ADVANCES IN FATIGUE AND FRACTURE MECHANICS ANALYSE

ADVANCESINFATIGUEANDFRACTUREMECHANICSANALYSESFORAIRCRAFTSTRUCTURESJ.C.Newman,Jr.*Thispaperreviewssomeoftheadvancesthathavebeenmadeinstressanalysesofcrackedaircraftcomponents,intheunderstandingofthefatigueandfatigue-crackgrowthprocess,andinthepredictionofresidualstrengthofcomplexaircraftstructureswithwidespreadfatiguedamage.Finite-elementanalysesofcrackedstructuresarenowusedtodetermineaccuratestress-intensityfactorsforcracksatstructuraldetails.Observationsofsmall-crackbehavioratopenandrivet-loadedholesandthedevelopmentofsmall-cracktheoryhasleadtothepredictionofstress-lifebehaviorforcomponentswithstressconcentrationsunderaircraftspectrumloading.Fatigue-crackgrowthundersimulatedaircraftspectracannowbepredictedwiththecrack-closureconcept.Residualstrengthofcrackedpanelswithsevereout-of-planedeformations(buckling)inthepresenceofstiffenersandmultiple-sitedamagecanbepredictedwithadvancedelastic-plasticfinite-elementanalysesandthecriticalcrack-tip-openingangle(CTOA)fracturecriterion.Theseadvancesarehelpingtoassurecontinuedsafetyofaircraftstructures.INTRODUCTIONIn1969,SchijveintheSecondFrederikJ.PlantemaMemorialLecture[1]statedthat“fatigueinaircraftstructuresisaproblemforwhichquantitativeandgenerallyacceptedsolutionsarenotavailable.”Duringthepast30years,manyadvanceshavebeenmadeinthestressanalysesofcrackedaircraftcomponents,inunderstandingthefatigueandfatigue-crackgrowthbehaviorinmetallicmaterials,andinthepredictionofresidualstrengthofcomplexbuilt-upaircraftstructureswithwidespreadfatiguedamage.Althoughthefailurerateinaircraftstructuresduetofatigueandstructuralfailurehasdroppedsignificantly[2]fromthemid-1950’s,thefatigueandfracturecommunitymuststayalert.Thetechnicalcommunityshouldcontinuetoimprovetheunderstandingofthefatigueandfractureprocessandtousetheadvancedanalysistoolstosafeguardthepublicagainstunexpectedfailuremodes,suchastheAlohaAirlinesfuselagefailurein1988duetowidespreadfatiguedamage.Swift,intheEleventhPlantemaLecture[3],discussedhowmultiple-sitedamagecrackingcouldreducetheresidualstrengthoffuselagestructures.TheSeventeenthPlantemaMemorialLectureisareviewofsomeofthetechnicaldevelopmentsandconceptsthathaveledtoabetterunderstandingofthefatigueandfractureprocessinmetallicmaterials.Advancesincomputertechnologyhasallowedmoreaccuratestressanalysestobeconductedonthree-dimensionalcrackconfigurations,morerealisticsimulationsoffatigueandfatiguecrackinginstructuralcomponents,andtheuseofmoreadvancedelastic-plasticfracture*MechanicsandDurabilityBranch,NASALangleyResearchCenter,Hampton,VA,USA23681mechanicsconceptstoassesstheresidualstrengthandfail-safecapabilityofaircraftstructures.Thisreviewislimitedinscopeandwillnotbeabletofullycoverthevastamountofresearchthathasbeenconductedoverthepast30yearsinthefieldsoffatigueandfracturemechanics.Theauthorrequeststhereadersindulgenceandforgivenessifsomemajoreventshavebeenomitted,orifreferenceisnotmadetoallofthosewhohavemadesignificantcontributionstothesubject.Inparticular,thispaperwillreviewsomeoftheadvancesthathavebeenmadeintheareaofstressanalysesofcrackedbodies,fatigue(crack-initiationandsmall-crackbehavior),fatigue-crackgrowth,andfractureofcomplexfuselagestructure.Thepaperwilldiscusstheglobalstressanalysesofcrackedfuselagelap-jointstructuretodeterminelocalfastenerstresses.Theselocalstressesarethenusedtodevelopstress-intensityfactorsforsurfaceandcornercracksatopenandfastener-loadedholesusingthree-dimensionalcodes.Observationsofsmall-crackbehavioratopenandrivet-loadedholesandthedevelopmentofsmall-cracktheoryhaveleadtothepredictionofstress-lifebehaviorforcomponentsunderconstant-andvariable-amplitudeloading.Someofthesepredictionsareshownformaterialscommonlyusedinaircraftconstruction.Fatiguecrackgrowthunderaircraftspectrumloadingcanbepredictedwiththecrack-closureconcept.Theimportanceofconstraint(three-dimensionalstressstatearoundthecrackfront)onfatigue-crackgrowthunderspectrumloadingwillbediscussed.Theabilityoftheadvancedfinite-elementcodestopredictthesevereout-of-planedeformationsofcrackedsheetsisdemonstrated.Predictionsofresidualstrengthofcrackedpanelswithsevereout-of-planedeformationsinthepresenceofstiffenersandmultiple-sitedamagecrackingwillbedemonstratedwithadvancedfinite-elementcodesandthecriticalcrack-tip-openingangle(CTOA)fracturecriterion.Inthelastdecade,theinternationalconferencesonagingaircraft[4-6]haveintensifiedthedevelopmentoftheseadvancedmethodologiesandtoolsforfatigueandfracturemechanicsanalyses,seeforexampleHarrisetal[7].Theseadvancesarehelpingtoinsurethecontinuedairworthinessofaircraftstructures.STRESSANALYSESOFCRACKEDBODIESDuringthepast30years,thestress-analysiscommunityhasdevelopedalargenumberoffinite-elementcodestoconductlinearandnon-linearstressanalysesofcomplexaircraftstructuralcomponents.Manyaircraftcompaniesdeterminestressanddeformationstatesinaircraftstructuresusingcodes,suchasABAQUS[8],ANSYS[9],MSC/NASTRAN[10]andSTAGS[11,12].Manyofthesecodeshavespecialfeaturestoanalyzecrackproblemsanddeterminethestress-intensityfactororthenon-linearequivalent,theJ-integral.Som