外文翻译--残余应力.doc

附录ResidualStressesAresidualstressisonethatexistswithoutexternalloadingorinternaltemperaturedifferencesonastructureormachine.Itisusuallyaresultofmanufacturingorassemblingoperations.Sometimesitiscalledinitialstress,andtheoperations,prestressing.Whenthestructureormachineisputintoservice,theserviceloadssuperimposestresses.Iftheresidualstressesaddtotheservice-loadstresses,theyaredetrimental；iftheysubtractfromtheservice-loadstressestheyarebeneficial.Intheplasticdeformationtheexternalforcedoesthemeritturnsintooutsidetheheatexceptthemajorityofextensions,butalsosomesmallpartbythedistortioncantheformstoresupinthedeformationmaterial.Thispartofenergynamedstorageenergy.Thestoragecantheconcretemanifestationwayis:Macroscopicresidualstress,microscopicresidualstressandlatticedistortion.Accordingtotheresidualstressbalancescopedifference,usuallymaydivideintoitthreekinds:(1)Firstkindofinternalstress,alsocalledthemacroscopicresidualstress,itiscausesbytheworkpiecedifferentpartmacroscopicdistortionnonuniformity,thereforeitsstressbalancescopeincludingentireworkpiece.Forexample,serveswithJinShubangthecurvingload,thenaboveispulledelongates,underreceivesthecompression；Thedistortionsurpasseswhenthelimitofelasticityhashadtheplasticdeformation,afterthentheexternalforceeliminationbyelongatedonesideontheexistencecompressedstress,thelegofrighttriangleisthetensilestress.Thiskindofresidualstresscorrespondsthedistortioncannotbebig,onlyaccountsforalwaysstoresupcanabout0.1%.(2)Secondkindofinternalstress,alsocalledthemicroscopicresidualstress,itisproducesbybetweenthecrystalgrainorthesubgraindistortionnonuniformity.Itssphereofactionandthecrystalgrainsizequite,namelymaintainthebalancebetweenthecrystalgrainorthesubgrain.Sometimesthiskindofinternalstressmayachievetheverygreatvalue,evenpossiblycreatesthemicrocrackandcausestheworkpiecedestruction.(3)Thirdkindofinternalstress,alsocallsthelatticedistortion.Itssphereofactionisseveraldozenstoseveralhundrednanometers,itisbecausetheworkpieceformsintheplasticdeformationthemassivelatticeflaw(forexamplevacancy,interstitialatom,dislocationandsoon)cause.Inthedistortionmetalthestoragecanthemajorpart(80%90%)usesinformingthelatticedistortion.Thispartofenergyenhancedthedistortioncrystalenergy,causesittobeatthethermodynamicsnon-steadystate,thereforeithasonekindtomakethedistortionmetaltorestoretothefreeenthalpyloweststablestructureconditionspontaneoustendency,andcausestheplasticdeformationmetalinheatingtimereplyandtherecrystallizationprocess.Onlyafewexamplesofdetrimentalresidualstresseswillbegivenhere.One,intheassemblyofmachinery,occurswhentwoshaftsarenotinlineorareafewthousandthsofaninchoutofparallel,andtheyareforcedintoconnectionbyrigidcouplings.Theresultingstressesintheshaftsbecomereversingstresseswhentheshaftsarerotated.Thecorrection,whenperfectalignmentcannotbeeconomicallyattained,asisfrequentlythecase,istouseflexiblecouplingsofatypenecessaryforthedegreeofmisalignment.Theprecedingcaseoccurswithelasticstressesonly,andtheresidualstressesaremaintainedbybearingconstraints.Inapplicationswheremechanicalworkcausesplasticyielding.stressesremainwhentheconstraintsareremoved.Forexample,theforgingofshaftsandcrankshaftsandthecoolingafterforgingmayinduceresidualstresses,theequilibriumofwhichidchangedinmachining,causingsomewarpingoftheshafts.Itisthencommonpracticetostraightentheshaftsinapressbeforethefinalmachiningoperation.Straighteningrequiresabendingmomentlargeenoughtocausepermanentsetoryielding.Detrimentalresidualstressescommonlyresultfromdifferentialheatingorcooling.Aweldisacommonexample,Theweldmetalandtheareasimmediatelyadjacentare,aftersolidification,atamuchhighertemperaturethanthemainbodyofmetal.Thenaturalcontractionofthemetalalongthelengthoftheweldispartiallypreventedbythelargeadjacentbodyofcoldmetal.Henceresidualtensilestressesaresetupalongtheweld.Ingeneral,localorshallowheatingwhichwouldexpandtheregionorsurface,ifitwerefree,adistancewellbeyondthatwhichtheadjacentlargervolumewillallowcausesyieldingandupsettingoftheheatedmaterial,Thisreadilyoccursbecauseofthereducedyieldstrengthatelevatedtemperatures.Thesamecoolervolumepreventstheupset,heatedregionfromfullycontractingduringitscooling,andtensilegeneralruleisthatthe“lasttocoolisintension,”althoughthereisanexceptionifcertaintransformationsofmicrostructureoccur.Methodsforminimizingorreversingthesestressesincludeannealingforstressreliefandhammerorshotpeeningoftheweakenedsurface.Annealingrequiresheatingmildsteelto11001200F,followedbyslowcooling,Somepreheatingofthepartstobejoinedmayminimizethetensilestressesinwelds.Athinbuthighlyeffectivesurfacelayerofcompressivestressmaybeinducedbycold-rolling,coining,andpeeningprocesses.Itisseenthattheseprocesseswork-hardenanouterlayer,thuscausingcompressivestressestoremain,togetherwithminortensilestressesinadjacentinteriorlayers.Sincethecompressivelayerisreadilyobtainedallaround,theseprocessesaresuitableforreversingloadsandrotatingcomponentswherethestressvariesbetweentensionandcompression.Theprocessesmustbecarefullycontrolledinrespecttorollerpressuresandfeeds,shotsizeandspeed,etc.,forwhichextensiveinformationisavailableinengineeringbooksandperiodicals.Cold-rollingisappliedprimarilytocylindricalandothershapesthatcanberotated,suchasthreadsandshaftfillets.Theshape,size,andpressureoftherollerandtheyieldstrengthoftheshaftdeterminethedepthofpenetration,whichcanbecalculated.Aspecialfixturemaybeattachedtothecarriagesofalatheandmadetoslowlytraversethedesiredrollingofboltsandscrewshaslongbeenpartofaformingprocessthatnotonlyformsbutstrengthensthethreadsbydeformationandgrainflowaroundtherootsandbyinducingcompressiveresidualstresses.Coiningofholes,alsocalledballdrifting,isamanufacturingprocessofforcingahard,tungstencarbideorAIDI52100steel,slightlyoversizeballthroughaholeinaplate,bushing,ortubingtogivetheholesfinalsizeandafinefinish.Thelengthoftheholemaybefrom1/20to10timesitsdiameter.Themachineisoftensetupforahighproductionofsmallpartswithunskilledlabor.Anincidentalresultisthattheprocessincreaseshardness,hencewearresistance,andinducesaroundtheholeacompressiveresidualstressthatisusuallyadvantageous,asinroller-chainlinks.Thelinksatehighlystressedinpulsatingtensionwithaconcentrationofthestressatandneartheholesurfaces.Withthecompressivestressfromballdrifting,thenettensilestressinserviceisdecreased,andfailureisminimized.Peeningisthemostwidelyusedmethodforprestressingbymechanicallyinducedyielding.Bytheimpactofroundedstrikingobjects,thesurfaceisdeformedinamultitudeofshallowdimples,whichintryingtoexpandputthesurfaceundercompression.Hammerpeening,usuallybyair-driventoolwitharoundedend,isusefulonlimitedareas,suchasaweldinshaftoronareasfoundweakenedbycorrosion,decarburization,orminorfatiguedamage.Withahardsphericalendtothetool,thedepthofthecompressedlayer,whichoccursbelowthesurface,isabouthalfthestrain-hardenedregion.Shotpeeningisdoneonsteelsbythehigh-velocityimpingementofsmall,round,steelorchilledcast-ironshotwithdiametersfrom0.007to