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International Journal of Solids and Structures

journalhomepage:www.elsevier.com/locate/ijsolstr

Effect of nonlinear multi-axial elasticity and anisotropic plasticity on quasi-static dent properties of automotive steel sheets

Jeong-Yeon Lee

^a^,^b

, Myoung-Gyu Lee

^a^,^b^,^∗

, Frédéric Barlat

^b^,^c

, Kyung-Hwan Chung

^d

, Dong-Jin Kim

^d

aDepartment of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea

bGraduate Institute of Ferrous Technology (GIFT), POSTECH, Pohang, Gyeongbuk 37673, Republic of Korea

cCenter for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193, Portugal

dProduct Application Center, POSCO, Incheon 21985, Republic of Korea

a rt i c l e i nf o

Article history:

Received 16 November 2015 Revised 15 January 2016 Available online 3 March 2016 Keywords:

Dent resistance Plastic anisotropy Bauschinger effect Elastic degradation Elastic anisotropy

a b s t ra c t

Thisstudy investigatesthe inﬂuenceoftheelasto-plastic propertiesofautomotivesteel sheetsonthe dentingbehavior and suggestsaconstitutivemodelingapproachfor reliabledentanalysis.The stress–

strainbehaviorsofthreekindsofsteelsheets weremeasuredinuniaxialtension,in-planebiaxialten- sionand forward-reversesimple shear tests.Advancedconstitutivemodels wereemployedto capture theplasticanisotropy,reverseloadingcharacteristicssuchas theBauschingereffect,andelasticmodu- lus degradation.Inparticular,the biaxialelasticmodulus andits degradationbehavior weremeasured andimplementedintheconstitutivemodel.Thesuggestedmodelsigniﬁcantlyimprovedtheprediction ofdentscomparedtotheconventionalmodelintermsoftheload-displacementcurve.Sensitivitystudies ontheconstitutivemodeldemonstratedthatmainlyplasticanisotropyandelasticbehaviorofamaterial inﬂuencethepanelstiffness,whereasthereverseloadingbehaviorstronglyaffectsthepermanentdent depth.

1. Introduction

Panel stiffnessand dent resistance are two important quality criteriaforautomotiveouter panels.Thesepropertiesindicatethe ability of a panel to withstand denting under the application of anexternalload.Automotivedentingoccursfrequentlyduringthe lifetimeofa vehicle,includingincidentssuch ashaildamageand door-to-door impact, but it is also a concern in the handling of partsduringmanufacturing.Theaccurateassessmentofpanelstiff- nessanddentresistanceisnecessarytoensuretherobustnessofa formedpanelthroughoutthelifetimeofthecomponent.

Thesepropertiesdependstronglyonthemechanicalproperties of the sheet, the deformation paths undergone during forming, panel geometry, and the supporting and external loading condi- tions. Therefore,with newly designed panel shapes orsubstitute

∗ Corresponding author at: Department of Materials Science and Engineering, Ko- rea University, Seoul 02841, Republic of Korea. Tel.: + 82 2 3290 3269.

E-mail addresses: [email protected] (M.-G. Lee), [email protected] (F. Barlat).

materials, the stiffness and dent properties must be examined again. This is particularly important considering the current re- placement of conventional mild steels by high strength steels (HSS) or advanced high strength steels (AHSS) with reduced thicknessforlightweightvehicles.

Thedirectmeasurementofdentpropertiesisexpensiveincost andtime,becauseacompletetoolsetisrequiredtoformeachtrial shape. For thisreason, a number of empirical relationshipshave beensuggestedtocorrelatedentresistancetotheinfluencingfac- tors. Usually, the static dent energy or dent load has been expressed as a function of elastic properties, yield strength, panel thickness and geometric factors (Dicello and George, 1974; van Veldhuizen et al., 1995). However, these simple expressions are onlyvalid forlimitedcases,because theyignore the couplingef- fect among the influencing factors. One example ofthe coupling effects is that the change of panel curvature affects the amount ofplasticdeformationappliedtothesheet,alteringboththeyield strengthandpanelthickness.Moreover,theeffectsofcomplexge- ometry and supporting condition of a real automotive part can- not be sufficiently represented by the few parameters appear- ing in the proposed empirical relationships. Consequently, these empiricalequationsareunsuitabletoquantitativelyassessthedent propertiesofapanel.

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Finiteelement(FE)analysisallowsarealisticmodelingofform- ing and denting procedures, enablinga morereliable assessment of dent properties. However, a proper constitutive description of the sheetis necessary for accurate simulations. First, the plastic anisotropy of the sheetshould be described, because automotive outer panelsare generallyformed underbiaxialstressconditions.

Forthisreason, thequadraticanisotropic Hillyield criterion(Hill, 1948) hasbeenusedfordentsimulations(HolmbergandNejabat, 2004; Shen etal., 2010). Second, thereverse hardening behavior, such as the Bauschinger effect, needs to be considered, because thesheetmayexperience reverseloadingwhena dentoccurson acurvedpanel.FEsimulationsusingtheChabochenonlinearkine- matichardeningmodel(Chaboche,1986)revealedthatthedescrip- tionoftheBauschingereffectsigniﬁcantlyinﬂuenceddentpredic- tions(Shenetal.,2010).

Theelasticpropertiesofthesheetarealsoan importantfactor becauseelasticdeformationdominatesthedentingprocess.When a dent load of 200N is applied to a dome-stretchedsteel panel, the permanent dent depth is only 0.2mm, whereas the maximum deﬂectionofthepanel is7.7mm(SAE,2004). Thissuggests that nearly 97% of the total deﬂection is elastically recovered whenthe loadisremoved.Previousdent studieshaveassumeda constant value of Young’s modulus measured in uniaxial tension tests, usually inthe rolling direction(RD)of the sheet. However, experiments on steelsheets have revealedelastic anisotropy and the reduction ofthe apparent elastic modulus orso-called chord modulus withaccumulated plastic deformation (Eggertsen et al., 2011; SunandWagoner,2011). The variation intheinitial elastic modulus from the rolling to transverse direction of the sheet is usually 5–10% forsteels (Eggertsenet al.,2011). The reduction of chordmodulus,asaresultofplasticdeformation,isabout15–25%

for steels, and this phenomenon is mainly attributed to the re- pulsive interactions betweenpiled-up dislocations(Cleveland and Ghosh,2002).Therefore,itisnecessarytoinvestigatetheinﬂuence ofelasticpropertiesondentpredictionandtodeterminewhether the use of the conventional Hooke’s law is appropriate in these analyses.

Thisstudyaimstounderstandthedeformationmechanismsun- dergone by the sheet throughout the forming and denting pro- cessesandtosuggestaconstitutivemodelingapproachforreliable dent analysis. Three kinds of steel sheets were selected for this study:deepdrawingquality (DDQ),bake-hardenable(BH340)and dual-phase (DP490) steels, which are used for automotive outer panels. The stress–strain behaviors of the materials were mea- suredunderdifferentloadingconditionsusinguniaxialtension,in- plane biaxial tensionandforward-reversesimple sheartests. Ad- vanced constitutive models were employed to describe the mea- suredstress–strainresponsesintermsofplasticanisotropy,reverse loading behavior and elastic modulus degradation. The adopted models arethe non-quadraticanisotropic yieldfunction Yld2000- 2d (Barlat et al., 2003), the anisotropic hardening model based on distortionalhardening,so-calledHAH(Barlatetal., 2011), and strain-dependentelasticitymodels(Yoshidaetal.,2002).Thesug- gestedconstitutivemodelwasveriﬁedusingthestandarddenttest applied inthe automotiveindustry (SAE,2004), inwhichthe geometry, loadingandboundary conditionsare preciselydeﬁnedto minimize experimental scatteringor prediction errors caused by factorsotherthanthematerialproperties.Thisstudyconcernsonly quasi-staticdenting,inwhichtheindentervelocityisslowenough toignorethestrainrateeffect.

Section 2presentsapreliminary numericalstudyonthestan- darddenttestandinvestigatesthedeformationmechanismofthe sheet. Section 3 describes the experimental procedures for the stress–strain measurement andSection 4 reviewsthe abovemen- tioned constitutive models. Section 5 presents the veriﬁcationof thesuggestedmodelinthestandard denttest andthe sensitivity

Holder Indenter Blank

(b)

Axis of symmetry Blank Die (a)

Stinger beads Punch

Fig. 1. Schematic for the standard dent test with a dome-stretched panel for (a) pre-stretching and (b) denting. Dimensions are provided in Appendix A .

Table 1

Summary of FE mesh optimization for the standard dent test. Both shell and solid elements are ﬁrst-order and use reduced integration.

Shell Solid Mesh size in the central region ( r = 50 mm) 0.1 mm 0.1 mm Mesh size in the other region 1–3 mm 0.25 mm Number integration points through thickness 9 – Number of mesh through thickness – 7

Relative computation time 1 5.3

analysisontheinﬂuenceofmaterialproperties.Section 6demon- stratestheusefulnessofthestandarddenttestasaveriﬁcationex- amplefortheabovementionedconstitutivemodels.Finally,Section 7gives the conclusions and suggestions for future work.

2. Preliminarystudyonthestandarddenttest

A preliminary FE simulation of the standard dent test (SAE, 2004) was conducted to observe the deformation history of the materialthrough theforming anddenting processes.The simula- tionassumedsimplematerialmodels,namely,thevonMisesyield criterion,isotropichardeningandaconstantelasticmodulus,using theuniaxialtensiondataofBH340.

Thetestprocedureconsistsoftwosteps,asschematicallyillus- trated inFig. 1. A 305mm×305mm ﬂat square sheetis formed intoadome-stretchedpanelwithapunchstrokeof12mm,which resultsinabout2%biaxialstrainprior todenting.Next,thepanel isﬁxedbyaholder,subjectedtodentingusingasphericalinden- ter of25.4mmdiameter, andunloaded.The tool geometryisde- scribed in detail in Appendix A. The standard procedure recom- mends an incremental increase of the dent load with successive loading-unloading cyclesbut, forsimplicity, only asingle loading of200Nisconsideredinthisstudy.

A FE model for the dent test was constructed in Abaqus/Standard (implicit) version 6.12. One-quarter of the whole geometry was considered. The tools were modeled using analyticalrigidsurfaces.Differentmeshsizeswereassignedintwo regionsof thesheet:thecentralregion witharadius of50.8mm (twice the radius of the indenter) required a ﬁne mesh because thedentwaslocalizedinthisregion,butarelativelycoarsemesh wasused forthe rest.The mesh option was optimized for both shell and solid elements through a series of convergence tests, as summarized in Table 1. The solid and shell elements yielded similar simulation resultsin terms ofthe dent proﬁle aswell as the load-displacement response of the indenter. This is because the plane stress state dominates during both the forming and dentingprocesses,allowingthestressinthethicknessdirectionto be safely ignored. (It is worth noting that this distinguishes the

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Fig. 2. Predicted von Mises equivalent stress (in MPa) and strain contours on the top surface (a) before denting and (b) under the dent load of 200 N.

deformation characteristics between the denting andindentation of a sheet, as the latter involves signiﬁcant through-thickness stress.)Because ofthissimilarity, shell elements(S4Rin Abaqus) were used for further simulations for computational eﬃciency.

Notethataveryﬁnemeshassmallas0.1mmisnecessaryinthe dentregion,asshowninTable1.

The friction coefficient was assumed to be 0.2 for all of the contactbetweenthesheetandthetools.Thisvalue isconsidered appropriatefornon-lubricatedconditions,asintheactualtests. A sensitivitystudyonthefrictioncoefficientintherangebetween0 and0.2revealedthatthesimulationresultsarerarelyaffectedby thefrictioncoefficient.Thehardcontactconditionwasimposedto prohibittheoverclosureofthesheetandtoolsurfacesbecausethis candirectlyinfluencethedisplacementandloadpredictionsofthe indenter.

Fig. 2(a)showsthepredictedstress andstrain distributions of the pre-stretched and unloaded sample before denting. The von Mises equivalent quantities are shown in the ﬁgure, where the toolsareremovedfromthedisplay forbettervisibility.The resid- ualstress isnegligibleandthe pre-strainis uniforminthedome plateauatthisstage.Fig.2(b)showsthesameplotsunderthedent loadof200N.Theequivalentstraindistributionatthisstageshows thatadditional plasticdeformation duetodenting is limitedtoa smallcentral region,inwhichthetotalaccumulatedplasticstrain islessthan3%onaverage.Therestofthepaneldeformsonlyelas- ticallyduringdenting.Theplasticstrainishigheraroundthedome cornersbutlessthan10%.Thestressandstrainquantitiesshownin Fig.2werecalculatedonthetopsurface,whichisincontactwith theindenter,andthoseonthebottomsurfaceareslightlydifferent.

Furthermore,theentirestresshistoryofanelement locatedat thecenterofthespecimenwasobserved.Fig.3(a)and(b)shows thehistoryon thetop surfaceforthe pre-stretching anddenting processes, respectively. The numbers in the ﬁgures indicate the sequence of stress history; for instance, the material on the top surfaceﬁrstexperiencesbiaxialtensionduringforming(points1–

3), is unloaded(points 3 and 4), subsequently undergoes biaxial compressionduringdenting (points 4–6), andis ﬁnally unloaded (points6and7).The elasticloadingandunloading responsesare alsointhebalanced biaxialstate.Points 4and6inFig.3(b)cor-

respondtothestatesshowninFig.2(a)and(b),respectively.The material on the bottom surface, in contactwiththe punch, does notexperiencereverseloading,becauseplasticdeformationoccurs in biaxialtension duringboth forming anddenting, as shownin Fig.3(c)and(d).The neighboringelements,which arenot atthe centerbutwithinthedentregion,donotfollowtheexactbalanced biaxialconditionbutbetweenthebalancedbiaxialandplanestrain modes.

Theseobservationssuggestthattheconstitutivemodelfordent analysisneedstoconsiderthefollowingaspects:

• Elasticbehavior,especiallyinthebiaxialstate

• Plasticanisotropy

• Monotonicandreversehardeningbehaviors

The hardening behaviors of the materials should be carefully measuredanddescribedatsmallplasticstrains(lessthan10%),as mentionedabove.

3. Stress–strainmeasurement 3.1. Materials

This studyused severalcandidate steel sheets forautomotive outer panels. DDQ is an ultra-low carbon interstitial-free steel, classiﬁed as mild steel, with high formability. BH340 is an HSS with superior dent resistance compared to mild steels because ofthe bake-hardening effect.DP490, an AHSS, exhibits thehigh- est strength of the tested materials, while maintaining suﬃcient formability.TheDP490sheetsusedinthisworkwereproducedat acustomorderformanufacturingautomotivedoorpanels,andex- hibitslightlylower yieldstrength andhigherelongationthan the otherDP490 productsofthesamemanufacturer. Allofthesheets wereprovidedwiththesamethicknessof0.7mm.

3.2. Uniaxialtensionandunloadingtests

UniaxialtensiontestswereperformedusingaZwick/Roellten- sile testing machine. The specimens were fabricated according to the ASTM E8 standard along 0°, 45° and 90° from the RD

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Fig. 3. Stress history of the center element on the top surface during (a) pre-stretching and unloading and (b) denting and ﬁnal unloading. Similarly, (c) and (d) show the stress history on the bottom surface. Numbers indicate the sequence of the stress history. YS: yield surface.

Fig. 4. Effective stress–strain curves measured in uniaxial and balanced biaxial tension for (a) DDQ, (b) BH340 and (c) DP490.

Table 2

Mechanical properties of the steel sheets.

E 0a(GPa) ν^b σ0c(MPa) σ45c(MPa) σ90c(MPa) σb(MPa) r 0 r 45 r 90 r b

DDQ 207 0 .3 152 158 155 161 1 .50 1 .51 1 .80 1 .83

BH340 216 0 .3 236 239 220 262 1 .60 0 .96 1 .62 1 .62 DP490 207 0 .3 264 262 264 278 0 .77 0 .92 0 .99 1 .29

a Measured in uniaxial tension along the RD.

bAssumed.

c0.1% offset.

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Fig. 5. True stress–strain curve of DP490 measured in uniaxial loading–unloading–

reloading. One cycle at a plastic strain of 0.127 is shown in this ﬁgure.

(ASTM2010).Themeasuredstress–straincurvesofallthreemate- rialsareshowninFig.4.Thebasicmechanicalpropertiesobtained fromthesecurvesaregiveninTable2.Theuniaxial yieldstresses weredetermined usingthe0.1% offsetmethodandr-valueswere takenat15%plasticstrainTheuseofthe0.1%offsetinsteadofthe typical0.2%allowedthedeﬁnitionoftheinitialyieldstresscloser totheborderofthelinearelasticregion.Ther-valuestendtosat- uratequicklywithin2–4%plasticstrainforallofthematerials.

Uniaxialloading–unloading–reloadingtestswereconductedus- ing thesame test setup. The unloading–reloading cycles, such as thatshownin Fig.5,were imposedat differentamountsofplas- tic strain. For each unloading cycle, the chord modulus was de- terminedastheslope ofthestraight lineconnecting thestarting andending pointsof the unloading stress–strain curves(see Fig.

5).TheinitialandchordmoduliareplottedinFig.6asafunction ofeffectivestrainforthe threeloadingdirectionsofRD,diagonal direction(DD)andtransversedirection(TD).Asshownintheﬁg- ure,thechordmodulus isreducedby 18%,22%and23%forDDQ,

BH340and DP490,respectively, in theRD. Thetendencies inthe RD,DDandTDaresimilarforallofthetestedmaterials.

3.3. In-planebiaxialtensionandunloadingtests

In-plane biaxial tests were conducted using cruciform specimens with the machine designed by Kuwabara et al., 1998, as shown in Fig. 7. These test results were particularly useful to this study because the biaxial stress–strain data were accurately measured at small strains and an arbitrary load history such as unloading-reloadingcould beimposed.Arbitrary loadratioscould beappliedbetweentheRDandTDofthesheet,butonlythebal- ancedbiaxialloadingconditionwasusedinthisstudy.Twopairs ofstraingaugeswereattachedtomeasurethestrainalongtheRD andTD on bothsurfaces ofthe sheet, asshowninFig. 7(b).The inﬂuenceof bendingstrain, whichoriginatesfromtheinitial cur- vatureofthesheet,waseliminatedbyaveraging thestrainsmea- sured on the top and bottom surfaces of the sheet. The biaxial stress and thickness strain, corresponding to the effective stress and strain, respectively, are compared with the uniaxial tension datain Fig.4. The hardeningcurves obtainedfromthe twotests havesimilarshapes,butthestress isslightlyhigherinthebiaxial tensioncurveforallofthetestedmaterials.

Thebiaxialyieldstress,

σ

b,wasdeterminedsuchthatitresults inthesameamountofplasticworkas

σ

0 inuniaxialtension.The plasticstrain ratio,r_b=

ε

yy^p/

ε

xx^p,wasdetermined bythe linearre- gressionofthemeasured plasticstrain ratiobetweentheRDand TD.ThesetwopropertiesarelistedinTable2.Notethattheplastic strain ratio,r_b,wasnearly constant throughoutthe testforall of thematerials.

In addition to the monotonic tests, the loading–unloading–

reloading tests were conducted at different amounts of applied biaxial stress. A pair of true stress–strain curves,

σ

xx−

ε

xx and

σ

^yy−

ε

^yy^, ^measured ^along ^the ^RD ^and ^TD, respectively, were ob- tainedandareshowninFig.8.Theinitial andchord slopes were

Fig. 6. Reduction of chord modulus as a function of effective strain measured in uniaxial tension (UT) and balanced biaxial tension (BB) for (a) DDQ, (b) BH340 and (c) DP490.

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Fig. 7. (a) In-plane biaxial tester and (b) cruciform specimen.

Fig. 8. True stress–strain components along the RD and TD in balanced biaxial loading–unloading–reloading tests for (a) DDQ, (b) BH340 and (c) DP490.

obtained fromeach curve, similar to the uniaxial case. It should benotedthattheseslopesdonotcorrespondtothebiaxialchord modulus.AssumingHooke’slawforisotropicelasticityintheplane stresscondition(

σ

^zz=0)

d

ε

^exx=

(

^d

σ

xx−

ν

^d

σ

yy

)

^/E ^(1a) d

ε

^eyy=

(

^d

σ

^yy⁻

ν

^d

σ

^xx

)

^/E ^(1b)

where the superscript e indicates the elastic component of the strain. When the balanced biaxial condition is imposed such that

σ

^xx=

σ

^yy^, ^the^above ^equations ^yield ^d

σ

^xx/d

ε

xx^e =d

σ

^yy/d

ε

^eyy= E/(¹−

ν

)^.^Although^the^measured^slopes,^d

σ

^xx/d

ε

^exxandd

σ

^yy/d

ε

yy^e , weredifferent(24%forDDQand11%forDP490),theiraveragewas usedasanapproximation

(

^d

σ

/d

ε

^e

)

BB =

d

σ

xx/d

ε

xx^e +d

σ

yy/d

ε

yy^e

/2 (2)

andthecorrespondingchordmoduluswasobtained,assumingthe aboveHooke’slawsandthePoissonratioof0.3

EBB=

(

¹⁻

ν ) (

^d

σ

/d

ε

^e

)

BB (3) Thebiaxialchord modulus,E_BB,isplottedasafunction ofthe effectivestrain inFig.6.Its initial value issimilar tothat ofuni- axialtensionbutitsdecreaseasafunctionofstrainismuchlower forallofthetestedmaterials.

3.4.Forward–reversesimplesheartests

Forward–reverse simple shear tests were performed using a simple shear device described by Choi (Choi, 2013). A 50mm (RD)×16mm (TD) rectangular specimen was sheared along the RD,asdescribedinthecitedpaper.Thereverseshearstress–strain curveswereobtainedatpre-strainsofapproximately2%and5%in logarithmic(Hencky)shearstrain, exceptforDDQ,forwhichonly the2%pre-straindatawere available.Themeasuredshearstress–

straindataareshowninFig.9,whichrevealstypicalreverseload- ingcharacteristics, suchastheBauschinger effect,transienthard- eningandpermanentsoftening.Similar reverseloadingbehaviors are observed forthe other two materials, except that permanent softeningisnotobservedforDDQ.

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Fig. 9. Monotonic and reverse shear stress–strain curves for (a) DDQ, (b) BH340 and (c) DP490. Note that the measured stress–strain data for DDQ and BH340 were scaled to match with the Yld20 0 0-2d prediction for parameter identiﬁcation.

4. Constitutivemodeling 4.1.Plasticanisotropy

The Yld2000-2dyieldfunctioninvolveseightparametersavail- able using two linear transformations of the stress tensor in the plane stress condition (Barlat et al., 2003). A brief review of the theory and formulation is provided in Appendix B. The anisotropyparametersweredeterminedusingtheuniaxialtension yieldstressesandr-valuesalongthethreedifferentloadingdirec- tions,i.e., RD, DD andTD, aswell asthe biaxialyield stress and plasticstrain ratio,which aregiven inTable 2.The anisotropy in boththeyieldstressandr-valuearesimultaneouslycapturedusing thismodel underthe associated ﬂow rule. The Newton–Raphson methodwas used to solve the eight nonlinear equations forpa- rameteridentiﬁcation.TheresultingparametersarelistedinTable B1(seeAppendixB)forallofthematerialsandthepredictedyield surface,forDP490only,isshowninFig.10.

4.2.Hardeningbehavior

The monotonic hardening behavior of the materials was de- scribedusingtheSwifthardeninglawwiththeparametersapprox- imatedusingtheuniaxial tensioncurves,asshowninFig.4.The stress–straindataupto10%effectivestrainwereusedforﬁttingto preciselycapturethe hardeningbehavior atsmall strains.Asdis- cussed in Section 2, the plastic deformation involved in denting rarelyexceeds10%.

Thereversehardeningbehaviorwasdescribedusingthehomo- geneous yieldfunction-based anisotropic hardening (HAH)model (Barlatetal.,2011).Thismodeldescribeshardeningbytheexpan- sion and distortion of the yield surface to capture the complex anisotropic hardeningresponse of metalsunder non-proportional loading,asexplainedindetailinAppendixC.ForDDQandBH340, because the monotonic shear stress–strain curve predictions did notmatch theexperimental data, all theshear stress–strain data werecalibratedbythesamefactortoﬁtthemonotoniccurve.The

Fig. 10. Evolution of yield surface for DP490 under biaxial tension (BT) – biaxial compression (BC), predicted using the Yld20 0 0-2d and HAH models. Yield surfaces are normalized by the initial yield stress in the RD.

HAHparameters, determined using thesecalibratedexperimental data,are giveninTable C1(see AppendixC).As showninFig.9, the Bauschinger effect, transient hardening and permanent soft- ening are all captured usingthe HAH model.The expansion and distortionoftheyieldsurfaceareillustratedinFig.10,wherethe evolutionoftheyieldsurfaceispredictedforDP490 foraloading sequenceof2%biaxialtension– 2%biaxialcompression.Asshown intheﬁgure,theyieldsurfaceuniformlyexpandsneartheloading directionbutﬂattensontheoppositeside.

4.3. Elasticbehavior

The strain-dependent elasticity model is adopted to describe thedegradationofthechordmodulusobservedinFig.6.Theuni- axial or biaxial moduli could be well ﬁtted by the exponential

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Fig. 11. Dent specimen with a dome height of 12 mm. The dent depth is less than 1 mm and hardly visible in the ﬁgure.

function(Yoshidaetal.,2002)

E=E₀−

(

^E0−E_S

)

^[¹⁻^exp

(

⁻

ξ ε

^¯

)

^] ⁽⁴⁾

where

ε

¯ îs^theêffective^strain. Ê0 denotesthe initialmodulus, E_S thesaturated modulusand

ξ

^the^rate^of^reduction.^These^param-

eterswere determinedusingeithertheuniaxialorbiaxialmoduli andaregiveninTable3.

5. Predictionofload-displacementinthestandarddenttest 5.1. Standarddenttests

The standarddent tests, discussedin Section2 andillustrated inFig.1,wereconductedatPOSCO(seeAppendixA).First,dome- stretched panels were formed with two different punch strokes of 12 and18mm, resulting inapproximately 2 and5% of biaxial strain,respectively. Then,adentloadof200Nwasapplied atthe center ofthe formed panel atan indentervelocity of 2mm/min.

AtestedsampleisshowninFig.11.Theloadanddisplacementof the indenterwere measuredduringthetest, asshowninFig.12. The dent depthwascalculated fromthe load-displacementcurve asthedifference betweentheindenter displacementattherefer- enceload(2N)duringloadingandunloading,asillustratedinFig.

12(a).ThecalculateddentdepthsaregiveninTable4.

5.2. Veriﬁcationofthesuggestedmodel

The FE model described in Section 2 wasused forthe simulations. For veriﬁcationpurpose,the simulations were performed usingtwosetsofconstitutivemodels:

• Conventionalmodel,assumingtheisotropicvonMisesyieldcri- terion, isotropic hardening and a constant elastic modulus as measured duringthe initial loading in uniaxial tension along theRD.

• Advancedmodel,assumingtheYld2000-2dyieldfunction,HAH hardeningandchord modelsbasedonthebiaxialelasticmod- ulus.

It is commonpractice to usethe uniaxial modulus to charac- terize the chord model.However, it seems moredesirable touse thebiaxialmodulusinthisexamplebecausetheelasticresponses in the dentregion are in thebiaxial stress state, asdescribedin Section2.

The predictedload-displacementresponses arecomparedwith the experimental data in Fig. 12. The simulations results show that theconventional modelsigniﬁcantly overestimatesthe panel

stiffness and underestimates the dent depth, especially for DDQ andDP490.Theadvancedmodelprovidesmuchimprovedpredic- tionscomparedtotheconventionalmodel.Thisisalsoclearfrom Table 5, which compares the measured and predicted displacements atmaximum(200N) andintermediate (100N) loads.Both the load-displacement history and the dent depth are predicted withgreateraccuracy formostcases,asshowninTable 4.These resultsclearlydemonstratetheimportanceofcomprehensivecon- stitutivemodelingindentanalysis.

Theinﬂuencesofplasticanisotropy,reversehardeningandelas- ticbehaviorswereseparatelyinvestigatedthroughadditionalsim- ulations.OnlytheresultsforDP490withasampleheightof12mm arepresentedhere,butsimilartrendswereobtainedfortheother materials.

Inﬂuence of plastic anisotropy: FE simulations were performed using the HAH and chord model, based on the biaxial modulus, butthe yield function was eitherthe isotropic von Mises or the anisotropic Yld2000-2d. The simulation results are compared in Fig.13(a),whichshowsthattheYld2000-2dmodelpredictshigher stiffnessthanthevonMises.Thisisattributedtothehigheryield stress predicted by Yld2000-2d in the balanced biaxial state, as showninFig.10.Thisindicatesthatthedescriptionofbiaxialyield stressdirectlyinﬂuencesthepredictionofpanelstiffness. Itisin- terestingthatforDP490 thebiaxialyieldstress isonly5% higher thantheuniaxialstress, butthiscausesanoticeabledifference in theload-displacementpredictions.However,thisamountofplastic anisotropydoesnotaffectthedentdepth.

Inﬂuence of reverse loading behavior: Simulations were performed using either isotropic hardening or the HAH model. The Yld2000-2dandchordmodelscompletedtheconstitutivedescrip- tion. The choice of the hardening model drastically changes the dentprediction, asshownin Fig.13(b),especially regarding dent depth. In this ﬁgure, the isotropic hardening prediction (dashed curve) can be seen more clearly in the inset. The plastic deformation caused onlyby denting is1%, implying that theaccurate description of stress–strain behavior early in the reverse loading stage is highly important. It should be mentioned that the reverse shear stress–strain curves measured in this work may not accuratelyrepresenttheactualbiaxialtension-compressionbehav- ior involved in the dent test. For instance, it has been reported thattheuniaxialtension-compressionandforward-reversesimple shearstress–straincurvesaredifferentatthebeginningofreverse loading(Choi,2013).

Inﬂuenceofelastic behavior: Simulationswere performedusing eitherthechordmodelbasedontheuniaxialmodulusorthecon- stantinitialmodulus,E₀,whilekeepingthechordmodelbasedon thebiaxial modulus asa basis forcomparisons. As expected, the elasticity model directly inﬂuences the panel stiffness, as shown inFig.13(c)and(d).Ifthechordmodelischaracterizedusingthe uniaxialmodulus,whichshowsmuchmoreseveremodulusreduc- tionthanthebiaxialmodulus(seeFig.6),thepredictedpanelstiff- nessdecreases,asshowninFig.13(c).Iftheinitialmodulusisused andthemodulusreductionisignored,thepredictedpanelstiffness increases,as shown in Fig.13(d).These results indicate that the elasticpropertiesshouldbedescribedusingamorecomprehensive modelthantheconventionalHooke’slawandtheelasticdegrada- tionmodelsbasedsimplyontheuniaxialmodulus.Notealsothat thedentdepthishardlyaffectedbythechoiceofelasticitymodel.

6. Discussions

The anisotropic (or kinematic) hardening and chord models have been widely used for springback analysis of sheet metals.

In fact, springback examples have been used frequently to ver- ify the validity of these models (Yoshida et al., 2002, Lee etal., 2012). However, model veriﬁcationby springback analysis is not

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Table 3

Parameters for the strain-dependent elasticity (chord) model.

Based on uniaxial modulus (RD) Based on biaxial modulus, E BBin Eq. (3) E 0(GPa) E S(GPa) ξ E 0(GPa) E S(GPa) ξ

DDQ 207 167 63 215 208 125

BH340 216 171 71 215 199 258

DP490 207 163 82 207 185 191

Fig. 12. Measured and predicted load-displacement responses in the standard dent tests for DDQ with sample heights (a) 12 mm and (b) 18 mm, for BH340 with (c) 12 mm and (d) 18 mm, and for DP490 with (c) 12 mm and (d) 18 mm.

Table 4

Measured and predicted dent depths, calculated using the reference load of 2 N. The numbers in parentheses indicate the prediction error:

(prediction – experiment)/(experiment) ×100%.

h = 12 mm H = 18 mm

Experiment (mm) Conventional (mm) Advanced (mm) Experiment (mm) Conventional (mm) Advanced (mm) DDQ 0 .98 0 .57 ( −42%) 0 .78 ( −20%) 0.59 0 .33 ( −44%) 0 .60 ( + 2%) BH340 0 .40 0 .34 ( −15%) 0 .40 ( + 0%) 0.26 0 .22 ( −15%) 0 .33 ( + 27%) DP490 0 .30 0 .13 ( −57%) 0 .31 ( + 3%) 0.19 0 .06 ( −68%) 0 .31 ( + 63%)

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Table 5

Measured and predicted indenter displacements at (a) the maximum load of 200 N, (b) 100 N during loading and (c) 100 N during unloading.

The numbers in parentheses indicate the prediction error: (prediction– experiment)/(experiment) ×100%.

(a) h = 12 mm h = 18 mm

Experiment (mm) Conventional (mm) Advanced (mm) Experiment (mm) Conventional (mm) Advanced (mm) DDQ 5 .36 4 .34 ( −19%) 4 .95 ( −8%) 5.80 5 .23 ( −10%) 5 .48 ( −5%) BH340 4 .66 4 .03 ( −13%) 4 .58 ( −2%) 5.31 4 .94 ( −7%) 4 .94 ( −7%) DP490 4 .77 4 .38 ( −8%) 5 .09 ( + 7%) 5.43 5 .18 ( −5%) 5 .46 ( + 0%)

(b) h = 12 mm h = 18 mm

Experiment (mm) Conventional (mm) Advanced (mm) Experiment (mm) Conventional (mm) Advanced (mm) DDQ 1 .56 0 .88 ( −44%) 1 .12 ( −28%) 1.69 1 .04 ( −39%) 1 .38 ( −18%) BH340 1 .08 0 .73 ( −32%) 0 .86 ( −20%) 1.24 0 .91 ( −27%) 1 .13 ( −9%) DP490 1 .15 0 .75 ( −34%) 1 .03 ( −10%) 1.32 0 .95 ( −28%) 1 .39 ( + 5%)

(c) h = 12 mm h = 18 mm

Experiment (mm) Conventional (mm) Advanced (mm) Experiment (mm) Conventional (mm) Advanced (mm) DDQ 3 .36 1 .70 ( −49%) 3 .31 ( −1%) 3.33 1 .56 ( −53%) 3 .69 ( + 11%) BH340 1 .94 1 .30 ( −33%) 1 .59 ( −18%) 1.89 1 .30 ( −31%) 1 .86 ( −2%) DP490 1 .96 1 .00 ( −49%) 1 .80 ( −8%) 1.96 1 .06 ( −46%) 2 .50 ( + 28%)

Fig. 13. Changes in dent prediction using different choices of (a) yield function, (b) hardening law, (c) and (d) elasticity model. The isotropic hardening (IH) prediction in (b) can be seen more clearly in the inset.

Fig. 14. (a) Predicted springback proﬁles for DP780 in U-draw/bending with a blank holding force of 20 kN and (b) predicted load-displacement in the standard dent test for DP490 (IH: isotropic hardening and E 0: initial elastic modulus). The von Mises yield criterion was used in both examples.

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straightforward andmight lead to erroneousconclusions. An ex- ampleisshowninFig.14(a),whichcomparesthespringbackpro- ﬁlesafterU-draw/bending(Numiform’93benchmark(Tayloretal., 1995)) predicted using two sets of models: (1) isotropic harden- ingandinitialelasticmodulusand(2)theHAHandchordmodels.

Althoughtheformercannot capturethereverse loadingandelas- ticdegradation behaviorsas the latter does, both generate simi- larspringbackpredictions. Thiscan be understoodsimply, know- ingthatamaterialwithhigherstrengthandlowerelasticmodulus generallyexperienceslargerspringback.Isotropichardening,which overestimates the reverse yield stress, predicts larger springback thantheHAH(orkinematichardening)model.Onthecontrary,the useoftheinitial elasticmoduluspredicts smallerspringbackthan thechord model.Therefore,when isotropic hardening andinitial modulus are used together, the errors induced by these models compensateforeach other,thusprovidingresultssimilartothose withthe combinationoftheHAHandchord models.This iswhy the conventional model, which captures neither the Bauschinger effectnorthechangeofelasticmodulus,sometimesprovidesasur- prisinglyaccurate,butcoincidental,springbackprediction.

Dent predictions also depend strongly on the choice of the hardeningandelasticitymodels. However, unlike springbackpre- dictions,the effects of these two models do not compensate for each other. As demonstrated in Fig. 13, the hardening model mainly inﬂuences the predicted dent depth, while the elasticity model does the overall stiffness. Therefore, combining isotropic hardening and initial modulus leads to results entirely different from those predicted by the HAH and chord models, as shown inFig. 14(b). An accurate prediction is possible only when com- prehensiveconstitutivemodelsare used,andthismakesthedent analysis a good example for model veriﬁcation. It is also worth mentioningthat thedentanalysisisnotassensitivetofrictionas thespringbackanalysis, which isstronglyaffected by thefriction model(Leeetal.,2015).

7.Conclusions

In this study, the quasi-static dent behaviors of DDQ, BH340 andDP490steelsheetswerepredictedusingtheconventionaland advanced constitutive models. The latter provided more accurate predictionsthantheformer.Thesensitivitystudyprovidedthefol- lowingconclusions:

• Accuratedescriptionsofplasticanisotropy,reverseloadingand elasticbehaviorsofthematerialareessentialforaccuratedent predictions.

• The plastic anisotropy andelastic behavior strongly inﬂuence thepanelstiffnessandthereverseloadingbehavioraffectsthe dentdepth.

• Itisparticularlyimportanttoconsiderthebiaxialelasticmod- ulus of thematerial, which isoften simpliﬁed to theuniaxial modulus.

Further improvementtotheconstitutive modelisexpectedby investigating the reverse hardening behavior in different stress states, such as biaxial tension–compression. In addition, the ex- tension of the elastic degradation model to incorporate elastic anisotropy,capturingboth uniaxial and biaxialmoduli, may con- tributetobetterpredictions.

Acknowledgments

This research was supported by POSCO. This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (no.2012R1A5A1048294)and(NRF-2014R1A2A1A11052889).FBap- preciatesthesupportofFEDERfundsthroughtheOperationalPro-

(a)

4 mm

90 mm 13 mm

103 mm R 1015 mm R 13 mm Punch

130.5 mm

6.5 mm R 13 mm

105 mm Upper die

Stinger beads

Indenter Holder

(b)

R 12.7 mm 124 mm

Supporting plate

Fig. A1. Tool geometry for the standard dent test for (a) pre-stretching and (b) denting. The dash-dotted line denotes the axis of axisymmetry.

gram forCompetitivenessFactors– COMPETEandNationalFunds through the Portuguese FCT – Foundation for Science and Tech- nologyundertheprojectsPTDC/EMS-TEC/2404/2012.JisikChoiand Ahmad Zadi Maad from GIFT-POSTECH are appreciated for their supports for the shear andin-plane biaxial tests. Jeong-Yeon Lee appreciatesthesupportfromKoreaUniversityGrant.

AppendixA. Toolgeometryforthestandarddenttest

Thetoolgeometryforthestandarddenttestusedinthiswork generally follows the descriptions given in the SAE J2575 standard(SAE,2004).However,thegeometryofthepunchusedinthe pre-stretching process is ambiguouslydefined in the cited docu- ment.Thetextdefinesthepunchradiusontheplateauas940mm, whereas the supplementary drawing defines it as 1015mm. The equipment used in this study was built according to the drawing, thus the FE model uses a punch radius of 1015mm. This is illustrated in Fig.A1. Notethat the corner radii ofthe lower die (forpre-stretching,seeFig.A1(a))andthatoftheholder(fordent- ing,seeFig.A1(b))donotrequirespecifications,becausethepanel doesnottouchthesecorners.

AppendixB.Non-quadraticanisotropicyieldfunctionforplane stress(Yld2000-2d)

The Yld2000-2d yield function is expressed asfollows (Barlat etal.,2003):

φ

⁼

φ

+

φ

2

1/m

=

σ

¯ ^(B1)

where

φ

⁼

X₁−X₂

^mand

φ

⁼

2X₂+X₁

^m₊

2X₁+X₂

^m (B2) Here, X₁ andX₂ are thetwo principal valuesof a linearlytrans- formed tensorX=L

σ

⁽

σ

^is^the ^Cauchy ^stress ^tensor). ^Similarly,

X₁ andX₂are deﬁned usingX=L

σ

^,

⎡

⎣

^X

xx X_yy X_xy

⎤

⎦

=

⎡

⎣

^L

11 L₁₂ 0 L₂₁ L₂₂ 0 0 0 L₆₆

⎤

⎦

⎡

⎣ σ

^xx

σ

^yy

σ

xy

⎤

⎦

(B3a)

⎡

⎣

^X

xx

X_yy X_xy

⎤

⎦

=

⎡

⎣

^L

11 L₁₂ 0 L₂₁ L₂₂ 0 0 0 L₆₆

⎤

⎦

⎡

⎣ σ

^xx

σ

^yy

σ

^xy

⎤

⎦

(B3b)

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Table B1

Yld20 0 0-2d yield function parameters. The exponent m was assumed to be 6.

α1 α2 α3 α4 α5 α6 α7 α8

DDQ 0 .9331 1 .1038 1 .1520 0 .9571 0 .9115 0 .6178 0 .9936 0 .9308 BH340 0 .8406 1 .2766 0 .8992 0 .9654 0 .9301 0 .6814 0 .9594 1 .1136 DP490 0 .8193 1 .1147 0 .9958 0 .9841 0 .9804 0 .7479 0 .9834 1 .1096

Table C1

Swift and HAH model parameters.

Swift HAH

K(MPa) ε0 n k 1 k 2 k 3 k 4 k 5

DDQ 626 .12 0 .009 0 .3 220 150 0 .3 1 .0 0 BH340 645 .18 0 .012 0 .245 250 80 0 .5 0 .9 20 DP490 1026 .82 0 .005 0 .267 110 150 0 .3 0 .9 17

The transformationoperators L andL contain three andﬁve independent anisotropy parameters, respectively, which are de- notedas

α

1−

α

8

⎡

⎢ ⎢

⎣

L₁₁ L₁₂ L₂₁ L₂₂ L₆₆

⎤

⎥ ⎥

⎦

⁼

⎡

⎢ ⎢

⎣

2/3 0 0

−1/3 0 0 0

0 0

−1/3 2/3

0 0 0 1

⎤

⎥ ⎥

⎦ α

1

α

²

α

⁷

(B4a)

⎡

⎢ ⎢

⎣

L₁₁ L₁₂ L₂₁ L₂₂ L₆₆

⎤

⎥ ⎥

⎦

⁼

1 9

⎡

⎢ ⎢

⎣

−2 2 8 −2 0

1 −4 −4 4 0

4 −4 −4 1 0

−2 8 2 −2 0

0 0 0 0 0

⎤

⎥ ⎥

⎦

⎡

⎢ ⎢

⎣ α

³

α

4

α

⁵

α

⁶

α

8

⎤

⎥ ⎥

⎦

^(B4b)

Theeightparameterscanbedeterminedusingtheuniaxialten- sionyieldstresses andr-valuesalongtheRD,DDandTD,aswell as the biaxial tension yield stress and plastic strain ratio (r_b=

ε

yy^p/

ε

xx^p). The parameters determined forDDQ, BH340 andDP490 aregiveninTableB1.Notethat,whenmequals2and

α

1through

α

8 equal1,theyieldfunctionreducestothevonMises.

AppendixC. Homogeneousyieldfunction-basedanisotropic hardening(HAH)model

TheoriginalHAHmodelexpressestheyieldfunctionasacom- binationofstable

φ

^and^ﬂuctuating

φ

h parts(Barlatetal.,2011)

(

^s

)

⁼

φ

^q⁺

φ

^qh

¹q

=

φ

^q⁺^f1^q

h^ˆ^s:s−

h^ˆ^s:s

^q₊f₂^q

h^ˆ^s:s+

h^ˆ^s:s

^q

¹q

=

σ

¯ (C1) Anyhomogeneous yieldfunctionofdegreeone,intheformof

φ

(^s)=

σ

¯^, ^can ^be ûsed ^for ^the^stable ^component. ^The ^fluctuating componentis introduced toinduce a yieldsurfacedistortion and is a function of the stress deviator s, microstructure deviatorhˆ^s, andstatevariables f₁ and f₂.Themicrostructuredeviatormemo- rizes theprevious loadinghistoryandis definedasanormalized tracelesstensoras

hˆ^s= h^s

(

⁸^/³

)

^h^s^:^h^s ^(C2)

When theplasticdeformationinitiates, h^s issetequalto s.As the deformation path changes, h^s also changes gradually toward thestressdeviatorcorrespondingtothenewloadingdirectionata ratecontrolledbyaparameterk

Ifs:hˆ^s≥0, dhˆ^s

d

ε

¯ ⁼^k

sˆ−8

3hˆ^s

(

^s^ˆ^:^h^ˆ^s

)

(C3a)

Ifs:hˆ^s<0, dhˆ^s

d

ε

¯ ⁼^k

−ˆs+8

3hˆ^s

(

^s^ˆ^:^ˆ^h^s

)

(C3b)

where sˆis equivalent to s butnormalized in the manner of Eq.

(C2).

Themicrostructuredeviatorcontrolsthedirectionofyieldsur- face distortion, and the state variables f₁ and f₂ control the amount andrate ofdistortion. These are functions ofg₁ and g₂, whichrepresenttheratioofthecurrentﬂowstress tothatofthe hypotheticalassociatedisotropichardeningmaterial

f1=

g⁻₁^q−1

1/q

and f2=

g⁻₂^q−1

1/q

(C4) Intheinitialstate,thefluctuatingcomponentdonotcontribute totheyieldfunction,andtheyieldfunctionîs^the^sameâs^the stablecomponent

φ

^. ^Therefore, ^f1= f₂=0, or equivalently, g₁= g₂=1.Ifthematerialplasticallydeforms,thestatevariablesevolve accordingtothefollowingrelationships:

Ifs:hˆ^s≥0, dg₁

d

ε

¯ ⁼^k²

k3

σ

¯

(

⁰

) σ

¯

( ε

^¯

)

⁻^g¹

(C5)

dg2

d

ε

¯ ⁼^k¹ g3−g2

g2

(C6)

dg4

d

ε

¯ ⁼^k⁵

⁽

^k⁴⁻^g⁴

⁾

^(C7)

Ifs:hˆ^s<0, dg1

d

ε

¯ ⁼^k¹ g4−g1

g1

(C8)

dg₂ d

ε

¯ ⁼^k²

k3

σ

¯

(

⁰

) σ

¯

( ε

^¯

)

⁻^g²

(C9)

dg3

d

ε

¯ ⁼^k⁵

⁽

^k⁴⁻^g³

⁾

^(C10)

Here,

σ

¯(

ε

^¯) ^is ^the ^monotonic ^hardening ^curve ^and ^k1−k₅ are thematerialparameters.Twoadditionalstatevariables,g₃andg₄, areintroducedtodescribepermanentsoftening.Ifk4=1(ork5= 1)andg₃=g₄=1,themodelpredictsnopermanentsoftening.

Themonotonichardeningcurve

σ

¯ =

σ

¯(

ε

^¯)^should^be^deﬁnedⁱⁿ themodel.Ifthematerialissubjectedonlytomonotonicloading, thestress–strainbehaviordescribedbytheHAHmodelisidentical to that by isotropichardening. In thiswork, the Swift hardening lawisemployedformonotonichardening

σ

¯

( ε

^¯

)

⁼^K

( ε

0+

ε

¯

)

ⁿ ^(C11)

Theparameters K,

ε

0 andn were obtainedby ﬁttingthe uniaxial tensiondata measured along the RD. Since thedent analy- sisdoesnot involvelarge strains,thestress–straindataupto10%

strainwereusedfortheﬁtting,asshowninFig.4.Theseparame- tersaregiveninTableC1.