flac3d之Interface

INTERFACES3-13INTERFACES3.1GeneralCommentsThereareseveralinstancesingeomechanicsinwhichitisdesirabletorepresentplanesonwhichslidingorseparationcanoccur—forexample:1.joint,faultorbeddingplanesinageologicmedium;2.aninterfacebetweenafoundationandthesoil;3.acontactplanebetweenabinorchuteandthematerialthatitcontains;4.acontactbetweentwocollidingobjects;and5.aplanar“barrier”inspace,whichrepresentsafixed,non-deformableboundaryatanarbitrarypositionandorientation.FLAC3DprovidesinterfacesthatarecharacterizedbyCoulombslidingand/ortensileandshearbonding.Interfaceshavethepropertiesoffriction,cohesion,dilation,normalandshearstiffnesses,tensileandshearbondstrength.Althoughthereisnorestrictiononthenumberofinterfacesorthecomplexityoftheirintersections,itisgenerallynotreasonabletomodelmorethanafewsimpleinterfaceswithFLAC3Dbecauseitisawkwardtospecifycomplicatedinterfacegeometry.Theprogram3DEC(Itasca1998)isspecificallydesignedtomodelmanyinteractingbodiesinthreedimensions;itshouldbeusedinsteadofFLAC3Dforthemorecomplicatedinterfaceproblems.Interfacesmayalsobeusedtojoinregionsthathavedifferentzonesizes.Ingeneral,theATTACHcommandshouldbeusedtojoingridstogether.However,insomecircumstancesitmaybemoreconvenienttouseaninterfaceforthispurpose.Inthiscase,theinterfaceispreventedfromslidingoropeningbecauseitdoesnotcorrespondtoanyphysicalentity.FLAC3DVersion2.13-2TheoryandBackground3.2FormulationFLAC3Drepresentsinterfacesascollectionsoftriangularelements(interfaceelements),eachofwhichisdefinedbythreenodes(interfacenodes).Interfaceelementscanbecreatedatanylocationinspace.Generally,interfaceelementsareattachedtoazonesurfaceface;twotriangularinterfaceelementsaredefinedforeveryquadrilateralzoneface.Interfacenodesarethencreatedautomaticallyateveryinterfaceelementvertex.Whenanothergridsurfacecomesintocontactwithaninterfaceelement,thecontactisdetectedattheinterfacenode,andischaracterizedbynormalandshearstiffnesses,andslidingproperties.Eachinterfaceelementdistributesitsareatoitsnodesinaweightedfashion.Eachinterfacenodehasanassociatedrepresentativearea.Theentireinterfaceisthusdividedintoactiveinterfacenodesrepresentingthetotalareaoftheinterface.Figure3.1illustratestherelationbetweeninterfaceelementsandinterfacenodesandtherepresentativeareaassociatedwithanindividualnode.interfaceelementinterfacenodenode’srepresentativeareaFigure3.1DistributionofrepresentativeareastointerfacenodesItisimportanttonotethatinterfacesareone-sidedinFLAC3D.(Thisdiffersfromtheformulationoftwo-sidedinterfacesintwo-dimensionalFLAC(Itasca2000).)ItmaybehelpfultothinkofFLAC3Dinterfacesas“shrink-wrap”thatisstretchedoverthedesiredsurface,causingthesurfacetobecomesensitivetointerpenetrationwithanyotherfacewithwhichitmaycomeintocontact.Thefundamentalcontactrelationisdefinedbetweentheinterfacenodeandazonesurfaceface,alsoknownasthetargetface.Thenormaldirectionoftheinterfaceforceisdeterminedbytheorientationofthetargetface.FLAC3DVersion2.1INTERFACES3-3Duringeachtimestep,theabsolutenormalpenetrationandtherelativeshearvelocityarecalculatedforeachinterfacenodeanditscontactingtargetface.Bothofthesevaluesarethenusedbytheinterfaceconstitutivemodeltocalculateanormalforceandashear-forcevector.TheconstitutivemodelisdefinedbyalinearCoulombshear-strengthcriterionthatlimitstheshearforceactingataninterfacenode,normalandshearstiffnesses,tensileandshearbondstrengths,andadilationanglethatcausesanincreaseineffectivenormalforceonthetargetfaceaftertheshear-strengthlimitisreached.Bydefault,porepressureisusedintheinterfaceeffectivestresscalculation.Thisoptioncanbeactivated/deactivatedusingthecommandINTERFACEieffective=on/off.Figure3.2illustratesthecomponentsoftheconstitutivemodelactingatinterfacenode(P).Figure3.2ComponentsofthebondedinterfaceconstitutivemodelThenormalandshearforcesthatdescribetheelasticinterfaceresponsearedeterminedatcalculationtime(t+t)usingthefollowingrelations.F(t+t)n=knunA+σnA(3.1)F(t+t)si=F(t)si+ksu(t+(1/2)t)siA+σsiAwhereF(t+t)nisthenormalforceattime(t+t)[force];F(t+t)siistheshearforcevectorattime(t+t)[force];FLAC3DVersion2.13-4TheoryandBackgroundunistheabsolutenormalpenetrationoftheinterfacenodeintothetargetface[displacement];usiistheincrementalrelativesheardisplacementvector[displacement];σnistheadditionalnormalstressaddedduetointerfacestressinitialization[force/displacement];knisthenormalstiffness[stress/displacement];ksistheshearstiffness[stress/displacement];σsiistheadditionalshearstressvectorduetointerfacestressinitialization;andAistherepresentativeareaassociatedwiththeinterfacenode[length2].Theinelasticinterfacelogicworksinthefollowingway:(1)Bondedinterface—Theinterfaceremainselasticifstressesremainbelowthebondstrengths:thereisashearbondstrengthaswellasatensilebondstrength.Thenor-malbondstrengthissetusingthetensioninterfacepropertykeyword.ThecommandINTERFACEnpropsbratio=sbrsetstheshearbondstrengthtosbrtimesthenormalbondstrength.Thedefaultvalueofsbratio(ifnotgiven)is100.0.Thebondbreaksifeithertheshearstressexceedstheshearstrength,orthetensileeffectivenormalstressexceedsthenorm