1FractureToughnessofSi3N4/S45CJointwithanInterfaceCrackLiedongFu,YukioMiyasitaandYoshiharuMutohCopyrightAD-TECH.；licenseeAZoM.comPtyLtd.ThisisanAZoOpenAccessRewardsSystem(AZo-OARS)articledistributedunderthetermsoftheAZoOARS/oars.aspwhichpermitsunrestricteduseprovidedtheoriginalworkisproperlycitedbutislimitedtonon-commercialdistributionandreproduction.Posted:September2005TopicsCoveredAbstractKeywordsIntroductionExperimentalSpecimenPreparationExperimentalResultsOscillatorySingularStressFieldofTheInterfaceCrackandTheMaximumNormalStressCriteriaElasto-PlasticSingularStressFieldatTheInterfaceCrackTipFemAnalysisandEvaluationofFracturePathandToughnessBasedontheElasto-PlasticStressIntensityFactorsConclusionsReferencesContactDetailsAbstractFracturetoughnesstestswerecarriedoutforSi3N4/S45Cspecimenswithinterfacecracksofdifferentlengths.Itwasfoundthatthespecimenwithacrackof4mmhashigherapparentfracturetoughnessthanthosewithcracksof1mmand2mmduetothereductionoftheresidualstress.FracturepropagatedintoSi3N4fromthecracktipinthedirectionof40oforcracksof1mmand2mmwhileitpropagatedalongtheinterfaceforcrackof4mm.Elasto-plasticanalysiswascarriedoutconsideringS45CasthelinearhardeningmaterialandSi3N4astheelasticmaterial.Itwasfoundthatthestressaroundthecracktipisdominatedbyanelasto-plasticsingularstressfield,whichissubstantiallythesameastheelasticsingularstressfieldofaninterfacecrack.Evaluationofthefracturepathandtoughnesswascarriedoutbasedonthestressintensityfactorsoftheelasto-plasticsingularstressfield.KeywordsInterfaceCrack,FractureToughness,Si3N4/S45CJoint,ThermalResidualStress,Elasto-plasticAnalysisIntroductionTheceramic/metaljointshavebeenincreasinglyappliedinawiderangeofengineeringfieldsbecausetheceramichasstablemechanicalpropertiesathightemperatureandgoodresistancetowear,erosionandoxidation.However,thedifferenceofmaterialpropertiesbetweenmetalandceramicinducesstress2singularitiesattheinterfaceedge.Moreover,highthermalresidualstresswillbeinducedduringthecoolingprocessduetothemismatchofthethermalexpansioncoefficients.Thestresssingularitytogetherwiththethermalresidualstressdegradesthestrengthofceramic/metaljointandmakestheevaluationofthestrengthdifficult.Manyworkshavebeendoneabouttheresidualstressandthestrengthevaluationofceramic/metaljoints.Forexample,Kobayashietal.1,2haveinvestigatedthebendingstrengthandresidualstressofSi3N4/S45Cjointandtheeffectofthesizeofthespecimenonthebendingstrength.Qiuetal.3haveinvestigatedtheinfluenceofresidualstressandcyclicloadonthestrengthofSi3N4/S45Cjoint.However,duetothecomplexityoftheproblem,ageneralizedevaluationmethodfortheceramic/metaljointhasnotyetbeenproposed.Theelasticsolutionofthesingularstressfieldoftheinterfacecrackhasbeenstudiedsince19594-9.Rice10hassummarizedtheworkinthisfieldandsetuptheelasticfracturemechanicsconceptsforinterfacialcracks.Yuukietal.11,12haveproposedthemaximumnormalstresscriteriaforpredictingfracturepathandstrengthofceramic/metaljointbasedontheelastictheory.Theplasticdeformationofmetalwillinevitablyappearnearthecracktipduetothestresssingularity.Formostoftheceramic/metaljoints,theplasticdeformationofmetalhasasignificantinfluenceonthestrengthoftheceramic/metaljoint.Duetotheanalyticalcomplexity,theevaluationofthefracturepathandstrengthofceramic/metaljointbasedontheelasto-plastictheoryhasnotyetbeenmade.Inthisstudy,fourpointbendingtestsofSi3N4/S45Cjointspecimenswithaninterfacecrackwerecarriedout.Evaluationofthefracturepathandfracturetoughnesswasattemptedbasedontheelasto-plasticanalysis.ExperimentalSpecimenPreparationFigure1showsthegeometryanddimensionsofSi3N4/S45Cjointspecimen.Thesilverbasedbrazingalloy(wt%is:Ag,71%,Cu,27%,Ti,2%)with60mthicknesswasusedforthebondingbetweenSi3N4ceramicsandS45Csteel.Brazingwascarriedinavacuumfurnace(2.5x10-5Torr).Thetemperatureofthefurnacewasincreasedatarateof20oC/minuptothebrazingtemperatureof850oCandkeptfor10min,thendecreasedatarateof10oC/min.Thejoiningsurfaceswerepolishedwithdiamondpowderof0.25mdiameter.Duringthebrazing,acontactpressureof0.002MPawasapplied.Afterbrazing,aninterfacecrackwasintroducedbytheelectricdischargemethodwiththecuttingwireof0.1mmdiameter.Fourspecimenswithdifferentcracklengthswereprepared.Twoofthespecimenshadcracklengthsof4.0mmandtheothertwospecimenshadcracklengthsof1.0mmand2.0mm.3Figure1.Fracturetoughnessspecimen.ExperimentalResultsFourpointbendingtestswerecarriedoutonthefracturetoughnessspecimensatacrossheadspeedof0.5mm/min.Table1showstheresultsofthefracturetoughness.Theapparentfracturetoughnessisdefinedas:(1)with(2)(3)WherePfisthefractureload,aisthecracklength,wthespecimenwidth,tthespecimenhighness,L2theouterspanandL1theinnerspan.Table1.Resultofthefracturetoughnesstests.No.Cracklengtha(mm)Pf(N)f(MPa)FIKIApparent(MPam)11.0285.417.1281.04360.980722.0237.814.271.05301.160734.01649.098.951.256112.431744.01744.2104.651.256113.1478AscanbeseeninTable1,thespecimenswithacracklengthof4.0mmindicateahigherfractureloadthanthosewithshortercracklengthsof1.0and2.0mm.Astheresidualstresswillredistributeaftercutting2,therelaxationofthermalresidualstressforlongercracklengthmaybeapossiblereason.Figure2showsthemacroscopicobservationofthefracturedspecimen.Forthespecimenswithacracklengthof1.0and2.0mm,crackpropagatedintoSi3N4directlyfromtheinitialcracktipinthedirectionofabout40o.Forthespecimenswithacracklengthof4.0mm,thecrackpropagatedalongtheinterface4forabout1.0mmandthenkinkedintoSi3N4inadirectionofabout10ototheinterface.(a)a=1.0mm(b)a=2.0mm(c)a=4.0mm(d)a=4.0mmF