机械制造英语LESSION2STRENGTHANDDUCTILITYOFMATERIALS

LESSION2STRENGTHANDDUCTILITYOFMATERIALSCapacityofamachinecomponentisrelatedtothemostsevereserviceconditionitcansustainwithoutachangewhichwillpreventthecomponentfromcontinuingitsintendedfunction.Inmostcases,loadsarethemainmanifestationsofcapacity.Toassesstheload-carryingcapacityofamachinecomponent,themaximumunitload(stress)hastobecomparedwiththeappropriatematerialproperty.Here,weshalldiscussthecapacityofengineeringmaterialstosustainloads.Amostinformativematerialtestisthesimpletensiletest.Aspecimenintheformofacylindricalbar,machinedtoacertainspecification,isslowlyloadedinatensiletestingmachineandloadFanddisplacement(extension)△Larerecorded.Theresultingload-displacementcurveisshowninFig.2-1foratypicalmildsteel.Assumingconstantcross-sectionoftherod,thesamecurverelatesstressandstrain.Thecurveofthisfiguredosenotcorrespondexactlytotherealstress-strainrelationshipbecauseofthewayitismade;thatis,wemeasureforceanddisplacementandinterpretthemasstressandstrainbasedontheinitiallengthandtheinitialcross-sectionofthetestspecimen.Thesepropertiescontinuouslychangeduringtheexperimentthereforetheresultsofsuchatestwillhaveonlyaformalvalue.Formostmaterialsusedinmachinedesignfromthisdiagramweobservealinearstress-strainrelationshipwhichextendsuptosomepoint,andshortlythereafteronecanobserveanincreasingdeformationwithoutaproportionalincreaseintheloadandthestress.Thisroughlycorrespondstothepointthatwehaveasubstantialyieldingofthematerialandwecallthisthe‘yieldpoint’.Thecorrespondingstress(σy)iscalledthe‘yieldstrength’ofthematerial.Wehavebeenintentionallyvagueintheprecisedefinitionoftheyieldpointowingtothefactthatinmostmaterialsitisnotpossibletoindentifyasinglepointwherewehavethetransitionfromelastictoplasticbehavior.Infact,inmostengineeringmaterialsthistransitionisnotabruptanditisamatterofdefinitiontospecifytheyieldpoint.Usuallywedefinetheyieldpointasthepointwhereacertainpercentageofplasticdeformationremainsafterloading.Ultimatetensilestrength(σu)isthemaximumnormalstresswhichcanbeobservedinthestress-straindiagram,whichcorrespondstothemaximumnormalstressthatmaterialcansustain,theratioofmaximumloadtotheoriginalcross-sectionalarea2.Inductilematerial,thereisasubstantialdifferencebetweenyieldandtensilestrength.Inmostmaterials,thestrengthisthesameintensionandincompression.Somematerials,however,haveverydifferentvaluesofstrengthintensionandcompression,suchascastironforexample.Micro-cracksexistinthestructureofthismaterialwhichgiverisetohighstressconcentrationduringtensileloadingwhileincompressiveloading,forgeometricalreasonsthesemicro-cracksareineffective3.Thereforethematerialcansustainmuchhigherloads.Insuchmaterialsthestrengthintensionandincompressionhavetoberecordedindependently.Inmaterialpropertytables,especiallyfordesignpurposes,onewillalsoobservedifferentstrengthsintensionandbending.Similardifferencesmightbeobservedinshearandtorsion.Althoughitappearsthatnomatterwhatloadscausethestresses,thestrengthtoaparticulartypeofstressmustbethesame,andthisisnotalwaysthecase.Takeforexamplethestrengthinpuretensionandinbending.Inbothcasesthedirectionofstressisthesame,namelytensilestress.Inflexure,however,onlytheouterfibresofthematerialhavehighstresswhilethestressesdiminishaswemovetowardstheneutralline.Since,themicro-cracksalreadymentionedareuniformlydistributedinthematerial,theprobabilityofhavingamicro–crackintheareaofhighstressissmallerinflexurethaninpuretension,thereforethestrengthintheflexureis,ingeneral,greaterthanthatinpuretension.Asimilarsituationexistsforstrengthsintorsionandshear.Anotherdeviationofthestress-straincurvefromrealitymustbepointedout:thehorizontalscaleisusuallyarbitrarilynonlinearsinceelasticdeformationatsmallstrainsisverysmallcomparedwithplasticdeformation.Forthisreason,thestrainscaleisenlargedforsmallstrains.Thisyieldpointinductilematerialsisusuallywelldefined.Incaseswherethereisnopronouncedyieldpointinthediagramtheyieldstrengthisdefinedasthestressatwhichthepermanentsetεpt=0.002or0.2%(Fig.2-2).Insomecasestheyieldstrengthisestablishedforεpl=0.5%.Todistinguishbetweentheyieldpointintensionandincompression,anadditionalsubscript‘t’or‘c’isintroducedinthenotationwhenitisnecessaryinsomematerials.ThusweobtainthesymbolsSytandSyefortheyieldpoint.Theyieldpointisoneofthemainmechanicalcharacteristicsofamaterialinmachinedesignwhereyieldingismostlyunwanted.Evenifthestructuralintegrityofthecomponentismaintained,becauseofgeometricand/orotherimplications.Asnotedabove,theratioofthemaximumforcethatthespecimeniscapableofsustainingtoitsoriginalcross-sectionalareaistermedthe‘ultimatetensilestrength’andisdenotedbyσu.Itisimportanttonotethatσu.isnotthestressatwhichthespecimenfractures.Ifthetensileforceisreferredtotheminimumsectionatagivenpointintimeratherthantotheoriginalcross-sectionalareaofthespecimenbeforeruptureisappreciablygreaterthanσu.Thustheultimatetensilestrengthisalsoaconventionalquantity.Owingtotheconvenienceandsimplicityofitsdetermination,theultimatetensilestrengthiswidelyusedindesignpracticeasabasiccomparativecharacteristicofthestrengthpropertiesofmaterials.Anotherimportantcharacteristicofamaterialfordesignisfromtensiontests,thepercentageelongationatrupture,whichistheaveragepermanentdeformationproducedinaspecifiedstandardlengthofthespecimenatthemomentofrupture.Theabilityofamaterialtoacquirelargepermanentdeformationswithoutfractureisknownas‘ductility’.Thepropertyofductilityisofprimeimportanceinsuchmanufacturingprocessasextrusion,drawing,bending,etc.Themeasureofductilityδisthepercentageelongationatrupture.Thegreaterδ,themoreductileisthematerial.Highlyductilematerialsincludeannealedcopper,aluminum,brass,low-carbonsteel,etc.Duraluminandbronzearelessductile.Slightlyductilematerialsincludemanyalloysteels.Apropertyoppositetoductilityisbrittleness.i.e.thetendencyofamaterialtofracturewithoutanyappreciablepermanentdeformation.Materialsprocessingthispropertyarecalled‘brittle’.Forsuchmaterials,theamountofelongationatrupturedoesnotexceed2%to5%,andinsomecasesitisexpressedbyafractionof1%.Brittlematerialsincludecastiron,high-carbontoolsteel,glass,brick,stone,etc.Thetensiontestdiagramforbrittlematerialshasnoyieldpointorstrainhardeningzone.NewWordsmanifestationinformativespecificationmilddisplacementsubstantialyieldingyieldvaguetransitionabruptnominalductileineffectiveflexurediminishneutraldeviationpronouncedsetsubscriptnotationintegrityruptureextrusiondrawingannealedduraluminbronzebrittlenessPhrasesandExpressionsneutrallinepermanentsetyieldstrengthcorrespondto...giveriseto...NotesCapacityofamachinecomponentisrelatedtothemostsevereserviceconditionitcansustainwithoutachangewhichwillpreventthecomponentfromcontinuingitsintendedfunction.(1).itcansustain...intendedfunction作定语从句修饰先行词condition，it前省略了关系代词。在定语从句中，介词短语withoutachange作状语。(2).which引导定语从句修饰先行词change2.Ultimatetensilestrength(σu)isthemaximumnominalstresswhichcanbeobservedinthestress-straindiagram,whichcorrespondstothemaximumnominalstressthatthematerialcansustain,theratioofmaximumloadtotheoriginalcross-sectionalarea.Micro-cracksexistinthestructureofthismaterialwhichgiverisetohighstressconcentrationduringtensileloadingwhileincompressiveloading,forgeometricalreasons,thesemicro-cracksareineffective.ExercisesTranslatethefollowingwordsintoChinese:themainmanifestationsofcapacitythemaximumunitload(stress)stress-straindiagramthesimpletensiletestthepercentageelongationatrupturetheendsoftensilespecimenspermanentdeformationtheresultingload-displacementcurveasubstantialyieldingofthematerialyieldpointthetransitionfromelastictoplasticbehaviormaterialpropertytableplasticdeformationaspecifiedstandardlengthofthespecimenatthemomentofruptureshortcylindricalspecimensductilematerialhighstressconcentrationultimatetensilestrengthstrainhardeningzoneTranslatethefollowingsentencesintoChinese:Atensiletestconsistsofslowlypullingasampleofmaterialwithatensileloaduntilitbreaks.Theendsoftensilespecimensareusuallyenlargedtoprovideextraareaforgrippingandtoavoidhavingthesamplebreakwhereitisbeinggripped.Theusualmannerofconductingthetestistodeformthespecimenataconstantspeed.Forexample,intheuniversaltestingmachines,thementionbetweenthefixedandmovingcrossheadscanbecontrolledataconstantspeed.Theloadthatmustbeappliedtoenforcethisdisplacementratevariesasthetestproceeds.ThisloadFmaybedividedbythecross-sectionalareaAtoobtainthestressinthespecimenatanytimeduringthetest.Howeveractualmeasurementof△Lispreferablewherethisisfeasible.Stressandstrainbasedontheinitial(undeformed)dimensions,AiandLi,arecalledengineeringstressandstrain.Ductileandbrittlematerialsbehavedifferentlyincompressiontestaswill.Thecompressiontestisconductedonshortcylindricalspecimensplacedbetweenparallelplates.Thecompressiontestdiagramforthesematerialsretainsthequalitativefeaturesofthetensiontestdiagram.Theultimatecompressivestrengthofabrittlematerialisdeterminedinsamewayasintension.Answerthefollowingquestions:1)Describetensiletestbriefly.2)Whatisyieldpoint?3)Howtogetthevalueofultimatetensilestrengthforacertainmaterial?4)Whatareofbrittlematerials?5)Whyisthestrengthusuallyhigherinbendingthanthatintensionformostmaterials?6).Foracertainmaterial,howmuchinformationcanbegotfromasimpletensiletest?READINGMATERIALS(A)MECHANICALPROPERTISOFMETALSMechanicalpropertiesarethecharacteristicresponsesofamaterialtoappliedforces.Thesepropertiesfallintofivebroadcategories:strength,hardness,elasticity,ductilityandtoughness.Strengthistheabilityofametaltoresistappliedforces.Strengthpropertiesarecommonlyreferredtoastensilestrength,bendingstrength,compressivestrength,torsionalstrength,shearstrengthandfatiguestrength.Tensilestrengthisthatpropertywhichresiststheforcesactingtopullthemetalapart.Itisoneofthemostimportantfactorsintheevaluationofametal.Compressivestrengthistheabilityofamaterialtoresistbeingcrushed.Compressionistheoppositeoftensionwithrespecttothedirectionoftheappliedload.Mostmetalshavehightensilestrengthandhighcompressivestrength.However,brittlematerialssuchascastironhavehighcompressivestrengthbutonlymoderatetensilestrength.Bendingstrengthisthatqualitywhichresistsforcesfromcausingamembertobendordeflectinthedirectioninwhichtheloadisapplied.Actuallyabendingstressisacombinationoftensileandcompressivestresses.Torsionalstrengthistheabilityofametaltowithstandforcesthatcauseamembertotwist.Shearstrengthreferstohowwellamembercanwithstandtwoequalforcesactinginoppositedirections.Fatiguestrengthisthepropertyofamaterialtoresistvariouskindsofrapidlyalternatingstresses.Forexample,apistonrodoranaxleundergoescompletereversalofstressesfromtensiontocompression.Bendingapieceofwirebackandforthuntilitbreaksisanotherexampleoffatiguestrength.Hardnessisthatpropertyinsteelwhichresistsindentationorpenetration.Hardnessisusuallyexpressedintermsoftheareaofanindentationmadebyaspecialballunderastandardload,orthedepthofaspecialindenterunderaspecificload.Elasticityistheabilitytospringbacktooriginalshape.Autobumpersandallspringsshouldhavethisquality.Ductilityistheabilitytoundergopermanentchangesofshapewithoutrupturing.Modern,deep-formedautobodiesandfenders,andotherstampedandformedproductsmusthavethisproperty.Toughnessistheabilitytoabsorbmechanicallyappliedenergy.Strengthandductilitydetermineamaterial’stoughness.Toughnessisneededinrailroadcars,automobileaxles,hammers,rails,andsimilarproducts.fallintotoughnessshearfatiguecastironmemberpistonaxleindentationindenterpenetrationdeep-formedbumperfenderstamp(B)HARDNESSANDHARDNESSTESTINGSeveraltypesofhardnesstestareavailableforaccuratelymeasuringthehardnessofallshapesofwroughtmaterials.ThesearetheBrinell,RockwellandVickers.TheBrinellTestTheBrinelltest(Fig.2-3)ismainlyusedformaterialswhosethicknessis0.25in.orgreater.Thetestingtechniqueistoimpressa10-mmdiameterhardenedsteelballintothesurfaceofthematerialunder3000Kgloadfor15~60sec.Asmallerloadisusedwhentestingsoftermetalsandalloys.Thediameter,inmillimeters,ofthedentmadebytheballismeasuredbyamicroscope,andtheareaiscalculated.Thebrinellhardnessnumber(HBS)maybecalculatedfromtheformulaHBS=WhereD=balldiameter(mm)d=diameterofcircularindentation(mm)F=load,which,inhardnesstesting,hasunitsofkilogramme-forcewhere1kgf=9.8NHowever,toavoidthistediouscalculation,theBrinellhardnessnumbermaybefoundfromtablesrelatinghardnesstothed