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.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.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.docx(11页珍藏版)》请在冰豆网上搜索。
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
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.Section5-9ofthischapterincludesmethodsofanalysisforloadingcasesinwhichthefluctuatingstressproducesastressratiodifferentfromR=