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8/4/2019 Seminar Linear Contact Analysis Notes
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LinearContactAnalysis:
Demystified
withFemapV10.1.1andNastran 7.0
AdrianJensen
MechanicalEngineer
Adrian.Jensen@PredictiveEngineering.com
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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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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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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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Regions:Solids,byElements . ,
selection.Thefollowingpagesprovidesexamplesofeach.
FaceID:
Eachfaceofanelementhasafaceid.Plateelementsusefaces1&2,tet elementsusefaces14andhexelementsusefaces16.
Thisoptionisgenerallytheleastusefulforasolidmodelbecausetheelementfacesonthesurfaceofthesoliddonotnecessarily
shareacommonfaceID.ThegraphicsbelowshowtheConnectionRegionscreatedbyselectingalloftheelementsandusingthe
FaceIDfaceselectionmethod.Thefirstgraphicshowsface1,thesecondshowsface2andsoforth.
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ConnectionRegions:Solids,byElements
NearSurface:ThisoptionisusefulifyouhavegeometrytoworkwithbutyoudontwanttolimittheelementsinyourConnectionRegion.The
graphicsbelowshowtheConnectionRegionscreatedbyselectingaclusterofelementsinthecorneroftheblockandusingthe
NearSurfacefaceselectionmethod.
NearCoordinates:
TheNearCoordinatesmethodisverysimilartoNearSurface.Insteadofspecifyingasurface,however,youchoosea
coordinatesystem,directionandposition.Thisdefinesaplanarsurface,whichisusedalongwiththetolerancetofindtheclosest
faces.
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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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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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ConnectionRegions:Plates,byElementsInsomecasesyoubeunabletoselectyourConnectionRegionsbysurfaces.Theotheroptionistoselectyourregionsby
elements.ThistechniquegivesyoumorecontroloverselectingyourConnectionRegion,butalsoopensupthepossibilityfor
error.StarttocreateyouConnectionRegionbyselectingtheElementsradiobuttonundertheDefinedBysectionofthe
ConnectionRegiondialoguebox.Youprobablydontwanttoselectelementsonebyone.UsetheMultiplebuttontospeed
thingsup.
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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 .
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Hint:TurnonelementthicknesstohelpyouchecktheConnectionRegionyouhavecreated.
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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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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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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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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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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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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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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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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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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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