外文翻译.docx

外文翻译.docx

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外文翻译

FailureAnalysis，DimensionalDeterminationAndAnalysis，ApplicationsOfCams——故障的分析、尺寸的决定以及凸轮的分析和应用

Itisabsolutelyessentialthatadesignengineerknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesigned．Sometimesafailurecanbeserious，suchaswhenatireblowsoutonanautomobiletravelingathighspeed．Ontheotherhand，afailuremaybenomorethananuisance．Anexampleisthelooseningoftheradiatorhoseinanautomobilecoolingsystem．Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant，aconditionthatisreadilydetectedandcorrected．

Thetypeofloadapartabsorbsisjustassignificantasthemagnitude．Generallyspeaking，dynamicloadswithdirectionreversalscausegreaterdifficultythanstaticloads，andtherefore，fatiguestrengthmustbeconsidered．Anotherconcerniswhetherthematerialisductileorbrittle．Forexample，brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved．

Manypeoplemistakinglyinterpretthewordfailuretomeantheactualbreakageofapart．However，adesignengineermustconsiderabroaderunderstandingofwhatappreciabledeformationoccurs．Aductilematerial，howeverwilldeformalargeamountpriortorupture．Excessivedeformation，withoutfracture，maycauseamachinetofailbecausethedeformedpartinterfereswithamovingsecondpart．Therefore，apartfails（evenifithasnotphysicallybroken）wheneveritnolongerfulfillsitsrequiredfunction．Sometimesfailuremaybeduetoabnormalfrictionorvibrationbetweentwomatingparts．Failurealsomaybeduetoaphenomenoncalledcreep，whichistheplasticflowofamaterialunderloadatelevatedtemperatures．Inaddition，theactualshapeofapartmayberesponsibleforfailure．Forexample，stressconcentrationsduetosuddenchangesincontourmustbetakenintoaccount．Evaluationofstressconsiderationsisespeciallyimportantwhentherearedynamicloadswithdirectionreversalsandthematerialisnotveryductile．

Ingeneral，thedesignengineermustconsiderallpossiblemodesoffailure，whichincludethefollowing．

——Stress

——Deformation

——Wear

——Corrosion

——Vibration

——Environmentaldamage

——Looseningoffasteningdevices

Thepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects，suchasgeometricdiscontinuities，residualstressesduetoformingofdesiredcontours，andtheapplicationofinterferencefitjoints．

Camsareamongthemostversatilemechanismsavailable．Acamisasimpletwo-memberdevice．Theinputmemberisthecamitself，whiletheoutputmemberiscalledthefollower．Throughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired．Someofthecommonapplicationsofcamsare

——Camshaftanddistributorshaftofautomotiveengine

——Productionmachinetools

——Automaticrecordplayers

——Automaticwashingmachines

——Automaticdishwashers

Thecontourofhigh-speedcams（camspeedinexcessof1000rpm）mustbedeterminedmathematically．However，thevastmajorityofcamsoperateatlowspeeds（lessthan500rpm）ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout．Ingeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachined．

DESIGNPROPERTIESOFMATERIALS

Thefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest．

Figure2.7

StaticStrength． Thestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunction．Thusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit，sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerial．

Stiffness． Stiffnessisthedeformation-resistingpropertyofamaterial．Theslopeofthemoduluslineand，hence，themodulusofelasticityaremeasuresofthestiffnessofamaterial．

Resilience． Resilienceisthepropertyofamaterialthatpermitsittoabsorbenergywithoutpermanentdeformation．Theamountofenergyabsorbedisrepresentedbytheareaunderneaththestress-straindiagramwithintheelasticregion．

Toughness． Resilienceandtoughnessaresimilarproperties．However，toughnessistheabilitytoabsorbenergywithoutrupture．Thustoughnessisrepresentedbythetotalareaunderneaththestress-straindiagram，asdepictedinFigure2．8b．Obviously，thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequal．

Brittleness． Abrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplace．Brittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders，holes，notches，orkeyways．

Ductility． Aductilitymaterialexhibitsalargeamountofplasticdeformationpriortorupture．Ductilityismeasuredbythepercentofareaandpercentelongationofapartloadedtorupture．A5%elongationatruptureisconsideredtobethedividinglinebetweenductileandbrittlematerials．

Malleability． Malleabilityisessentiallyameasureofthecompressiveductilityofamaterialand，assuch，isanimportantcharacteristicofmetalsthataretoberolledintosheets．

Figure2.8

Hardness． Thehardnessofamaterialisitsabilitytoresistindentationorscratching．Generallyspeaking，theharderamaterial，themorebrittleitisand，hence，thelessresilient．Also，theultimatestrengthofamaterialisroughlyproportionaltoitshardness．

Machinability． Machinabilityisameasureoftherelativeeasewithwhichamaterialcanbemachined．Ingeneral，theharderthematerial，themoredifficultitistomachine．

COMPRESSIONANDSHEARSTATICSTRENGTH

Inadditiontothetensiletests，thereareothertypesofstaticloadtestingthatprovidevaluableinformation．

CompressionTesting． Mostductilematerialshaveapproximatelythesamepropertiesincompressionasintension．Theultimatestrength，however，cannotbeevaluatedforcompression．Asaductilespecimenflowsplasticallyincompression，thematerialbulgesout，butthereisnophysicalruptureasisthecaseintension．Therefore，aductilematerialfailsincompressionasaresultofdeformation，notstress．

ShearTesting． Shafts，bolts，rivets，andweldsarelocatedinsuchawaythatshearstressesareproduced．Aplotofthetensiletest．Theultimateshearingstrengthisdefinedasthestressatwhichfailureoccurs．Theultimatestrengthinshear，however，doesnotequaltheultimatestrengthintension．Forexample，inthecaseofsteel，theultimateshearstrengthisapproximately75%oftheultimatestrengthintension．Thisdifferencemustbetakenintoaccountwhenshearstressesareencounteredinmachinecomponents．

DYNAMICLOADS

Anappliedforcethatdoesnotvaryinanymanneriscalledastaticorsteadyload．Itisalsocommonpracticetoconsiderappliedforcesthatseldomvarytobestaticloads．Theforcethatisgraduallyappliedduringatensiletestisthereforeastaticload．

Ontheotherhand，forcesthatvaryfrequentlyinmagnitudeanddirectionarecalleddynamicloads．Dynamicloadscanbesubdividedtothefollowingthreecategories．

VaryingLoad． Withvaryingloads，themagnitudechanges，butthedirectiondoesnot．Forexample，theloadmayproducehighandlowtensilestressesbutnocompressivestresses．

ReversingLoad． Inthiscase，boththemagnitudeanddirectionchange．Theseloadreversalsproducealternatelyvaryingtensileandcompressivestressesthatarecommonlyreferredtoasstressreversals．

ShockLoad． Thistypeofloadisduetoimpact．Oneexampleisanelevatordroppingonanestofspringsatthebottomofachute．Theresultingmaximumspringforcecanbemanytimesgreaterthantheweightoftheelevator，Thesametypeofshockloadoccursinautomobilespringswhenatirehitsabumporholeintheroad．

FATIGUEFAILURE-THEENDURANCELIMITDIAGRAM

ThetestspecimeninFigure2.10a．，afteragivennumberofstressreversalswillexperienceacrackattheoutersurfacewherethestressisgreatest．Theinitialcrackstartswherethestressexceedsthestrengthofthegrainonwhichitacts．Thisisusuallywherethereisasmallsurfacedefect，suchasamaterialflaworatinyscratch．Asthenumberofcyclesincreases，theinitialcrackbeginstopropagateintoacontinuousseriesofcracksallaroundtheperipheryoftheshaft．Theconceptionoftheinitialcrackisitselfastressconcentrationthatacceleratesthecrackpropagationphenomenon．Oncetheentireperipherybecomescracked，thecracksstarttomovetowardthecenteroftheshaft．Finally，whentheremainingsolidinnerareabecomessmallenough，thestressexceedstheultimatestrengthandtheshaftsuddenlybreaks．Inspectionofthebreakrevealsaveryinterestingpattern，asshowninFigure2.13．Theouterannularareaisrelativelysmoothbecausematingcrackedsurfaceshadrubbedagainsteachother．However，thecenterportionisrough，indicatingasuddenrupturesimilartothatexperiencedwiththefractureofbrittlematerials．

Thisbringsoutaninterestingfact．Whenactualmachinepartsfailasaresultofstaticloads，theynormallydeformappreciablybecauseoftheductilityofthematerial．

Figure2.13

Thusmanystaticfailurescanbeavoidedbymakingfrequentvisualobservationsandreplacingalldeformedparts．However，fatiguefailuresgivetowarning．Fatiguefailmatedthatover90%ofbrokenautomobilepartshavefailedthroughfatigue．

Thefatiguestrengthofamaterialisitsabilitytoresistthepropagationofcracksunderstressreversals．Endurancelimitisaparameterusedtomeasurethefatiguestrengthofamaterial．Bydefinition，th