Fatigue and Fracture

Dexter,R.J.andFisher,J.W.“FatigueandFracture”StructuralEngineeringHandbookEd.ChenWai-FahBocaRaton:CRCPressLLC,1999FatigueandFractureRobertJ.DexterandJohnW.FisherDepartmentofCivilEngineering,LehighUniversity,Bethlehem,PA24.1Introduction24.2DesignandEvaluationofStructuresforFatigueClassificationofStructuralDetailsforFatigue•ScaleEffectsinFatigue•DistortionandMultiaxialLoadingEffectsinFatigue•Low-CycleFatigueDuetoSeismicLoading24.3EvaluationofStructuralDetailsforFractureSpecificationofSteelandFillerMetal•FractureMechanics24.4Summary24.5DefiningTermsReferencesFurtherReading24.1IntroductionThischapterprovidesanoverviewofaspectsoffatigueandfracturethatarerelevanttodesignorassessmentofstructuralcomponentsmadeofconcrete,steel,andaluminum.Thischapterisintendedforpracticingcivilandstructuralengineersengagedinregulation,design,inspection,repair,andretrofitofavarietyofstructures,includingbuildings;bridges;sign,signal,andluminairesupportstructures;chimneys;transmissiontowers;etc.Establishedproceduresareexplainedfordesignandin-serviceassessmenttoensurethatstructuresareresistanttofatigueandfracture.Thischapterisnotintendedasacomprehensivereviewofthelatestresearchresultsinthesubjectarea;therefore,manyinterestingaspectsoffatigueandfracturearenotdiscussed.Thedesignandassessmentproceduresoutlinedinthischaptermaybeappliedtoothersimilarstructures,evenoutsidethetraditionaldomainofcivilengineers,includingoffshorestructures,cranes,heavyvehicleframes,andships.Themechanicalengineeringapproach,whichworkswellforsmoothmachineparts,givesanoverlyoptimisticassessmentofthefatiguestrengthofstructuraldetails.Therearemanycasesoffailuresofthesetypesofstructures,suchasthecraneinFigure24.1orthevehicleframeinFigure24.2,whichwouldhavebeenpredictedhadthestructuralengineeringapproachbeenapplied.Thepossibilityoffatiguemustbecheckedforanystructuralmemberthatissubjectedtocyclicloading.Amongthefewcaseswherecrackinghasoccurredinstructures,thecracksareusuallyonlyanuisanceandmayevengounnoticed.Onlyincertaintrulynon-redundantstructuralsystemscancrackingleadtostructuralcollapse.Theloadingformoststructuresisessentiallyunderfixed-cancrackingleadtostructuralcollapse.Theloadingformoststructuresisessentiallyunderfixed-connectionsinredundantstructuresareessentiallyunderdisplacement-controlboundaryconditions.Inotherwords,becauseofthestiffnessofthesurroundingstructure,theendsofthememberhavetoc
1999byCRCPressLLC•TheEffectiveStressRangeforVariable-AmplitudeLoadingAnalysisFIGURE24.1:Fatiguecrackingatweldeddetailincraneboom.deforminawaythatiscompatiblewithnearbymembers.Underdisplacementcontrol,amembercancontinuetoprovideintegrity(e.g.,transfershear)afterithasreachedultimatestrengthandisinthedescendingbranchoftheload-displacementcurve.Thisbehaviorunderdisplacementcontrolisreferredtoasloadshedding.Inorderforloadsheddingtobefullyeffective,individualcriticalmembersintensionmustelongatetoseveraltimestheyieldstrainlocallywithoutcompletelyfracturing.Goodshort-termperformanceshouldnotleadtocomplacency,becausefatigueandstress-corrosioncrackingmaytakedecadestomanifest.Corrosionandotherstructuraldamagecanpre-cipitateandacceleratefatigueandfracture.Also,fabricationcracksmaybebuiltintoastructureandneverdiscovered.Thesedormantcrackscanfractureifthestructureiseverloadedintotheinelasticrange,suchasinanearthquake.FatiguecrackinginsteelbridgesintheU.S.hasbecomeamorefrequentoccurrencesincethe1970s.Figure24.3showsalargecrackthatwasdiscoveredin1970attheendofacoverplateinoneoftheYellowMillPondmultibeamstructureslocatedatBridgeport,Connecticut.Between1970and1981,c1999byCRCPressLLCFIGURE24.2:Fatiguecrackingatweldeddetailinvehicleframe.numerousfatiguecrackswerediscoveredattheendsofcoverplatesinthisbridge[19].Fatiguecrackinginbridges,suchasshowninFigure24.3,resultedfromaninadequateexperimentalbaseandoverlyoptimisticspecificationprovisionsdevelopedfromtheexperimentaldatainthe1960s.Theassumptionofafatiguelimitattwomillioncyclesprovedtobeincorrect.Asaresultofextensivelarge-scalefatiguetesting,itisnowpossibletoclearlyidentifyandavoiddetailsthatareexpectedtohavelowfatiguestrength.Thefatigueproblemswiththeolderbridgescanbeavoidedinnewconstruction.Fortunately,itisalsopossibletoretrofitorupgradethefatiguestrengthofexistingbridgeswithpoordetails.Low-cyclefatigueisapossiblefailuremodeforstructuralmembersorconnectionsthatarecycledintotheinelasticregionforasmallnumberofcycles.Forexample,bracingmembersinabracedframeorbeam-to-columnconnectionsinaweldedspecial-momentframe(WSMF)maybesubjectedtolow-cyclefatigueinanearthquake.Insectionsthatarecyclicallybuckling,thelow-cyclefatigueislinkedtothebucklingbehavior.ThisemergingareaofresearchisbrieflydiscussedinSection24.2.5.Theprimaryemphasisinthischapterisonhigh-cyclefatigue.Trucktrafficcauseshigh-cyclefatigueofbridges.Fatiguecrackingmayoccurinindustrialbuildingssubjectedtoloadsfromcranesorotherequipmentormachinery.Althoughithasnotbeenaprobleminthepast,fatiguecrackingcouldoccurinhigh-risebuildingsfrequentlysubjectedtolargewindloads.Windloadshavecausednumerousfatigueproble