Seminar Linear Contact Analysis Notes

8/4/2019 Seminar Linear Contact Analysis Notes

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LinearContactAnalysis:

Demystified

withFemapV10.1.1andNastran 7.0

AdrianJensen

MechanicalEngineer

[email protected]

2010 AllRightsReserved


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3 /??LinearContactAnalysis:Demystified

WORKINPROGRESS

Introduction

Frustrated?

Usin linear contact can be frustratin . In fact it is robabl one of the most fre uent sub ects of calls for tech su ort staff. But

whenyoubreakitdownintoitsmostsimplecomponents,itdoesnthavetobefrustrating.Itcanbeaquick,easyanduseful

techniqueinyourFEAtoolbox.Itallboilsdowntoelementfacesintersectingotherelementfaces.Ifyoucorrectlyspecifythecontactregionsandtheinitialcontactconditionsyouwillhavesuccesswithlinearcontactanalysis.

Whatislinearcontact?

Linearcontactiscalledsuchbecausethereisnogeometricormaterialnonlinearityintheanalysis.Thatis,thestiffnessmatrixis

notupdatedandthematerialbehavesinalinearelasticfashion.Takeforexampleasmallsteelpunchpressingagainstasurface.

Letsassumeanominalelasticmodulusof29,000,000andayieldstrengthof60,000psi.Whenthesteelreachestheyieldstress,

ithasastrainof0.2%(60,000psi/29,000,000psi).Ifthatpunchwasoriginally1long,ithascompressed0.002.Onecould

safelyassumethatthebehaviorofthesystemhaschangedverylittle.Whatdoesthisallmean?Forproblemswithstiff

. . ,

analysismechanism.

Howdoesitwork?

Thiswhitepaperwillcoverawiderangeoflinearcontactproblemsfromthesimpletaskofsettingupcontactregionstothe more

dauntin char eofmanuall ad ustin the enalt factors.Theke toan linearcontact roblemittostartsim le.Before outr

torunthat500,000nodemodelwith20contactregions,buildsmall,manageablemodelsofthedifferentcontactscenariosforeasydebugging.



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WORKINPROGRESS

ConnectionRegions

Whenyoubreakalinearcontactproblemdownintoitsmostsimplecomponent,youhaveaConnectionRegion.Itmightseemlikeanelementarystepintheanalysisbutifitisnotdonecorrectly,therewillbeproblemsthatcantbefixedbytoying withthe

moreadvancedcomponentsoflinearcontact.

ConnectionRegions:Solids,bySurfacesTheeasiestlinearcontactanalysistosetupisforasimplesolidmodelusingsurfaces.WhencreatingyourConnectionRegionby

surfacesyoullnoticeanoptiontoselectthepositiveornegative.Forsolidmodels,thisoptionisnotnecessary.Ifyoucheckthe

normalsofthesurfacesofthesolidbody,theyreallfacingoutward.Theoptiontomodifysurfacenormalsonlyworksforsheet

solids.Additionally,evenifyoudoselectthenegativefaceofasurface,yourresultswillchangenegligibly.



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WORKINPROGRESS

ConnectionRegions:Solids,byElementsInsomecasesyoubeunabletoselectyourConnectionRegionsbysurfaces.Theotheroptionistoselectyourregionsby

elements.ThistechniquegivesyoumorecontroloverselectingyourConnectionRegion,butalsoopensupthepossibilityfor

error.StarttocreateyouConnectionRegionbyselectingtheElementsradiobuttonundertheDefinedBysectionofthe

ConnectionRegiondialoguebox.

ssum ngyou on twanttose ecte ementsone yone,c c t e u t p e utton. erearetwostagestot ereg onse ect on

byelements:theelementselectionandtheelementfaceselection.Theelementselectionissimple;graballelementsthathave

facesyouwanttoincludeintheConnectionRegion.



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WORKINPROGRESS

Regions:Solids,byElements . ,

selection.Thefollowingpagesprovidesexamplesofeach.

FaceID:

Eachfaceofanelementhasafaceid.Plateelementsusefaces1&2,tet elementsusefaces14andhexelementsusefaces16.

Thisoptionisgenerallytheleastusefulforasolidmodelbecausetheelementfacesonthesurfaceofthesoliddonotnecessarily

shareacommonfaceID.ThegraphicsbelowshowtheConnectionRegionscreatedbyselectingalloftheelementsandusingthe

FaceIDfaceselectionmethod.Thefirstgraphicshowsface1,thesecondshowsface2andsoforth.



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WORKINPROGRESS

ConnectionRegions:Solids,byElements

NearSurface:ThisoptionisusefulifyouhavegeometrytoworkwithbutyoudontwanttolimittheelementsinyourConnectionRegion.The

graphicsbelowshowtheConnectionRegionscreatedbyselectingaclusterofelementsinthecorneroftheblockandusingthe

NearSurfacefaceselectionmethod.

NearCoordinates:

TheNearCoordinatesmethodisverysimilartoNearSurface.Insteadofspecifyingasurface,however,youchoosea

coordinatesystem,directionandposition.Thisdefinesaplanarsurface,whichisusedalongwiththetolerancetofindtheclosest

faces.



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WORKINPROGRESS

ConnectionRegions:Solids,byElements

AdjacentFaces:Apowerfulmethodforchoosingfaces,especiallyforcomplexsolidandplanarelementmodels.Youchoosejustoneinitialface

(andtheassociatedelementID).Thiscanbedoneveryeasilybygraphicallyselectingtheface.Youthenspecifyatolerance angle.

FEMAPwillsearchallselectedelementsforfacesthatareconnectedtothefacethatyouchoseandthatarewithinthespecified

tolerancefrombeingcoplanarwithanalreadyselectedface.

ModelFreeFaces

TheModelFreeFaces methodsimplyincludesevery"freeelementface"inyourelementselection.Agoodtechniqueifyoucan

rabexactl theelements ouwantwithintheelementselectiondialo uebox.



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WORKINPROGRESS

ConnectionRegions:Plates,bySurfacesPlatemodelsaregenerallyrequiremorecarewhenselectingyour

regions.

When

selecting

you

plate

model

Connection

Regions

by

surfaces,youneedtopayspecialattentiontothesurfacenormals

andelementnormals.Sincebothfacesofaplateelementare

linkedtothesamesurface,youwillneedtoturnthePositive

e opt ononan o w ereappropr ate.

Thesurfacenormalsareshownintheuppermostgraphiconthe

right.ThemiddleimageshowsthefirstConnectionRegion.The

surfacewasselectedwiththePositiveSideoptionunchecked.

.

surfacewasselectedwiththePositiveSideoptionchecked.

NotethattheConnectionRegiondirectionsfaceeachotherina

properlyconfiguredcontactanalysis.Youcancheckthisbyusing

Window>ShowEntities.SelectRegionforyourentitytypeand

checktheShowNormalsstyleoption.



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WORKINPROGRESS

ConnectionRegions:Plates,byElementsInsomecasesyoubeunabletoselectyourConnectionRegionsbysurfaces.Theotheroptionistoselectyourregionsby

elements.ThistechniquegivesyoumorecontroloverselectingyourConnectionRegion,butalsoopensupthepossibilityfor

error.StarttocreateyouConnectionRegionbyselectingtheElementsradiobuttonundertheDefinedBysectionofthe

ConnectionRegiondialoguebox.Youprobablydontwanttoselectelementsonebyone.UsetheMultiplebuttontospeed

thingsup.



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WORKINPROGRESS

Regions:Plates,byElements , .

elements,onlytheFaceIDandAdjacentFacesoptionsshouldbeused.

FaceID:

Asmentionedearlier,plateelementsusefaces1&2forConnectionRegions.Beawarethatafoursidedplateelementhassix

facesandFemapwillallowyoutoselectanyofthesefacesbutonlyfaces1&2aresupportedforcontact.TheFaceIDmethod

allowsyoutoenterthefaceIDnumericallyorselectafacefromthemodel.Thismodelismoreeffectivewithplatemodels

because

the

faces

of

the

elements

are

more

commonly

aligned.

AdjacentFaces:

ThismethodisusefulforcreatingConnectionRegionswhenelementsdonothaveconsistentorientationorwhenthemodelhas

comp exgeometry see acent aces nt eso e ementsect on .


Hint:TurnonelementthicknesstohelpyouchecktheConnectionRegionyouhavecreated.


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WORKINPROGRESS

Connectors

Once

you

have

properly

set

up

your

Connection

Regions,

you

must

establish

contact

pairs,

called

Connectors

in

the

Femap

interface.AConnectorspecifiesacontactmechanisminthemodelwithaSourceConnectionRegion,aTargetConnection

RegionandaConnectionProperty.

SettingupaConnectorisquitesimple,selectaConnectionPropertyfromthedropdownlistorusethequickbuttontotherighttocreateanewone.LeavethefieldsoftheConnectionPropertyblankfornow.ConnectionPropertieswillbediscussedingreater

detaillaterinthiswhitepaper.Next,selectMaster(Target)andSlave(Source)ConnectionRegionsfromthedropdownlistsor

selectbyclickingonthedesiredConnectionRegionsinthemodel.TheConnectionRegionswillhighlightasyouplacethecursor

overthemtomaketheselectionprocesseasier(seethegraphicabove).



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WORKINPROGRESS

Connectors:SourceRegionsandTargetRegions,fromtheNXNastran7UsersGuide

It's

important

to

understand

how

contact

element

are

created

when

selecting

which

region

will

be

the

source

and

which

the

target,sincethetwocanbeinterchangeable.Thesolverprojectsvectornormalsfromthesourceregiontothetargetregion. It

thencreatescontactelementswhenthesenormalsintersectelementsinthetargetregionandarewithinthesearchdistance

criteriaforthecontactpair.Thismeansthatwhenthetworegionsofapairdonothavecorrespondingonetooneelements,the

numberofcontactelementsthatthesolvercreatescanchangedependingonwhichregionitprojectstheelementsfromand

whichregionitprojectsthemto.

Ingeneral,ofthetwocontactregionsyouuseforthepair,choosetheonewiththefinermeshforthesourceregion.Whenthe

sourceandtargetregionshavedifferentmeshdensities,moreelementsonthesourceregionwillmeanthatmorecontact

elementsarecreated,whichwillproduceamoreaccuratesolution.

, .

region(B)hasfourelements.

Whencreatingthecontactelementsbetweentheseregions,thesoftwareprojectscontactelementsfromthesingleelementon

.

ontheleft).

However,ifyouweretousetheregionwithfourelementsasthesource(C)andtheregionwithoneelementasthetarget(D),the

solutionwillcreate4contactelements(seethegraphicontheright).



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WORKINPROGRESS

AfteryouhavecreatedandcheckedyourConnectionRegionsand

ConnectorsyoucansetuptheConnectionProperty.

AlthoughaConnectionPropertyisrequiredtocreateaConnector,

itisrecommendedthatfull confi urin theConnectionPro ert

shouldbesavedforlast.Thisisbecauseitiseasytoget

overwhelmedwiththeoptionsprovidedonaConnectionProperty

card.Additionally,unlesstheConnectionRegionsandConnectors

are

configured

correctly,

none

of

the

options

of

the

Connection

Propertywillallowforaaccuratecontactanalysis.

TheConnectionPropertycardisdividedintothreesections:

ContactPair(BCTSET),ContactProperty(BCTPARAM)and

CommonContactParameters(BCTPARAM).CommonGlue

Parameters(BGPARAM)andGluedContactProperty(BGSET)will

.



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WORKINPROGRESS

ConnectionProperties:ContactPair(BCTSET)

The

options

in

this

portion

of

the

dialog

box

can

be

set

individually

for

each

Connector

(contact

pair)

that

is

created

in

the

model.

TheseoptionswillbewrittenouttotheBCTSETentryforeachindividualcontactpair.Eachcontactpairwillbedesignatedinthe

graphicswindowwithasinglelinegoingfromoneConnectionRegiontoanotherandthislineisacontactelement.

Friction EntersavalueintheFRICi fieldontheBCTSETentry.DesignatestheStaticCoefficientoffrictionforcontactpair"i".

Note: Ingeneral,ifdifferentfrictionvaluesareNOTneededthenthecontactpairsshouldallreferencethesamecontactproperty.

MinContactSearchDist EntersavalueintheMINDi fieldontheBCTSETentry.DesignatestheMinimumsearchdistancefor

contactpair"i".Note:Theminimumdistancecanbenegativeandusedforaninterferencefitconditionmodeledasoverlappingsurfaces.

.

contactpair"i".

Note: Themaxdistancemustbedefinedforallcontactproblems.ThisisthedistancethatNXNastranwillsearchforcontactfrom

theelementnormal.



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WORKINPROGRESS

ConnectionProperties:ContactProperty(BCTPARM)

These

options

need

to

only

be

defined

once

for

a

contact

analysis,

regardless

of

how

many

Connectors (contact

pairs)

are

defined

inthemodel.EachConnectorhasanIDassignedtoitandcanreferenceadifferentConnectionProperty.FEMAPwillusetheConnectionPropertyreferencedbytheConnectorwiththelowestIDtodefinetheBCTPARMentryfortheentiremodel.Forexample,ifamodelhas2Connectors (contactpairs)withIDnumbers101and102,theConnectionPropertyvaluesdefinedinthepropertyassociatedwithConnectorID101wouldbeusedfortheanalysis.

ax orce era ons rea es e e on e en ry. es gna es emax mumnum ero era ons ora orce

(inner)loop(Default=10).

MaxStatusIterations CreatestheMAXSfieldontheBCTPARMentry.Designatesthemaximumnumberofiterationsforastatus

(outer)loop(Default=20).

ForceConvergenceTol CreatestheCTOLfieldontheBCTPARMentry.DesignatestheContactForceconvergencetolerance

(Default=0.01).

ConvergenceCriteriaandNumForConvergence Together,thesetwovaluescreatetheNCHGfieldontheBCTPARMentry.The

value

and

type

of

number

(real

or

integer)

entered

for

NumFor

Convergence

depends

on

the

option

set

for

ConvergenceCriteria:

0..NumberofChanges Whenthisoptionisset,NumforConvergence mustbeaninteger>1.Thisvaluedefinestheallowablenumberofcontactchanges.

1..PercentageofActive Whenthisoptionisset,NumforConvergence mustbeenteredasapercentage(between1and99,whichwillappearas0.01to0.99intheNXNastraninputfile).Thesolvertreatsthisvalueasapercentageofthenumberofactive

contactelementsineachouterloopofthecontactalgorithm.Thenumberofactivecontactelementsisevaluatedateachouter

loopiteration.



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WORKINPROGRESS

ConnectionProperties:ContactProperty(BCTPARM)ShellZOffset AllowsyoutochooseiftheZOffsetonshellelementsshouldbeincludedindetermining"GluedContact."Creates

ZOFFSETfieldinBGPARMentryandgivesyou2choicesforthecorrespondingValuefield:

0..IncludeZOffset(Default) Zoffsetofshellsisincludedfordetermininggluedsurfaces.

1..NonotIncludeZOffset ZoffsetofshellsisNOTincludedfordetermininggluedsurfaces.Note:

TheDefaults buttonwillautomaticallyfillinthedialogboxwiththedefaultvaluessuggestedbyNXNastran.Itmaybehelpfulto

tryandruntheanalysiswiththedefaultsandthenrunitagainifanymodificationsareneededtocreatemoreaccurateresults or

ac eveconvergence.

AdaptiveStiffness Thisisaflagtoindicatewhetheradaptivestiffnessisactivated.CreatesPENADAPTfieldonBCTPARMentry

(Default=0).Whennotchecked,itplacesa"0"(NoAdaptiveadjustment)intothePENADAPTfield,whencheckedplacesa"1"

(Adaptivity adjustscontactstiffness)intothePENADAPTfield.

PenetrationFactor CreatesthePENETFACfieldontheBCTPARMentry(Default=1.0E4).Designatesthepenetrationfactorfor

adaptivepenaltystiffnessadjustment.OnlyusedwhenAdaptiveStiffnessis"on"andshouldusuallyonlybesettoalowervalue

toreducetheamountofpenetrationallowedtooccurinananalysis.



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WORKINPROGRESS

ConnectionProperties:CommonContact(BCTPARM)andGlue(BGPARM)ParametersThissectioncontainsoptionsforGluedcontactandseveraloptionsavailableforbothGluedContactandLinearContact.

Eval Order Determinesthenumberof"LinearContactorGluePoints"forasingleelementonthesourceregion.CreatesINTORD

fieldinBCTPARMorBGPARMentryandgivesyou4choices:

0..Default DoesNOTwritetheINTORDfieldorcorrespondingvaluefieldtotheBCTPARMorBGPARMentry.Simplyusesthe

defaultvaluefor"Linear"or"Glued"contactbuiltintotheNXNastransolver.

.. ow owes or ero po n sonsourcereg on.

2..Medium Mediumorderofpointsonsourceregion.Thisisthedefault.

3..High Highestorderofpointsonsourceregion.Thehighertheintegrationorder,thelongerthesolvewilltake.

" " " " .

BCTPARMorBGPARMentryandgivesyou2choicesforthecorrespondingValuefield:

0..DoNotRefine Doesnotrefinethe"LinearContact/Glue"sourceregionbasedontargetsurfacedefinition.

1..RefineSourcetoTarget(Default) Refinesthe"LinearContact/Glue"sourceregionbasedontargetsurfacedefinition.

2..NXN

7.0

Method

Refines

the

"Linear

Contact/Glue"

source

region

using

the

NX

Nastran

7.0

method.



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WORKINPROGRESS

ConnectionProperties:CommonContact(BCTPARM)andGlue(BGPARM)ParametersPenaltyFactorUnits CreatesthePENTYPfieldontheBCTPARMorBGPARMentry.Specifieshowcontactelementstiffnessis

calculated.

WhensettingpenaltyfactorsforlinearcontactorgluedcontactwhenGlueType=1..Spring(GLUETYPE=1)

1..1/Length(Default) NormalPenaltyFactor(PENN)andTangentialPenaltyFactor(PENT)areenteredinunitsof1/Length.

2..Force/(LengthxArea) NormalPenaltyFactor(PENN)andTangentialPenaltyFactor(PENT)areenteredinunitsof

Force/(LengthxArea).

ense ngpena y ac ors org ue con ac w en ue ype= .. e =

1..ScaleFactor GlueFactor(PENGLUE)isaunitless value.

2..F/L^2 GlueFactor(PENGLUE)hastheunitsofF/Lengthsquared.

AutoPenaltyFactor Thisisaflagtoindicatewhethernormalandtangentialpenaltyfactorswillbeautomaticallycalculated.

, . .

NormalFactor CreatesthePENNfieldontheBCTPARMorBGPARMentry.Designatesthepenaltyfactorforthenormaldirection

(Default=10.0forBCTPARM,100forBGPARM).

TangentialFactor CreatesthePENTfieldontheBCTPARMorBGPARMentry.Designatesthepenaltyfactorforthetangential

direction(Default=1.0forBCTPARM,100forBGPARM).



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WORKINPROGRESS

ConnectionProperties

ConnectionProperties:Friction

ConnectionProperties:ContactSearchDistance

ConnectionProperties:Refinement

ConnectionProperties:Refinement,EvaluationOrder

ConnectionProperties:Refinement,RefineSource

ConnectionProperties:PenaltyFactor

ConnectionProperties:PenaltyFactor,Units

ConnectionProperties:PenaltyFactor,Automaticvs.Manual


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Seminar Linear Contact Analysis Notes

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