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1、Chapter9FailureofMaterials Introduction Failureofengineeringmaterialsisabadthing causinglifeandeconomiclosses andalsointerferingwiththeavailabilityofproductsandservicesUsualreasonsforthefailure impropermaterialsselectionandprocessing inadequatedesignofthecomponent misuseItistheresponsibilityofengine
2、erstoexpectandplanforpossiblefailure andwhenfailureoccurs tofindoutitsreasonsandtakepropermeasuresagainstfutureincidentsTopics simplefracture fundamentalsoffracturemechanics impactfracturetesting ductile to brittletransition fatigue andcreep FundamentalsofFracture Simplefractureistheseparationofabod
3、yintotwoormorepiecesinresponsetoanappliedstressatlowtemperatures relativetothemeltingtemperature Theappliedstress tensile compressive shear ortorsionalTwofracturemodes ductileandbrittle dependingontheabilityofamaterialtoexperienceplasticdeformationAnyfractureprocessinvolvestwosteps crackformationand
4、propagation Ductilefractureiscorrespondingtoextensiveplasticdeformationnearadvancingcracksandrelativelyslowcrackpropagationrate stable showingconsiderabledeformationonfracturesurfaces rough brittlefracturetolittleplasticdeformationandfastcrackpropagationrate unstable showingnoapparentdeformationonfr
5、acturesurfaces smooth Brittlefractureoftencausedisastersbecausethereisnowarningsignalbeforefractureanditisaveryfastprocess However ductilefracturedevelopsslowlyandthereiswarningsignal considerableplasticdeformation beforefractureDuctilefractureisalwaysdesirable Undernormalconditions mostmetalalloysa
6、reductile butceramicsareconsiderablybrittleandpolymersmayexhibitbothtypesoffracture DuctileFracture 1 Featuresofductilefracture Macroscopically Neckingdowntoapoint Somenecking DuctileFracture 2 Microscopically Initialnecking Finalshearfractureata45oanglerelativetothetensiledirection Thefracturesurfa
7、cehasafibrousappearance DuctileFracture 3 Cup and conefractureinaluminum DuctileFracture 4 Typicalductilefracturesurfaceappearance microscopically 1 Sowingmanydimples DuctileFracture 5 Typicalductilefracturesurfaceappearance microscopically 2 BrittleFracture 1 Nonecking noapparentplasticdeformation
8、Brittlefractureinasteel BrittleFracture 2 Fracturesurfacemarkingsforbrittlefracture Withnakedeyes V shapedmarkingsnearthecenteroffracturesurfacewhichpointbacktowardstheoriginofthecrack Withnakedeyes ridgeswhichradiatefromtheoriginofthecrack BrittleFracture 3 Formostbrittlematerials crackpropagationi
9、salongspecificcrystalplanes Theplanesarecalledcleavageplanesandthefractureiscallcleavagefracture ThistypeoffractureisTransgranularfracture GrainyorfacetedtextureseenintheSEMmicrograph SEMfractographforacleavagefracture Transgranularfracture Crackpropagationisalonggrainboundaries Intergranularfractur
10、e BrittleFracture 5 d SEMfractographsforintergranularfracture 3 Dnatureofgrainsispresent BrittleFracture 6 Anoiltankthatfracturedinabrittlemannerbycrackpropagationarounditsgirth PrinciplesofFractureMechanics 1 Brittlefractureofnormallyductilematerialsrequiresustohaveagoodunderstandingofthemechanisms
11、offracture Todothis weneedtoknowtheknowledgeoffracturemechanics Concernedwiththerelationshipbetweenmaterialproperties stresslevel crack producingflaws andcrackpropagationmechanisms StressConcentration Measuredfracturestrengthsformostbrittlematerials muchlowerthantheirtheoreticallycalculatedvaluesont
12、hebasisofatomicbondingenergies PrinciplesofFractureMechanics 2 Themaximumstressatthecracktip Foramicrocrack a t 1 2maybeverylarge leadingtoaverylarge m i e averylargestressconcentration Theratio m oisdenotedasthestressconcentrationfactorKt PrinciplesofFractureMechanics 3 Itisseenfromtheabovethatstre
13、ssconcentrationoccursNotonlyatmicroscopicflawsorcracksbutalsoatmacroscopicinternaldefectslikevoids atsharpcornersoratnotches Forabrittlematerial thereisnearlynoplasticdeformationoccurringatthecracktip Thusthetheoreticalstressconcentrationfactorwillresult Foraductilematerial plasticdeformationoccursw
14、henthemaximumstress mexceedstheyieldstrength PrinciplesofFractureMechanics 4 Forabrittlematerial whenanappliedtensilestressexceedsthecriticalstress thecrackwillpropagate leadingtofailure Forthedefect freemetallicorceramicwhisker whataboutitsmeasuredfracturestrength Question PrinciplesofFractureMecha
15、nics 5 FractureToughness Inthelightofprinciplesoffracturemechanics therelationofthecriticalstressforcrackpropagation c tothecracklength a canbeexpressedas whereKcisthefracturetoughnessinunitsof MPam1 2 andYisaparameterassociatedwithbothcrackandspecimensizesandgeometriesaswellasthemannerofloadapplica
16、tion Kcisamaterialproperty whichisthemeasureofthematerial sresistancetobrittlefracturewhenacrackexists PrinciplesofFractureMechanics 6 Aninternalcrackinaplateofinfinitewidth Asurfacecrackinaplateofsemi infinitewidth Y 1 0 Y 1 1 PrinciplesofFractureMechanics 7 Whenthespecimenisnotthickenoughcomparedwiththecracklength thevalueofKcchangeswithspecimenthickness Whenthespecimenissufficientlythick thevalueofKcwillnolongerchangewithspecimenthickness aconstant Undertheseconditions aconditionofplanestrain
