冲压工艺中几何及内圆角对模具应力产生的影响毕业论文外文翻译Word格式文档下载.docx
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冲压工艺中几何及内圆角对模具应力产生的影响毕业论文外文翻译Word格式文档下载.docx
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Keywords:
Stamping;
Metalforming;
Finiteelementmethod;
Diefailure
1.Introduction
Inmetalformingprocesses,diefailureanalysisisoneofthemostimportantproblems.Beforethebeginningofthisdecade,mostresearchfocusedonthedevelopmentofthe-oreticalandnumericalmethods.Upperboundtechniques[1,2],contact-impactprocedures[3]andthefiniteelementmethod(FEM)[4,5]arethemaintechniquesforanalyzingstampingproblems.Withthedevelopmentofcomputertechnology,theFEMbecomesthedominanttechnique[6-12].
Altanandco-workers[13,14]discussedthecausesoffailureinforgingtoolingandpresentedafatigueanalysisconceptthatcanbeappliedduringprocessandtooldesigntoanalyzethestressesintools.Inthesetwopapers,theyusedthepunchingloadastheboundaryforcetoanalyzethestressstatesthatexistintheinsertsduringtheformingprocessanddeterminedthecausesofthefailures.Basedontheseconcepts,theyalsogavesomesuggestionstoimprovediedesign.
Inthispaper,linearstressanalysisofathree-dimensional(3D)diemodelispresented.Thestresspatternsarethenanalyzedtoexplainthecausesofthecrackinitiation.Somesuggestionsaboutoptimizationofthedietoreducethestressconcentrationarepresented.Inordertooptimizethedesignofthedie,theeffectsofgeometryandfilletradiusarediscussedbasedonasimplifiedaxisymmetricmodel.
2.Problemdefinition
Thisstudyfocusesonthelinearelasticstressanalysisofthedieinatypicalmetalformingsituation(Fig.1).Thedie(Fig.2)withahalf-moonshapedingotonthetopsurfaceispuncheddowntowardstheworkpiecewhichisheldinsidethecollar,andthepatternismadeontotheworkpiece.Crackswerefoundinthedieafterrepeatedoperation:
(i)whenthediepunchedtheworkpiece,thereiscrackinitiationbetweenthetipofthemoonshapedpatternandoneoftheedges(CrackI);
and(ii)afterrepeatedpunching,thereisalsoacrackatthefilletofthedie(CrackII).
Thepresentworkwascarriedoutwiththefollowingobjectives:
(i)toestablishthecausesofthecrackinitiation;
and(ii)tostudytheeffectsofgeometryandfilletradius.
3.Simulationandanalysis
3.1.3Dsimulation
ThesimulationisperformedwiththeFEMcodeAbaqus[15].TwomeshesarecreatedforthedieshowninFig.3aandb.The3DsolidelementsfortheworkpieceareC3D8(8-nodelinearbrick)elements.Thereareabout4000nodesand3343elementsinthecoarsemeshmodel,and7586nodesand6487elementsinthefinemeshmodel.Theboundaryconditioninvolvesfixingthebottomofthedie,i.e.,U2=0forallthenodesonthediebottom.Apressureof200MPaisappliedonthetopsurfaceofthehalf-moonpattern.Young'
smodulusis200GPaandPoisson'
sratiois0.3.
InordertoanalyzetheprincipalstressconcentrationareaintheregionofCrackI,differentcasesarestudied.LetthemodelsshowninFig.3aandbbeCase1.Anew3Dmodel(Case2)isusedasshowninFig.3c.Thedieisseparatedintothreeparts.TheAbaquscommand*CONTACTPAIR,TIEDisusedtotieseparatesurfacestogetherforjoiningdissimilarmeshes.Theadvantageofthismodelisitsconvenienceinchangingthemeshofthehalf-moonpatternanditsposition.First,thehalf-moonpatternismoved6mmtowardsthecenter(Case3)asshowninFig.3d.Second,thefilletradiusofthehalf-moonpatternischangedfrom0to0.5mm(Case4)asshowninFig.3e.
3.2.Resultsanddiscussion
ForthetwomeshesusedinCase1.Themaximumprincipalshearstress(S12)distributionattheregionoffilletareshowninFig.4aandb.Theresultsshowthatthestressdistributionpatternsarethesameforthetwodifferentmeshes,andtherefore,theconvergenceofthesolutionsisestablished.
Altanandco-workers[14]havepresentedthestressanalysisofanaxisymmetricupperdie.Intheirwork,whenthematerialoftheworkpieceflowstofillthevolumebetweenthediesandcollar,thecontactsurfaceofthedieisstretched.Attheareaofthetransitionradius,theprincipalstresseschangedirectionandreachhightensilevalues.
Accordingtotheiranalysis,thefatiguefailureisduetotwofactors:
(i)whenthestressexceedstheyieldstrengthofthediematerial,alocalizedplasticzonegenerallyformsduringthefirstloadcycleandundergoesplasticcyclingduringsubsequentunloadingandreloading,thusmicroscopiccracksinitiate;
and(ii)tensileprincipalstressescausethemicroscopiccrackstogrowandleadtothesubsequentpropagationofthecracks.
TheVonMisesstressdistributionisshowninFig.5a.Veryhighstressoccurinthehalf-moonandfilletregions.Ifthecontactpressurekeepsincreasing,plasticzoneswillformfirstinthesetworegions.
Fig.5bshowsthemaximumprincipalstress(SP3)distributionpattern.InordertoshowtheareaofCrackIinitiation,Fig.5cprovidesazoomedviewofthearea.Itisclearthatatensileprincipalstress(SP3)concentrationof25.5MPaexistsbetweenthehalf-moonpatternandthefreeedgeandisthecauseofcrackinitiation.
SinceCrackIpropagatesnearlynormaltothe1-2plane,thedirectionofthestresseswhichcausethecrackinitiationmustbeparalleltothatplane.Fig.5dshowsthedirectionofthemaximumprincipaltensilestressatnode145andconfirmsCrackIisnormaltothe1-2plane.
Afterrepeatedpunching,CrackIIinitiatesinthefilletregion,andgivesrisetofatiguefailure.ThegeometryinthelocalareaisverysimilartothecasewhichAltanandco-workers[14]haveanalyzed.However,therearenocontactstressesinthatareaforthepresentcase,andFig.5bshowsthatthemaximumprincipalstressesareallcompressiveatthefillet.Fig.5eshowsthatthereishighshearstress(S12)concentrationatthefilletwhichisabout30MPa.Theshearstressesseemtobethestresseswhichleadtotheinitiationandpropagationofcracks.
Theresultsofthefourcases(Cases1-4)forthelargestmaximumprincipalstressesarelistedinTable1.
Whenthenumberofelementsforthehalf-moonpatternisincreasedfrom10to70,thelargestprincipalstressatthepositionofCrackIinitiationisincreasedby(30.5-25.5)/30.5=16%(Case2).Theprincipalstressesareverysensitivetothehalf-moonpattern.
Cases2-4showtheeffectoflocationofthehalf-moonanditsfilletradius.Ifthehalf-moonpatternismoved6mmtowardsthecenter,thelargestprincipalstressatthepositionofCrackIisreducedby(25.3-30.5)/30.5=-17%(Case3).Ifthefilletradiusofthehalf-moonpatternischangedto0.5mm,theprincipalstressisreduced(28.5-30.5)/30.5=-7%(Case4).Thereforeboththesemethodscanreducethestressconcentration,thefirstbeingmoreeffective.
4.Effectsofgeometryandfilletradiusondiestressdistribution
4.1.2Dmodeling
Inordertooptimizethedie,theeffectsofgeometryandfilletradiusondiestressdistributionarediscussedfurther.Anaxisymmetricmodelisused(Fig.6)fortheanalysis.
Initially,theradiusr1oftheinnercylinderissetto10mm,theheighthoftheinnercylinderissetto5mm,andtheheightHoftheoutercylinderissetto25mm.Also,r2istheradiusoftheoutercylinder,andtheratior2/r1ischangedfrom1.2to1.5,2.0,3.0and4.0.TheradiusRofthefilletischangedfrom2.0to0.5mm,andhischangedfrom5to2and0mm.Thepressureisgivenas200MPaatthetopsurface.Thenodesatthebottomedgearefixed,andallothersarefreetotranslate(exceptthoseontheaxisintheradialdirection).
4.2.Resultsanddiscussion
Atotalof30caseswerestudied.Parametersthatarevariedincluder2/r1ratio,h,andfilletradiusR.These30casesareshowninTable2.Forallcases,r1isfixedat10mmandHisfixedat25mm.
4.2.1.Effectofr2/r1
Theeffectofvaryingther2/r1ratioisexaminedforcaseswiththevalueofhfixedat5mm.Fig.7a-cwiththevalueofhfixedat5mmandvaryingratioofr2/r1showsthatthemaximumvalueoftheprincipal
stress(SP3)reduceswithincreasingr2/r1,andchangesinpositionfromapointonthesurfacetobelowthesurface.ThistrendisreflectedinFig.8a.
Ontheotherhand,Fig.8bindicatesthatthemaximumshearstress(S12)becomeslargerwithincreasingratioofr2/r1.Therateofthisincreasedropswithincreasingr2/r1.TheshearstresspatternsforsomecasesareshowninFig.7f-h.
4.2.2.Effectofheighth
Theeffectofheighthoftheinnerportionisexaminedforthreecaseswithh=0,2and5mmwithRfixedat2mm.FromFig.8a,itcanbeseenthatthemaximumprincipalstress(SP3)increasesmarginallywithincreasinghuptor2/r1of2,afterwhichthetrendisreversed.However,forlargeh,theeffectbecomeslessimportant.Ontheotherhand,themaximumshearstressishigherwithincreasinghforthesamer2/r1ratio.StresspatternsareshowninFig.7a,d-f,iandj.
4.2.3.EffectoffilletradiusR
TheeffectoffilletradiusRisexaminedfortwocaseswithR=0.5and2mm.TheresultsareshowninFig.8cand
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