vehicle crashworthiness and occupants protection 4

FundamentalPrinciplesforVehicle/OccupantSystemAnalysisPage159FFFFFundamentalPrinciplesfundamentalPrinciplesfundamentalPrinciplesfundamentalPrinciplesfundamentalPrinciplesforororororVVVVVehicle/OccupantSystehicle/OccupantSystehicle/OccupantSystehicle/OccupantSystehicle/OccupantSystemsemsemsemsemsAnalysisAnalysisAnalysisAnalysisAnalysisCliffordC.Chou4.1IntroductionRestraintsystemsaresafetydevicesthataredesignedtoassistinrestrainingtheoccupantintheseatingposition,andhelpreducetheriskofoccupantcontactwiththevehicleinterior,thushelpingreducetheriskofinjuryinavehicularcrashevent.Thischapterdescribesfundamentaldynamicsprinciplesforoccupantrestraintsystemsanalysis.Theseprinciplesareessentialandaretheestablishedbasisonwhichrestraintsystemsaredesignedandanalyzedfortheproblemsinvolvedinautomotivesafety.Atitssimplest,automotivesafetyisamatterofreducingtherelativevelocitybetweentheoccupantsandthevehicleinteriortohelpreducetheriskofinjurytotheoccupantduringacollision.Newton’sLawsindicatethatachangeinanoccupant’svelocityrequiresaforcetobeappliedtotheoccupant.Toreducetheriskofinjurytotheoccupant,safetyengineersanalyzethemaximumforcethatcanbeappliedtothedriverorpassengerwithoutinjury.Consequently,itisnecessarytoquantifytheinjuryparametersinamannerthatpermitsthemtobeusedasperformancenumbersinsaferdesigns.SinceautomobilemanufacturersmustdesigntheirproductstocomplywithcertainFederalMotorVehicleSafetyStandards(FMVSS)foroccupantprotection,theinjurycriteriaspecifiedinthesestandardsmustbemet.StandardspertainingtooccupantprotectionareFMVSS208forfrontalimpactprotectionandFMVSS214forsideimpactprotection,whichwillbedescribedlater.AllnewcarstobesoldintheUnitedStatesarealsorequiredtomeetarangeoffederalsafetystandardsthatmeettheneedformotorvehiclesafetyfeaturessuchaswindshieldsthatresistpenetrationandsteeringwheelassembliesdesignedtoabsorbcrashenergyandhelpcushiontheoccupantsuponimpact.Themostfamiliarsafetyrestraintfeaturesrequiredbyfederalstandardaresafetybeltsandsupplementalairbagrestraintsystems.Othernon-regulatedrestraintsystemsFundamentalPrinciplesforVehicle/OccupantSystemAnalysisPage160includeenergy-absorbingE-A(orcollapsible)steeringcolumnandE-Akneebolsters.Thereisabundantliteraturedealingwiththeeffectivenessofrestraintsystemssuchasbeltsandsupplementalairbagsinprovidingoccupantprotectioninautomobilecrashes.Statisticsindicatethatmandatoryseatbeltusageandairbagshavesignificantlyreducedoccupantfatalityandinjury(seeChapter7-AnthropomorphicTestDevices).Inaddition,eachvehicledesignmustcomplywithFMVSS208and214forfrontalandsideimpactoccupantprotection,respectively.TherequirementsoftheFMVSS208standardincludea30mph(48k/hr)impacttoanon-movablerigidbarrier.NewFMVSS208requiresalsotheoffsetimpactandtherigidbarrierimpactatdifferentvelocities.ThischapteraddressesonlytherigidbarrierimpacttestsinwhichthepotentialinjurytotheoccupantisassessedbyusingHybridIIIanthropomorphicdummies(refertoChapter7–AnthropomorphicTestDevicesfordetails).HybridIII,themostadvancedtestdummyforfrontalimpact,possesseshuman-likeimpactresponseincriticalbodyregionssuchasthehead,neckandthorax.InjuryseverityindicesmeasuredfromHybridIIIinbarriertestsincludetheHeadInjuryCriterion(HIC),chestG,chestdeflection,femurloadandneckloadsandmoments.TherequirementsofFMVSS214areassessedusingamovingdeformablebarrier(MDB)impactingthedriversideofatargetvehicle.PotentialinjuryincludespeakpelvicaccelerationandTTI(ThoracicTraumaIndex).Ifanyoftheseinjuryseverityindicesexceedsthedummyperformancenumberlimitsspecifiedbythestandards,thevehicledesignmustbemodifieduntiltherequirementsaremet.However,relyingexclusivelyoncrashtestsfordesignimprovementisveryinefficient.Furthermore,intheearlystageofdesign,notallhardwarehasbeenfabricated,yetcompetingdesignconceptsmustbeevaluated.WithrapidadvancementindigitalcomputertechnologyandCAEmethodology,simulationhasbecomeanessentialandusefultoolinthedesignstage.Mathematicalmodelscanprovidequickassessmentofvariousdesignconceptsandexplorenewdesigndirections.AreviewofmathematicalmodelswidelyusedbytheautomotiveindustryforoccupantsimulationsisgivenbyPrasadandChou[1].DetailedoccupantmodelingtechnologyforrestraintsystemsisalsopresentedinChapter5–DesignTools:HumanBodyModeling.Todesignaneffectiverestraintsystemthatstopstheoccupantwithinthevehiclecompartmentwithoutinjuringtheoccupant,itisessentialtoknowtherequiredstoppingdistance.Anexample:considerthestoppingdistancerequiredbythechestifthetolerancelevelof60g’sisnottobeexceeded.Notethata“g”isusuallyusedintheautomotivesafetyfieldtoindicateaforcebeingappliedtoanoccupant,aswillbedescribedlater.First,theoptimumorminimumstoppingFundamentalPrinciplesforVehicle/OccupantSystemAnalysisPage161distancewouldbeachievedwithasquarewave(uniform)decelerationpulse,a.k.a.EquivalentSquareWave(ESW).Thiswouldrequirea6-instoppingdistancefrom30mph,ora24-instoppingdistancefrom60mph.Thesevaluesareoptimumandwillneverbereachedinpractice.Amorereasonablestoppingpulseshapeisahalf-sinedecelerationpulse,whichrequires9.7infrom30mphand37.7infrom60mphwithoutexceeding6