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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