文献翻译影响机械结构设计的材料失效类型.docx
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文献翻译影响机械结构设计的材料失效类型.docx
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文献翻译影响机械结构设计的材料失效类型
原文:
影响机械结构设计的材料失效类型
Chapter1Introduction
1.1INTRODUCTION
1.2TYPESOFMATERIALFAILURE
1.3DESIGNANDMATERIALSSELECTION
1.4TECHNOLOGICALCHALLENGE
1.5ECONOMICIMPORTANCEOFFRACTURE
1.6SUMMARY
OBJECTIVES
Gainanoverviewofthetypesofmaterialfailurethataffectmechanicalandstructuraldesign.
Understandingeneralhowthelimitationsonstrengthsandductilityofmaterialsaredealtwithinengineeringdesign.
Developanappreciationofhowthedevelopmentofnewtechnologyrequiresnewmaterialsandnewmethodsofevaluatingthemechanicalbehaviorofmaterials.
Learnofthesurprisinglylargecostsoffracturetotheeconomy.
1.1INTRODUCTION
Designersofmachines,vehicles,andstructuresmustachieveacceptablelevelsofperformanceandeconomy,whileatthesametimestrivingtoguaranteethattheitemisbothsafeanddurable.Toassureperformance,safety,anddurability,itisnecessarytoavoidexcessdeformation---thatis,bending,twisting,orstretching---ofthecomponents(parts)ofthemachine,vehicle,orstructure.INaddition,crackingincomponentsmustbeavoidedentirely,orstrictlylimited,sothatitdoesnotprogresstothepointofcompletefracture.
Thestudyofdeformationandfractureinmaterialsiscalledmechanicalbehaviorofmaterials.Knowledgeofthisareaprovidesthebasisforavoidingthesetypesoffailureinengineeringapplications.Oneaspectofthesubjectisthephysicaltestingofsamplesofmaterialsbyapplyingforcesanddeformations.Oncethebehaviorofagivenmaterialisquantitativelyknownfromtesting,orfrompublishedtestdata,itschancesofsuccessinaparticularengineeringdesigncanbeevaluated.
Themostbasicconcernindesigntoavoidstructuralfailureisthatthestressinacomponentmustnotexceedthestrengthofthematerial,wherethestrengthissimplythestressthatcausesadeformationorfracturefailure.Additionalcomplexitiesorparticularcausesoffailureoftenrequirefurtheranalysis,suchasthefollowing:
1.Stressesareoftenpresentthatactinmorethanonedirection;thatis,thestateofstressisbiaxialortriaxial.
2.Realcomponentsmaycontainflawsorevencracksthatmustbespecificallyconsidered.
3.Stressesmaybeappliedforlongperiodsoftime.
4.Stressesmayberepeatedlyappliedandremoved,orthedirectionofstressrepeatedlyreversed.
Intheremainderofthisintroductorychapter,wewilldefineandbrieflydiscussvarioustypesofmaterialfailure,andwewillconsidertherelationshipsofmechanicalbehaviorofmaterialstoengineeringdesign,tonewtechnology,andtotheeconomy.
1.2TYPESOFMATERIALFAILURE
Adeformationfailureisachangeinthephysicaldimensionsorshapeofacomponentthatissufficientforitsfunctiontobelostorimpaired.Crackingtotheextentthatacomponentisseparatedintotwoormorepiecesistermedfracture.Corrosionisthelossofmaterialduetochemicalaction,andwearissurfaceremovalduetoabrasionorstickingbetweensolidsurfacesthattouch.IFweariscausedbyafluid(gasorliquid),itiscallederosion,whichisespeciallylikelyifthefluidcontainshardparticles.Althoughcorrosionandwearalsoofgreatimportance,thisbookprimarilyconsidersdeformationandfracture.
ThebasistypesofmaterialfailurethatareclassifiedaseitherdeformationorfractureareindicatedinFig.1.1.Sinceseveraldifferentcausesexist,itisimportanttocorrectlyidentifytheonesthatmayapplytoagivendesign,sothattheappropriateanalysismethodscanbechosentopredictthebehavior.Withsuchaneedforclassificationinmind,thevarioustypesofdeformationandfracturearedefinedandbrieflydescribednext.
Figure1.1Basictypesofdeformationandfracture.
Figure1.2Axialmember(a)subjecttoloadingandunloading,showingelasticdeformation(b)andbothelasticandplasticdeformation(c).
1.2.1ElasticandPlasticDeformation
Deformationsarequantifiedintermsofnormalandshearstraininelementarymechanicsofmaterials.Thecumulativeeffectofthestrainsinacomponentisadeformation,suchasabend,twist,orstretch.Deformationsaresometimesessentialforfunction,asinaspring.Excessivedeformation,especiallyifpermanent,isoftenharmful..
Deformationthatappearsquicklyuponloadingcanbeclassedaseitherelasticdeformationorplasticdeformation,asillustratedinFig.1.2.Elasticdeformationisrecoveredimmediatelyuponunloading.Wherethisistheonlydeformationpresent,stressandstrainareusuallyproportional.Foraxialloading,theconstantofproportionalityisthemodulusofelasticity,E,asdefinedinFig.1.2(b).Anexampleoffailurebyelasticdeformationisatallbuildingthatswaysinthewindandcausesdiscomforttotheoccupants,althoughtheremaybeonlyremotechanceofcollapse.Elasticdeformationsareanalyzedbythemethodsofelasticityandstructuralanalysis.
Plasticdeformationisnotrecovereduponunloadingandisthereforepermanent.ThedifferencebetweenelasticandplasticdeformationisillustratedinFig.1.2(c).Onceplasticdeformationbegins,onlyasmallincreaseinstressusuallycausesarelativelylargeadditionaldeformation.Thisprocessofrelativelyeasyfurtherdeformationiscalledyielding,andthevalueofstresswherethisbehaviorbeginstobeimportantforagivenmaterialiscalledtheyieldstrength,
.
Materialscapableofsustaininglargeamountsofplasticdeformationaresaidtobehaveinaductilemanner,andthosethatfracturewithoutverymuchplasticdeformationbehaveinabrittlemanner.Ductilebehavioroccursformanymetals,suchaslow-strengthsteelscopper,andlead,andforsomeplastic,suchaspolyethylene.Brittlebehavioroccursforglass,stone,acrylicplastic,andsomemetals,suchasthehigh-strengthsteelusedtomakeafile.(Notethatthewordplasticisusedbothasthecommonnameforpolymericmaterialsandinidentifyingplasticdeformation,whichcanoccurinanytypeofmaterial.)
Figure1.3Tensiontestshowingbrittleandductilebehavior.Thereislittleplasticdeformationforbrittlebehavior,butaconsiderableamountforductilebehavior.
Tensiontestsareoftenemployedtoassessthestrengthandductilityofmaterials,asillustratedinFig.1.3.Suchatestisdonebyslowlystretchingabarofthematerialintensionuntilitbreaks(fractures).Theultimatetensilestrength,
whichisthehigheststressreachedbeforefracture,isobtainedalongwiththeyieldstrengthandthestrainatfracture,
.Thelatterisameasureofductilityandisusuallyexpressedasapercentage,thenbeingcalledthepercentelongation.Materialshavinghighvaluesofboth
and
aresaidtobetough,andtoughmaterialsaregenerallydesirableforuseindesign.
Largeplasticdeformationsvirtuallyalwaysconstitutefailure.Forexample,collapseofasteelbridgeorbuildingduringanearthquakecouldoccurduetoplasticdeformation.However,plasticdeformationcanberelativelysmall,butstillcausemalfunctionofacomponent.Forexample,inarotatingshaft,aslightpermanentbendresultsinunbalancedrotation,whichinturnmaycausevibrationandperhapsearlyfailureofthebearingssupportingtheshaft.
Bucklingisdeformationduetocompressivestressthatcauseslargechangesinalignmentofcolumnsorplates,perhapstotheextentoffoldingorcollapse.Eitherelasticorplasticdeformation,oracombinationofboth,candominatethebehavior.Bucklingisgenerallyconsideredinbooksonelementarymechanicsofmaterialsandstructuralanalysis.
1.2.2CreepDeformation
Creepisdeformationthataccumulateswithtime.Dependingonthemagnitudeoftheappliedstressanditsduration,thedeformationmaybecomesolargethatacomponentcannolongerperformitsfunction.Plasticandlow-melting-temperaturemetalsmaycreepatroomtemperature,andvirtuallyanymaterialwillcreepuponapproachingitsmeltingtemperature.Creepisthusoftenanimportantproblemwherehightemperatureisencountered,asingas-turbineaircraftengines.Bucklingcanoccurinatime-dependentmannerduetocreepdeformation.
Figure1.4Atungstenlightbulbfilamentsaggingunderitsownweight.Thedeflectionincreaseswithtimeduetocreepandleadtotouchingofadjacentcoils,whichcausesbulbfailure.
Anexampleofanapplicationinvolvingcreepdeformationisthedesignoftungstenlightbulbfilaments.ThesituationisillustratedinFig.1.4.Saggingofthefilamentcoilbetweenitssupportsincreaseswithtimeduetocreepdeformationcausedbytheweightofthefilamentitself.Iftoomuchdeformationoccurs,theadjacentturnsofthecoiltouchoneanother,causinganelectricalshortandlocaloverheating,whichquicklyleadstofailureofthefilament.Thecoilgeometryandsupportsarethereforedesignedtolimitthestressescausedbytheweightofthefilament,andaspecialtungstenalloythatcreepslessthanpuretungstenisused.
1.2.3FractureunderStaticandImpactLoading
Rapidfracturecanoccurunderloadingthatdoesnotvarywithtimeorthatchangesonlyslowly,calledstaticloading.Ifsuchafractureisaccompaniedbylittleplasticdeformation,itiscalledabrittlefracture.Thisisthenormalmodeoffailureofglassandothermaterialsthatareresistanttoplasticdeformation.Iftheloadingisappliedveryrapidly,calledimpactloading,brittlefractureismorelikelytooccur.
Ifacrackorothersharpflawispresent,brittlefracturecanoccureveninductilesteelsoraluminumalloys,orinothermaterialsthatarenormallycapableofdeformingplasticallybylargeamounts.Suchsituationsareanalyzedbythespecialtechnologycalledfracturemechanics,whichisthestudyof
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