Fatigue & Damage Tolerance of Metallic Aircraft St

Fatigue&DamageTolerance(F&DT)ofMetallicAircraftStructuresAerospaceEngineeringDepartmentSchoolofEngineering,CranfieldUniversity,U.K.Lecture2OutlineMaterialselection:strengthvs.toughnessLimitationsofLEFM:sizeeffectinfractureLEFMmethodsfornewmanufacturingprocesses(a)Frictionstirwelding(b)AdhesivebondingDamagetolerance&materialselection:Theroleofstrengthlevelandtoughness[Source:PEIrving,CranfieldUniversitylectures.2007]Influenceofmaterialstrengthoncrackinitiation(safelife).EffectsofmaterialstrengthoncrackgrowthratesEffectsonstaticfractureCombinedeffectEffectofstrengthonsmoothspecimenfatiguepropertiesHighcyclefatiguestrengthisdependentalmostexclusivelyonthestaticultimatetensilestrength.Forquenchedandtemperedsteels,thefatiguestrengthat106cyclesisabout0.5ofthestaticUTS.ForAlalloys,thefatiguestrengthat106cyclesisabout0.3ofthestaticUTS.Lowcyclefatigue(102–104cycles)athighappliedstressismostlyinfluencedbytheductilitybecauseplasticityisoccurringinthesample.Undertheseconditions,thegreaterthestaticductility,thebetterthefatiguelife.Atlowcyclesplasticstrainrangeisabetterpredictoroflifethantheappliedstressrange.Variableamplitudeloading,relevanttoserviceapplications,willcontaincyclesofrangescausingbothHCFandLCF,andbothwillcontributetofatiguedamage.ThebestmaterialforaparticularspectrumwilldependonthematchoftheS-Nore–Ncurvetothespectruminquestion.ThepredictioncanbedoneusingaMiner’stypecumulativedamagerule.[Source:PEIrving,CranfieldUniversitylectures.2007]EffectofstrengthoncrackgrowthratesIncreasesinmaterialstrengthleveldonotimproveresistancetofatiguecrackgrowthratesatcracklengthsgreaterthan1–2mm.HighstrengthlowductilitymaterialstendtohavemarginallyfastergrowthratesforthesamevalueofDKthanthatoflowerstrengthvariantsofthesamealloysystem.[Source:PEIrving,CranfieldUniversitylectures.2007]EffectofstiffnessoncrackgrowthratesMaterialstiffnessorelasticmodulusEhasthemosteffectonfatiguecrackgrowthrates.Fatiguecrackgrowthratesareinverselyproportionaltothisparameter,andaplotofDK-Eforthecommonengineeringmaterialsofsteels,aluminiumalloysandtitaniumalloysrevealsacommonline.Generally,steelshavelowestcrackgrowthratesfollowedbytitaniumandwithaluminiumalloythefastest.[Source:PEIrving,CranfieldUniversitylectures.2007]1.0E-101.0E-091.0E-081.0E-071.0E-061.0E-05110100StressIntensityrangeMPam1/2crackgrowthratem/cycle2024-T3AluminiumTi-6-4Titanium300MSteelFatiguecrackgrowthrates–selectionofoptimummaterialsFatiguecrackgrowthrates–selectionofoptimummaterials(Cont.)Thechartshowsacomparisonbetweensteel,titaniumandaluminium.Therankingofthematerialsisinverselyaccordingtotheirstiffness,withaluminiumbeingtheworstandsteelthebestincrackgrowthrates.Datasource:RORitchie,MetalsScience,Vol.11,1977,p368.Atlowda/dN,therewillbesubstantialeffectsofmicrostructure,environmentandmeanstress;differentmaterialswillhaveverydifferentgrowthrates.TherearesubstantialexperimentaldataonawiderangeofmaterialsandenvironmentswhichareavailableindatabasessuchastheESDUdatabase.Thesecanbeusedtoselectthebestmaterialtogivethelongestlife,bycomparisonofcurvessuchasthatabove.Selectingamaterialwithhightoughnessallowsalongercracklengthtobepresentbeforecatastrophicfailureoccurs.Thisisoftenbeneficialinincreasingtheprobabilityofcrackdetection,priortocatastrophicfailure.Howeveroveralllifedoesnotbenefitverymuch.Materialproperties:2024-T3:E=72GPa,YS=365MPa,UTS=450MPa,KIC=36MPa(m)1/2(Plane-strain),KC=72MPa(m)1/2(Plane-stress),ParislawconstantsC=1.545x10-10,n=3.28(source:AFGROW/NASGROW)Ti-6Al-4v:E=110GPa,YS=951MPa,UTS=1007MPa,KIC=55MPa(m)1/2(Plane-strain),KC=82MPa(m)1/2(Plane-stress),ParislawconstantsC=4.8197x10-11,n=3.00(source:AFGROW/NASGROW)EffectofstaticfracturetoughnessStaticfracturetoughnesswillinfluencefatiguelivesinfatiguecrackgrowthdominatedregimesintwoways:•Kincreasesasthecrackgrows.WhenKmaxapproachesKIC,crackaccelerationoccurs.Failuremodeislikelytobecleavageandvoidcoalescence,andintergranularfailureaddstothenormalcyclicfatiguecrackgrowthincrement.ThisprocesswilloccuratsmallervaluesofKmaxforlowKICmaterials.•FinalfailurewilloccuratsmallervaluesofKandshortercriticalcracklengths,whenKmaxapproachesKIC.Thiseffecthaslittleinfluenceonthelifeasthecyclestofailureareasmallfractionofthetotal.However,ithasgreatimplicationsondamagetolerance,aslongcrackshavemuchgreaterdelectabilitythanshortcracks.[Source:PEIrving,CranfieldUniversitylectures.2007]ExampleProblem2ass2WGeometry:M(T)sampleunderuniformstresss.Panelwidthisinfinite(2W=infinite)Henceb=1.0Initialcracklength2a=4mmThickness=1.6mm(planestressstate)Materials:2024-T351aluminiumalloyTi-6Al-4vtitaniumalloyCyclicload:smax=33%YS,R=0.1Find:(1)Criticalcracklengthatthelimitload(assumingslimit=67%YS)(2)Crackgrowthlives.For2024-T3:slimit=0.67xYS=244MPaKC=72MPa(m)1/2(planestress)FromK=KC,wehave:Hence:(1)CriticalCrackLengthunderLimitLoadCcritLimitKa=s)(28)(12limmmKaitCcrit==sForTi-6Al-4v:slimit=0.67xYS=637MPaKC=82MPa(m)1/2(planestress)FromK=KC,wehave:Hence:CcritLimitKa=s)(3.5)(12limmmKaitCcrit==sComments:Underthesamenormalisedstresslevel,i.e.s