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FractureofMaterialsFracture/Rupture/断裂：Solidmaterialsaredecomposedintoseparatepartsunderloading/force,indicatingcompletelymechanicalfailureofmaterials.固体材料在力的作用下分成若干部分的现象。意味作材料的彻底失效。断口分析是金属材料断裂失效分析的重要方法。记录了断裂产生原因，扩散的途径，扩散过程及影响裂纹扩散的各内外因素。Ductilefracture&brittlefracture/金属的韧性断裂与脆性断裂a.Macro-characteristics/宏观特征(a),(b)韧性断裂：断裂前产生较大的塑性变形;(c)脆性断裂：断裂前没有明显的塑性变形，断口平齐。１）Micro-mechanismofductilefracture/韧断机制b.Micro-characteristicsandmechanism/微观特征及机制Process：Necking/constriction－Maximumuniaxialstressatthecenter－Formationofmicro-cracks－growing,aggregatingandpropagating－centralcracks－expandingtransversely－collapsenearthesurfacealong45overticallyAggregationandpropagatingofmicro-cracks/微孔聚合Macro-characteristics/韧性断裂：graydarkandfibrous/呈暗灰色的纤维状。Dimples/韧窝Brittlefracture/脆性断裂：Characteristics：Thereisusuallynoanysignsbeforerupture,andthefractureappearsbrightandflatwithoutvisibleplasticdeformation断裂前没有明显的塑性变形，断口平齐，呈光亮的结晶状。（1）lowstressfailure,commonlymuchlowerthantheyieldingpoint（2）Usuallyhappensatrelativelylowtemperature(20℃)（3）nosign,andoccursrapidly!（4）cracksformanddevelopoutofstressconcentration.Transgranular&intergranularfracture/穿晶与沿晶断裂(裂纹发生途径)穿晶断裂沿晶断裂TransgranularIntergranularCleavage/解理断裂:cleavagefracturehappensalongwithoralongsideofacrystallineplane,calledcleavageplane，forexample(100)planeofBCCmetals。。１.Cleavage(Brittlefracture/总是脆断)Characteristics:streamlinetypefracture,andthelinesjoin/convergetothedirectionofcracksdeveloping!Cleavage/解理断裂FractureToughness/断裂韧性6Fracturetoughnessisanindicationoftheamountofstressrequiredtopropagateapreexistingflaw.Itisaveryimportantmaterialpropertysincetheoccurrenceofflawsisnotcompletelyavoidableintheprocessing,fabrication,orserviceofamaterial/component.Flawsmayappearascracks,voids,metallurgicalinclusions,welddefects,designdiscontinuities,orsomecombinationthereof.Sinceengineerscanneverbetotallysurethatamaterialisflawfree,itiscommonpracticetoassumethataflawofsomechosensizewillbepresentinsomenumberofcomponentsandusethelinearelasticfracturemechanics(LEFM)approachtodesigncriticalcomponents.WhydidTitanicsinkandwhatisthereason?Thisapproachusestheflawsizeandfeatures,componentgeometry,loadingconditionsandthematerialpropertycalledfracturetoughnesstoevaluatetheabilityofacomponentcontainingaflawtoresistfracture.linearelasticfracturemechanics(LEFM)approachAparametercalledthestress-intensityfactor(K)isusedtodeterminethefracturetoughnessofmostmaterials.ARomannumeralsubscriptindicatesthemodeoffractureandthethreemodesoffractureareillustratedintheimagetotheright.ModeIfractureistheconditioninwhichthecrackplaneisnormaltothedirectionoflargesttensileloading.Thisisthemostcommonlyencounteredmodeand,therefore,fortheremainderofthematerialwewillconsiderKIThreetypes张开型(或称拉伸型)裂纹滑开型(或称剪切型)裂纹撕开型裂纹实际工程构件中裂纹形式大多属于I型裂纹，也是最危险的一种裂纹形式，最容易引起低应力脆断。所以我们重点讨论I型裂纹。Thestressintensityfactorisafunctionofloading,cracksize,andstructuralgeometry.Thestressintensityfactormayberepresentedbythefollowingequation:Where:KIisthefracturetoughnessinsistheappliedstressinMPaorpsiaisthecracklengthinmetersorinchesBisacracklengthandcomponentgeometryfactorthatisdifferentforeachspecimenandisdimensionless.SpecimenshavingstandardproportionsbutdifferentabsolutesizeproducedifferentvaluesforKIRoleofMaterialThicknessThestressstatesadjacenttotheflawchangeswiththespecimenthickness(B)untilthethicknessexceedssomecriticaldimension.Oncethethicknessexceedsthecriticaldimension,thevalueofKIbecomesrelativelyconstantandthisvalue,KIC,isatruematerialpropertywhichiscalledtheplane-strainfracturetoughness.Therelationshipbetweenstressintensity,KI,andfracturetoughness,KIC,issimilartotherelationshipbetweenstressandtensilestress.Thestressintensity,KI,representsthelevelof“stress”atthetipofthecrackandthefracturetoughness,KIC,isthehighestvalueofstressintensitythatamaterialunderveryspecific(plane-strain)conditionsthatamaterialcanwithstandwithoutfracture.AsthestressintensityfactorreachestheKICvalue,unstablefractureoccurs.Aswithamaterial’sothermechanicalproperties,KICiscommonlyreportedinreferencebooksandothersources.Plane-StrainandPlane-StressPlaneStrain-aconditionofabodyinwhichthedisplacementsofallpointsinthebodyareparalleltoagivenplane,andthevaluesofthesesdisplacementsdonotdependonthedistanceperpendiculartotheplanePlaneStress–aconditionofabodyinwhichthestateofstressissuchthattwooftheprincipalstressesarealwaysparalleltoagivenplaneandareconstantinthenormaldirection.Whenamaterialwithacrackisloadedintension,thematerialsdevelopplasticstrainsastheyieldstressisexceededintheregionnearthecracktipinitially,butthen:1.Materialwithinthecracktipstressfield,situatedclosetoafreesurface,candeformlaterally(inthez-directionoftheimage)becausetherecanbenostressesnormaltothefreesurface.Thestateofstresstendstobiaxialandthematerialfracturesinacharacteristicductilemanner,witha45oshearlipbeingformedateachfreesurface.Thisconditioniscalled“plane-stressanditoccursinrelativelythinbodieswherethestressthroughthethicknesscannotvaryappreciablyduetothethinsection.2.However,materialawayfromthefreesurfacesofarelativ