vehicle crashworthiness and occupants protection 2

DesignofVehicleStructuresforCrashEnergyManagementPage11DesignofVehicleStructuresforCrashEnergyManagementHikmatF.MahmoodandBahigB.Fileta2.1IntroductionHistorically,stylingandpackagingconsiderationsdrovethedesignofvehiclebodystructures.Thefinaldesignwastheproductofalongevolutionguidedprimarilybytesting,supportedbysimplelinearstrengthofmaterialmethods.Withtheadvancesincomputerhardwareandsoftware,severalanalyticaldesigncapabilitiesevolved,providingengineerswithavarietyoftoolstodesignmodernvehiclestructuresthatcanmeetthegrowingcustomerdemandsforbettercrashworthinessperformance,quieterrideandreliability.Thesetoolsincludesimplespring-massmodels,beamelementmodels,hybridmodelsandfiniteelementmodels.Althoughthesetoolsvaryincomplexity,eachisbasedonstructuralmechanicsprinciplesthatrequireconservationofmass,momentumandenergy.Theselectionofaparticularanalysistooldependsonthetaskathandandontheparticulardesignphase.Forexample,asimplebeammodelmaybemoreappropriateatconceptinitiationtodeterminesectiongeometry,sizeandthickness,whereasafull-scalefiniteelementanalysisofthevehiclemaybemoresuitablewhenaprototypeisbuilt.Correspondingly,thetimerequiredtodevelopamodelandtheamountofinformationprovidedvarybymodelcomplexity.Inrecentyears,theautoindustryhasexperiencedthegreatestdemandfromcustomers,regulators,andthemediatoprovidesafervehicles.Thistranslatesintobettercrashworthinessofthevehiclestructureandeffectiverestraintsystems.Examplesare:FederalMotorVehicleSafetyStandard(FMVSS),NewCarAssessmentProgram(NCAP)test,InsuranceInstituteforHighwaySafety(IIHS)tests,compatibilitytesting,andtestingtoinsureprotectionofchildrenandsmalladultoccupants.Inotherwords,abroaderviewofvehicles’crashworthinessisnowneeded.Inaddition,competitionintheindustrytobuildproductsofhighqualitywithinshortdesigncyclesisdrivingtheuseofanalyticaltoolsandreplacingtestingexceptwhenabsolutelynecessary.Thisdramaticchangeinvehiclestructuraldesign,especiallyinvehiclecrash,signalstheneedfordesigntoolsthatareeffectiveinanalyzingthecrashphenomena.UnderstandingtheeffectofthePage12VehicleCrashworthinessandOccupantProtectioncollapsingstructureonthevehicle’sdeceleration-timehistory,andlearningabouttheinteractionbetweentheoccupantsandthevehicleduringcrashareexamplesoftheneededknowledge.Efficientmethodsandtoolsarenecessarytoenableinitiationofasounddesignattheearlyconceptphasesofdevelopment.Inthischapter,methodsandtoolsfordesigningstructuresforcrashworthinessaredescribedwithemphasisonfrontalcrashes.Thereasonforemphasisondesigningforfrontalcrashesisthattheseareconsideredtoberesponsibleformoretrafficfatalitiesandinjuriesthananyothercrashmode.InSection2.2–CurrentDesignPractice,thecurrentmodelinganddesignprocessesarereviewedwhileidentifyingtheshortcomingsforearlydesignstages.InSection2.3–Crash/CrushDesignTechniquesforFrontStructures,techniquesareexaminedforanalyzingthefront-endsystemandcomputingtheperformancedesignobjectivesofsubsystemsandcomponents.Section2.4–AnalyticalDesignTools,introducescomponentdesignmethodsthatenablethedesignertocreatestructuralconceptsforenergyabsorptionandstrength.Section2.5–VehicleFrontStructureDesignforDifferentImpactModes,discussesstrategyfordesigningstructuresfordifferentfrontalimpactmodes,includingvehicle-to-vehiclecrashes.2.2CurrentDesignPracticeTheprimaryaimofthecrashworthinessdesignprocessistosecuredummyresponseresultsthatmeasurebeloworatacceptableinjuryriskvalues.Thecrashpulse,typicallygeneratedfromafrontalcrashofthevehicleintoarigidbarrier,istheessentialfeatureinthedesignprocess.Itisusedasinputtooccupantmodels.Currently,thedesignprocessreliesoncalculatingthecrashpulsefromeitherLumpedMass-Spring(LMS)modelsorFiniteElement(FE)models.Figures2.2.1and2.2.2provideaschematicdescriptionofthetwoprocesses.LMSmodelswereintroducedintheearly1970’s,andreliedonstaticcrushteststoestablishthespringstiffness.NonlinearFEmodelswereintroducedinthemid-1980’sandrapidlygainedacceptanceamongstructuralanalysts.Thisacceptancewasfueledbycontinuallyincreasingcomputercapabilitiessuchasspeed,memory,storagespace,graphics,andnumericalstructuralmechanicstechniques.Otherfactors,suchasdetaileddesignofcomponentslayoutandsurfaces,facilitatedthedevelopmentofvehiclemodelsbyshellfiniteelementssincemostofthegeometryofthestructuralsurfaceswasalreadyoncomputergraphicfiles.Inadditiontotheadvancementincomputertechnology,manyenhancementshavebeenmadetothedynamiccrashanalysiscomputercodes.Theseincludeefficientcomputationforspeedaswellasuser-friendlysoftwareforpreparinginputdataDesignofVehicleStructuresforCrashEnergyManagementPage13andprocessingtheanalysisresults.Theseenhancementsincreasedthepopularityofusingfiniteelementcodesfortheanalysisanddesignofvehiclestructuresforcrash.Infact,theuseofthefiniteelementmethodbecomesevenmoreappealingwhenchangesfromthebasevehicledesigntothenewvehicledesignaresmall.Inthiscase,validatedfiniteelementmodelsaswellastunedoccupantsimulationmodelsareusuallyavailable.MoredetailsonFEanalysisofvehiclecrashworthinessandoccupantsimulationmodelsarepresentedinotherchapters.2.2.1Compar