BondedBoron/EpoxyDoublersforReinforcementof

MetallicAircraftStructures

PRESENTATIONOUTLINE

•  BoronDoublerDescription–  ReinforcementConcept–  Advantages

•  InstallationProcess–  SurfacePreparation–  MaterialsandBondingProcess–  Inspection

•  Applications–  Military–  Commercial

•  TestPrograms

BoronEpoxyDoublers

Metal Structure

Applied Stress

Applied Stress Structural Damage

(Stop-Drilled Crack Shown)

Film Adhesive Boron Fiber Direction

Boron Doubler Reinforcement Concept Multi-ply Bonded Boron/Epoxy Doubler

ADVANTAGESOFBORON/EPOXYDOUBLERS

•  BondedInstallation– NoAdditionalHolesinAircraft– NoFastener‐AssociatedStressRisers– OnlyOneSideAccessRequired– CanReinforceWhereRivetingisNotPossible

•  HighSpeciQicModulus– EfQicientLoadTransfer– Thinner,Lighter

ADVANTAGESOFBORON/EPOXYDOUBLERS

•  Non‐MetallicMaterial– Conformable– DoesNotCorrode– GalvanicallyInert

•  UsedforDamageRepairandStructuralEnhancement

DoublerInstallationProcess

INSTALLATIONPROCESS

•  Lay‐Up– DesignforSpeciQicLoadConQiguration–  StandardLaminateConvention– CanBeAssembledAheadofTime– DoublersCanBePre‐curedforSpeciQicConQigurations

INSTALLATIONPROCESS

•  AluminumSurfacePreparation MostCriticalStep

–  PaintRemovalPerConventionalProcess–  CleanandAbradeSurfaceSealUnderlyingFasteners–  SurfaceTreatment

•  PhosphoricAcidAnodize­PACSProcess•  Silane•  Others

–  ApplyandCurePrimer

INSTALLATIONPROCESS

•  BondOntoAircraft–  StructuralFilmAdhesive–  PortableCureEquipment–  VacuumBagPressure–  DoublerCanBeCo‐cured

•  Inspect–  UltrasonicsforBond/CompositeQlaws–  EddyCurrentforUnderlyingCrackGrowth

•  Seal/Paint

Boron/Epoxy Doubler

Adhesive Primer

Aluminum Surface Treatment (e.g., Anodize)

Epoxy-Coat Rivet-Head

Load Load

Cover Ply

Aluminum Alloy Skin

Stop-Drilled Crack Underlying Rivet

Boron Doubler Installation Schematic

Applications

MILITARYAPPLICATIONS

MILITARYAPPLICATIONSUNDERDEVELOPMENT

Aircraft DoublerLocation

F‐15,F‐14 LowerWingSkinB‐1 HorizontalStabilizerSparC‐130 OuterWingStringersB‐52 UpperWingSkinF‐16 LowerwingFuelVentT‐38 LowerWingdoorFrameC‐5 NoseLandingGearDoorKC‐135 UpperWingSkin

COMMERCIALAPPLICATIONS

PerformanceTestPrograms

PERFORMANCETESTPROGRAM

•  PerformedByBoeingTechnologyServices•  Objectives:

–  InstallationProcessSpeciQicationDevelopment–  StructuralAnalysis‐BondedLineStresses–  PerformanceTesting

• ValidationofStructuralEnhancement‐StaticTests• ValidationofCrackGrowthSuppression‐Fatigue

PROCESSSPECIFICATIONDEVELOPMENT

•  SpeciQicationNumberD658‐10183‐1–  DetailedDocumentationofMaterials,EquipmentandProcessingSteps

–  OrientedTowardBoeingProcessSpeciQications–  CriticalStepsValidatedThroughEmpiricalTesting

STRUCTURALANALYSIS

•  INCAP‐LaminateAnalysisProgramUsedtoSizeDoublersforTesting

•  COSMOS‐2DFEAUsedtoAnalyzeInternalStressesDuetoCureTemperatures

•  NIKE‐3DFEAUsedtoEvaluateSpecimenGeometries

•  ANSYS‐FEAUsedtoCharacterizeThermalandResidualStresses

STRUCTURALANALYSIS

•  InformationGainedfromAnalysis–  ShearandPeelStressesPeakatDoublerEdge–  StressesinFrontofDoublerare25%HigherThanAppliedAxialStress

–  ResidualStressesfromBondingOperations(thermal)OpposeAxialStressesDuringService‐ReducingPeakStressinDoubler

PERFORMANCETESTING‐STATICTESTS

•  Objective:DetermineifDoublerRestoresUltimateAluminumStrength(78ksi,538MPa)toCrackedSpecimen

•  TestsonBaselineSpecimensBeforeandAfterFatigueTesting

PERFORMANCETESTING‐STATICTESTRESULTS

•  Pre‐Fatigue‐Quantity:12–  AllSpecimens>78ksi(538MPa)Requirement–  MostBrokeinAluminumOutsideDoubler

•  Post‐Fatigue‐Quantity:91–  87Tests>78ksi(538MPa)–  4Tests42to76ksi(290to524MPa)

• AluminumFailurewithDoublerIntact

PERFORMANCETESTING

•  Tension‐TensionFatigue–  PrimaryObjectives:–  DetermineifDoublerRestoresCyclicCapabilityofAircraftStructure

–  BaselineSpecimensatRoomTemperature–  ParametricStudiesRelatingtoDoublerDesign

•  Sub‐Ambient(‐65oF,‐54oC)• GeometrySensitivities•  Impact

FATIGUETARGETS

Design Design Test Test Aircraft Stress Cycles Stress Cycles

737 0 to 15 ksi 75,000 3 to 20 ksi 300,000 (0 to 103 MPa) (21 to 138 MPa)

747 0 to 18 ksi 20,000 3 to 20 ksi 300,000 (0 to 124 MPa) (21 to 138 MPa)

FATIGUERESULTS

•  UnpatchedSpecimensFailat4000Cycles•  Baseline:15Specimens

–  12Specimens‐NoFailure‐NoCrackGrowth–  1‐CrackGrowthat161,385cycles‐NoFailure–  1‐CrackGrowthat17,464cycles‐NoFailure–  1‐Failureat89,454–  FailureandCrackGrowthaResultofImperfectionsinStopDrillHole

FATIGUERESULTS

•  ObservationsFromParametricStudies–  BaselinePerformanceMinimallyAffectedBy:

• VariationsinDoublerGeometries• ChangesinCurePressure•  IncreasedCrackLength(1”,2.54cm)• MoistureandSolventImmersion•  Impact@100and300Lb‐in• EdgeDisbonds0.5”in(1.27cm)Diameter

FATIGUETESTING

•  ObservationsfromParametricStudies–  CrackRe‐initiationMoreLikelytoOccur:

•  WithIn‐LineRivetsNearCracks•  DisbondsBeneathCrackedArea•  ‐65DegreeF(53.8oC)Environment•  NoStopDrillHole•  TooThinaDoubler

–  WhenCrackRe‐initiationOccurstheCrackGrowthisLinear(NotCatastrophic)

CONCLUSIONS

•  BoronEpoxyDoublersHaveBeenSuccessfullyUsedonAircraftForManyYears

•  ProcessHasBeenDeQinedandDocumented

•  WhenProperlyDesignedandApplied,BoronEpoxyDoublersRestoreStructuralIntegrity