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.docx

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.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

附录A外文文献

Effectsofgeometryandfilletradiusondiestressesinstampingprocesses

Abstract:

Thispaperdescribestheuseofthefiniteelementmethodtoanalyzethefailureofdiesinstampingprocesses.Forthedieanalyzedinthepresentproblem,thecracksatdifferentlocationscanbeattributedtoacoupleofmechanisms.Oneofthemisduetolargeprincipalstressesandtheotheroneisduetolargeshearstresses.Athree-dimensionalmodelisusedtosimulatetheseproblemsfirst.Themodelisthensimplifiedtoanaxisymmetricproblemforanalyzingtheeffectsofgeometryandfilletradiusondiestresses.2000ElsevierScienceS.A.Allrightsreserved.

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