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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
——Printingmachines
——Automaticwashingmachines
——Automaticdishwashers
Thecontourofhigh-speedcams(camspeedinexcessof1000rpm)mustbedeterminedmathematically.However,thevastmajorityofcamsoperateatlowspeeds(lessthan500rpm)ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout.Ingeneral,thegreaterthecamspeedandoutputload,thegreatermustbetheprecisionwithwhichthecamcontourismachined.
DESIGNPROPERTIESOFMATERIALS
Thefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest.
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.
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.
Thusmanystaticfailurescanbeavoidedbymakingfrequentvisualobservationsandreplacingalldeformedparts.However,fatiguefailuresgivetowarning.Fatiguefailmatedthatover90%ofbrokenautomobilepartshavefailedthroughfatigue.
Thefatiguestrengthofamaterialisitsabilitytoresistthepropagationofcracksunderstressreversals.Endurancelimitisaparameterusedtomeasurethefatiguestrengthofamaterial.Bydefinition,theendu