无机非金属材料工程专业英语 第8章

FundamentalsofMaterialsScienceandEngineeringChapter8FailureWhatistherealnatureforthedifferentappearanceoffracturesection?FundamentalsofMaterialsScienceandEngineeringWhystudyfailure?Airplaneorcarcrash,bridgebreaking,machinebreakingdownareall,tosomesense,relatedtothefailureofengineeringmaterials.Thebestwaytopreventsuchin-servicefailuresistostudythemechanicsofthevariousfailuremodesanddesignthestructuresorcomponentsappropriately.FundamentalsofMaterialsScienceandEngineeringWhatshouldyoubeabletodoafterstudyingthischapter?Havethebasicknowledgeofthemajortypesofengineeringmaterialsfailure.Understandthemechanicsofvariousfracturemodes.FundamentalsofMaterialsScienceandEngineeringImportanttermsandconcepts:Failure失效Fracture断口Brittlefracture脆性断裂Ductilefracture塑性断裂Ductile-to-brittletransition塑脆转变Casehardening表面强化Fatigue疲劳Low-cyclefatigue低周疲劳High-cyclefatigue高周疲劳Corrosionfatigue腐蚀疲劳Origin裂纹源Beachmarks沙滩印Fatiguestriations疲劳辉纹Fibroustexture纤维组织Meanstress平均应力FundamentalsofMaterialsScienceandEngineeringThermalfatigue热疲劳Fatiguecrack疲劳裂纹Fatiguelife疲劳寿命Fatiguelimit疲劳极限Endurancelimit容许极限Fatiguestrength疲劳强度Creep蠕变Neck颈缩cup-and-cone杯突形Charpytest夏氏试验Crackinitiationsite裂纹源Residualcompressivestress残余压缩应力Constantload恒载荷Primarycreep/transientcreep第一级蠕变Secondarycreep/Steady-statecreep第二级蠕变Tertiarycreep第三级蠕变FundamentalsofMaterialsScienceandEngineeringPlanestrain平面应变Planestrainfracturetoughness平面应变断裂韧性Stressintensityfactor应力强度因素Stressraiser应力集中源Fracturemechanics/mode断裂机制/模式Transgranularfracture穿晶断裂Intergranularfracture沿晶断裂Fracturetoughness断裂韧性Impactenergy冲击能Stressratio应力比Stressamplitude应力幅S－NcurveS-N曲线FundamentalsofMaterialsScienceandEngineeringCrackinitiation裂纹萌生Crackpropagation/growth裂纹扩展Coalescencen.合并smallcavities-microvoids微孔jeopardy－perilordangerellipticalcrack椭圆形裂纹Dimples韧窝Chevron人字形Ridgelike山脊状Patterns花样FundamentalsofMaterialsScienceandEngineeringKeyknowledgepoints:INTRODUCTIONThefailureofengineeringmaterialsisalmostalwaysanundesirableeventforseveralreasons;thesehumanlivesthatareputinjeopardy(Riskoflossorinjury;perilordanger),economiclosses,andtheinterferencewiththeavailabilityofproductsandservices.Eventhoughthecausesoffailureandthebehaviorofmaterialsmaybeknown,preventionoffailureisdifficulttoguarantee.FundamentalsofMaterialsScienceandEngineeringTheusualcausesareimpropermaterialsselectionandprocessingandinadequatedesignofthecomponentoritsmisuse.Itistheresponsibilityoftheengineertoanticipateandplanforpossiblefailureand,intheeventthatfailuredoesoccur,toassessitscauseandthentakeappropriatepreventivemeasuresagainstfutureincidents.FundamentalsofMaterialsScienceandEngineering•Fracture-theseparationofbodyintotwoormorepiecesinresponsetoaimposedstressthatisstaticandatlowtemperature.•Therearetwofracturemode:ductileandbrittlemodes,bothofwhichinvolvetheformationandpropagationofcracks.Fatigue-aformoffailurethatoccursinstructuressubjectedtodynamicandfluctuatingstresses,acommontypeofcatastrophicfailure.Creep-deformationcausedbywhichmaterialsareplacedinserviceatelevatedtemperaturesandexposedtostaticmechanicalstresses.FundamentalsofMaterialsScienceandEngineeringDUCTILEFRACTUREDuctilefracturesurfaceswillhavetheirowndistinctivefeaturesonbothmacroscopicandmicroscopiclevels.Figure9.1showsschematicrepresentationsfortwocharacteristicmacroscopicfractureprofiles.TheconfigurationshowninFigure9.1aisfoundforextremelysoftmetals,suchaspuregoldandleadatroomtemperature,andothermetals,polymers,andinorganicglassesatelevatedtemperatures.Thesehighlyductilematerialsneckdowntoapointfracture,showingvirtually100%reductioninarea.ThemostcommontypeoftensilefractureprofileforductilemetalsisthatrepresentedinFigure9.1b,whichfractureisprecededbyonlyamoderateamountofnecking.FundamentalsofMaterialsScienceandEngineeringFundamentalsofMaterialsScienceandEngineeringThefractureprocessnormallyoccursinseveralstages(Figure9.2).First,afterneckingbegins,smallcavities,ormicrovoids,formintheinteriorofthecrosssection,asindicatedinFigure9.2b.Next,asdeformationcontinues,thesemicrovoidsenlarge,cometogether,andcoalescetoformanellipticalcrack,whichhasitslongaxisperpendiculartothestressdirection.Thecrackcontinuestogrowinadirectionparalleltoitsmajoraxisbythismicrovoidcoalescenceprocess(Figure9.2c).FundamentalsofMaterialsScienceandEngineeringFinally,fractureensuesbytherapidpropagationofacrackaroundtheouterperimeteroftheneck(Figure9.2d),bysheardeformationatanangleofabout45withthetensileaxis—thisistheangleatwhichtheshearstressisamaximum.Sometimesafracturehavingthischaracteristicsurfacecontouristermedacupand-conefracturebecauseoneofthematingsurfacesisintheformofacup,theotherlikeacone.Inthistypeoffracturedspecimen(Figure9.3a),thecentralinteriorregionofthesurfacehasanirregularandfibrousappearance,whichisindicativeofplasticdeformation.FundamentalsofMaterialsScienceandEngineeringFig.9.2stagesinthecup-and-conefracture.(a)Initialnecking.(b)Smallcavityformation.(c)Coalescenceofcavitiestoformacrack.(d)Crackpropagation.(e)Finalshearfractureata45°an