影响耐久性强度的因素 机械设计及制造专业外文翻译中英文对照Word文档格式.docx

影响耐久性强度的因素 机械设计及制造专业外文翻译中英文对照Word文档格式.docx

《影响耐久性强度的因素 机械设计及制造专业外文翻译中英文对照Word文档格式.docx》由会员分享，可在线阅读，更多相关《影响耐久性强度的因素 机械设计及制造专业外文翻译中英文对照Word文档格式.docx（11页珍藏版）》请在冰豆网上搜索。

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<

R<

0

■Repeated,one-directiontensilestress（Figure5-6）;

R=0

■Fluctuatingtensilestress【Figure5-4（a）】;

0<

1.0

■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.Section5-9ofthischapterincludesmethodsofanalysisforloadingcasesinwhichthefluctuatingstressproducesastressratiodifferentfromR=