Tutorials In Finite Element Analysis Using MSC-Pat

2MSC/PATRANTUTORIAL#1MODELINGABARPROBLEMI.THEPHYSICALPROBLEMInthesimplebarproblembelow,therearethreeseparatesectionsofthebar.Eachsectionhasdifferentproperties.Thefollowingpropertiesapply,AlàAluminum,StàSteel,EforSteel=200E9Pa,EforAl=70E9PaAllBarshavesquarecrosssectionandtherightandleftendsofthebararebuiltin.TheforceF=9000NewtonsThe2-dmodeloftheproblemisshownbelow.StAlAl5cm5cm10cm2cm1cm5cmFFFAlStAl3II.THINKINGABOUTTHEMECHANICSTheanalyticsolutionforstressesanddisplacementsforthisproblemisreadilyavailable.AnyMechanicsofMaterialstextwillprovideequationsforthedisplacementsandstressesthroughoutthebar.Theproblemisindeterminantbecausetherearetworeactions(oneateachwall)andonlyonerelevantequilibriumequation(0=∑xF).Therefore,itisnecessarytousetheMechanicsofmaterials(stressandordisplacement)equationsaswellastheforceequilibriumequationstosolvetheproblem.Thenormalstressduetoaxialloadingisgivenby:APxx=s,wherePistheinternalforceintheaxialdirectionandAisthecrosssectionalareaofthebar.ThedisplacementsarecomputedfromAEPLu=hereListhebar’slengthandEistheElastic(Young’s)modulus.Somebasicquestionstoconsiderbeforecreatingthecomputationalmodelare:1.Wherewillthestressesbetensileandwherewilltheybecompressive?2.Whatwillbethemagnitudeanddirectionofthereactionforces?3.Wherewillthedisplacementsbegreatest?4.Howdothedisplacementsvaryalongthelength(linear,quadraticetc.)?5.Whatwillthelocaleffectoftheconcentratedloadbeonthestresses?6.Isthemodelfullyconstrainedfromrigidbodyrotationsanddisplacements?Answeringthesequestionsqualitatively,alongwiththequantitativeanalyticalsolutionsforthestressesanddisplacements,willprovidereinforcementthatyourcomputationalmodeliscorrectlyconstructed.III.GEOMETRICANDFINITEELEMENTMODELSomegeneralnotesonPATRAN:Ageneralfiniteelementanalysiscanbebrokendowninto3principletasks;preprocessing,analysisandpostprocessing.Thepreprocessingtaskincludesbuildingthegeometricmodel,buildingthefiniteelementmodel,givingtheseelementsthecorrectproperties,settingtheboundaryconditionsandloadingconditionsandfinally,assemblingtheseelementsintoaconnectedstructureforanalysis.Theanalysisstagesimplysolvesfortheunknowndegreesoffreedom,aswellasreactionsandstresses.Inthepostprocessingstage,theresultsareevaluatedanddisplayed.Theaccuracyoftheseresultsispostulatedduringthispostprocessingtask.ThePatranandNastransoftwaretogetherperformall3oftheprincipletasksofafiniteelementanalysis.ThepreandpostprocessorsareuniquetoPATRANitself.However,thispackageallowstheusertodotheactualsolutionanalysisonavarietyofdifferentpackages.AtmanysitesyouhavetheoptionofusingtheMSC/Nastranpackage,whichisprobablythemostwidelyusedsolverinindustry.Manyoftheotherpackagescommonlyusedinindustrialsettings(ABAQUAS,ANSYS,MARC)arealsocompatiblewithPATRAN.4IV.FINITEELEMENTTHEORYTheexactdetailsoftheformulationoftherodelementsinMSC/NastranisgivenintheMSC/Nastranmanualsandissomewhatlengthy.However,thebasicformulationofanisoparametric2noderodelementisnotdifficultandwillprovideuswithsufficientbackgroundinformationtobegintounderstandtheconvergenceandotheraccuracystudies.Thisbasicformcanbefoundinanystandardtextoffiniteelementanalysis.ForExampleseeFiniteElementModelingforStressAnalysis,byR.D.Cook,JohnWiley&Sons,1995.V.STEPBYSTEPINSTRUCTIONSFORMODELINGTHEBARPROBLEMUSINGMSC/PATRANUnlessyouhaveusedthePATRANsoftwarenumeroustimesinthepast,thestepsshownbelowshouldbefollowedexactly.However,inordertoprepareyoutodoindependentfiniteelementworkusingPATRANinthefuture,youareencouragedtogobackafteryouhavecompletedtheassignmentandinvestigatemodelingoptionsusingdifferentPATRANselections.Also,Iencourageyoutotakenotesasyougothroughthisexerciseinordertoprepareforthetimewhenyouwillbeaskedbuildacertaingeometricstructureorapplyacertaintypeofboundaryconditionwithoutbeinggiventhespecificstepsforcarryingoutthistask.TheMSC/PatranprogramismenudrivenmuchinthesamewaythatmostWindowsprogramsaredriven.Selectingacategoryfromamenumayresultinapulldownsetofoptionsorinasubordinatemenu.Selectionsinmenusmaybeintheformofbuttonstoturnonoroff,orintheformofboxeswhichrequiretext.Textenteredintoboxesmaybechangedbypositioningthecursoratthepointoftextinsertionandeithertypingthenewtextorerasingtheincorrecttext.AstandardfiniteelementanalysisnormallyproceedsacrossthetopmenusstartingwithGeometryandendingwithResults.Selectingoneofthesetopmenusresultsinasetofmenuswhichallowyoutocompletethattaskintheanalysisprocess.Generally,itisbesttoattempttoproceedfromthetopofthesemenustowardthebottom,answeringquestionsasyougo.PreliminariesforusingMSCPatranandNastrannormallyinclude:1)Logintothemachine.2)Changetothedirectorythatyouwishtocontainyourresults.3)TostarttheprogramMSC/Patran,clickonStart/Programs/MSC(common)andchooseMSCPatran90.Intheinstructionsbelow,thefollowingabbreviationsandtermswillbeused:5TM=TopMenu.ThisreferstothehorizontalmenuoptionsresidingatthetopofthescreenafterPATRANhasbeeninitiated.RM=RightMenu.Thisreferstothemenusthatpopupafteranoptionhasbeenchosenfromthetopmenu.ThesemenusresideonthefarrightsideofthePATRANdesktop.SM=SubordinateMenu.Thisrefer