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International Journal of Mechanical Sciences
journalhomepage:www.elsevier.com/locate/ijmecsci
Calibration of the modified Mohr-Coulomb fracture model by use of localization analyses for three tempers of an AA6016 aluminium alloy
Henrik Granum
a,∗, David Morin
a,b, Tore Børvik
a,b, Odd Sture Hopperstad
a,baStructural Impact Laboratory (SIMLab), Department of Structural Engineering, NTNU – Norwegian University of Science and Technology, Trondheim, Norway
bCentre for Advanced Structural Analysis (CASA), NTNU, Trondheim, Norway
a r t i c le i n f o
Keywords:
Ductile fracture AA6016 Uncoupled damage Crack propagation Numerical simulations
a b s t r a ct
ThispaperpresentsanovelcalibrationprocedureofthemodifiedMohr-Coulomb(MMC)fracturemodelbyuse oflocalizationanalysesandappliesitforthreetempersofanAA6016aluminiumalloy.Thelocalizationanalyses employtheimperfectionbandapproach,wheremetalplasticityisassignedoutsidethebandandporousplasticity isassignedinsidetheband.Ductilefailureisthusassumedtooccurwhenthedeformationlocalizesintoanarrow band.Themetalplasticitymodeliscalibratedfromnotchtensiontestsusinginversefiniteelementmodelling.The porousplasticitymodeliscalibratedbyuseoflocalizationanalyseswherethedeformationhistoriesfromfinite elementsimulationsofnotchandplane-straintensiontestsareprescribedasboundaryconditions.Subsequently, localizationanalysesareusedtoestablishthefailurelocusinstressspaceforproportionalloadingconditionsand thustodeterminetheparametersoftheMMCfracturemodel.Finiteelementsimulationsofnotchtensionand in-planesimplesheartestsaswellastwoloadcasesofthemodifiedArcantestareusedtovalidatethecalibrated fracturemodel.Thepredictionsbythesimulationsareingoodagreementwiththeexperiments,eventhough somedeviationsareseenforeachtemper.Theresultsdemonstratethatlocalizationanalysesareacost-effective andreliabletoolforpredictingductilefailure,reducingthenumberofmechanicaltestsrequiredtocalibratethe MMCfracturemodelcomparedtothehybridexperimental-numericalapproachusuallyapplied.
1. Introduction
Modellingandsimulationofductilefractureinmetallicmaterialsis anactiveresearchfieldwheresignificantprogresshasbeenmadeover thelastdecades.Thisresearchisimportantsinceindustriesliketheau- tomotiveindustrywanttoutilizethematerialstothebrinkoffailure.
Thus,thedemandforaccuratepredictionsoffracturebynumericalsim- ulationsisincreasing.Reliabledesignofstructuralcomponentsagainst ductilefracturerequiresarobustnumericalframeworkabletoaccu- ratelydescribethedamageandfracturepropertiesofthematerial.In manylightweightmetals,whichhavereceivedspecialattentionbythe automotiveindustryinrecentyears,strengthandductilityareinversely proportionalproperties.Asstrengthisoftenfavouredinthiscase,the ductilityimposesagreatchallengein designofsafetycomponentsof suchmaterials.
Nucleation,growthandcoalescenceofmicroscopicvoidsatvarious lengthscalesisknowntobethephysicalmechanismgoverningductile failure.Studieshaveagreedthatthestressstateaffectstheductilityofa metallicmaterial[1–3].Theinfluenceofthehydrostaticstressstatewas discoveredearlyandhassincebeenincludedinseveralfracturemodels.
∗Correspondingauthor.URL:https://www.ntnu.edu/kt/fractal. E-mailaddress:henrik.granum@ntnu.no(H.Granum).
URL:http://www.ntnu.edu/kt/fractal(H.Granum)
Morerecently,theinfluenceofthedeviatoricstressstateonductilityhas beenproventhroughexperiments,see[4],forexample.Thisledtopro- posalsofbothnewandmodifiedversionsofexistingfracturemodelsto incorporatethisdependence.Avarietyofapproachestomodelductile fracturearecurrentlyavailable.Notablementionsareporousplasticity, continuumdamagemodels,forminglimitcurvesanduncoupleddam- agemodels.Thelatterapproachispopularduetoitssimplicity,where thedamageevolutionis uncoupledfromtheconstitutiveequationin contrasttoporousplasticityandcontinuumdamagemodels.Material degradation isthusnotaccountedforandthedamageismerelyrep- resentedbyascalarvariable.Thiscomeswiththeadvantagethatthe fracturemodelmaybecalibratedindependentlyoftheplasticitymodel, simplifyingtheidentificationofmodelparameterssignificantly.Theun- coupledfracturemodelsareusuallypresentedonlocusformwherethe failurestrainisdefinedbythestressstate.Bythisapproach,thevalid- ityofthefailurestrainisconfinedtoproportionalloadingpaths.Dam- ageisoftenaccumulatedbyanintegral-basedapproachwheredamage evolveswithincrementsoftheequivalentplasticstrainovertheplastic strainpath.Byemployingsuchadamageaccumulationapproach,the modelisjustifiedintheliteraturetobevalidinsimulationsinvolving
https://doi.org/10.1016/j.ijmecsci.2020.106122
Received29June2020;Receivedinrevisedform15September2020;Accepted27September2020 Availableonline2October2020
0020-7403/© 2020TheAuthor(s).PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)
curvesfromthesimulationstothoseoftheexperiments.Theaccuracy ofthisapproachreliesonexperimentsthatcoverarangeofstressstates andexhibitclosetoproportionalloadingpathsallthewaytofracture.
Thelatterrequirementisdifficulttofulfilformoststressstates.Asanal- ternativetothisapproach,localizationanalysesmaybeusedtopredict ductilefailure.Anunderlyingassumption hereisthatstrainlocaliza- tionisaprecursortofailure,andthusmayberegardedastheonsetof fracture.Mechanicaltestsarethenonlyneededtocalibratetheconstitu- tiveequationsusedinthelocalizationanalysesandthereisnorequire- mentforproportionalloadingpaths.Morinetal.[7]combinedunitcell simulationsandlocalizationanalysestopredictfailureofasteelunder non-proportionalloading.Thenumericalresultswerevalidatedagainst experimentalresultsreportedbyBasuandBenzerga[8]andfoundtobe ingoodagreement.Theversatilityandeffectivenessofthelocalization analysesweredemonstratedbyMorinetal.[9]wherefailureloci of metalsweregeneratedfromlocalizationanalysesandappliedtoanad- vancedhigh-strengthsteelsubjectedtoproportionalloadingpaths.The resultswereevaluatedagainst3DunitcellanalysesbyDunandandMohr [10]andproventogivecomparableresultsinafractionofthecompu- tationaltime.Grubenetal.[11]appliedanexperimental-numericalap- proachtodeterminethestrainlocalizationandductilefractureoftwo dual-phasesteels.Fourteststhatcoveredstressstatesfromsimpleshear toequi-biaxial tensionwereconducted.Numericalsimulationsofthe testswereperformed,andthefailurestrainswereestimatedbycom- parisontotheexperimentaldata.Localization analysesbyuse ofthe imperfectionbandapproachwereconductedtopredicttheonsetoflo- calization.Theresultsindicatedthatthelocalizationanalysesprovided conservativevalues ofthefailurestrainsandthatthestressstatein- sidethebandtendstomovetowardsageneralizedshearstatepriorto localization.Bergoetal.[12]usedunitcellsimulationsandlocaliza- tionanalysestocalibratefailurelociforthreedifferentsteels.Thestudy wasconfinedtogeneralizedtensionstressstatesandthusonlythede- pendenceonstresstriaxialitywasincluded.Theuncoupledplasticity andfracturemodelswerecalibratedbasedonasingleuniaxialtension testandmicromechanicalsimulationsusingunitcells,metalandporous plasticityandlocalizationtheory.Thepredictedductilitywassomewhat conservativeforWeldox460Eandnon-conservativeforWeldox900E, butaccurateforWeldox700E.Itwasemphasizedthattheaccuracyof thelocalizationanalysesreliesheavilyonanaccuratecalibrationofthe porousplasticitymodel.However,itwasnotedthatthisapproachis wellsuitedtoreducetheexperimentalprogrammerequiredtocalibrate fracturemodels.
Wierzbickietal.[13],Lietal.[14],Grubenetal.[15]andBaietal.
[16]haveallpresentedstudiescomparingthepredictivecapabilitiesof differentuncoupledfracturecriteriaforvarioussteelsandaluminium alloys.Severalofthecriteriaevaluatedareheuristicextensionsofwell- knowncriterialike theMohr-Coulomb(MC),Cockcroft-Latham (CL), Rice-Tracey(RT)andWilkinscriteriatonameafew.AsWierzbickietal.
[13]pointedout,thequalityofafracturecriterionintendedforindus- trialapplicationmayberoughlymeasuredbytheperformanceandthe cost.Here,performanceisdefinedbytheaccuracyofsimulationscom- paredtoexperimentaltests,whilethecostisrelatedtothenumberof mechanicaltestsneededtocalibratethemodelandthecomplexityre- latedtothis.Amongthecriteriawithonlyonemechanicaltestrequired forcalibrationistheCLcriterion.Ithasbeenusedwithsuccessinmany
T351aluminiumalloyandaTRIP690steel,andvalidateditagainstvar- iousmechanicaltests.Accuratepredictionsoffractureinitiationwere obtained,butitwasnotedthatthepredictionswerelessaccurateingen- eralizedtension.DunandandMohr[20]investigatedthepredictiveca- pabilityoftheMMCfracturemodelbycomparingpredictionstofracture experimentsonTRIP780steel.Ninedifferentexperimentswereusedin thecomparisoncoveringwiderangesofstresstriaxialityandLodepa- rameter.Fractureinitiationwascorrectlypredictedinallsimulationsof thetests.Itwassuggestedthattheunderlyingphysicsofthefracture modelisoflessimportancethanitsmathematicalflexibility,implying thateventhoughphenomenologicalfracturemodelsaremotivatedby micromechanicalobservations,theirabilitytofitexperimentaldatais asuperiorcharacteristic.However,afracturemodelwithhighflexibil- ityallowserroneouscalibrationandrequiresdetailedknowledgebythe user.
A modificationof theMMC fracturemodeldenotedthe Hosford- Coulomb (HC) fracturemodelwas proposed byMohr andMarcadet [5], wherethevonMisesequivalentstresswas replacedbytheHos- fordequivalentstressincombinationwiththenormalstressactingon theplaneofmaximumshear.TheHCfracturemodelisbasedonthe extensivestudyon3DunitcellsbyDunandandMohr[10],thusithas amicromechanicalfoundationincontrasttotheMMCfracturemodel.
TheHCfracturemodelwaspresentedonlocusform,anddamageac- cumulationwastakencareofbyanintegral-basedapproach.Fracture experimentsonthreedifferentsteelswereconductedandthreeexper- iments wereused tocalibratethefracturecriterion.Whencompared againsttheexperiments,thesimulationsshowedgood agreementfor thethreematerialsandfracturewasaccuratelypredictedinallcases.
GorjiandMohr[21]andZhangetal.[22]investigatedductilefracture inthealuminiumalloyAA6016.InGorjiandMohr[21],theHCfrac- turemodelwasemployedincombinationwithananisotropicplasticity modeltopredictshearfractureindeepdrawingtests.Eightcupdraw- ingexperimentswereusedinthecalibrationprocesstoincreasethero- bustnessofthefracturemodel.Theresultsshowthattheplasticityand fracturemodelscanpredictthelocationandtheonsetoffracturewith goodaccuracy.InZhangetal.[22],ananisotropicDruckeryieldfunc- tionandafracturecriterionproposedbyLouetal.[23]wereemployed, whereasimplesheartestandtwonotchtensiontestswithdifferentradii wereusedinthecalibrationofthefracturecriterion.Bycomparingthe experimentstothesimulationsitwasfoundthattheonsetoffracture wasaccuratelypredictedintestsrangingfromsimplesheartouniaxial tension.
Inthepresentstudy,weapplyanisotropicplasticityandfracture modeltopredictductilefractureinvariousexperiments,wherespeci- mensaretakenfromAA6016aluminiumalloysheetsinthreedifferent tempers.TheMMCfracturemodelwasselectedandcalibratedbyuseof localizationanalysesbasedontwomechanicaltests.Thestudyisanat- uralextensiontotheworkbyBergoetal.[12]whereastresstriaxiality dependantfracturemodelwasinvestigatedtogetherwithanisotropic plasticitymodel.Theaimofthestudyistoassesstheaccuracyofthe calibratedMMCfracturemodelbycomparisonagainstmechanicaltests, wherethemodel’sabilitytopredictfractureinitiationandcrackpropa- gationisevaluatedforarangeofstressstates.Theresultsdemonstrate thattheuseoflocalizationanalysestocalibrateafracturemodelhas
Table1
ThechemicalcompositionofAA6016inwt%.
Si Mg Fe Cu Mn Cr Zn Ti Al
1.3160 0.3490 0.1617 0.0081 0.0702 0.0025 0.0084 0.0175 Balance
thepotentialtobeacost-effectiveandaccuratewayofpredictingduc- tilefractureandcrackpropagation.
2. Materialsandmechanicaltests 2.1. Materials
Experimentswereconductedonthreedifferenttempersofthealu- miniumalloyAA6016.Thematerialsweredeliveredas1.5mmthick plateswith in-plane dimensions625mm ×625 mm in tempers T4, T6andT7byHydroAluminiumRolledProductsinBonn.Thisalloy ismainlyusedintheautomotiveindustryasouterbodypanelsdueto itsexcellentsurfacequality,goodformability,andhighstrength.Toob- tainthevarioustempers,allplateswerefirstsolutionheat-treatedat 530°CbeforebeingairquenchedtoreachtemperT4.TempersT6and T7werethenobtainedforsomeoftheplatesbyartificialageingfor5h at185°Candfor24hat205°C,respectively.Thechemicalcomposi- tionofthealloyisgiveninTable1.Theyieldstrengthofthetempers rangesfrom about135MPaforT4to245MPafor T6,andtheulti- matetensilestrengthrangesfromroughly200MPaforT7tojustbelow 300MPaforT6.Allmechanicaltestswerecarriedoutwiththelongi- tudinalaxisalongtherollingdirection,unlessspecifiedotherwise.The initialthicknessofallspecimenswasmeasuredandfoundtobesimilar tothenominalplatethicknessof1.5mm.AnInstron5985seriesuniver- saltestingmachinewasusedinalltests,wheretheforcewasmeasured bya30kNloadcellattachedtotheactuator.AProsilicaGC2450camera orientatedperpendiculartothein-planeaxesofthespecimencaptured imagesfromalltests.Allspecimenswerespray-paintedwithaspeckle patterntoenable2Ddigitalimagecorrelation(2D-DIC)byuseofthe in-housesoftwareeCorr[24].
2.2. Uniaxialtensiontests
InGranumetal.[25],uniaxialtensiontestsinthreedifferentdirec- tionswithrespecttotherollingdirection(0°,45° and90°)oftheplates wereconducted.Additionaluniaxialtensiontestsintherollingdirec- tionoftheplatewereconductedonlyfortemperT4inconjunctionwith thematerialtestprogrammepresentedinthisstudy.Thiswasdoneto monitorthenaturalageingthatoccursintemperT4underprolonged roomtemperaturestorage,resultinginsoluteclustering.Thiseffectis knowntoslightlystrengthenthealloy.Thespecimenhadagaugelength of70mmandawidthof12.5mm,andisdepictedinFig.1a).Thetests wereconductedwithacrossheadvelocityof2.1mm/min,resultingin aninitialstrainrateof5 ×10−4s−1inthegaugeregion.Avirtualex- tensometerwithaninitiallengthof60mmwasusedtoextractdisplace- mentsbyuseof2D-DIC.
2.3. Notchtensiontests
Notchtensiontestsintherollingdirectionwithtwodifferentnotch radiiwereconducted,withgeometryinspiredbythespecimensusedin Ericeetal.[26].ThetwospecimensaredenotedNT10andNT3,and thegeometriesaredepictedinFig.1b)andFig.1c),respectively.The NT10specimenhadanotchradiusof10mm,whiletheNT3specimen hadanotchradiusof3.35mm.Theminimumwidthofthenotchwas 5mmforbothgeometries.Thespecimensweretestedwithacrosshead velocityof0.6mm/min.Forcemeasurementfromtheloadcellandim- agesfromthecamera wererecordedat2Hz.Twosetsofvirtualex- tensometersavailableineCorrwereusedinthepost-processingofthe
experiments,oneglobalandonelocal.Theinitiallengthoftheglobal andlocalvirtualextensometerswas15mmand2mm,respectively,for bothtestgeometriesandthevirtualextensometerswereplacedcentric tothenotchradius.
2.4. Plane-straintensiontests
Thegeometryoftheplane-straintension(PST)specimenisdepicted inFig.1d).Ithadalengthof100mmandawidthof40mm.Theopening ofthenotchwas10mmandthewidthatthenarrowestpointinsidethe notchwasmeasuredto17.33mm.Theforcewasmeasuredbytheload cellandimagesweretakenbythecameraat2Hz.Thetestswerecon- ductedwithmechanicalclamps,wheretheclampedregiononeachside ofthegaugewasapproximately40mm×35mm.Priortotesting,the clampedregionsofthespecimenweresandeddowntoensuregoodgrip betweentheclampsandthespecimen.Thetestswereconductedwith acrossheadvelocityof0.4mm/min.Twoglobalvirtualextensometers withaninitiallengthof10mmpositionedapproximately16mmfrom thecentreofthenotchwereusedtoextractthedisplacementsintheDIC calculations.ThedisplacementsfromtwovirtualextensometersineCorr wereusedtoensurethatnorotationswereenforcedduringloading.A localvirtualextensometerwithagaugelengthof2mmplacedcentric acrossthenotchwasusedtoobtainlocalmeasurementsfromthetests.
2.5. In-planesimplesheartests
Thein-planesimpleshear(ISS)specimenhadagaugelengthof5mm andthegeometryisdepictedinFig.1e).Thetestswereconductedwith acrossheadvelocityof0.15mm/mininanattempttoobtainaninitial strainrateinthegaugeregionof 5 ×10−4s-1.Avirtualextensome- terineCorrspanningacrossthegaugeregionwithaninitiallengthof 10.05mmwasusedtoextractdisplacements.Acameraaimedperpen- diculartothein-planesurfacecapturedimagesandforcemeasurements wererecordedbytheloadcell,bothat1Hz.
2.6. ModifiedArcantests
SixmodifiedArcanspecimenswerecutfromaplateofeachtem- per.ThegeometryofthespecimenisgiveninFig.1f).Thespecimen wasclampedbyfourloadingbracketsusing12M6-boltsasshownin Fig.2.Twodifferentloadcaseswereapplied byalteringtheloading direction𝛽;three with𝛽 = 90° andthree with𝛽 = 45° asshown in Fig. 2a)andFig. 2b), respectively. All tests wereconducted witha crossheadvelocityof1mm/min,similartothosecarriedoutin[27]. Thetestsareabbreviated“Arcan𝛽” todistinguishbetweenthetwoload cases.IntheArcan90tests,thespecimenisloadedinatensionmode wheretheloadaxiscoincideswiththespecimen’slongitudinalaxis.The pinconnectingthemountingbracketstothetestmachineallowsthe mountingbracketsandspecimentorotatewhenloaded,enablingthe specimentotearopen.IntheArcan45tests,thespecimenissubjected toamixed-modeloading.Thewidthatthenarrowestpointinthegauge sectionwasmeasuredto32mmwithnegligiblevariationsbetweenthe specimens.Avirtualextensometerofinitiallength10.5mmspanning acrossthenotchalongthelongitudinalaxisofthespecimenineCorr wasusedtoextractdisplacementsbyuseof2D-DIC.
Fig.1. Geometryoftestspecimenswithmeasuresinmm:a)uniaxialtension,b)andc)notchtension,d)plane-straintension,e)in-planesimpleshearandf)modified Arcan.
Fig.2. TestsetupofamodifiedArcanspecimenwitha)𝛽=90° andb)𝛽=45°
3. Experimentalresults 3.1. Uniaxialtensiontests
Engineeringstress-straincurvesofrepresentativetestsfromGranum etal.[25]areplottedinFig.3a).Aslightvariationinelongationat fracturebetweenthedifferenttensiledirectionsisseenforeachofthe tempers,whiletheflowstressshowedamaximumdeviationof3%.The LankfordcoefficientsR𝛼 werecalculatedfor alltensiontests andare listedinTable2,where𝛼denotestheanglewithrespecttotherolling direction.Allcoefficientsarebelowunityandsomewhathigherinthe rollingdirection.Thesimilarityinflowstressbetweenthethreedirec- tionssuggeststhatthealloyexhibitsisotropicpropertieswithrespect tostrengthandstrainhardening.However,theLankford coefficients suggestthatthematerialisprone tothinningandexhibitsmoderate
Table2
LankfordcoefficientsR𝜶forrepre- sentativetensiontests.
Temper R 0 R 45 R 90
T4 0.58 0.45 0.44 T6 0.69 0.48 0.55 T7 0.77 0.57 0.62
anisotropyinplasticflow.Theequivalentstrainattheonsetoffracture wasfoundbyuseofDICwhereacharacteristicelementsizeof0.57mm wasused.MultipleDICsimulationswithaslightlyshiftedpositionof themeshwereconductedtoavoidresultsbiasedbytheDICmesh.For thetestsintherollingdirection,theaveragelogarithmicfracturestrain cameoutas0.70,0.33and0.73fortempersT4,T6andT7,respectively.
Fig.3. Engineeringstress-straincurvesfrom:a)representativetestspresentedinGranumetal.[25]andb)representativetestsintemperT4conductedinconjunction withthemechanicalteststomonitortheeffectofnaturalageing.
Thesevaluesrepresentthestrainwherefractureinitiatesatthesurface ofthespecimen.
Theengineeringstress-straincurvesforthreesetsoftemperT4tests arepresentedinFig.3b).TheT4–1curveisarepresentativetestinthe rollingdirectionfromFig.3a).T4–2isatestconductedinconjunction withthemodifiedArcantestsandT4–3isatestconductedinconjunc- tionwiththeplane-straintensionandin-planesimplesheartests.The numberingofthetestsindicatestheordertheywereperformedin,i.e., theT4–1testwasconductedatanearlierpointintimethantheT4–2 test,whichwasperformedpriortotheT4–3test.Itisseenthatnatural ageingincreasesthestrengthastheT4–2andT4–3curvesareshiftedup- wardscomparedtotheT4–1curve.However,thenaturalageingseems tohavereachedsaturationwhentheArcantestsweredone,asthedif- ferencebetweentheT4–2andT4–3curvesisnegligible.Thiseffectwas alsoinvestigatedbyEngleretal.[28]forthesamematerialandthe readerisreferredtothisworkforathoroughdiscussionofthis phe- nomenon.DuetothenegligibledifferencesbetweentheT4–2andT4–3
curves,theneedformultiplecalibrationsoftheplasticitymodelforthis temperwasdeemedunnecessary.
3.2. Notchtensiontests
TheexperimentalresultsfromtheNT10andNT3testsareshownin Fig.4.Therepeatabilityofthenotchtensiontestswasexcellent,and thusonlyonetestperconfigurationisplotted.Thedisplacementwas extractedfrom theglobalvirtualextensometerwhilethelogarithmic strainiscalculatedfromthedisplacementmeasuredbythelocalvir- tualextensometer.Theforcelevelsbetweenthetwotestgeometriesare similarwhilethedisplacementsareslightlylargerforthelargestnotch radius.Thelocalstrainisalsoseentobehigherinthetestswiththe largestnotchradius.Thisisexpectedasthesharpernotchradiuscon- finesthegaugesectionmorethanthelargernotchradius,resultingin higherstresstriaxialitieswithinthisregion.
Fig.4. Experimentalresultsfromthea)NT10andb)NT3testsintermsofforce-displacementandlogarithmicstrain-displacementcurves.
Fig.5. Fracturesurfacesofa)NT10andb)NT3testsforthethreetempers.
Fig.6. Force-displacementandlogarithmicstrain-displacementcurvesfrom plane-straintension(PST)tests.
Fracturedspecimensfromthesixdifferentnotchtensiontestswere examinedbyvisualinspectionandareshowninFig.5.Onlyonetest perconfigurationisshownasnegligibledifferenceswereseenbetween fracturesurfacesofrepeatedtests.Aslantfracturesurfacewasfound foralltests,eventhoughsomeNT3testsdisplayed roughshearlips.
Ingeneral,thefracturesurfaceswererougherfortemperT7thanfor tempersT4andT6, andshear lipsweremoreprominentin theNT3 teststhanintheNT10tests.
3.3. Plane-straintensiontests
Theforce-displacementandlogarithmicstrain-displacementcurves fromrepresentativeplane-straintensiontestsareshowninFig.6.Du- plicatetestsarenotshownduetotheexcellentrepeatability.Theresults areinaccordancewiththetrendsseenforthenotchtensiontests,where themostprominentdifferencebeingthesimilarityinelongationatfrac- turebetweentempersT6andT7.Thefractureinitiatedin thecentre ofthespecimenforalltestsandpropagatedinastraightlinetowards thefreeedges,perpendiculartotheloadingdirectionasseeninFig.7. ForthetestsintemperT6,thecrackpropagatedinstantlyresultingin asuddenlossofload-carryingcapacity,whilethetestsin temperT4 exhibitedslightlyslowercrackpropagation.OnlytheT6–3testexperi- encedcompletefracture,wherethespecimenwaspulledapart.Inthe restofthetests,theforceleveldroppedbelowathresholdlimitatwhich thetestwasstoppedautomatically.ForthetestsintemperT7,thecrack propagatedslowlyandittookapproximately40sfrominitiationtocom- pletion.ByinspectionofthefracturedT6–3specimenshowninFig.7,
aslantfracturesurfacewasobserved,wherethecrackwasseentoflip totheotheradmissibleshearband.
3.4. In-planesimplesheartests
Theforce-displacementcurvesfromthesheartestsareshowntothe leftinFig.8wherethenumberedmarkerscoincidewiththenumbered imagesontheright-handside.Owingtoslightscatter,resultsfromall repeattestsarepresented.Thestrainfieldsobtainedby2D-DICreveal thatstrainslocalizedinathinbandacrossthegaugesectioninalltests (notshownforbrevity).Byinspectionoftheimages,fractureseemingly occurredsimultaneouslywithinthisband,astheoriginoffractureiniti- ationwasdifficulttopinpointexactly.In-planerotationswereobserved inalltests,resultinginanangleofthelocalizeddeformationbandcom- paredtotheloadingdirection,asevidentfrom theimagesin Fig.8. Theangleofthebandwithrespecttotheloadingdirectionwassimilar between repetitionsandtempers.Thedropin theforce-displacement curves,particularlyfortemperT7,ispresumedtooccurduetothecom- binedeffectofmaterialsofteningandareareductioninthelocalized deformationband.ThesheartestsindicatedthattemperT7hassupe- rior ductilitycomparedtotempersT4andT6, andthehighductility makes itdifficulttopinpointtheonsetof fracturein thetemper T7 tests.Byinspectionoftheimages,theonsetoffractureisanticipated tooccursomewherebetweenpoint2and3intherepresentativeforce- displacementcurvefortemperT7inFig.8.Allspecimensdisplayeda smoothandflatfracturesurfacethroughthethickness,andtherewere negligibledifferencesamongrepetitionsandtempers.
3.5. ModifiedArcantests
Theforce-displacementcurvesfromthemodifiedArcan45andAr- can90tests areshownin Fig.9a)andFig.9b),respectively.Overall, thetrendsareinaccordancewiththeothermechanicaltestspresented showingthattemperT6givesthehighestpeakforcefollowedinturn bytempersT4andT7.Thepeakforcesareconsistentlyhigherinthe Arcan90teststhanintheArcan45tests,andtheratiobetweenthepeak forcesinthetwoloadcasesisalmostidenticalamongstthetempers.The displacementatpeakforceissmallerintheArcan45teststhaninthe Arcan90tests,butthefinaldisplacementislargerintheformer.This islinkedtothecrackpathsbeinglongerintheArcan45teststhanin theArcan90tests,especiallyfortempersT4andT6.AsFig.9b)indi- cates,theArcan90-T6testexperiencedasuddenlossofload-carrying capacityasthecrackpropagatedinstantlyacrossthespecimenbetween twoimagesofthetest.EventhoughtemperT7isthemostductileal- loycondition,thetestsintemperT4exhibitsthelargestdisplacements.
Thisislinkedtothecombinationofadequatestrength,work-hardening andductilityintemper T4,whichseemstobemorefavourable than thehighductilityandlowwork-hardeningseenfortemperT7inthese tests.
Fig.7.Fracturedplane-straintensionspecimensandfracturesurfaceoftestspecimenT6–3.Theredlinesonthepicturesindicatethecrackpath.(Forinterpretation ofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
Fig.8.Force-displacementcurvesfromthein-planesimpleshear(ISS)testswherethenumberedmarkerscoincidewiththeimagesontheright-handside.
Fig.9. Force-displacementcurvesfroma)theArcan45testsandb)theArcan90tests.
Fig.10. FracturedspecimensshowingthedifferentfracturemodesofthemodifiedArcantestswithcorrespondingfracturesurfaces.
Fracturedspecimens fromthe modifiedArcan tests areshownin Fig.10.Intheupperpartofthefigure,thethreedifferentfracturemodes observedinthetestsaredisplayed.Acurvedcrackpathwasobservedin boththeArcan45-T4andtheArcan45-T6tests.IntheArcan45-T6tests, thecrackwasarrestedapproximately10mmfromtheedgeandthetests werestoppedastheforceleveldroppedbelowalowerthreshold.Inthe Arcan45-T4tests,thecrackwasarrestedapproximately5mmfromthe edgewhenstopped,butduringthedismantlingofthespecimensfrom theloadingbrackets,thespecimenswerepulledapart.Byinspectionof thefracturesurfacesfromtheArcan45-T4tests,allthreetestsexhib- itedthealternatingslantfracturephenomenon.Thecrackinitiatedand propagatedinaslantfracturemodeuntilarrested.Byinspectionofthe Arcan45-T6specimen,thefractureinitiatedinaV-modeandpropagated inthismodeforapproximately3mmbeforeittransitionedtoaslant fracturemode.Thealternatingslantfracturephenomenonwasnotob- servedinanyofthesetests.TheV-modewasalsoseeninGrubenetal.
[29]forArcan45specimensmadeofDocol600DLsteel.IntheArcan45- T7tests,fractureinitiatedinaV-modewithinthesameareaasforthe Arcan45-T4andArcan45-T6tests,andthecrackinitiallyfollowedasim- ilarcurvedpath.However,thecrackpathdeflectedabruptlyafterafew millimetresandpropagatedperpendicularlytothelongitudinalaxisof thespecimenasshowninFig.10.Thefracturesurfaceswereroughwith evidentshearlips.AlltheArcan90testsexhibitedasimilarcrackpath forallthreetempers,butdifferencesinthefracturesurfaceswereseen.
IntheArcan90-T4tests,thefractureinitiatedandpropagatedinaslant fracturemodewherethecrackwasseentoflipabruptlythroughoutthe deformationinalltests.OneoftheArcan90-T6testsinitiatedinaV- modebeforeatransitiontoslantfracturewasseen,whilethetwoothers initiatedinaslantfracturemode.BoththeArcan90-T4andArcan90-T6 testsshowedasmoothfracturesurface,whiletheArcan90-T7testshad arougherfracturesurface,similartowhatwasseenintheArcan45-T7 tests.
4. Modellingandsimulation 4.1. Stressstateparameters
Anyadmissiblestressstatecanbe expressedbythethreeordered principalstresses𝜎1≥𝜎2≥𝜎3givenby
𝜎1= 2
3𝜎vMcos(𝜃)+𝜎h (1a)
𝜎2= 2 3𝜎vMcos
(𝜃−2𝜋 3
)
+𝜎h (1b)
𝜎3= 2 3𝜎vMcos
(𝜃+2𝜋 3
)
+𝜎h (1c)
where0≤𝜃≤π6 isthedeviatoricangle,𝜎vM=√
3𝐽2 isthevonMises equivalentstress,and𝜎h=I1/3isthehydrostaticstress.Here,J2andI1 arethesecondprincipaldeviatoricstressinvariantandthefirstprinci- palstressinvariant,respectively.Thestressstatemaybeconveniently describedbythestresstriaxialityTandtheLodeparameterL.Thestress triaxialityisdefinedastheratiobetweenthehydrostaticstress𝜎hand thevonMisesequivalentstress𝜎vM,viz.
𝑇= 𝜎h
𝜎vM
(2)
TheLodeparameterdescribesthedeviatoricstressstate,andisde- finedeitherintermsofthedeviatoricangle𝜃ortheorderedprincipal stresses(𝜎1,𝜎2,𝜎3)as
𝐿=√ 3tan
(𝜃−𝜋 6 )
= 2𝜎2−𝜎1−𝜎3
𝜎1−𝜎3
(3)
TherangeoftheLodeparameterisL∈[−1,+1],whereL=−1,0 and+1representstatesofgeneralizedtension,generalizedshearand generalizedcompression,respectively.
4.2. Plasticitymodel
Theconstitutiverelationisgivenbythehigh-exponentyieldsurface proposedbyHershey[30]andHosford[31],theassociatedflowrule andanextendedVocehardeningrule.Eventhoughthematerialexhibits moderateplasticanisotropyaccordingtotheLankford coefficient,an isotropicplasticitymodelisapplied.Theequivalentstressisgivenby theprincipalstressesas
𝜎eq=(1
2(||𝜎1−𝜎2||𝑎+||𝜎2−𝜎3||𝑎+||𝜎3−𝜎1||𝑎))1𝑎
(4) whereaisaparametercontrollingthecurvatureoftheyieldsurface.
Thisparameteris settoa= 8in thisstudyassuggestedbyHosford [32]basedonpolycrystalplasticitycalculations.Theyieldfunctionis expressedas
𝜙= 𝜎eq− 𝜎M= 𝜎eq−(
𝜎0+𝑅(𝑝))
≤0 (5)
where𝜎Misthematrixmaterialflowstress,𝜎0istheyieldstress,Risthe hardeningvariableandpistheequivalentplasticstrain.Thehardening variableisdefinedbyanextendedVocehardeningruleontheform 𝑅(𝑝)=
∑3 𝑖=1
𝑅𝑖(𝑝)=
∑3 𝑖=1
𝑄𝑖( 1−exp(
−𝐶𝑖𝑝))
(6)
whereRiarehardeningtermsthatsaturateatdifferentlevelsofplastic strain.ThehardeningparametersQiandCiworkinpaircontrollingthe strainhardeningofthematerial.
4.3. MMCfracturemodel
FractureinthesimulationsisgovernedbyamodifiedMohr-Coulomb fracturemodel.TheversionoftheMohr-Coulombmodelusedin this studywasinspiredbytheoneproposed byBaiandWierzbicki[19]. Theytransformedtheoriginalmodelintolocusformwherethefailure strain̄𝜀f,i.e.,theequivalentplasticstrainatfailure,wasdefinedinterms ofthestresstriaxialityTandtheLodeangleparameter ̄𝜃 .Thelatter isanormalizedparameteroftheLodeangleandiswithin therange
̄𝜃∈ [−1,+1].Inthisstudy,theLodeparameterLisusedinsteadofthe Lodeangleparameter ̄𝜃,andtheexpressionforthefailurestrain ̄𝜀f is thengivenas[19]
𝜀f(𝐿,𝑇)=
⎧⎪
⎨⎪
⎩ 𝐾̂𝐶2
[
̂𝐶3+
√3 2−√
3 (̂𝐶4∗− ̂𝐶3
)(sec(−Lπ 6
)
−1)]
×
⎡⎢
⎢⎣
√ 1+ ̂𝐶12
3 cos(−𝐿𝜋 6
) + ̂𝐶1
(𝑇+1 3sin(−𝐿𝜋
6 ))⎤⎥
⎥⎦
⎫⎪
⎬⎪
⎭
−1 𝑛
(7)
where
̂𝐶4∗=
{1 for −1≤𝐿≤0
̂𝐶4 for 0<𝐿≤1 (8)
Themodelhassixparametersthatmustbecalibrated: ̂𝐶1 governs thepressuredependence; ̂𝐶2andKcontroltheoverallductility; ̂𝐶3de- terminestheLodeparameterdependence; ̂𝐶4governstheasymmetryof thefailurelocusbetweenstatesofgeneralizedtensionandcompression;
andincreasingvaluesofnshifttheductilityupwardsanddecreasethe stresstriaxialityandLodeparameterdependence[19].
Damageis introduced by the damage variableD which is setto evolvewithincrementsoftheequivalentplasticstrainp,givenas 𝐷(𝑝)=
∫
𝑝 0
d𝑝
𝜀f(𝐿,𝑇) (9)
Thematerialisundamagedinitsinitialconfiguration,i.e.,D=0, andfractureinitiateswhenD=1.Whereasthefailurelocusisvalidfor proportionalloadingonly,thedamageaccumulationruleisassumedto accountfornon-proportionalloadpathsinanapproximateway.
4.4. Finiteelementmodelling
The finiteelement (FE) simulations of themechanical tests used in the calibration process were conducted with the implicit solver of Abaqus[33]withdisplacement-controlledloading.Allsimulations withtheMMCfracturemodelwereconductedusingtheexplicitsolver of Abaquswith velocity-controlledloading. Thespecimensweredis- cretizedbyuseof8-nodelinearbrickelementswithselectivereduced integration,denotedC3D8inAbaqus.Fractureismodelledbyelement erosion,whereelementsareremovedwhenDinEq.(9)reachesunity.In thesimulationsoftheNT3,NT10andPSTtests,threesymmetryplanes wereutilized,andthereducedmodelswereoptimizedwithrespectto thenumberofelements.Thevalidityofthereducedmodelswithop- timizedmeshdesignwas verifiedagainstselected simulationsof the fullspecimenwithadense,uniformmesh.Thedifferencesinthepre- dictedcrackinitiationandpropagationbetweensimulationswiththe optimizedanduniform meshdesignswerenegligible.Themaximum timestepintheimplicitsimulationswasselectedsothateachsimula- tionhadaround200increments.Allsimulationswereconductedwith 5elementsoverthehalf-thicknessandanin-planediscretizationinthe gaugeregionwithacharacteristicelementsizeof0.15mm.Thisresulted ininitiallycubic-shapedelementsinthegaugeregion.
Anin-planeviewoftheFEmodelswiththemeshdepictedonthe initialgeometryofthespecimensisshowninFig.11.Inallsimulations, exceptforthesimulationsofthePSTtests,theloadwasassignedtoa referencenodepositionedinthecentreofthepinhole.AnMPCbeam constraintwasusedtoconnectthereferencenodetotheelementset ontheboundaryofthespecimen,tomimictheeffectofapinpulling thespecimen.Thisisvisualizedbytheredlinesandthebluereference nodesinFig.11.Thisapproachlimitsthenumberofelementsinthe FEmodelssignificantlyandpresumesthattheomittedpartmovesasa rigidbody.Also,therotationsinducedbythepinnedlinkisrecognized inthismodellingapproachbyallowingthereferencenodetorotatein- plane.InthesimulationofthePSTtest,theclampedareawasassumed tobehaveasarigidbodyandthusomittedinthemodel.Theboundary conditionswerethusassignedtotheedgesborderingtotheclamped regionsofthemodel.TheISSandmodifiedArcantestspecimenswere modelledaccordingtoFig.11d)-e),whereonlythrough-thicknesssym- metrywasutilized.IntheISSmodel,tworeferencenodesconnectedto theedgesofthespecimenwereappliedtoallowforin-planerotations.
InthemodifiedArcanmodel,thepartofthespecimengrippedbythe clampingframewaspresumedtomoveasarigidbodyandwasthus omittedfromthemodel.In-planerotationswereaccountedforbyin- sertingreferencenodescoincidingwiththecentreofthetwoloading pinsshowninFig.2.Thereferencenodeswereconnectedtotheedges ofthespecimenbyanMPCbeamconstraint.Thisapproachsimplifies themodelsubstantiallyandenablesfeasiblecomputationaltimeswith thedesireddiscretization.Inallexplicitsimulations,thevelocitywas rampedupoverthefirst10%ofthesimulationtime,andtheenergy balancewascarefullycheckedtoensurequasi-staticloadingconditions.
4.5. Localizationanalyses
The localizationanalyses were conductedusing the imperfection bandapproach,followingtheworkbyRice[34].Adetaileddescription oftheapproachusedinthisstudycanbefoundinMorinetal.[9],and onlyabriefoverviewisgivenherein.Asolid,homogenousbodythatis homogeneouslydeformedisconsidered.Withinthisbody,athinplanar bandisassumedtoexistwherestressandstrainratesareallowedtobe differentfromtheirvaluesoutsidetheband.However,continuingequi- libriumandcompatibilityacrosstheimperfectionband areenforced.
Thenormaltothebandisdenotednandasketchofasolidbodywith aplanarbandisdepictedinFig.13.Localizationissettooccurwhen thestrainrateinsidethebandbecomesinfinite.Thecriticalorientation ofthebandisnotknownonbeforehandandlocalizationanalysescov- eringarangeofbandorientationsmustbeconductedtofindtheone
Fig.11. Finiteelementmeshesoftestspecimen:a)NT3,b)NT10,c)PST,d)ISSande)modifiedArcan.
Fig.12. Experimental(crosses)andnumericalforce-displacementandlogarithmicstrain-displacementcurvesofthethreetempersfora):NT10andb):NT3.
exhibitingthelowestductility.Thefailurestrainistakenastheequiva- lentplasticstrainoutsideoftheimperfectionbandatlocalizationinside theband.
The solid body is governed by theplasticity model described in Section4.2.Insidetheband,aporousplasticitymodelisusedtorepre- sentthematerialbehaviour,whichenablesasimpleapproachtointro- duceanimperfectionbynucleationandgrowthofvoids.Theheuristic extensionoftheGurson-Tvergaardmodel[35,36]proposedbyDæhli etal.[37]isadopted,wheretheyieldconditionisgivenby
Φ = (𝜎eq
𝜎M
)2
+2𝑞1𝑓cosh (3𝑞2
2 𝜎h
𝜎M
)
−( 1+𝑞3𝑓2)
=0 (10)
where𝜎eqistheHershey-HosfordequivalentstressdefinedinEq.(4), 𝜎MistheflowstressofthematrixmaterialaccordingtoEq.(5),q1,q2,q3 aretheTvergaardparameters,𝜎histhehydrostaticstress,andfisthe voidvolumefraction.Theevolutionofvoidvolumefractionisdefined as
̇𝑓= ̇𝑓g+ ̇𝑓n+ ̇𝑓s=(1−𝑓)tr𝐃p+𝐴ṅ𝑝+𝑘s𝑓𝜅(
𝝈′)𝝈′∶𝐃p 𝜎eq
(11)
where ̇𝑓gisthevoidgrowthrate, ̇𝑓nisthevoidnucleationrateand ̇𝑓s
representsthecontributionfromvoidsofteninginsheartotheporosity evolution[38].TheparametersAnandksgovernvoidnucleationand
voidssofteninginshear,respectively.Furthermore,𝝈′isthedeviatoric stresstensor,Dpistheplasticrate-of-deformationtensordefinedbythe associatedflowruleand𝜅(𝝈′)isafunctionofthesecondandthirdin- variantofthedeviatoricstresstensor,J2andJ3,respectively,viz.
𝜅( 𝝈′)
=1−27 4
𝐽32
𝐽23 (12)
Byincludingthetermforvoidsofteninginshearintheevolution equation,thephysicalmeaningofthevoidvolumefractionfislostandit shouldbeinterpretedasadamageparameter,assuggestedbyNahshon andHutchinson[38].ThereaderisreferredtoDæhlietal.[37]fora detailedaccountoftheporousplasticitymodelusedinthelocalization analyses.
5. Calibration
5.1. Calibrationofhardeningparameters
Thecalibrationprocedurefollowsasimilarapproachasemployed ine.g.MohrandMarcadet[5].Theuniaxialtensiontestswereusedto obtainaninitialestimateofthehardeningparameters.Aspreadsheet wasusedtofittwoofthethreehardeningtermstothetruestress-strain curveuptonecking.InversemodellingoftheNT10testsbyuseofthe
Fig.13. CalibrationapproachfortheporousplasticitymodelparametersAnandks.TheplotshowsdataforthetemperT6calibration.
Fig.14. Comparisonbetweenresultsfromthelocalizationanalyses(SLM)andthecalibratedMMCfracturemodel.
Table3
MaterialparametersoftheextendedVocehardeningrule.
Temper 𝜎0(MPa) Q 1(MPa) C 1 Q 2(MPa) C 2 Q 3(MPa) C 3 T4 135.0 19.04 87.05 142.22 10.06 75.00 3.08
T6 245.1 6.45 438.98 109.39 11.13 2.58 9.05
T7 152.8 3.76 2316.10 57.11 38.34 25.81 4.59
optimizationtoolLS-OPT[39]wasthenemployedtocalibratethehard- eningparameters.Theinitialestimatewasusedasastartingpointin theoptimization,wherethefirsthardeningtermwaskeptfixedandthe secondandthirdhardeningtermscouldchange.SequentialFEsimula- tionswerethenconductedwithdifferenthardeningparameterswhere LS-OPTemployedageneticoptimizationalgorithmtofindtheoptimal setofparameters.Theforce-displacementcurvesfromtheNT10tests wereusedastargetsintheoptimizations.Thefiniteelementmodelem- ployedispresentedinSection4.4andthecalibratedhardeningparam- etersaredisplayedinTable3.Theforce-displacementandlogarithmic strain-displacementcurvesfromthesimulationsareplottedassolidlines togetherwiththeexperimentsascrossesforthetwonotchtensiontests inFig.12.Thegoodagreementbetweenthesimulationsandexperi- mentsforbothtestsillustratesthevalidityofthecalibratedplasticity model.
5.2. CalibrationoftheMMCfracturemodel
Thepredictivecapabilityof thelocalizationanalysesrelies onan accuratedescriptionofthematerialbehaviour insideandoutsidethe imperfectionband.Theplasticitymodelusedoutsidethebandwascal- ibratedasdescribedintheprevioussection.Fortheporousplasticity
modelusedinsidetheband,theparametersintroducedbyTvergaard [36]weregivenstandardvalues,i.e.,q1=1.5,q2=1.0,𝑞3=𝑞21=2.25. ThenucleationrateAnandthevoidshearingparameterkswerecal- ibratedbasedonlocalizationanalysesfollowingtheprocessdepictedin Fig.13,whereastheinitialvoidvolumefractionf0wassettozero.The deformationgradientF(t)wasextractedfromthecriticalelementinthe through-thicknesscentreofanNT10andaPSTsimulationandassigned asboundaryconditionsinlocalizationanalyses.Aseriesoflocalization analyses withvaryingnucleationrateAn andvoid shearingparame- terkswasconducted.AccordingtoNahshonandHutchinson[38],the voidshearingparameterissuggestedtobeintherange1≤ ks≤3for structuralalloys,thusthreevalueswithinthisrangewereinvestigated:
ks={1.0, 2.0, 3.0}.TheoptimalvalueofAnwasfoundwhenlocaliza- tionoccurredforastrainoutsidethebandsimilartotheexperimental failurestrainforagivenks,asdepictedintheplotinFig.13.Theoptimal valueofkswasnotsearchedforandthecalibratedvalueofkswascho- senfromthethreevaluesinvestigated.Theexperimentalfailurestrain wastakenfromthecriticalelementinasimulationbasedonbothglobal andlocalmeasurementsfromDICresults.Ingeneral,failurewasfound toinitiatejustbeforethefinaldipintheforcelevels,asitwasassumed thatstrainlocalizationinitiatedatthispoint.TheplotinFig.13shows theresultsfromthelocalizationanalysesfortemperT6wheretheopti-
Fig.15. FailurelocioftheMMCfracturemodelforthethreedifferenttempersofAA6016.
Table4
Calibratedvaluesofthepa- rametersintheporousplas- ticitymodel.
Temper A n k s
T4 0.006 2.0 T6 0.0125 2.0 T7 0.005 2.0
malvalueofthenucleationrateAnisdepicted.Thecalibratedvaluesof theparametersintheporousplasticitymodelaregiveninTable4.
Withthemetalandporousplasticitymodelscalibrated,localization analyseswithproportionalloadingconditionswereconducted,i.e.,as- signingadeformationwherethestresstriaxialityTandtheLodepa- rameterLareconstantduringtheentiresimulation.Fromtheseanal- yses,failurestrainscoveringaconsiderableregionofthestressspace wereobtained,eveniflocalizationwasnotachievedforalltheapplied
stressstates.TheparametersoftheMMCfracturemodelwerefinally optimizedagainstthiscloudofpointsinaPythonscript.Approximately 100pointswereusedineachoptimizationtoensureasolidbasisfor theidentification.Thebasin-hopping algorithm[40]availableinthe SciPypackage[41]wasemployedtodeterminetheoptimalsetofmodel parameters.Thealgorithmaimstofindtheglobalminimumofacost function,heredefinedasthedifferencebetweenthefailurestrainscal- culatedwithMMCfracturemodelandlocalizationanalysesforawide rangeofstressstates.Withintheglobalsteppingalgorithm,alocalmin- imizationiscarriedoutusingasequentialleastsquaresprogramming (SLSQP)method[42].Approximately60basin-hoppingiterationswere performedtofindtheoptimalsetofparametersusingthedefaultpa- rametersofthealgorithm(thereaderisreferredto[41]formoredetails uponthesenumericalaspects).Thecalibratedparametersaregivenin Table5.
ThecalibratedMMCfracturemodelandthetargetpointscalculated bythelocalizationanalyses(giventheabbreviationSLM)areshownin Fig.14forselectedvaluesofthestresstriaxiality.Fromthefigureitisev- identthatthemaintrendsarecapturedinthecalibrationofthefracture
Fig.16. Comparisonbetweenexperimental(crosses)andnumerical(solidlines)force-displacementandlocalstrain-displacementcurvesoftemperT4:a)NT10,b) NT3,c)PSTandd)ISS.Allcurvesareplottedtofracture.
Table5
CalibratedparametersofthemodifiedMohr-Coulombfracturemodelbased onlocalizationanalyses.
Temper K ̂𝐶 1 ̂𝐶 2 ̂𝐶 3 ̂𝐶 4 n
T4 0.9969 0.01000 0.5075 0.8820 1.0056 0.01122 T6 0.9988 0.01135 0.5081 0.8847 1.0066 0.01000 T7 0.9611 0.00100 0.4815 0.8672 1.0005 0.00138
model,eventhoughthefitaroundgeneralizedshear(L=0)isnotalways good.ThedependenceonthestresstriaxialityaroundL=0issmallac- cordingtothelocalizationanalyses,resultinginthepointsforL=±0.2 insomecasesoverlappingeachother.Thisbehaviourisnotaccurately capturedbythecalibratedfracturemodel,whereanevidentdependence onthestresstriaxialityisseenaroundL=0.Thefitisaccurateforgen- eralizedtensionandcompressionforallthreetempers.Thesomewhat flatfailurelocuspredictedbythelocalizationanalysesisamongstother linkedtotheuseofaHersheyyieldsurfacewithexponenta=8.Thein- fluenceoftheyieldsurfacecurvatureonductilefailurewasinvestigated byDæhlietal.[37],whereHersheyyieldsurfaceswithexponenta=2 (i.e.,equaltothevonMisesyieldsurface)anda=8wereinvestigated byuseoflocalizationanalyses.Theresultssuggestthatayieldsurface
witha=8displaysaflatterfailurelocuscomparedtotheyieldsurface witha=2.ThereaderisreferredtoDæhlietal.[37]forfurtherdetails ontheinfluenceoftheyieldsurfacecurvatureonductilefailure.
The resultsfrom thecalibration of theMMC fracturemodelsare showninFig.15.Theoverallshapeofthethreefracturesurfacesinthe leftcolumnofthefigureissimilar,wherebothanevidentstresstriaxial- ityandLodeparameterdependenceisseen.Themonotonicdecreasein ductilityforincreasingstresstriaxialityisshowninthemiddlecolumn ofthefigureforgeneralizedcompression(L=1),generalizedtension (L=−1)andgeneralizedshear(L=0).Therateofdecreaseinductil- ityforincreasingstresstriaxialityissimilarforgeneralizedcompression andgeneralizedtensionforalltempers.However,therateofdecrease inductilityforincreasingstresstriaxialityismuchlowerforgeneralized shear,especiallyfortempersT4andT6.Generalizedshearexhibitsthe lowestductilityfollowedbygeneralizedtensionandgeneralizedcom- pression,wherethedifferencebetweenthetwolatterissmall.Temper T7exhibitsamuchstrongerdependenceonthestresstriaxialityforgen- eralizedshearcomparedtotempersT4andT6.Thisisclearlyvisualized intherightcolumnwherethefailurelociareplottedasafunctionof theLodeparameterforselectedvaluesofthestresstriaxiality.Itshould benotedthatthisstrongstresstriaxialitydependencewasnotpredicted bythelocalizationanalysesandisaresultofthecalibrationoftheMMC
Fig.17. Comparisonbetweenexperimental(crosses)andnumerical(solidlines)force-displacementandlocalstrain-displacementcurvesoftemperT6:a)NT10,b) NT3,c)PSTandd)ISS.Allcurvesareplottedtofracture.
fracturemodel.Theductilityinsimpleshearisconsiderablyhigherfor temperT7thanfortemperT6,whiletemperT4issomewherein be- tween.Theasymmetryofthefailurelociisevidentforalltemperswhere thelowestductilityforselectedvaluesofthestresstriaxialityisfound forslightlynegativevaluesoftheLodeparameter.Thehighductilityfor T=0andL=±1fortemperT6,higherthanforbothtempersT4andT7, issomewhatpeculiar.Thelocalizationanalysesdidnotprovideresults forthesestressstatesandthispartofthefailurelocusisobtainedby extrapolation.However,forthestressstatesachievablebyexperiments, thefailurelocusfortemperT6exhibitslowerductilitythanthefailure locifortempersT4andT7.
Thehighest andlowestductility ontheplane stressfailurelocus is found forthe stress statesrepresenting uniaxialtension (L = −1, T=0.33)andplane-straintension(𝐿=0,𝑇=1∕√
3),respectively.Itis worthnotingthattheductilityishigherinuniaxialtensionthaninequi- biaxialtension(L=1, T=0.67)foralltempers.Thisdifferencewould notbepossible toexpresswiththeHosford-Coulombfracturemodel, wheretheductility isforcedtobeequalinuniaxialandequi-biaxial tension.Thecuspontheplane-stressfailurelocusforuniaxialtension andthevalleytowardssimpleshear(L=0,T=0)foralltemperscatego- rizesthesematerial’sfracturebehaviourasLodeparameterdominated.
Astresstriaxialitydominatedfracturebehaviourwouldexhibithigher
ductilityinsimpleshearcomparedtouniaxialtension,andthusnocusp wouldappearintheplane-strainfailurelocusforuniaxialtension.
6. Numericalresultsanddiscussion 6.1. Materialtests
Theresultsfromthesimulationsofthematerialtestsonthenotch tension(NT),plane-straintension(PST)andin-planeshear(ISS)speci- menswiththeMMCfracturemodelareshowninFigs.16–19.Theex- perimentalresultsarepresentedasdiscretecrosses,whilethesolidlines representthesimulations.Bycomparisonoftheresponsecurvesfortem- perT4showninFig.16,thepredictionsbytheMMCfracturemodelare ingeneralfoundtobegood.Whencomparingtheforce-displacement curvesforthefourtests,theagreementforthenotchtensiontestsisex- cellent,whiletherearesomedeviationsforthePSTandISStests.These deviationsareexpectedwhenmodellingamoderatelyanisotropicma- terialwithanisotropicyieldsurface.Theonsetoffractureisaccurately predictedfortheNT10test,whileitisslightlyconservativefortheNT3 andPSTtests.FortheISStest,theresponsecurvesdeviatealreadyat yieldingandtheonsetoffractureispredictedforalargerdisplacement thanintheexperiment.Thereasonforthisdeviationislinkedtothe
Fig.18. Comparisonbetweenexperimental(crosses)andnumerical(solidlines)force-displacementandlocalstrain-displacementcurvesoftemperT7:a)NT10,b) NT3,c)PSTandd)ISS.Allcurvesareplottedtofracture.
textureofthealloy,requiringananisotropicyieldcriteriontocapture thebehaviourasdiscussedinAchanietal.[43].Engleretal.[28]in- vestigatedthemicrostructureandtextureofanAA6016sheetintemper T4whereacharacteristiccuberecrystallizationtexturewasfound.This textureleadstoarelativelyhighyieldstressinshearcomparedtouni- axialtension[43].Thus,theyieldstressinasheartestisnotexpected tobeaccuratelypredictedwithanisotropicyieldsurface.Considering thatthetextureisnotalteredbytheheat-treatment,thisbehaviourisex- pectedforalltempers.TheconflictingfracturepredictionfromthePST andISStestsillustratesthedifficultiesoffindingasetofparametersthat accuratelydescribestheonsetoffractureinbothtests.
TheresultsfromsimulationswiththeMMCfracturemodelfortem- perT6areshowninFig.17.ThepredictionoffractureinthePSTtestis slightlyconservative,whilethefracturepredictionsfortheNT10,NT3 andISStestsareslightlynon-conservative.Theimpressiveaccuracyin thepredictionsofthetemperT6testsisevidentastheleastaccuratepre- dictionisobtainedfortheNT10test,whichwasusedinthecalibration.
Asexpected,thedeviationsbetweentheforce-displacementcurvesfor theISStestinitiatedalreadyatyield.Despitethis,accurateprediction ontheonsetoffractureisalsoobtainedforthistest.
TheresultsfromthesimulationswiththeMMCfracturemodelfor temperT7areshowninFig.18.Theagreementbetweentheexperimen- talandnumericalresponsecurvesandthepredictedonsetoffractureis
excellentfortheNT10,NT3andPSTtests.Theonlynotabledeviation amongstthesetestsistheshiftinthelocalstrainforthePSTtests,re- sultinginaslightlyhigherstrainattheonsetoffractureinthesimula- tioncomparedtotheexperiment.Asmentionedearlier,theexactonset offractureintheISStestisdifficulttodeterminebasedontheforce- displacementcurves.Thedisplacementatwhichtheonsetoffractureis predictedinthesimulationappearstobeareasonableestimatewhen inspectingimagesfromthetestatasimilardisplacement.
The predictive capability of the MMC fracture model has been demonstratedintermsofglobalandlocalresponseparametersforfour different materialtests.InFig.19,thestressstatehistoriesextracted fromsimulationsofthesameexperimentsarepresented.Thesolidlines aretakenfromsimulationswherethecriticaldamageparameterisset artificiallyhighandtheendofthelinesindicatetheonsetoffracturein theexperiments.Fractureintheexperimentswasdeterminedbasedon bothlocalandglobalmeasurementsforNT10,NT3andPSTtests,while globalmeasurementswereusedforISS.Thedotsindicatetheonsetof fracturepredictedbytheMMCfracturemodel.Thestressstatescovered bytheexperimentsincludemainlynegativevaluesoftheLodeparame- terandpositivevaluesofthestresstriaxiality.Thestressstatehistories aretakenfromthethrough-thicknesscentreelementfortheNT10,NT3 andPSTtests,whichcorrespondstotheelementsubjectedtothelargest equivalentplasticstrain.InthesimulationsoftheISStests,theelement
Fig.19. Evolutionofthestressstate(i.e.,Lodeparameterandstresstriaxiality)asfunctionoftheequivalentplasticstrainextractedfromthecriticalelement.The predictedfracturebytheMMCfracturemodelisindicatedbythedotswhiletheendofthesolidlinesgivestheonsetoffractureinthetests.
Fig.20. Experimentalandnumericalforce-displacementcurvesfortheArcan45testsina),c)ande)andcorrespondingcrackpathsontheundeformedconfiguration inb),d)andf).
Fig.21. Experimentalandnumericalforce-displacementcurvesforthea)Arcan90-T4,b)Arcan90-T6andc)Arcan90-T7tests.
subjectedtothelargestvalueofthedamageparameterDatthedisplace- mentoffractureintheexperimentistakenasthecriticalelement.The damageparameterwasusedtodeterminethecriticalelementsincethe largestvalueoftheequivalentplasticstrainwasfoundonthethrough- thicknesssurfacewithinthenotch.Thisregionis heavilyaffectedby thein-planerotationsthatoccurinthetests,whichmakestheelement subjectedtothelargestvalueofequivalentplasticstrainanunsuitable choicefortheISStests.ThechosencriticalelementinallISStests is locatedonthein-planesurfacewithinthegaugeregionwherestrains localize.Amongtheeightintegrationpointswithinthecriticalelement, theonesubjectedtothelargestvalueofequivalentplasticstrainischo- seninalltests.WhencomparingthepredictionsbytheMMCfracture modelwiththeexperimentalvaluesinFig.19,i.e.,comparingthedots totheendpointofthesolidlines,thetrendsaresimilartotheonesin Figs.16–18.ByinspectionofFig.19,itisevidentthatthestressstate historiesarequitesimilaramongthedifferenttempersapartfrominthe ISStest.Thereasonforthisisthevaryingpositionofthecriticalelement amongthesimulationsoftheISStest.Thedifferenceinstrength,work-
hardeningandductilitybetweenthethreetempersresultsindifferent deformationprocesseswhichaffectthepositionofthecriticalelement, emphasizing thedifficultiesfacedwithanin-planesimplesheartest.
RothandMohr[44]investigatedthechallengesrelatedtodetermining thestraintofailureforsimpleshearforawiderangeofsheetmetals.
Amongstthestudy’sconclusions,itwasstatedthattheshapeofthespec- imenplaysasignificantroleandthatdifferentmaterialpropertiessuch asstrength,work-hardening andductility requiredifferent shapesof thespecimen.ByinspectionofthestressstatehistoriesfortheISStests, temperT6isclosesttoexhibitaproportionalloadingpath,suggesting thatthegeometryoftheISStestspecimenissuitableforthematerial propertiesofthistemper.Ideally,boththestresstriaxialityandtheLode parametershouldbeequaltozeroallthewaytofractureinasheartest.
EspeciallytheISStestfortemperT7exhibitsaloadingpathwhereTand Lvarymarkedlythroughoutthedeformation,makingitlesssuitableto useastargetinacalibrationprocess.Thisisoneofthereasonswhythe PSTtestwaschoseninordertocalibratethevoidshearingparameter ks intheporousplasticitymodelandnottheISStest.Consideringthe
Fig.22. ThestrainfieldsofArcan45-T6andArcan90-T7fromDICandFEsimulationstakenwhenthecrackhadpropagatedapproximatelyhalfwaythroughthe specimen.
rathersimpleplasticitymodelchosenandthatonlytwomaterialtests wereusedinthecalibration,thepredictionsbytheMMCfracturemodel aredeemedsatisfactory.
6.2. ModifiedArcantests
Tofurtherassess thepredictive capabilities of theMMC fracture model,themodifiedArcantestswith𝛽=45° and𝛽=90° weresimulated.
Here,themodel’sabilitytopredictbothcrackinitiationandpropagation istested.InFig.20theforce-displacementcurvesfromexperimentsand simulationsoftheArcan45testsareshowntotheleft,withcorrespond- ingcrackpathsontheundeformedconfigurationtotheright.Despite thesmallinaccuraciesseeninthepredictionsforthematerialtests,ex- cellentagreementbetweentheexperimentalandnumericalresultsis seenfortheArcan45tests,bothintermsofforce-displacementcurves andcrackpaths.Theonsetoffractureisinitiatedatthecorrectdisplace- mentandpositiononthespecimenforallthreetempers.Additionally, thesimulatedcrackpropagationoccursmostlyalongthecorrectpaths atsimilarvelocitiesastheexperimentalones.Eventhesomewhatsur- prisingstraightcrackpathseenintheArcan45-T7testwaspredicted accurately.Theslanted fracturesurfaceobservedin theexperiments wasnotpredictedinanyofthesimulations.Topredictslantedfracture, thethrough-thicknesssymmetrymustbeabandonedandamuchdenser meshaccompaniedbyacoupleddamagemodelismostlikelyrequired [45].However,thecrack inthetemperT7simulationpropagatedin atunnellingmodefrominitiationtocompletefracture.Thestresstri- axialityinsidethenotchwherefractureinitiatedwasbetween0.3and 0.4,whiletheLodeparameterwasapproximatelyequalto−1forall tempers.Justinfrontofthepropagatingcrack,aregionwithstresstri- axialitybetween0.6and0.7andaLodeparameterclosetozerowas presentforalltempers.
Fig.21 showstheexperimentalandnumericalforce-displacement curvesfortheArcan90tests.Theonsetoffracturewasaccuratelypre- dictedfortempersT4andT7,whilefortemperT6fractureoccurred slightlylater inthesimulation thaninthe experiment.Additionally,
therewasaslightdeviationintheforcelevelinpartsoftheresponse curvebeforetheonsetoffracturefortemperT6ofunknownreasons.
ThecrackpropagationwasaccuratelypredictedfortempersT4andT7, wheretheagreementbetweentheexperimentalandnumericalresponse curveswasgoodthroughoutthewholedeformationprocess.Theveloc- ityofthepropagatingcrackfortemperT6wasnotaccuratelycaptured in thesimulations, wherealowervelocitythaninthetestswaspre- dicted.
Theonsetof fracturein thesimulationwas foundtooccurafew millimetreswithinthenotchandnotatthefreesurface.Thisoccurred eventhoughthelargestvalueoftheequivalentplasticstrainwasfound on thefree surface.However,byinspection ofthestressstateatthe onsetoffracture,theregioninsidethenotchwasfoundtobesubjected toahigherstresstriaxialityandaLodeparameterclosertozerothan onthefreesurface.Fractureinitiatedinthisregionbeforepropagating perpendicularly totheloadingdirection.Theagreementbetweenthe crack patternin theexperimentandsimulationwasexcellent forall tempersandisnotshownforbrevity.
ThestrainfieldsofanArcan45-T6andArcan90-T7testareshown in Fig.22 fromboth DICandFEsimulations. Thestrainfields were takenwhenthecrackhadpropagatedapproximatelyhalfwaythrough thespecimeninbothtests.Themagnitudeofthestrainsisconsistently higherintheFEsimulationsthanintheDICsimulationsowingtothe densermeshusedintheformer.However,thequalitativetrendsare similar between thetwosetsof simulationsforbothtests.Anarrow zonewithlocalizedstrainsinfrontofthepropagatingcrackiscorrectly predictedinbothcases. IntheArcan45-T6test,thereisaband with slightlyhigherstrainsacrossthespecimenwhichisnotfullydeveloped intheFEsimulationsandthusonlypartiallypredicted.
7. Conclusions
Thispaperhaspresentedanovelcalibrationprocedureofthemod- ifiedMohr-Coulomb(MMC)fracturemodelbyuseoflocalizationanal- ysesoftheimperfectionbandtypeandapplieditforthreetempersof