Integration of AutoFEM Analysis With ShipConstructor _ AutoFEM

9/4/2015 IntegrationofAutoFEMAnalysiswithShipConstructor|AutoFEM

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IntegrationofAutoFEMAnalysiswithShipConstructorBasedonAutoCAD,ShipConstructoristheleadingsoftwareproductforshipdesignandshipbuilding,adoptedworldwide.ShipConstructorprovidesthetoolsneededtodesign,modelandprepareproductiondocumentationforanytypeofvessel,beitaship,asubmarineoranoffshoreplatform.Inthecourseofdesigningavesseloranoffshorestructure,itisnecessarytoverifyitsstrengthandstiffnesscharacteristics,preferablyusingFiniteElementAnalysis.ThankstodirectintegrationofAutoFEMAnalysisandAutoCAD,ShipConstructoruserscannowuseapplyAutoFEMtotransparentlyrunFEAanalysisofShipConstructormodels.AsofAutoFEMAnalysisv2.2,theintegrationwithShipConstructoriscomplete.TheShipConstructor2014stockcataloguesupportsthemechanicalpropertiesofmaterialsneededforFEA,andAutoFEMreadsthefullmodeltopologydirectly,including:NamesofpartsNamesofAutoCADlayers(forpartsinthecurrentdocument)Dataaboutthematerial(name,grade,etc.).Iftheuserhasnotspecifiedthee,theywillbetakenfromthenativeAutoFEMAnalysismaterialslibrary,usinganamematchingtechnique.

SettingsofintegrationwithShipConstructor

ShipConstructorpartsandallrelatedinformationareacquiredbyAutoFEMAnalysis,andifneeded,ShipConstructormaterialsarenamematchedwithmaterialsintheAutoFEMlibrary.



CalculationexampleofaShipConstructorAssemblyDownloadtutorialvideoaboutAutoFEM&ShipConstructorintegration

DetailedarticleaboutjointworkofAutoFEM&Shipconstructor(pdf)AutoFEMAnalysis&ShipConstructorSoftware:WorkingTogether

AutoFEMAnalysis&ShipConstructorSoftware:WorkingTogetherPerformingthefiniteelementanalysisofShipConstructorstructuralmodels

ContentsShipConstructor(SSI,Canada)AutoFEMAnalysis(AutoFEMSoftware,UnitedKingdom)JointuseofAutoFEMAnalysisandShipConstructorsoftwareShipConstructor&AutoFEM:Howdoesitwork?InputdataPreparationofthecalculationmodelSettingsofintegrationwithShipConstructor1.DirectreadingofdatafromtheShipConstructormaterialslibrary2.MappingShipConstructormaterialstoAutoFEMAnalysismaterialsStagesoffiniteelementanalysisCreatingasetofobjectsforfiniteelementstudyCreatingthedesiredstudyGeneratingthefiniteelementmeshAssigningmaterialsDefiningconstraints/restraintsApplyingloadsSolvingthestudyStudyingtheresultsReviewofthecolordiagramsAnimationoftheresultonscreenCreatinganimationvideosCreatingReportsConclusionSupplement.HelpfultipsandadviceCreatingmaterialsintheAutoFEMAnalysismateriallibraryStepstocreateanewmaterial:Validatingofthemodelandfixingintersections3Dmodeldiagnostics(study)MultiplevolumebodiesNoncontactinggroupsofbodiesIntersectingbodiesContactbetweenbodiesCheckingthePLCrepresentationCreatingfiniteelementmeshesDecreasingthefiniteelementsizeIncreasingthecurvatureforcylindrical3DmodelsFlagThinwalledstructureMeshgenerationerrorsNodetolerancefor3DsurfacemodelsUseoftheoption"Stabilizetheunfixedmodel"Whichisthebetterequationsolvingmethod?WorkingwithShipConstructorPipes



Shipbuilding,engineeringandcalculationsShipbuildingrelatestothefieldofindustrialactivity,inwhich,inadditiontodevelopingtheproject,takingintoaccountallthedetails,designcalculationsforstrength,stability,anddynamiceffectsplaysanimportantrole.Thetechniquesoffiniteelementanalysis(FEM)hasbecometheprincipaltoolofmodernengineerswhousecomputeraideddesign(CAD).FEAhasbeenactivelydevelopedduringthepast50yearsanditsdevelopmenthasgainednewimpetuswiththeemergenceandspreadingofavailablecomputeraideddesignsystems(CADsystems).

ShipConstructor(SSI,Canada)TheShipConstructorsuiteofproductshasbeendeliberatelydesignedtomatchstandardindustryprocesses,terminologyandconceptsofmodernshipbuilding.Withtechnologydesignedtoscaleuptothelargestshipbuildingprojects,associativeandparametricshipspecific3Dmodelingcapabilities,andindustryspecificproductionoutputthatpromotesmodularconstruction,ShipConstructortrulythinksshipbuilding.AutoCADprovidestheunderlyingCADengineanddraftingtoolsforShipConstructorproducts.MoreinformationabouttheShipConstructorsoftwarecanbefoundatwww.SSIcorporate.com.

AutoFEMAnalysis(AutoFEMSoftware,UnitedKingdom)AutoFEMAnalysisisacomprehensivesystemoffiniteelementanalysis,which,likeShipConstructor,usesAutoCADasitsgeometrymodelingkernel.ThesystemisfocusedonthewidercommunityofusersofAutoCADandallowscarryingoutthemostrequiredcalculationswithoutleavingAutoCAD,therebyenjoyingallthebenefitsofanintegratedsolution.MoreinformationonAutoFEMAnalysissoftwarecanbefoundatwww.autofemsoft.com.

JointuseofAutoFEMAnalysisandShipConstructorsoftwareTakingadvantageofthefactthatbothsoftwareprogramsarebasedonthesamesystem(AutoCAD),ShipConstructorusershaveanaturalabilitytouseAutoFEMAnalysisforstrengthanalysisandotherFEAtasksinshipbuilding.TobettermeettheneedsofshipbuildersworkingwithShipConstructor,AutoFEMAnalysisaddedadditionalcapabilitiesinversion2.2toprocessdatastructuresdevelopedandmodelledinShipConstructor,forexamplethedirectacquisitionparts,theirnamesandmaterials,etc.inthefiniteelementstudy.

ShipConstructor&AutoFEM:Howdoesitwork?

InputdataThe3DShipConstructorstructuremodelandthecorrespondingmetadata(propertiesandattributes,etc.)constitutethefundamentalinputdatarequiredbyAutoFEMAnalysis.Additionally,AutoFEMAnalysis(v2.2andlater)hastheabilitytoapplymaterialpropertiesfromitslibrarytoShipConstructorobjects,basedonanamematchingtechnique.And,tocompletethepicture,theusercanaddelementsandadditionalmaterialswithintheAutoFEMAnalysisenvironment.Thus,inordertoproceedwiththefiniteelementstudy,usermust:haveasolidorsurface3Dstructuralmodel,createdinShipConstructor(obligatory)assignthematerialstothepartsofthestructureinShipConstructororinAutoFEM.

PreparationofthecalculationmodelGenerally,anyShipConstructormodeldrawing,i.e.anAutoCADfilethatcontainsShipConstructorparts(PlanarGroups,Assemblies,Arrangements,etc.),canbeusedasthebaseFEAmodel.TheShipConstructoruserhimselfcreatesthe3DmodelswhichwillthenbeanalyzedbyAutoFEM.Thereareseveralwaystopreparea3DmodelforFEAbyAutoFEM,suchas:UsingtheProductHierarchyDrawingProductHierarchydrawingsincludeShipConstructorpartsandcanthereforebeusedforFEAbyAutoFEM.However,theyalsocontaindatanotusedforFEA,anditmaythereforebeconvenienttoenableordisablecertainportionofit.

ProductHierarchyDrawingcanbededicatedtogrouppartsspecificallyforFEAUsingtheUnitAUnitdrawingmayalsobeusedforFEAmodelling.



AFEAstudyaddressingaUnit

UseofAssemblyProductiondrawing(Assemblies,etc.)maybeusedforfiniteelementmodelling,too.

FEAcarriedoutdirectlyonaproductionassemblyUsingtheModelLinkcommandTheusermaycombineseveraldrawingsandevenUnitstocreatea3DFEAmodel,usingShipConstructor'sModelLink.

CreatingastudyencompassingseveralUnits

"Exporting"the3Dmodelto"external"dwgfilesFinally,onecanevenexportaShipConstructormodeltoagenericdwgformat,andruntheFEAoutsidetheShipCostructorenvironment.



Finiteelementmodellingofan"exported"ShipConstructor3DmodelThus,theShipConstructorusercanchoosethemostappropriatestrategyforhisFEApurposesfromavarietyofoptions.But,inallcases,withouteverleavingtheAutoCAD3Denvironment.

SettingsofintegrationwithShipConstructorAutoFEMAnalysisisuniqueinitsabilitytointegratewiththeShipConstructorenvironmentand,mostespecially,projectdatabase.ThecorrespondingfunctionalitiesareofferedautomaticallytotheuserwhenopeningaShipConstructordrawing.

AutoFEMShipConstructorintegrationsettingsarefoundinthecommanddialog"SetofobjectsforFEA"TheIntegrationofAutoFEMAnalysiswithShipConstructorsoftwareincludesthefollowingfeatures:AutoFEMacquirestheShipConstructormodelunits,andwillusethemforFEAAutoFEMacquirestheShipConstructorpartnames,andusesthemtoidentifythecorrespondingFEobjectsinFEAstudyAutoFEMacquiresthemechanicalpropertiesofShipConstructorpartsfromtheprojectdatabase,foruseinFEA(firstintegrationalternative)AutoFEMacquiresthematerialnameofShipConstructorpartsfromtheprojectdatabaseandusesthemechanicalpropertiesofthematerialbearingthesamenamefoundintheAutoFEMlibrary(secondintegrationalternative).Letusnowmorecloselyconsiderthetwointegrationalternatives.

1.DirectreadingofdatafromtheShipConstructormaterialslibraryusingthe"takepropertiesfromSCbase"mode.Firstly,theuserwillspecifythemechanicalpropertiesofthedesiredmaterialsintheShipConstructorprojectdatabase.Forstructuralstrengthandbucklinganalysis,theminimumrequiredsetofpropertiesincludesthemodulusofelasticity(Young'smodulus,forexampleinN/mm2)andPoisson'sratio(dimensionless).ForFrequencyanalysis(calculationofresonancefrequencies),materialdensityisrequiredaswell.



StructuralmaterialpropertiesshownintheShipConstructordatabaseTofacilitatethereviewandprocessingofresultsinvolvingsafetyfactorsinstaticstructuralFEA,itisalsorecommendedtodefine,foreachmaterial:tensilestrength,compressivestrength.yieldstrength(separatelyfortensionandcompression,asmaybethecase),

When"takepropertiesfromSCbase"isselected,thesystemwillreadthematerialpropertiesfromtheShipConstructorprojectdatabaseandwillusethemfortheFEAcalculation.Thisapproachisverypowerfulfordesignandscantlingsizingwork,asmaterialpropertiesaresetonlyonce,intheShipConstuctorprojectdatabase(andareavailabletootherprojectdatabases).

2.MappingShipConstructormaterialstoAutoFEMAnalysismaterialsusingthe"useAutoFEMmaterialsbyname"mode.TheuserwillensurethatthedesiredmaterialsintheAutoFEMandShipConstructorlibrariesbearthesamename.ThematerialpropertiesarethendefinedonlyintheAutoFEMlibrary,andthesewillbeusedforFEAcalculations.

CreatingamaterialwhosenameisthesameintheAutoFEMandShipConstructormateriallibraries

AssigningmaterialtoapartinShipConstructorThisapproachisconvenientbecauseitallowstousematerialswithcomplexproperties(e.g.,anisotropic)fortheanalysisofShipConstructormodels,andalsotosolvetheproblemsofheatconductionandfatigue,inwhichthenonlinearmaterialpropertiesarenotstoredintheShipConstructormaterialdatabaseandinfactcandependonvariousparameterswhichidentifiedandquantifiedduringcalculation.



AutoFEMAnalysislibraryofmaterialsallowsspecifyingmaterialswithcomplexpropertiesandapplyingthemtoShipConstructormodelsforFEA.

StagesoffiniteelementanalysisThetypicalsequenceofanAutoFEMAnalysisFEArunincludesthefollowingsteps:

CreatingasetofobjectsforfiniteelementstudyCreatingthedesiredstudy(varioustypesareavailable)GeneratingthefiniteelementmeshAssigningmaterials(unlessalreadyacquiredfromShipConstructorDefiningboundaryconditions(constraints,loads)CalculationAnalysisofresultsThefollowingparagraphspresentthestepslistedabove,basedonastaticstrengthanalysisexample.

CreatingasetofobjectsforfiniteelementstudyThefirststepinvolvedthecreationofthedesiredobjectsforfiniteelementstudy,asetwhichmayincludeallthemodel'scomponents,oronlysomeofthem.TocreatetheFEobjects,theAutoFEMPreprocessoracquiresthe3DgeometryofthestructurefromAutoCAD,whichisuseddirectly,therebyguaranteeingabsolutegeometricfidelity.OnlyonesetofobjectsforFEAcanbedefinedinadocument,andallsubsequentstudieswillbebasedonthissetandusesomeoralloftheobjects.Therefore,differentstudiescaninvolvedifferentgroupsofobjects,allfromthesameset.Therearetwotypesofobjectsets:setmadeof3Dsolids(AutoCAD3Dsolids)setmadeofsurfaceobjects(AutoCAD3Dsurfaces)ShipConstructoruserswillgenerallyworkwithsetsof3Dsolidobjects,asillustratedbelow.IntheShipConstructordrawing,gototheAutoFEMtabandrunthe"Createastudy"command.IfthedocumentdoesnothaveanexistingsetofobjectsforFEA,adialogappears,tocreateit.

Runningthe"Createastudy"command



ObjectsforFEAcanbecreatedviatheSetofobjectsforFEAdialog

Checkingforobject/objectintersections(clashes)InordertocreatethecorrectandrepresentativeFEAmesh,object/objectintersections(clashes)mustbefoundandprocessed.Ifanyarefound,thefollowingmessageappears:

Thesystemofferstoautomaticallyfixallobject/objectintersections



Ifuserrefusestheremovalofclashes,thismaythendestroythemeshgenerationprocess.Thepresenceofclashesisadmissibleinthecasewhenonewantstoworkwiththeplatesurfacestudy,basedonthefacesof3Dsolidmodels.However,volumetric(tetrahedral)meshesforeseethateachmeshpointbelongstoasingleobjectandtherearenointersectionsbetween3Dsolids.Inpractice,intersectionsbetweenobjects(clashes)shouldbeavoidedwhenusingvolumetricFEelements(tetrahedrons).Onceintersectionsarefixed(eliminated),thestudycreationprocesscontinues.

CreatingthedesiredstudyAfterthesuccessfulcreationofasetofobjects,thecommanddialog"CreatingStudy"appearsautomatically.AutoFEMallowstwoprincipalFEformulations:1.Volumetricformulation:modelgeometryisapproximatedwithtetrahedralfiniteelements.2.Plate(shell)formulation:usedinthecaseofthinwalledstructures,themodelgeometryisapproximatedwithtriangularfiniteelementsofplate/shelltype.Suchformulationsarefurtherdistinguishedas:3DAutoCADsurfacemodels(AutoCADsurfaces)3DAutoCADsolidmodels,thefacesofwhichareusedtobuildtheFEmeshmodelThestudycanincludealltheobjectsinthesetofobjectsforFEA,oronlysomeofthem,userselected.Thegroupofobjectsselectedforastudycanbemodifiedbyaddingorremovingobjects.Studiescanbecopied,andtheirtypemaybemodified.Forexample,onecancreateandrunaStaticAnalysisstudy,thenchangethetypetoBucklingAnalysisandrunitagainwithnofurtheraction:objects,boundaryconditions,etc.areallpersisted.Thisstrategysavesconsiderabletime,eliminatesmanysourcesoferror,etc.

TheStudycreationdialog

GeneratingthefiniteelementmeshOncetheobjectgroupofastudyisdefined,themeshcreationdialogisautomaticallybeingavailable.Formodelsbasedon3Dsolids,atetrahedralfiniteelementmeshwillbegenerated.Formodelsbasedon3Dsurfaces,AutoFEMwillusetriangularfiniteelements.

ThemeshiscreatedintheMeshingParametersdialog.FortypicalShipConstructormodels,whosepartswhichareusuallyrelativelythinandspacedapart,itisrecommendedtobuildthefiniteelementmeshusingthe"Thinwalledstructure(regular)"option,whichwillresultinabalancedandrepresentativeFEmesh.Oncetheothermeshparametersareselected,themesh'squalitycanbeevaluatedandthecreationprocesscompleted:

ThetetrahedralgridmodelofaShipConstructormodel



AssigningmaterialsTheAutoFEMAnalysismaterialslibraryincludesthematerialsmostcommonlyusedinvariousindustries.AutoFEMsupportsanisotropicmaterials,andallowstosetgraphsofnonlineardependenceofultimatestrengthvsnumberofloadingcycles(SNcurves)forthedesiredmaterials.Asnotedabove,AutoFEMcanalsoacquirematerial'snamedirectlyfromtheShipConstructormaterialslibraryandaddpropertiesonlypresentintheAutoFEMmateriallibrary(ShipConstructordoesnotneedallstrengthrelatedparametersofagivenmaterial).Ingeneral,ifamaterialispresentintheShipConstructormaterialslibrary,itspropertiesshouldbemaintainedthere.

TheAutoFEMmaterialslibrary

Materialsdisplayednexttotheparts'namesineachstudy

Definingconstraints/restraintsFEAgenerallyrequiresforcebalancing,hencethemodelbeinganalyzedmustbefixedinspace.AutoFEMprovideddedicatedcommands(Fixture,PlaneofSymmetry,Elasticbase)forthis,allowingthespecificationofaconstraint'snature(fullorpartial).Specialfilters(selectors)facilitatetheselectionofthemodelelement(s)towhichtheconstraintsareapplied:faces,edges,vertices.Whenrequired,usercoordinatesystemsareprovidedtospecifytheorientationofthedegreesoffreedom.Constraintsareshowngraphicallyonthemodel.

TheRestraintdialog

DefiningtherestraintsofaShipConstructormodel



ApplyingloadsManydifferenttypesofloadsareavailable:force,pressure,acceleration,torquehydrostaticload,centrifugalforce,andtheadditionalmass.Theuserdeterminesthemagnitudeanddirectionofloadsandtheirapplicationlocations.EachloadisidentifieduniquelyinthePreprocessorwindow,andthesymbol'ssizeandcolorareadjustableintheForcedialogue.

ApplyingloadstoastructuralmodelThe"AutoFEMPalette"liststhedefinedstudies,andeachstudy'spertinentdata,meshcharacteristics,materials,boundaryconditionsandcalculationsresults.Additionally,contextualmenusarealsoavailableforeachiteminthelist.TheAutoFEMPaletteandthestudytreeareAutoFEM'sprincipalworkmanagementtools.

StudyTree,showingthecalculationruns

SolvingthestudyOncethefiniteelementmeshhasbeencreated,thematerialsassigned,restraintsandloadsimposed,thecalculationscanberun.Priortorunningthecalculations,theuserselectstheequationsolvingmethod,alistofresultstobemadeavailable,andfinallyspecifiesadditionalanalysisoptionswhichdependonthetypeofanalysistoberun(buckling,nonlinear,thermoelasticity,etc.).



Runningcalculationsfromthecontextualmenu(left)andfromtheAutoFEMRibbon(right)

Oncethecalculationstarts,theStudyPropertiesmayrequestadditionalinformationspecifictothatrun:

PropertiesofaStaticAnalysisstudy

Solvingdialog,reportingoncalculationstatusandprogress

StudyingtheresultsOncearuncompletessuccessfully,thelistofresultsappearsintheStudyTreeand,fromthecontextualmenuofagivenresult,theuseropensthecorrespondingPostprocessorwindow.Resultsaredisplayedascolormapsdirectlyappliedtothestructuralmodel,andtheuserhasadditionaltoolsavailabletoinvestigatetheresults:tooltipsshowtheresultvalueasaflyoutlabelswiththeresultvalue(doubleclick)sensorsspecialtooltoassessresultsatapreselectedpointgraphics,allowingtoreflectchangesintheresultsection,allowingtolookintotheinteriorofthemodelandreviewresultsforotherwiseinvisiblepointsreportcreatesanindependenthtmlbaseddocument,containingtheprincipalresults,withcolordiagramsand3DVRMLmodelsoftheresults



onscreenresultsanimationcreatingavideooftheresultsanimation,inseveralvideoformats

ReviewofthecolordiagramsInstaticstrengthanalysis,reviewoftheresultsbeginswiththeinvestigationofdisplacements.Themustmakesurethatthecalculateddisplacementscorrespondtotheexpectedandhaveaphysicalsense.Then,onemayproceedtoreviewingstresses,etc.

Displayingthecalculateddeflections

Safetyfactorsexpressedasequivalentstress

AnimationoftheresultonscreenResultscanbeanimatedonscreen,onepurposebeingtoreviewchangesduetoloadingchanges..



Toolsforresultsanimationonscreen

CreatinganimationvideosTheusercancreatevideooftheresultanimations.

Creatingvideoofanimatedresult

CreatingReportsThe"Report"commandcreatesanhtmldocumentthatincludeswithprincipaldatapertainingtothestudyathand.Thedocumentcanbesaved.



"Report"commandinthecontextualmenu,anditsdialogbox

Sectionsofthereport

Resultsoverlaidonthe3Dmodel,VRMLformat,inthehtmlreportAtthispoint,theuserhasidentifiedmaximumdisplacementsandsafetyfactors,andcanascertainthedesign'sapplicabilitytoitsintendeduse.Moreover,presentationmaterialshavebeengenerated,includingvideosanddetailedhtmlreports.

ConclusionShipConstructorthishaveavailableafullyintegrated,fullscopeFEAtooltoanalyzetheirShipConstructormodels.FormoreinformationonAutoFEMAnalysis,pleasevisitwww.autofemsoft.com.

Supplement.Helpfultipsandadvice

CreatingmaterialsintheAutoFEMAnalysismateriallibraryAswasmentionedearlierinthisdocument,intheShipConstructor/AutoFEMintegratedenvironmentmaterialscanbeassignedtostructuralelementsinvariousways.InordertoapplymaterialpropertiesfoundinAutoFEMmaterialslibrarybutinShipConstructor's,thenameofthematerialathandmustbethesameinthetwolibraries.Thestepstoensurethisare:

Stepstocreateanewmaterial:1.Createanewusersmaterialslibraryoropenanexistingone.



Creatingtheusersmaterialslibrary2.Intheusersmaterialslibrary,selectanexistinggroupofmaterialsorcreateanewone.

Creatinganewgroupofmaterials3.Whenthecursorisonthegroupofmaterials,select"Createanewmaterial"inthecontextmenu.

Creatinganewmaterial4.Choosethematerialtype:Isotropic,OrthotropicorTransversallyisotropic.Then,enterthenameofthematerial,andclickOK..AfterOKispressed,thenewmaterialwillappearinthecurrentgroupofmaterials.

Typingthenameofthematerial5.Selectthenewlycreatedmaterialfromthelistofmaterialsandfillinitsparameters,choosingthemfromtheavailableselectionlist6.PressOKtocompletecreationofthematerialandclosethedialog.

ValidatingofthemodelandfixingintersectionsThemodel'sgeometrymustbecheckedtomakesureitisuseableinanFEAexercise.SomeofthecommonproblemstolookforinAutoCADmodelsinclude:

Intersections(clashes)of3Dsolids.IntersectingelementsdonotmodelreallifefromaFEAstandpoint.AnypointinaFEAmodelmustbelongtoasingleelementor,inourcase,physicalShipConstructorpart.Sometimes,eliminatingintersectionsmayresultsmallchangestotheoverall3Dgeometry,differenceswhicharegenerallysmallenoughtobenegligible.Therefore,theautomaticremedyingprocessofferedbyAutoFEMvastlyoutweighsthemarginallymoreaccuratebutalsopotentiallyimpossiblemanualcorrectionapproach.



AutomaticdetectionandresolvingofintersectionsWarning!IntersectionsareresolvedusingAutoCADcommands,aprocesswhichmayrequireconsiderabletimeforverylargeShipConstructormodels.Inextremecases,AutoCADmayactuallycrash.

"Flying"parts.Theseareobjectsdisconnectedfromthemodel,whichprovokeerrorsinmechanicalstudiessuchasstaticstrengthandbucklinganalysis,butareadmissibleforfrequencyandthermalstudies.

Multiplevolumebodies.IntermsofFEAmeshing,eachstructureobjectmustsubtendasingle,closedvolume.However,multiplevolumeobjectscanbecreatedinAutoCAD,forexampleexecutingaBooleanuniononoutofcontactobjects,orbycreatingasolidarray,etc.IftheAutoFEMtetrahedralmodellingstrategyisadopted,suchinstancesmustberesolvedintodiscreteobjectsbeforemeshing.

Outofcontactsolids.Sometimes,itisdifficulttodeterminewhethertheadjacentfacesofsolidsareindeedtouching.Thismayresultinmeshgenerationorcalculationerrors.Intheinterestofcleanruns,theusersmustensureclearcontactbetweenobjects.

Errorsinthepiecelinearcomplex(PLC)representationofAutoCAD3Dmodels.Theseerrorsmayoccurinthecaseofverycomplex3Dgeometry,generallydueto3Dmodellingoperations.Sucherrorsmaypreventthecreationofatetrahedralmesh,andthe3DCADmodelmustbecorrectedinordertorunanFEA.

3Dmodeldiagnostics(study)TheStudyDiagnosticscommandisdesignedtohelptheuserfindspuriousorFEAunfitobjectsinastudyobjectgroup.Thecommandrunsautomatically,butitisalsoavailableintheStudyTreecontextualmenu.

TheDiagnosticscommandintheSetofobjectsforFEA(left)andStudy(right)contextualmenus

Inallcases,thediagnosticscommandidentifiesthespuriousobjects,whichbecomeavailableinthedropdown"Groupofthebodies"list,andwhosenamesareavailableintheGeometrylist.Thefollowingdiagnosticmodesareavailable:

MultiplevolumebodiesTheMultivolumebodiesmodepertainstomultiplevolumeobjects,ifanyshouldbepresent.AMultiplevolumeobjectisonemadeoftwoormoresolidswhichdonottouch.WhichAutoCADcanoncontrarytreatasingleobject(body),AutoFEMrequiresthatobjectsandvolumes(bodies)areunivocallyassociated.



ExampleofamultiplevolumebodyTheAutoCADcommandExplodegenerallysufficestoseparatethemultiplevolumesintodiscreetobjects.InthecasewhereaBooleanoperationgeneratedamultiplevolumeobject,theAutoCADcommandSOLIDEDIT/BODY/SEPARATEmaybemoreappropriateifnotrequired.Insomecases,multiplevolumeobjectsmaybecreatedduringtheresolutionofintersectionsbyAutoFEMitself.Multiplevolumeobjectsmustberesolvedintosinglevolume,discreetobjectusingAutoCADtools.

Exampleofamultiplevolumeobjectengenderedbytheresolutionofanintersection,aconditionwhichmustberemedied

NoncontactinggroupsofbodiesThe"outofcontactbodies"modeaddressesobjectswhichareseparatefromthemodel.Iftheseobjectsarenotadequatelyconstrainedand/ordonotbearappropriateboundaryconditions,thealgebraicsystemofequationsattheheartofFEAmayprovetobedegenerative,andasolutionmaynotbefound.Therefore,ingeneral,suchobjectsshouldnotbeincludedintheStudygroupofobjectsinstaticandbucklinganalysis,thoughtheyareadmissibleinthermalandfrequencyanalysisstudies.

NoncontactingpartsinthePreprocessorwindow

IntersectingbodiesTheIntersectionsofthebodiesmodeidentifiespairsofintersectingobjects,whicharenotallowedinaFEmodelasacorrespondingmeshcannotbecreated.AlthoughAutoCADandotherCADsystemsallowclashingsolids,inFEAanypointinspacemustbeassociatedwithasingleobject.Therefore,FEAIncompatiblegeometrysuchasthiswouldadverselyandunpredictablyaffectthequalityofresults,andisnotallowedbyAutoFEM.ThemodelmustbecorrectedusingAutoCADtools.InadditiontoAutoFEM'sintersectionsearchtools,onemayalsouseAutoCAD'sINTERFEREcommand.



Identifyinganddocumentingpairsofintersectingobjectsinthediagnosticswindow

ContactbetweenbodiesTheShowthecontactsbetweenthebodiesmodeidentifiesallcontactfaces(twoandonlytwoobjectsshareacontactface),whichareshowninthepreprocessorwindow.Thismodeallowstheusertocheckmodelconnections.Sometimes,whenmodellinginAutoCAD,invisiblegapsmayoccurbetweenobjects.ThesegapswillbetreatedbyAutoFEMasaseparation,whichwillleadtounreliablecalculationresults.Gapsmayalsobeengenderedbytheresolvingofintersectionsmakingthistoolthatmuchmoreusefulandnecessary.

Thecommonfacesofcontactingpartsareshowningreen.

Therearenovisiblecontactfaces,themodelrequireschecking.

CheckingthePLCrepresentationCheckPLCrepresentationchecksthe3DfromaPLCstandpoint.InaccuraciesareshowninthePreprocessorwindowwhilethe"Geometry"windowindicatesthenumberofsuspiciousobjectsfound.Theseerrorsaregenerallyremediedbyadjustingthemeshconstructionparameters,otherwisetheinaccuraciesdocumentedbyAutoFEMmustberemediedatthemodellevel,inAutoCAD.

PossibleproblemareasarehighlightedonthemodelNotethattheFEMAMESHcommandcannotberunifanymultiplevolumebodiesarepresentinthemodel.



CreatingfiniteelementmeshesItiscrucialthattheFEAmeshapproximatesmodelgeometryofthestructureveryaccurately.Sometimes,modelmodificationsarerequiredtoachieveameshappropriateforstructuralFEA,inwhichcasethemodelmustbeadaptedtothepurpose.Moreover,meshgenerationparametersmayalso,onoccasion,hamperthegenerationofacongruentmesh.AutoFEMoffervarioustools,suchasitsdiagnosticstools,formodelreviewsandproblemareaidentification.

Parameter"Edgelength"

Parameter"Curvature"

DecreasingthefiniteelementsizeNormally,thesystemofferstoolargesizeofthefiniteelementofmesh.Usershoulddefineacceptablesizeofthefiniteelementusingaspecialslider(Edgelength)ortypingthevalueinthetextbox.

Increasingthecurvatureforcylindrical3DmodelsFor3Dmodels,whichhaveintheircompositionroundorcylindricalgeometricalelements,oftenisusefultoincreasetheaccuracyofcurveelementsrepresentation.

FlagThinwalledstructureFlag"Thinwalledstructure(regular)"enablesameshingprocedurethatusessimplifiedqualitycontrolofmeshelements.Ifthe"Meshquality"parameterissetto"Dis",themeshwillbegeneratedonthebasisoftheinitialPLCmodelwithminimalmodifications(usuallyitlookslikea"regular"meshwithsquarecells).Suchmeshescontainfewerelementsandmaybeusedtocomputethinwalledstructures,butalsoincaseswhereareducedsizemeshisdesired.

MeshgenerationerrorsIfmeshgenerationshouldfail,AutoFEMwillthrowanerrormessageandidentifythepointsand/orgeometryareainquestion.Theusercanthenresolvetheprobleminthe3Dmodelorexcludethe"bad"object(s)fromthefiniteelementstudy.

Indicatingthelocationofaproblempointonthe3Dmodel

Nodetolerancefor3DsurfacemodelsNodalcouplingtoleranceisusercontrolled,veryusefulinthecaseofsurfacemodelsbuiltonwireframes,whichgenerallyexhibitsmallgapsbetweensurfaceedges.

Agapinthemeshmodel



Removingthegapsbyincreasingthenodecouplingtolerance

Useoftheoption"Stabilizetheunfixedmodel"WhenanalysinglargeShipConstructormodels,itmaybethatmodelorelementconstraintsproveinadequateandthemodeloranelementbecomeunbalanced.AutoFEMwillthenstopcalculationsandopenthefollowingdiagnosticmessagedialogs:

Diagnosticmessageofunbalancedsystem

The"Stabilizetheunfixedmodel"toolanditsoptionarefoundintheStudyPropertiesdialogOften,theoption"Stabilizetheunfixedmodel"helpsfindinadequatelyconstrainedobjectsorsuccessfullyperformthecalculation.

Whichisthebetterequationsolvingmethod?Thetwomainmethodsforthesolutionofsystemsofequationsaredirectanditerative.AutoFEMAnalysiswillautomaticallyattempttoselectthemostefficientmethodfortheanalysisathand.IfthenumberofequationsexceedsthemaximumsetintheProcessorsettings(default100,000),theiterativemethodisusedforsolving.Ifsmaller,thedirectmethodisused.Theusercancontrolthischoicebydefiningselectionparameters,orbyselectingthedesiredmethodjustpriortolaunchingtheanalysis.



SelectingthesystemofequationsolvingmethodandnumberofequationsthresholdGeneralrecommendationsandcriteriaapplytothechoiceonemethodortheother.Thefirstcriterionisthepowerofthecomputersystem.TheiterativemethodrequireslessRAMthanthedirectmethod.Othercriteriaarethequalityofthefiniteelementmeshandthe"thickness"oftheproblem.ThedirectmethodisbasedontheGaussmethodanditsevolutions,andcarriesoutafullmatrixinversionrequiringlargeamountsofRAM.CalculationtimeincreasesasN3,whereN="number"ofequations.Usuallythedirectmethodworkswellanditispreferablewhenonehas:

Arelativelysmallequationsystemorapowerfulcomputersystem(x64platform,min8Gbram)allowingtoinverttheFEmatrixinRAM(asopposedtousingdiscswapping).Problemswithupto1.52milliondegreesoffreedomgenerallyconstitutea'thin"3Dmodel,andaregenerallysolvablebythedirectmethodmanyofthemeshelementsare"stretched"(veryhighaspectratio)thestudyhascontactrestrictions

Theiterativemethod'smainadvantageovertheiterativemethodisitslowRAMrequirement,whichiscomparabletotheamountofRAMneededfortheinitialstiffnessmatrixlocation.Therefore,evenrelativelyweakcomputersystem(suchasanofficecomputerwithWindows32bitoperationalsystemandnomorethan3GBRAM)maysolvesystemsofupto1millionequationsorso.Ontheotherhand,performancedependsoncertainpropertiesoftetrahedralfiniteelements,suchasaspectratio.And,themoresuchelementsthereare,themoresolutiontimewillincrease,too.Then,theiterativemethodusuallyworkswellandispreferredwhenonehas:

agoodqualitymeshwhereelementaspectratioislessthan84andonlyfewstretchedelements."thick"model,meaningthatthereareseverallayersoftetrahedralelementspersection.Thestudyhasnocontactrestrictions.

A"thick"finiteelementmodel:thereareseverallayersofthefiniteelements.TheIterativemethodworksbetterthanthedirectmethodformediumandlargemodels.

A"thin"finiteelementmodel.Themeshconsistsofoneortwolayersoffiniteelements.Thedirectmethodworksbetterthantheiterativemethod,especiallyifmeshqualityispoor(manyhighaspectratiomeshelements).

WorkingwithShipConstructorPipes



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SomeShipConstructorparts,suchaspipes,donothavecorresponding3DsolidgeometryintheirShipConstructordrawing.TheuserwillthenexportsuchpartstoDWGfile,andthensimulatethemasordinaryafterwhichAutoFEMwilltreatthemasgenericAutoCADobjects.

ShipContstructorPipesdonothave3DsolidgeometryandcannotbeuseddirectlyinFEA

FEAcarriedoutonexportedShipConstructorPipeparts

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Integration of AutoFEM Analysis With ShipConstructor _ AutoFEM