影响耐久性强度的因素 机械设计及制造专业大学外文翻译中英文对照学位论文.docx

影响耐久性强度的因素 机械设计及制造专业大学外文翻译中英文对照学位论文.docx

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影响耐久性强度的因素机械设计及制造专业大学外文翻译中英文对照学位论文

外文翻译

FACTORSAFFECTINGTHEENDURANCESTRENGTH

Publisheddataforendurancestrengtharedeterminedbyspecialfatiguetestingdevices,whichtypicallyuseapolishedspecimensubjectedtoareversedbendingload,similarthatsketchedinfigure.Iftheactualoperatingconditionsofapartinamachinearedifferent.andtheyusuallyare,thefaiguestrengthmustbereducedfromthereportedvalue.Someofthefactorsthatdecreasetheendurancestrengtharediscussednext.

Thisdiscussionrelatestotheendurancestrengthformaterialssubjectedonlytotensilenormalstresses,thatis,tensilestressesresultingfrombendingactionsoraxialtension.Casesinvolvingfluctuatingtorsionalshearstressesarediscussedseparately.Fatiguefailuresaremostlikelytooccurinregionsofhightensilestressratherthancompressivestress.

Sizeofthesection

Thetestspecimenisusually0.30inch（7.6mm）indiameter.Largersectionsizesexhibitlowerstrengths,havealessfavorablestressdistribution,andhavelessuniformityofproperties,particularlywithheat-treatedparts.Reference1includesasuggestedmethodofdetermingthesizefactorforrotatingshaftsupto10.0inches（250mm）indiameter.Wewillusethatmethodherealsobecausewearebasingouranalysisonthereversedbendingphenomenon.Tableshowsthesuggestedrelationshipsfordetermingasizefactor,tobeappliedtotheendurancestrengthtoaccountforthesizeofthecrosssection.FigureshowsplotsoftheequationsfromTable,withsomeblendingofthecurves,Readingfromthecurvesshouldprovideacceptableaccuracy.

Whenthecomponentbeingdesignedisnotcircularlikeashaft,judgmentisrequiredtodeterminethecharacteristicdimensiontouseintheformulas.Forflat,rolledshapes,thethicknessshouldbeused.Notedthattheuseoftheseequationsisapproximate.

SurfaceFinish

Anydeviationfromapolishedsurfacereducesendurancestrength.Figureshowsroughestimatesfortheendurancestrengths,comparedwiththeultimatetensilestrengthofsteelsforseveralpracticalsurfaceconditions,Itiscriticalthatpartssubjectedtofatigueloadingbeprotectedfromnicks,scratches,andcorrosionbecausetheydrasticallyreducefatiguestrength.

StressConcentrations

Suddenchangesingeometry,especiallysharpgroovesandnotcheswherehighstressconcentrationsoccur,arelikelyplacesforfatiguefailurestooccur.Careshouldbetakeninthedesignandmanufactureofcyclicallyloadedpartstokeepstressconcentrationfactorstoalowvalue.Wewillapplythestressconcentrationfactors,asfoundfromthemethodsofSection,tothecomputedstresses,ratherthantotheallowablestrengths.

Flaws

Internalflawsofthematerial,especiallylikelyincastparts,areplacesinwhichfatiguecracksinitiate.Criticalpartscanbeinspectedbyx-raytechniquesforinternalflaws,Iftheyarenotinspected,ahigher-than-averagedesignfactorshouldbespecifiedforcastparts,andalowerendurancestrengthshouldbeused.

Temperature

Mostmaterialshavealowerendurancesstrengthathightemperatures.Thereportedvaluesareforroomtemperatures.Operationabove160°F（72°C）willreducetheendurancestrengthofmostductilematerials.

NonuniformMaterialProperties

Manymaterialshavedifferentmaterialpropertiesindifferentdirectionsbecauseofthemannerinwhichthematerialwasprocessed.Rolledsheetorbarproductsaretypicallystrongerinthedirectionofrollingthantheyareinthetransversedirection.

Fatiguetestsarelikelyhavebeenrunontestbarsorientedinthestrongerdirection.Stressingofsuchmaterialinthetransversedirectionmayresultinlowerendurancestrength.

Nonuniformpropertiesarealsolikelytoexistinthevicinityofweldsbecauseofincompleteweldpenetration,slaginclusions,andvariationsinthegeometryofthepartattheweld.

Also.weldingofheat-treatedmaterialsmayalterthestrengthofthematerialbecauseoflocalannealingneartheweld.

Someweldingprocessesmayresultinthproductionofresidualtensilestressesthatdecreasetheeffectiveendurancestrengthofthematerial.

Annealingornormalizingafterweldingisoftenusedtorelievethesestresses,buttheeffectofsuchtreatmentsonthestrengthofthebasematerialmustbeconsidered.

ResidualStresses

Fatiguefailurestypicallyinitiateatlocationsofrelativelyhightensilestress.Grindingandmachining,especiallywithhighmaterialremovalrates,alsocauseundesirableresidualtensilestress.Weldinghasalreadybeenmentionedasaprocessthatmayproduceresidualtensilestress.

Anymanufacturingprocessthattendstoproduceresidualstresswilldecreasetheendurancestrengthofthecomponent.Criticalareasofcyclicallyloadedcomponentsshouldbemachinedorgroundinagentlefashion.Processesthatproduceresidualcompressivestressescanprovetobebenefical.Shotblastingandpeeningaretwosuchmethods.Shotblastingisperformedbydirectingahighvelocitystreamofhardenedballsorpelletsatthesurfacetobetreated.Peeningusesaseriesofhammerblowsonthesurface.Crankshafts,springs,andothercyclicallyloadedmachinepartscanbenefitfromthesemethods.

CorrosionandEnvironmentalFactors

Endurancestrengthdataaretypicallymeasuredwiththespecimeninair.Operatingconditionsthatexposeacomponenttowater,saltsolutions,orothercorrosiveenvironmentscansignificantlyreducetheeffectiveendurancestrength.Corrosionmaycauseharmfullocalsurfaceroughnessandmayalsoaltertheinternalgrainstructureandchemistryofehematerial.Steelsexposedtohydrogenareespeciallyaffectedadversely.

Nitriding

Nitridingisasurface-hardeningprocessforalloysteelsinwhichthematerialisheatedto950°F（514°C）inanitrogenatmosphere,typicallyammoniagas,followedbyslowcooling.Impvovementofendurancestrengthof50%ormorecanbeachievedwithnitriding.

WroughtversusCastMaterials

Metalalloyshavingsimilarchemicalcompositionscanbeeitherwroughtorcasttoproducethefinalform.Wroughtmaterialsareusuallyrolledordrawn.Wroughtmaterialsusuallyhaveahigherendurancestrengththancastmaterialsofsimilarcompositioninregionswherenosignificantstressconcentrationexits.However,inthevicinityofnotchesandotherdiscontinuities,theendurancestrengthofwroughtandcastmaterialsismorenearlyequal.Onepossibleexplanationofthisphenomenonisthatthecastmaterialislikelytohavemoreisotropicmaterialpropertiesthanthewroughtmaterialandislessaffectedbythepresenceofthestressconcentration.

Tousethemoreconservativeapproach,itisrecommendedthatafactorof0.8beappliedtothebasicendurancestrengthifacaststeelisused.Forcastiron,afactorof0.70isrecommended.

TypeofStress

Endurancestrengthdataareobtainedfromtherotatingbeamtestthatproducescompletelyreversedandrepeatednormalstresses.Themaximumstressisproductedatthesurfaceofthespecimen,andthestressvaneslinearlytozeroatthecenterofthecircularcrosssection.Iftheactualloadingisdifferentfrombending,afactorforthetypeofloadingshouldbeappliedtotheendurancestrength.

AxialTension

Underpuretension,allofthematerial--notjustthesurface—issubjectedtothemaximumstress.Afactorof0.80issuggestedtobethebendingendurancestrengthtoreflectthisdifferentbehavior.

EffectofStressRatioonEnduranceStrength

Figure5-10showsthegeneralvariationofendurance-strengthdataforagivenmaterialwhenthestressratioRvariesfrom-1.0to+1.0,coveringtherangeofcasesincludingthefollowing:

■Repeated,reversedstress（Figure5-3）;R=-1.0

■Partiallyreversedfluctuatingstresswithatensilemeanstress【Figure5-4（b）】;-1.0

■Repeated,one-directiontensilestress（Figure5-6）;R=0

■Fluctuatingtensilestress【Figure5-4（a）】;0

■Staticstress（Figure5-1）;R=1

NotethatFigure5-10isonlyanexample,anditshouldnotbeusedtodetermineactualdatapoints.Ifsuchdataaredesiredforaparticularmaterial,specificdataforthatmaterialmustbefoundeitherexperimentallyorpublishedliterature.

Themostdamagingkindofstressamongthoselistedistherepeated,reversedstresswithR=-1.（SeeReference2.page27.）RecallthattherotatingshaftinbendingasshowninFigure5-2isanexampleofaload-carryingmembersubjectedtoastressratioR=-1.

FluctuatingstresseswithacompressivemeanstressasshowninParts（c）and（d）ofFigure5-4donotsignificantlyaffecttheendurancestrengthofthematerialbecausefatiguefailurestendtooriginateinregionsoftensilestress.

NotethatthecurvesofFigure5-10showestimateoftheendurancestrength,Sn,asafunctionoftheultimatetensilestrengthforsteel.Thesedataapplytoidealpolishedspecimensanddonotincludeanyofetheotherfactorsdiscussedinthissection.Forexample,thecurveforR=-1.0（reversedbending）showsthattheendurancestrengthforsteelisapproximately0.5timestheultimatestrength（0.50×Sn）forlargenumbersofcyclesofloading（approximately10orhigher）.Thisisagoodgeneralestimateforsteels.ThechartalsoshowsthattypesofloadproducingRgreaterthan-1.0butlessthan1.0havelessofaneffectontheendurancestrength.Thisillustratesthatusingdatafromthereversedbendingtestisthemostconservative.

WewillnotuseFigure5-10directlyforprobleminthisbookbecauseourprocedureforestimatingtheactualendurancestrengthstartswiththeuseofFigure5-9whichpresentsdatafromreversedbendingtests.Therefore,theeffectofstressratioisalreadyincluded.Section5-9ofthischapterincludesmethodsofanalysisforloadingcasesinwhichthefluctuatingstressproducesastressratiodifferentf