving in the crack takes place across an extended crack tip, or cohesive zone, and is resisted by cohesive tractions (Dugdale 1960, Barenblatt 1962)

Cohesive traction

Cohesive zone (Fracture process zone)

Cohesive zone length

Physical crack length

Crack tip

CohesiveZoneModels

• Materialseparation&fractureisresistedbyinternal“cohesive”forcesoverextendedprocesszone.

• ImplementedintheFEMusingcohesiveinterfaceelements(CIEs)placedatinter-elementboundaries.

CohesiveZoneModeling– TractionSeparationLaw(TSL)

u The behavior of CZEs is governed by traction-separation law • Exponential, bi-linear, trapezoidal, …

Tmax : Maximum traction

d0, df : Initial and final separation

Gc : Fracture energy

K : Initial stiffness

Tmax

d0 df

GcK

Traction

Separation

GeneralFormulation:CZM• Representsamajoradvancementoverconventionalfracturemechanics

(LEFM)– Crackgrowthwithoutremeshing orcracktipelements– Brittle,quasi-brittle, andductilefracture– Noinitialcrackrequired

• CohesiveInterfaceElements(CIEs)areplacedwithinameshalongcontinuumelementboundaries

• CIEsaregenerallycollapsedtoformazero-thickness“interface”betweenbulkelems

• DamagedCIEsrepresentalldamageandthecohesiveforcesinfracturezone

• Therefore,theCIEsizemustbesmallenoughtoresolvethefractureprocesszone

41

Cohesiveelements

BulkelementsCracktip

CohesiveZone

1

2

3

4

5

6

7

8

1,5

2,6

3,7

4,812

3

MONTANASTATEUNIVERSITY

MontanaStateUniversity’sUniqueApproach

42

• Multi-pointConstraintCohesiveZoneMethod(developedatMTState,Boeingawardedusamulti-yearcontractbecausetheydon’thavethiscapability)

• EliminateDOFfromIntrinsicCohesiveElements– InsertionofCIEs

• CIEsareinsertedpriortorunningsolver(IntrinsicScheme)• Increasesnumberofelements,nodes&correspondingDOF

– DefinitionofMaster-SlaveMultipointConstraints(MPCs)• EliminatesslavenodeDOF• Returnssystemtooriginalsize• CIEsbecome“dormant”untilactivatedbyspecifiedcriterion

– DeactivateMPCs(activateCIEs)wheredamagecriterionsatisfied• CIEsare“activated”onlyasneededduringsolution(ExtrinsicScheme)

top element

bottom element

Midsurface, CIE insertion

node

MPC, tied nodes

12

3 unconstrained nodes

constrained nodes

Traction-Separation LawDefines the evolution of the traction across the CIE surfaces relative to their

separation.Tr

actio

n (N

/m2 )

Separation (m)

1

2

3

4

5

6

7

8

1,5

2,6

3,7

4,812

3Tc

Gc

Intrinsic TSL: Initial Penalty Stiffness controls the amount of interfacial separation that occurs prior to damage.

Nor

mal

Tra

ctio

n, T

(N/m

2 )

Normal Separation, Δ (m)

K03

TSL

Tc

Δ03Δ02Δ01

Gc

ΔF

Extrinsic TSL: The interface is “initially rigid”. Achieved by adaptively inserting CIEs with non-zero initial tractions.

Nor

mal

Tra

ctio

n, T

(N/m

2 )

Normal Separation, Δ (m)

T0 = Tc

Gc

TSL

ΔF

Note:We have done sensitivity studies on how the TSL affects the results:

• Tc• Gc• ∆F• Shapes other than triangular

MPC Release Visualization

Note: CIEs are active in red areas only.

Multicrack Model

20mm

20m

m

Geometry, BC, Mesh

Thickness=2mm

Multicrack Growth (Pt 1)Displacement Max Principal Stress MPC Release

0

50

100

150

200

250

300

0 0.001 0.002 0.003 0.004

Load

(N)

Load Point Displacement (mm)


0

50

100

150

200

250

300

0 0.001 0.002 0.003 0.004

Load

(N)


Multicrack Growth (Pt 7)

0

50

100

150

200

250

300

0 0.001 0.002 0.003 0.004

Load

(N)


Displacement Max Principal Stress MPC Release

Multicrack Growth (Final)Displacement Max Principal Stress MPC Release

0

10000

20000

30000

40000

50000

0 1 2 3 4 5 6 7 8 9

Num

ber o

f Act

ive

DO

F

Sample Point Along Load History

A Case Study for “Spin-On” of Advanced Technology from Wind Research

• Aerospace does not have our advanced progressive damage analysis with our unique CZM formulations

• Boeing has awarded a large research contract to implement results from the BRC program.

Bondline

1.0

8.0

0°

90° 45°

-45°

3.0

Non-stickInsert

0.125

Loadingblocks/pinhole

0.079

A Case Study for “Spin-On” of Advanced Technology from Wind Research (cont.)

Thick Adhesive Joint Modeling Conclusions• The CZM is a versatile tool for modeling crack growth under complex load

and boundary conditions, but suffers from some drawbacks.

• Selective Activation of cohesive elements appears to be a natural improvement.– Removes or minimizes artificial compliance– Reduces the number of active DOF– Reactivates CIEs as needed “on-the-fly”

• It is not without exaggeration to claim that we are at the cutting edge of progressive damage modeling with cohesive zone elements.

Composite Repair Characterization

• Tammy Ritchey• Femi Ibitoye• Daniel Samborsky

TypicalBladeDamage• Wear

– Environmental– LifeCycleFatigue

• ImpactDamageConstructionTransportationBird&LightningStrikes

• ManufacturingFlawsPorosityFiberWaviness

CompositeRepairCharacterization

• TammyRitchey• FemiIbitoye• DanielSamborsky

RepairMethods

• StructuralRepair:ExternalPatchExternal Patch

Adhesive

Filler Parent Laminate

• SpotRepair:MatrixFill

RepairMethods• StructuralRepair:ScarfPatch

ElementsinaScarfJoint

TypicalLoadsExperiencedinAdhesiveBondline

AdhesiveBondlineFailureModes

AdhesiveFailureMode

CohesiveFailureMode

SubstrateFailureMode

(A.)DamagedCoupon (B.)RepairPatch

(C.)Repairprocess(Infusion orPasteAdhesive(D.)RepairedandSanded

Damage and Repair Procedures

DamagedandUnrepaired(Case1)

ARAMIS Digital Image Correlation – Full Field Strain

DamagedandUnrepaired(Case2)ARAMIS Digital Image Correlation – Full Field Strain

DamagedandRepaired(Case1)


DamagedandRepaired(Case2)


movie

Finite Element Analysis Captures Damage Progression

Simulationofdamagepropagationofunrepairedandrepairedcompositespecimen

Configuration

300mm52mm

10mm

Thickness=1.85mm

1/4Symmetrymodel

52mm

Materialproperty

• Elastic– E11=40.6GPa,E22=E33=16.3GPa,n12=n13=0.27,n23=0.35– G12=G13=16.8GPa,G23=6.22GPa

• Cohesive– Fiber(Q/P):T1=950MPa,GIC=30N/mm,K=5x107 MPa/mm,p=2– Matrix(Q/P):T1=40MPa,T2=60MPa,GIC=0.3N/mm,GIIC=0.6N/mm,K=107 MPa/mm,p=2

– Delamination(Q/P):T1=40MPa,T2=60MPa,GIC=0.3N/mm,GIIC=0.6N/mm,K=107 MPa/mm,p=2

– Patchadhesive(Q/P):T1=20MPa,T2=30MPa,GIC=0.05N/mm,GIIC=0.3N/mm,K=107 MPa/mm,p=2

(Q:QUADSdamageinitiation,P:PowerLawdamageevolution)

Crackpropagationhistory– Unrepaired– STEP1

Crackpropagationhistory– Unrepaired– STEP2

Fractureshape– Unrepaired

Matrixcracking,FibersplittingMatrixcracking,

Fibersplitting,Delamination

…

Fiberfracture

…

Stress-straincurve– Unrepaired

0

100

200

300

0 0.005 0.01 0.015 0.02 0.025Strain

Stress(M

a)

ExperimentsAnalysis

Crackpropagationhistory- Repaired

Fractureshape– Repaired

Matrixcracking,Fibersplitting

Matrixcracking,Fibersplitting,Delamination

Fiberfracture… Laminate-patch

separation

Stress-straincurve– Repaired

0

100

200

300

0 0.005 0.01 0.015 0.02 0.025Strain

Stress(M

a)

ExperimentsAnalysis

Materialproperty

• Elastic– E11=40.6GPa,E22=E33=16.3GPa,n12=n13=0.27,n23=0.35– G12=G13=16.8GPa,G23=6.22GPa

• Cohesive– Fiber(Q/P):T1=950MPa,GIC=30N/mm,K=5x107 MPa/mm,p=2– Matrix(Q/P):T1=40MPa,T2=60MPa,GIC=0.3N/mm,GIIC=0.6N/mm,K=107 MPa/mm,p=2

– Delamination(Q/P):T1=40MPa,T2=60MPa,GIC=0.3N/mm,GIIC=0.6N/mm,K=107 MPa/mm,p=2

– Patchadhesive(Q/P):T1=20MPa,T2=30MPa,GIC=0.05N/mm,GIIC=0.3N/mm,K=107 MPa/mm,p=2

(Q:QUADSdamageinitiation,P:PowerLawdamageevolution)

0

100

200

300

400

500

0 0.005 0.01 0.015 0.02 0.025Strain

Stress(M

a)

UsedmatrixCZEpropsforpatchadhesive

Current

Possibilityofincreasedrepairstrength

PENDINGWORK1. Determinemoreeffectiverepairmethods.2. Extendrepairstocouponsofdifferentplyconfigurations.3. Conductexperimentalrepairsonbeamstructures.4. FiniteElementModelingtocorrelatewithexperimental

results.

1. Withindustryrepairrecommendations,Ultimatestrengthinrepairedcaseislowerthanundamagedstrength• RepairEffectiveatlowstrainlevels• Needtodevelopbetterfieldrepairmethods

2. Stressconcentrationsarounddamageregionsisreducedinrepairedcasewhichwillinvariablyreducefracturetendencyatlowstress.

PreliminaryConclusions

• Wehavenotyetfoundanyrepairwithwindturbineindustryrecommendedproceduresthatwillrestoretheundamagedstrengthofhighlyunidirectionallaminatestoundamagedlevels.

• Ifyoudoarepair,youneedtoensurethatitactuallyimprovesthestrengths

• Anassessmentneedstobemadethattheworkingstrainoftherepairedregionisbelowthestrainatwhichitmaydisbond fromtheparentlaminate.• Static• Fatigue

NotesonBondedRepair

-100

0

100

200

300

400

500

600

700

800

900

1000

-0.5 0 0.5 1 1.5 2 2.5 3 3.5

Strain

Stress_StrainPlot

Undamaged_6(VE6)

Unrepaired_2(VE1)

Infusion_Repaired_8(VE2)

Infusion_Repaired_7(VE1)

Adhesive_Repaired_1(VE2)

Adhesive_Repaired_3(VE1)

Stress(M

Pa)

0.00.51.01.52.02.53.0

Strain,%

WorkingStress-Stainneedstobewellbelowonsetofrepairfailure

ScarfedPatchFailure

UndamagedLaminate

ScarfedandUnrepaired

Stress,M

Pa

NominalStress-Strain

Ongoing Work• Effects of Defects – moving on to carbon laminates• Thick Adhesive Joints

– Continuing Cohesive Zone Model Development– Carbon Fiber Adherands– Comparisons to aerospace adhesive joints (collaborative project

with Boeing• Multi-Scale, Multi-Axial Test Facility

– Building database, more replications– Using for Repair studies. Answer basic question: How high can a

composite laminate repair be compared to undamaged structures?

• Continuing with repair studies – progressive damage analysis

• ThisworkwassponsoredbyDOEundertheSandiaLedBladeReliabilityCollaborativeandtheMSUDOEWindTurbineDevelopmentprogram.

• JoshPaquette,BrianNaughton,andToddGriffithofSandiaNationalLaboratories– WindandWaterEnergyTechnologywereinstrumentalinthetechnicalguidanceofthiswork.

• TheentireMSUCompositesGroupwasinstrumentalinsupportingthiswork.

ThankYou!!

Acknowledgments

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