材料科学基础英文版课件 (7).ppt

Chapter9FailureofMaterials,Introduction,Failureofengineeringmaterialsisabadthingcausinglifeandeconomiclosses,andalsointerferingwiththeavailabilityofproductsandservicesUsualreasonsforthefailure:impropermaterialsselectionandprocessing,inadequatedesignofthecomponent,misuseItistheresponsibilityofengineerstoexpectandplanforpossiblefailure,andwhenfailureoccurs,tofindoutitsreasonsandtakepropermeasuresagainstfutureincidentsTopics:simplefracture,fundamentalsoffracturemechanics,impactfracturetesting,ductile-to-brittletransition,fatigue,andcreep,FundamentalsofFracture,Simplefractureistheseparationofabodyintotwoormorepiecesinresponsetoanappliedstressatlowtemperatures(relativetothemeltingtemperature)Theappliedstress:tensile,compressive,shear,ortorsionalTwofracturemodes:ductileandbrittle,dependingontheabilityofamaterialtoexperienceplasticdeformationAnyfractureprocessinvolvestwostepscrackformationandpropagation.Ductilefractureiscorrespondingtoextensiveplasticdeformationnearadvancingcracksandrelativelyslowcrackpropagationrate(stable),showingconsiderabledeformationonfracturesurfaces(rough);brittlefracturetolittleplasticdeformationandfastcrackpropagationrate(unstable),showingnoapparentdeformationonfracturesurfaces(smooth)Brittlefractureoftencausedisastersbecausethereisnowarningsignalbeforefractureanditisaveryfastprocess.However,ductilefracturedevelopsslowlyandthereiswarningsignal(considerableplasticdeformation)beforefractureDuctilefractureisalwaysdesirable.Undernormalconditions,mostmetalalloysareductile,butceramicsareconsiderablybrittleandpolymersmayexhibitbothtypesoffracture,DuctileFracture(1),Featuresofductilefracture,Macroscopically,Neckingdowntoapoint,Somenecking,DuctileFracture(2),Microscopically,Initialnecking,Finalshearfractureata45oanglerelativetothetensiledirection,Thefracturesurfacehasafibrousappearance,DuctileFracture(3),Cup-and-conefractureinaluminum,DuctileFracture(4),Typicalductilefracturesurfaceappearance(microscopically)-1,Sowingmanydimples,DuctileFracture(5),Typicalductilefracturesurfaceappearance(microscopically)-2,BrittleFracture(1),Nonecking,noapparentplasticdeformation,Brittlefractureinasteel,BrittleFracture(2),Fracturesurfacemarkingsforbrittlefracture,Withnakedeyes:V-shapedmarkingsnearthecenteroffracturesurfacewhichpointbacktowardstheoriginofthecrack,Withnakedeyes:ridgeswhichradiatefromtheoriginofthecrack,BrittleFracture(3),Formostbrittlematerials,crackpropagationisalongspecificcrystalplanes.Theplanesarecalledcleavageplanesandthefractureiscallcleavagefracture.ThistypeoffractureisTransgranularfracture,GrainyorfacetedtextureseenintheSEMmicrograph,SEMfractographforacleavagefracture,Transgranularfracture,Crackpropagationisalonggrainboundaries-Intergranularfracture,BrittleFracture(5),(d),SEMfractographsforintergranularfracture(3-Dnatureofgrainsispresent),BrittleFracture(6),Anoiltankthatfracturedinabrittlemannerbycrackpropagationarounditsgirth,PrinciplesofFractureMechanics(1),Brittlefractureofnormallyductilematerialsrequiresustohaveagoodunderstandingofthemechanismsoffracture.Todothis,weneedtoknowtheknowledgeoffracturemechanics,Concernedwiththerelationshipbetweenmaterialproperties,stresslevel,crack-producingflaws,andcrackpropagationmechanisms,StressConcentration,Measuredfracturestrengthsformostbrittlematerialsmuchlowerthantheirtheoreticallycalculatedvaluesonthebasisofatomicbondingenergies,PrinciplesofFractureMechanics(2),Themaximumstressatthecracktip:,Foramicrocrack,(a/t)1/2maybeverylarge,leadingtoaverylargem,i.e.,averylargestressconcentration,Theratiom/oisdenotedasthestressconcentrationfactorKt,PrinciplesofFractureMechanics(3),ItisseenfromtheabovethatstressconcentrationoccursNotonlyatmicroscopicflawsorcracksbutalsoatmacroscopicinternaldefectslikevoids,atsharpcornersoratnotches,Forabrittlematerial,thereisnearlynoplasticdeformationoccurringatthecracktip.Thusthetheoreticalstressconcentrationfactorwillresult,Foraductilematerial,plasticdeformationoccurswhenthemaximumstressmexceedstheyieldstrength,PrinciplesofFractureMechanics(4),Forabrittlematerial,whenanappliedtensilestressexceedsthecriticalstress,thecrackwillpropagate,leadingtofailure,Forthedefect-freemetallicorceramicwhisker,whataboutitsmeasuredfracturestrength?,Question:,PrinciplesofFractureMechanics(5),FractureToughness,Inthelightofprinciplesoffracturemechanics,therelationofthecriticalstressforcrackpropagation(c)tothecracklength(a)canbeexpressedas,whereKcisthefracturetoughnessinunitsof(MPam1/2),andYisaparameterassociatedwithbothcrackandspecimensizesandgeometriesaswellasthemannerofloadapplication,Kcisamaterialproperty,whichisthemeasureofthematerialsresistancetobrittlefracturewhenacrackexists,PrinciplesofFractureMechanics(6),Aninternalcrackinaplateofinfinitewidth,Asurfacecrackinaplateofsemi-infinitewidth,Y=1.0,Y=1.1,PrinciplesofFractureMechanics(7),Whenthespecimenisnotthickenoughcomparedwiththecracklength,thevalueofKcchangeswithspecimenthickness,Whenthespecimenissufficientlythick,thevalueofKcwillnolongerchangewithspecimenthickness(aconstant).Undertheseconditions,aconditionofplanestrainexistsaroundthecracktip,i.e.thereisnostraincomponentperpendiculartothefrontandbackfaces,TheKcvalueinthisplanestrainconditioniscalledtheplanestrainfracturetoughnessKIc,whichisdefinedas,PrinciplesofFractureMechanics(8),ModeI,openingortensilemode,ModeII,slidingmode,ModeIII,tearingmode,Threemodesofcracksurfacedisplacement,PrinciplesofFractureMechanics(9),Mainfactorsinfluencingtheplanestrainfracturetoughness,TemperatureKIcdecreaseswithdecreasingtemperatureStrainrateKIcdecreaseswithincreasingstrainrateMicrostructureKIcincreaseswithreducinggrainsize(toughening).Anincreaseinyieldstrengthbroughtaboutbysolidsolutionstrengthening,precipitationhardening,ors