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Towards advanced bicycle helmet test methods

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This paper exposes a critical analysis of current bicycle helmet standard tests and proposes an advanced test method. Different key aspects are considered consecutively, i.e. the bicyclist's head impact conditions in terms of velocity vector, head form boundary conditions, the headform itself with its instrumentation, the geometry of the impacted surface, the head impact location and finally the head injury criteria. Based on in deep analysis of bicycle accidents it has been shown that a significant component of the head velocity vector at the time of impact is the tangential velocity. On the other hand it has been shown that the head boundary condition at neck level does not play a significant role in the head response following to impact. Therefore it is suggested to consider tangential impacts against a helmeted Hybrid III dummy head, eventually connected to a Hybrid III neck. A critical analysis of the geometry of the impacted surface and the temperature at the time of accidents will be presented as well. The current ISO head form presents a rigid contact surface with the helmet liner which is quite fare from human scalp-helmet interface. Therefore it is recommended in this proposal, to use a head form with a skin such as the Hybrid III dummy head. This improved head surrogate also presents the advantage to be fitted easily with rotational accelerometers, and to be connected to the Hybrid III neck without further modifications. Finally this head form presents mass and inertia properties much closer to the human head as ISO head form does. The location of head impact in the current standard presents two key issues: The difficulty to guarantee an impact in line with the head form center of mass, and the recommended test line which excludes impacts to the temporal region which is often impacted in real world accidents. It is therefore suggested to prescript specific impact points in a similar way as for motorcycle helmets. A final and acute issue with current bicycle helmet standard test is the head injury taken into consideration and related to results from the 1950's, as the threshold is still expressed in terms of acceleration amplitude and duration. In order to take into account the linear and rotational acceleration of the head after impact as well as improved model based head injury criteria, a coupled experimental versus numerical method is suggested in order to assess the head injury risk. In this method the helmeted head form is impacted in the previously descript impact conditions and the six head form acceleration versus time curves are implemented into a FE head model in order to compute the intra-cranial head response and to compare it to the model based head injury criteria. It is believed that the proposed approach would permit the evaluation and optimization of bicycle helmets against biomechanical criteria and under realistic impact conditions.
: Illustration of the WSU head tolerance curve (1950's) and expression of HIC criterion (1970's) In the literature, rotational acceleration limits are proposed at 8 to 10 krad/s2. It must however be recalled that head tolerance limit to rotation is strongly time and direction dependant so that today there is no single limit known. Within EU project APROSYS SP5 and French PREDIT projects PROTEUS and BIOCASQ, improved head injury criteria to specific injury mechanisms have been defined taking into consideration the time evolution of both linear and rotational head acceleration as recalled hereafter. Today, state of the art FE head models exist and have been used for the definition of injury criteria to specific injury mechanisms. These models became much more powerful injury prediction tools that HIC, so the present proposal is to implement improved, model based head injury criteria into a new helmet impact test procedure. Based on the simulation of nearly 100 well documented head trauma, tolerance limits have been identified with respect to moderate and severe neurological injury. Human head tolerance limits relative to neurological injuries with a risk of occurrence of 50 % were established as follows:  A brain Von Mises strain reaching 20 % for moderate neurological injury.  A brain Von Mises s strain reaching 35 % for severe neurological injury.  Finally a global strain energy of the sub-arachnoid space exceeding 4.2 J generates subdural and sub-arachnoid haematoma, and a local strain energy in the skull of 439 mJ for a 50% risk of a skull fracture.
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Proceedings,InternationalCyclingSafetyConference2014
1819November2014,Göteborg,Sweden
ABSTRACT
Thispaperexposesacriticalanalysisofcurrentbicyclehelmetstandardtestsandproposesan
advancedtestmethod.Differentkeyaspectsareconsideredconsecutively,i.e.thebicyclist's
headimpactconditionsintermsofvelocityvector,headformboundaryconditions,thehead
formitselfwithitsinstrumentation,thegeometryoftheimpactedsurface,theheadimpactlo
cationandfinallytheheadinjurycriteria.Basedonindeepanalysisofbicycleaccidentsithas
beenshownthatasignificantcomponentoftheheadvelocityvectoratthetimeofimpactis
thetangentialvelocity.Ontheotherhandithasbeenshownthattheheadboundarycondition
atneckleveldoesnotplayasignificantroleintheheadresponsefollowingtoimpact.There
foreitissuggestedtoconsidertangentialimpactsagainstahelmetedHybridIIIdummyhead,
eventuallyconnectedtoaHybridIIIneck.Acriticalanalysisofthegeometryoftheimpacted
surfaceandthetemperatureatthetimeofaccidentswillbepresentedaswell.
ThecurrentISOheadformpresentsarigidcontactsurfacewiththehelmetlinerwhichisquite
farefromhumanscalphelmetinterface.Thereforeitisrecommendedinthisproposal,touse
aheadformwithaskinsuchastheHybridIIIdummyhead.Thisimprovedheadsurrogatealso
presentstheadvantagetobefittedeasilywithrotationalaccelerometers,andtobeconnected
totheHybridIIIneckwithoutfurthermodifications.Finallythisheadformpresentsmassand
inertiapropertiesmuchclosertothehumanheadasISOheadformdoes.
Thelocationofheadimpactinthecurrentstandardpresentstwokeyissues:Thedifficultyto
guaranteeanimpactinlinewiththeheadformcenterofmass,andtherecommendedtestline
whichexcludesimpactstothetemporalregionwhichisoftenimpactedinrealworldaccidents.
Itisthereforesuggestedtoprescriptspecificimpactpointsinasimilarwayasformotorcycle
helmets.
Afinalandacuteissuewithcurrentbicyclehelmetstandardtestistheheadinjurytakeninto
considerationandrelatedtoresultsfromthe1950's,asthethresholdisstillexpressedinterms
ofaccelerationamplitudeandduration.Inordertotakeintoaccountthelinearandrotational
accelerationoftheheadafterimpactaswellasimprovedmodelbasedheadinjurycriteria,a
coupledexperimentalversusnumericalmethodissuggestedinordertoassesstheheadinjury
risk.Inthismethodthehelmetedheadformisimpactedinthepreviouslydescriptimpactcon
ditionsandthesixheadformaccelerationversustimecurvesareimplementedintoaFEhead
modelinordertocomputetheintracranialheadresponseandtocompareittothemodel
basedheadinjurycriteria.Itisbelievedthattheproposedapproachwouldpermittheevalua
tionandoptimizationofbicyclehelmetsagainstbiomechanicalcriteriaandunderrealisticim
pactconditions.
Keywords:bicyclehelmet,testmethod,impactconditions,headinjurycriteria
Towardsadvancedbicyclehelmettestmethods
R.Willinger1,C.Deck1P.Halldin2,D.Otte3
1ICUBE,CNRSBiomechanics
UniversityStrasbourg
2rBoussingault,Strasbourg,France
email:remy.willinger@unistra.fr
2KTHandMIPS
UniversityStockholm
Stockholm,Sweden
email:peter.halldin@kth.se
3MUH
UniversityofHannover
Hannover,Germany
email:dietmar.otte@MUH.de
2
1INTRODUCTION
Itiswellknowninthescientificcommunitythatheadrotationalaccelerationisacritical
headloadingwhichcanleadtobraininjury.Concerningneurologicalinjuries,Holbourn(1943)
[1]suggestedthattherotationalaccelerationinducedbyagivenimpactcauseshighshear
strainsinthebrain,thusrupturingthetetheringcerebralbloodvessels,neoandsubcortical
tissue.Thisauthorwasthefirstwhosuggeststheimportanceofrotationalaccelerationinthe
appearanceofcerebralconcussion.In1967,Ommayaetal.[2]proposedamethodinorderto
extendtheresultsofexperimentsonconcussionproducingheadrotationsonlowerprimate
subjectstopredicttherotationsrequiredtoproduceconcussionsinman.Achartofangular
accelerationrequiredtoreproduceconcussionintherhesusmonkeyindicatesthatanacceler
ationof40000rad/s²willhavea99%probabilityofproducingconcussionwhichwasexpectto
correspondstoanangularaccelerationof7500rad/s²forhuman.
Ommayaetal.(1968)[3]studiedtheeffectofwhiplashinjuryonrhesusmonkeysand
showedthatiftheheadwassubjectedtoarotationalaccelerationaboveathresholdvalue,
subduralandsubarachnoidinjurieswereobtained.Inastudybasedonprimates,Gennarelliet
al.(1982)[4]proposedthatarotationalaccelerationexceeding175000rad/s²wouldproduce
SDHintherhesusmonkey.Withtheobjectivetoinvestigatetheinfluenceoftheheadrota
tionalaccelerationontheintracerebralmechanicalparametersunderaccidentalheadimpact,
atotalof69realworldheadtraumaweresimulatedwithandwithoutconsideringtheangular
rotationbyDecketal.2007[5].Thenumericalsimulationoftheseheadtraumabyconsidering
linearandrotationalaccelerationontheonehandandlinearheadaccelerationonlyonthe
otherhandpermittedittodemonstrateandtoexpressquantitativelythedramaticinfluence
oftherotationalaccelerationonbothintracerebralloadingandbrainskullrelativemotion,
supposedtoleadrespectivelytoneurologicalinjuriesandsubduralhaematomarespectively.In
thisstudytheeffectofangularaccelerationwasfoundtoincreasetheintracerebralshearing
stressforallaccidentcasesconsideredofabout50%whatevertheimpactseveritywas.Kleiv
enetal.2007[6]aswellasZhangetal.2001[7]demonstratedthattheangularkinematicsof
theheadwasthemostimportantfactorindeterminingthebrainstrain,basedonnumerical
simulationofrealworldheadtrauma.MorerecentlyTakhountsetal[8],[9]usedaheadFE
modelinordertoestablishaheadinjurycriteriaforrotationalaccelerationcalledBRIC.
Inparallelwiththedemonstrationofthecriticalroleofheadangularaccelerationinbrain
injury,anumberofstudiesfocussedontheheadkinematicsinrealworldaccidentinorderto
demonstratethatatangentialloadingoftheheaddoesexistinadditiontothenormalimpact
velocity.Millsetal.(1996)[10]showedthatobliqueimpactsarethemostcommonsituations
inmotorcyclecrashes.MorerecentlyBourdetetal.(2011,2012)[11],[12]quantifiedthehead
rotationalaccelerationduetothetangentialcomponentoftheheadimpact,byreconstructing
realworldandvirtualmotorcycleaswellasbicycleaccidents.
Despitethiswidelyrecognizedunderstandingofheadrotationalloadingandeffectofthe
inducedrotationalaccelerationtothebrain,noheadprotectionstandardiscurrentlyconsider
ingheadrotationalacceleration.OnlyECER2205EU[13]motorcyclehelmetstandardconsid
ersatangentialimpactconditionbuthelmetevaluationislimitedtotherecordingofthetan
gentialforcewhichinformsaboutfrictioncharacteristicsofthehelmetbutnotaboutits
protectioncapabilityagainsttangentialimpacts.Onepossiblereasonforthecurrentsituation
besidetheincreasedcomplexityandcostofthetestdeviceitselfisthatnoacceptedheadro
tationthresholdhasbeenestablishedyet.Anumberofmaximumheadrotationalaccelera
tionshavebeenproposedintheliterature[10],[14]–[16]butnoneofthemconsiderthetime
evolutionofthisparameter.Moreover,itisobviousthatthemaximumheadrotationalaccel
erationisafunctionofrotationaxisandcombinedlinearacceleration,twoaspectswhichare
nottakenintoaccountinexistingproposals.Totheauthor’sopinion,theonlywaytointegrate
thecomplexityofbraingeometryandbrainmaterialpropertiesistoprogresstowardstissue
levelbraininjurycriteriaasproposedinexistingFEmodelbasedheadinjurycriteria[6],[7],
[17].In2004Deckdemonstratesthathelmetoptimizationstronglydependsontheheadsub
stituteandinjurycriteriatakenintoaccount.Morerecentlyfirstattemptsweremade(Tinard
etal.2012[18])inordertooptimizenewhelmetsagainstbiomechanicalcriteriabycoupling
thehumanheadmodeltoahelmetFEmodel.Advancedmodelbasedheadinjurycriteriahave
3
alsobeensuggestedinrecentattemptstoimprovebicyclehelmettestmethods(Decketal
2012)[19].
InthedomainofbicyclehelmetevaluationMilneetal2012and2013[20]suggestedanew
helmetassessmentmethodusingmodelbasedheadinjurycriteriaunderbothlinearandtan
gentialimpactconditions,exactlyasHansenetal2013[21]inthecontextofthedevelopment
ofanadvanced'honeycomb"bicyclehelmet.Inasimilarway,butinthecontextofhockey
helmetevaluationPostetal.2013[22]suggestedtoimpactahelmetedHybridIIIheadneck
systemandtointroducethelinearandrotationalaccelerationintoanexistingheadFEmodel
inordertoassesstheinjuryrisk.
Inordertoprogressinthefieldofhelmetprotectionagainsttangentialimpactsanumberof
attemptswereproposedintheliterature.Aldmanetal.1970[14]droppedahelmetedhead
formfixedtoadummyneckagainstarotatingsteeldisc.In2001Halldinetal.[15]designeda
newobliqueimpacttestformotorcyclehelmetsbasedonaninstrumentedfreeHybridIII
dummyheaddroppedverticallyagainstahorizontallymovingplate.MorerecentlyPangetal.
2011[16]publishedanovellaboratorytestinordertoinvestigateheadandneckresponses
underobliquemotorcyclehelmetimpactsusingamobileanvil.Thisproposalisbasedonatest
rigconsideringahelmetedHybridIIIheadfittedtotheHybridIIIneckitselffixedtoa20kg
masswhichdropsagainstaslidingplate.Thistestpermitstherecordingoftheheadkinematics
aswellasneckloading.Reportedtestconditionsarecharacterizedbya5.4to7.7m/simpact
velocityandaplatespeedadaptedtoprovidea45to90°impactangle.Impactdirectionsare
eitherfrontalorlateral.
ConcerningHokeyhelmets,Gerberichetal.(1987)[23]andFlicketal.2005[24]investigat
edhokeyheadtraumaandreconstructedexperimentallytypicalimpactconditionsappliedto
theHybridIIIheadandnecksystem.Itwasshownthatbothlinearandrotationalheadaccel
erationsaresignificantandcanpotentiallyleadtobraininjuryaslongasheadinjurycriteria
proposedbyZhangetal.2001[7]areconcerned.Morerecently,Rousseauetal.2009[25],
[26]developedahokeyhelmettestbenchwherethehelmetedHybridIIIheadwasfixedtoa
HybridIIIneckandimpactedfrontallyorlaterallywithanimpactor.Linearandrotationalhead
accelerationintherangeof100to120gand3to6krad/s²wererecordedrespectively.This
methodwasappliedtohelmetmaterialevaluationandshowedthatspecifichelmetstructure
canhaveverydifferentoutcomeintermsoflinearversusrotationalheadacceleration.Inaddi
tion,Rousseauetal.2009[27],investigatedtheinfluenceoftheimpactpointdeviationinre
gardstothecenterofmassaswellastheneckrigidityonthelinearandrotationalheadaccel
eration.Thisstudydemonstratedthatheadrotationalaccelerationisverysensitivetoimpact
positionrelativetothecenterofmassandthattheincreaseofneckrigidityleadstoaheadro
tationdecreasing.Moreover,Walsh2010[28]investigatedtheeffectofimpactdirectionina
5‐ 15°rangeonHybridIIIheadkinematic,anditappearedthatthisparameterinfluences
muchmoretherotationalresponseasthelinearone.Inordertofurtherinvestigatetheseas
pectsWalshetal.2009[29],investigatedhelmetedHybridIIIheadkinematicwhenfixedon
HybridIIIneckandimpactedatanumberofpointsaroundtheheadandbyconsideringseveral
impactangles.Firstresultisthatevenwhendirectedalongthecenterofmass,rotationalac
celerationcanbeashighas10krad/s².Moregenerally,itwasshownthatimpactangleinflu
encessignificantlybothlinearandrotationalacceleration.FinallyWalshetal.2011[30]consol
idatedtheseresultswith20furtherimpactsanddemonstratedthathighestlinearacceleration
wasobtainedforradialdirectedimpactforallimpactpoints.Inthisstudytheauthorsplotted
linearversusrotationalaccelerationforallimpacts.Purecorrelationwasfund(=0.4)
demonstratingthatbothinjuryparametersmustberecordedduringtestasonecannotbees
timatedmeanstheother.
Aslongasbicyclehelmetsareconcernednoimprovementofcurrentstandardtestshas
beenproposedtotheauthor’sknowledge.Thereforethepresentpaper’objectiveistopresent
aproposalontheimprovementofthedifferentaspectsofbicyclehelmettestmethod.
PresentedisaglobalcriticalanalysisoftheexistingEN1078bicyclehelmettestmethodand
theproposalofitsevolution.Atotaloffourseparateaspectswillbediscussed,i.e.headimpact
conditions,headsubstitute,headimpactlocationandheadinjurycriteria.
4
2HEADIMPACTCONDITIONS
Inthecurrentstandardtest,impactconditionsarecharacterizedbyalinearvelocityof5.42
m/sand4.57m/s,twotemperatures(20°C,+50°C)andonewet(+20°C)condition.Thehel
metedheadformimpactstwoanvils,flatat5.42m/sandcurbstoneat4.57m/sanditiswell
knownthattheheadisfreeatnecklevelforEN1078standard.
Iftheheadinitialvelocityseemstobereasonableinregardofrealworldaccidentsituation,
andfallalonesimulations,thefactthatthisvelocityhasonlyanormalcomponentisnotac
ceptable.Ithasbeendemonstratedby(Bourdetetal2012[10])thatasignificanttangential
velocityexistswhichleadstoheadrotationalaccelerationinadditiontothelinearaccelera
tion.Comingtothetemperatureitisimportanttomentionthatveryfewbicyclist’saccidents
occurattemperaturesaslowas‐20°Corat+50°C.WithinCOSTactionTU1101,ithasbeen
shown,basedon7180realworldbicycleaccidentsfromGIDAS,that92.6%oftheaccidentoc
curredunderdryweatherconditionsandthat96.8%happenedattemperaturesbetween1
and30°C(figure1).Itisthereforenolongeracceptablethathelmetoptimizationincludesma
terialbehavioratextremelowandhightemperatures.Inasimilarwayaccidentanalysisshows
thatbicyclistheadsonlyrarelyimpactcurbstoneandthatthemostfrequentimpactedsurfaces
areclearlyflatrigidsurfaces.WithinCOSTT1101andbasedontheGIDASdatabaseonly4.4%
ofthecasesrevealedaheadimpactagainstanangularsurface.
Followingtothepreviouscriticalanalysisofheadimpactcondition,thehereafterproposalis
made.First,justtwotemperatures(0°Cand+30°C)andthewetconditionat+20°Ccouldbe
considered.Inadditionitissuggestedtocanceltheimpactsagainstthecurbstoneandto
maintainonlyimpactsagainstflatanvils.Nochangeissuggestedforthelinearimpactvelocity
whichcouldremainat5.42m/s.However,basedonrealworldaccidentanalysisandsimulated
accidentsitisproposedtoincludethreetangentialimpacts,characterizedbyavelocityof6.5
m/sagainsta45°inclinedanvil.Theimpactpointswillbefurtherdefinedinthefollowingsec
tions.
Figure1:ResultsbasedonGIDASandillustratingthat96.8%oftheaccidentsoccurbetween1
and30°C.
5
3HEADFORMCHARACTERISTICS
Thecurrentheadformsarecharacterizedbyanondeformable“headshaped”masse.The
valueofthemassincludes“some”neckeffectsanditsinertiaisnotcontrolled.Itshouldbe
mentionedthatexistingsimpleheadformssuchasthePedestrianISOheadformpresentsin
ertiaquitefarfromhumanheadinertiaasshownintable1.Importantimprovementwouldbe
possiblebyreplacingtheseISOheadformsbyHybridIIIhead.Mainargumentstodosoare:
Morerealisticheadmass
Morerealisticheadinertia,anessentialaspectifrotationisconsidered
Deformableskin,anessentialaspectforhelmetoptimisation
EasylinktoHybridIIIneck
Possibilitytofixrotationaltransducers
Acriticalpointwhichhastobemanagedisthesizingaspect,ascurrentlyanumberofsizes
areavailablefortheISOcyclistormotorcyclisthelmetheadform.Thiscanbesolvedthrough
thedifferentversionoftheHybridIIIdummyheadsasillustratedintable2.Asitiswellknown
thatsizesA,C,E,J,MandOrepresent95%ofsizesusedinstandard,onlythesesizeswould
showinterest.Itappearsintable2thatthesesizeswouldadequatelybecoveredbyfivesizes
oftheHybridIIIheadsfamily.
Mass[kg]Ixx [kg.m²] I
yy
[kg.m²] Izz[kg.m²]
ISOPedestri
an4.511.10311.103110.5.103
HybridIII50t
h
4.517.088.10318.872.10322.685.103
HumanHead4.517.996.10318.360.10321.902.103
ISOHelmet5.7Notcontrolled
Table1.Synthesisofheadforminertialpropertiesandcomparisonwithhumanheadcharac
teristics.
EN960headform
size
Headcircum
ference[mm]DummymodelHeadcircum
ference[mm]
A500HybridIII3YearOld508
B510
C520HybridIII6YearOld520.7
D530
E540HIII5thFemale(or10years)538.5
F550HIII5thFemale(or10years)538.5
G560
J570HybridIII95thLargeMale584
K580
L590
M600HybridIII50thMale597
N610
O620HybridIII50thMale597
P630
Q640
Table2.ComparisonbetweentheEN960headformscircumferencesandHybridIIIdummy
heads.TheA,C,E,J,MandOsizesrepresent95%ofsizeusedinglobalstandardsandarecov
eredbytheHybridIIIheadsfamily
6
Afurtherquestionconcerningtheheadformisrelatedtoitsboundaryconditionatnecklevel.
AsforthetangentialimpactitisrecommendedtousetheHybridIIIheadanumericalsimula
tionhasbeenperformedwiththreehelmetedheadformmodels,i.e.ISOheadform,HybridIII
headandHybridIIIheadconnectedtoHybridIIIneckinframeworkofCostTU0111.Impacts
weresimulatedunderoccipitalimpactagainsta45°inclinedanvil.Resultsshowninfigure2
demonstratethatISOheadformhasaverynonrealisticresponseduetoitsnoncontrolledin
ertia.OntheotherhandbothHybridIIIandcoupledHybridIIIheadandneckpresentsimilar
resultsintermsofrotationalaccelerationduringthe10firstmilliseconds.Obviouslytheneck
influencestheheadkinematiclongeraftertheimpactitself.
Figure2:Rotationalaccelerationcomputedwiththreedifferenthelmetedheadformsand
headboundaryconditionsunderoccipitalimpactagainsta45°inclinedanvil.
4IMPACTPOINTS
InthecurrentstandardimpactpointsarechosenanywhereabovetheRR’testlineasillus
tratedinfigure3.Accidentinvestigations(Bourdetetal2012[10])clearlyshowthatacritical
areaisthetemporalzoneandthisaspectshouldbeimproved.Thereforethepresentproposal
suggeststolowerthetestlineinasimilarwayassuggestedbyOtteetal2013[31]inframe
workofCostactionTU0111(figure3).TheselinearimpactsshouldbeconductedwithHybrid
IIIheadaloneandstrictlyinlinewithG(centreofmassofthehelmetedhead).Gyrometer
shouldcontrolthisaspectandaverylowacceptedrotationalaccelerationshouldbeimposed.
7
Figure3:Illustrationofcurrenttestlinea),impactpointstothehead(Bourdetetal2012b)
andproposalofnewtestlineasfromOtteetal2013c)
Comingtothetangentialimpacttests,theISOheadformshasbeenreplacedbytheHybrid
IIIhead,asthisdummyheadhasmuchmorerealisticinertiaproperties.Forthesetestsitis
suggestedtofreelydropthehelmetedheadwitha6.5m/sinitialvelocityagainsta45°inclined
anvilandtorecordrotationalaccelerationinadditiontothelinearacceleration.Thefirstpro
posedimpactisatangentialimpactinthesagittalplan(pointA)leadingtorotationaroundthe
Yaxis(lateralrighttoleft)direction.Thetwonexttangentialimpactsarelocatedatparietal
level(pointsBandC)andwillbeappliedinthefrontalplane,oneintroducingrotationaround
theX(posteroanterior)directionandoneintroducingarotationaroundtheZ(verticalas
cendant)direction(figure4).
Front_xLat_xLat_z
Figure4:Illustrationofthethreetangentialimpactconditions.Fromlefttoright,impactintro
ducingangularaccelerationaroundYaxes(calledFront_x),Xaxes(calledLat_x)andfinallyZ
axes(calledLat_z)
8
5HEADINJURYCRITERIA
Currentlythresholdsconcerninghelmetperformancearesetintermsofmaximumhead
formacceleration(fixedat250G)accordingtotheWSUtolerancecurvereportedinfigure5
andproposedinthe1950’s.Toprotecttheheadinanautomotiveenvironment,HIChasbeen
introducedinthe1970’sasrecalledinfigure5.Thiscriteria,isbasedonthelinearheadaccel
erationevolutionovertimeandhasbeensetataround1000forlinearfrontaloroccipitalim
pact.Formotorcyclehelmets,thiscriterionhasbeensetatHIC2400whichhasnosenseina
biomechanicalpointofview.ForbicyclehelmetsHICisnotconsidered.Inadditionitmustbe
recalledthatHICdoesnotintegratelateraldirectionorrotationalaccelerationandisunableto
predictskullfracture.Itisthereforeaverylimitedheadinjurycriterion.
Figure5:IllustrationoftheWSUheadtolerancecurve(1950’s)andexpressionofHICcriterion
(1970’s)
Intheliterature,rotationalaccelerationlimitsareproposedat8to10krad/s2.Itmusthow
everberecalledthatheadtolerancelimittorotationisstronglytimeanddirectiondependant
sothattodaythereisnosinglelimitknown.
WithinEUprojectAPROSYSSP5andFrenchPREDITprojectsPROTEUSandBIOCASQ,im
provedheadinjurycriteriatospecificinjurymechanismshavebeendefinedtakingintoconsid
erationthetimeevolutionofbothlinearandrotationalheadaccelerationasrecalledhereaf
ter.
Today,stateoftheartFEheadmodelsexistandhavebeenusedforthedefinitionofinjury
criteriatospecificinjurymechanisms.Thesemodelsbecamemuchmorepowerfulinjurypre
dictiontoolsthatHIC,sothepresentproposalistoimplementimproved,modelbasedheadin
jurycriteriaintoanewhelmetimpacttestprocedure.Basedonthesimulationofnearly100
welldocumentedheadtrauma,tolerancelimitshavebeenidentifiedwithrespecttomoderate
andsevereneurologicalinjury.Humanheadtolerancelimitsrelativetoneurologicalinjuries
withariskofoccurrenceof50%wereestablishedasfollows:
AbrainVonMisesstrainreaching20%formoderateneurologicalinjury.
AbrainVonMisessstrainreaching35%forsevereneurologicalinjury.
Finallyaglobalstrainenergyofthesubarachnoidspaceexceeding4.2Jgenerates
subduralandsubarachnoidhaematoma,andalocalstrainenergyintheskullof439
mJfora50%riskofaskullfracture.
Intheproposedapproachtheexperimentalheadlinearandrotationalheadaccelerationwill
constitutetheinputswhichwilldrivetheheadFEmodel,inchargeofthelattertocomputethe
injuryparametersrelatedto,subduralhaematomaandneurologicalinjury.Bythismethodolo
gyitwillbepossibletopredictheadinjuryriskmeansacoupledexperimentalversusvirtual
testingprocedureasillustratedinfigure6.

2
1
2.5
21 21
1
()
t
t
HIC t t adt
tt





9
Figure6:Illustrationofthecoupledexperimentalversusnumericalheadimpacttestmethod
basedonnovelmodelbasedheadinjurycriteria.
6CONCLUSION
ThispaperpresentsaproposalforapossibleevolutionofcurrentEN1078bicyclehelmet
standard.Atotaloffourkeyaspectshavebeenreviewedinacriticalway,i.e.headimpact
conditions,headsurrogate,headimpactlocationandheadinjurycriteria.Foreachoftheseis
suesaconcreteimprovementproposalhasbeenmadeinordertoopenadiscussiononfurther
researchneeded.Atheadimpactconditionslevelitisproposedtoimplementattangential
headimpacttestswithahelmetedHybridIIIhead.Thisimprovedheadhasalsotheadvantage
ofmorerealisticinertialpropertiesandinterfacecharacteristicsbetweenheadformandhel
met.Itisalsosuggestedtoremovetestsunderextremetemperatureoragainstthecurbstone
anvil.Furtherimprovementconcernstheimpactlocationasthecurrenttestlineexcludesany
impacttothetemporalregionwhichhasbeenshowntobeacriticalarea.Afinalkeyevolution
whichisproposedconcernstheassessmentoftheheadinjuryriskforwhichacoupledexperi
mentalversusnumericmethodisproposedinordertointroducemodelbasedheadinjurycri
teria.Itisexpectedthattheevolutionofhelmetstandardtestmethodwillenableadvanced
helmetevaluationandoptimizationagainstbiomechanicalcriteria.
Acknowledgement:TheauthorswishtoacknowledgetheFrenchDepartmentofTransport
fortheirsupportinframeworkofPREDITBICTETEproject,theECforCostactionTU1011and
FondationMAIFfortheirsupport.
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... Specifically, we first introduce the conventional bicycle helmets, discuss the current widely adopted testing standards for bicycle helmets, and these are subsequently followed by a detailed review of the recent innovation designs of the current commercially available bicycle helmets, and their protective performance. We will also discuss about the latest anti-rotational features in the helmet designs since rotational acceleration was found to be more dominant in terms of brain injury [36][37][38]. Current challenges, limitations and future directions for exploration will be identified at last. ...
... In particular, the test standards solely consider the linear kinematics of the head without taking the rotational acceleration into account. However, it is generally believed that rotational acceleration is more dominant in terms of damaging intracranial injuries, such as diffuse axonal injury (DAI) caused by shearing of brain tissues, as well as tearing of parasagittal bridging veins [36][37][38]42]. As doubted by Stigson [43], current bicycle helmets tested under these standards, may not have the capability in reducing rotational acceleration. ...
... Stigson [43] also argued that the linear acceleration threshold could be relatively high, as research demonstrated that a lower value of linear acceleration (i.e. from 250 g to 180 g) would lead to a lower risk of skull fracture [44]. Therefore, researchers in the field are proposing various advanced test methods for bicycle helmet, taking into account the translational and rotational effects [37]. These newly proposed tests will still need further improvement and standardizations before being considered mandatorily in all the certified bicycle helmet testing standards by legislative regulations. ...
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Cycling becomes more and more popular for recreation, exercising and commuting in many countries. Despite the popularity of cycling, it often comes with high level of risk. Conventional bicycle helmets, with expanded polystyrene foam (EPS) liners, can effectively mitigate impact by linear acceleration. However, there has been an urgent call for further improvement in bicycle helmets, as rotational acceleration has been found to be more dominant in severe head injuries. Recent advancements in manufacturing technologies have enabled various novel conceptual ideas and designs of bicycle helmets, which are previously deemed impracticable due to complexities in structures and materials, to become feasible. There are various bicycle helmet designs in terms of structures and materials used in both the helmet shell and liner, targeting at reducing both linear and rotational acceleration. Moreover, inspired by biological structures in nature, bio-inspired structures have been developed rapidly with excellent energy absorption capacity. As a piece of protective equipment, Bicycle helmet is a representative example where researchers are attempting to apply bio-inspired structures. The objective of the paper is to review the development of bicycle helmets and recent exploration for improvement. This includes the history of bicycle helmets, current test standards, designs of conventional bicycle helmets and the latest research (e.g. material replacement and novel structures), as well as application of bio-inspired structures to helmets. This review also identifies the limitations of current designs and standards, and challenges for future investigations.
... This does not fully reflect the scenario in a bike accident. In a fall or a collision, the impact to the head will be oblique (Willinger et al. 2014;Fahlstedt 2015;Bland et al. 2018). The intention was to simulate this in the test since it is known that angular acceleration is the dominating cause of brain injuries. ...
... Two helmets were tested in each test configuration to minimize variations. The test set-up used in the present study corresponds to a proposal from the CEN Working Group's 11 "Rotational test methods" (Willinger et al. 2014). ...
Technical Report
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... 30,35 This headform has been used in a number of previous studies as its mass and moment of inertia better represents the 50th percentile human head. 1,29,57 The helmet was fitted onto the HIII headform based on the helmet fitting requirements of ECE22.06. The helmeted headform was then placed on a free-falling U-shape testing platform (3.2 kg). ...
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Motorcyclists are at high risk of head injuries, including skull fractures, focal brain injuries, intracranial bleeding and diffuse brain injuries. New helmet technologies have been developed to mitigate head injuries in motorcycle collisions, but there is limited information on their performance under commonly occurring oblique impacts. We used an oblique impact method to assess the performance of seven modern motorcycle helmets at five impact locations. Four helmets were fitted with rotational management technologies: a low friction layer (MIPS), three-layer liner system (Flex) and dampers-connected liner system (ODS). Helmets were dropped onto a 45° anvil at 8 m/s at five locations. We determined peak translational and rotational accelerations (PTA and PRA), peak rotational velocity (PRV) and brain injury criteria (BrIC). In addition, we used a human head finite element model to predict strain distribution across the brain and in corpus callosum and sulci. We found that the impact location affected the injury metrics and brain strain, but this effect was not consistent. The rear impact produced lowest PTAs but highest PRAs. This impact produced highest strain in corpus callosum. The front impact produced the highest PRV and BrIC. The side impact produced the lowest PRV, BrIC and strain across the brain, sulci and corpus callosum. Among helmet technologies, MIPS reduced all injury metrics and brain strain compared with conventional helmets. Flex however was effective in reducing PRA only and ODS was not effective in reducing any injury metrics in comparison with conventional helmets. This study shows the importance of using different impact locations and injury metrics when assessing head protection effects of helmets. It also provides new data on the performance of modern motorcycle helmets. These results can help with improving helmet design and standard and rating test methods.
... Helmets are being improved for protecting the brain against rotational effects of head impacts, which produce axonal injuries and longterm effects (Khosroshahi et al., 2019;Siegkas et al., 2019;Abayazid et al., 2021). This is further driven by new methods for assessing helmet performance in mitigating rotational effects of oblique impacts (Ghajari et al., 2013;Willinger et al., 2014;Stigson et al., 2017). Our results show that most of the simulated falls lead to an oblique impact with the ground. ...
Article
E-scooters are the fastest growing mode of micro-mobility with important environmental benefits. However, there are serious concerns about injuries caused by e-scooter accidents. Falls due to poor road surface conditions are a common cause of injury in e-scooter riders, and head injuries are one of the most common and concerning injuries in e-scooter falls. However, the head-ground impact biomechanics in e-scooter falls and its relationship with e-scooter speed and design, road surface conditions and wearing helmets remain poorly understood. To address some of these key questions, we predicted the head-ground impact force and velocity of e-scooter riders in different falls caused by potholes. We used multi-body dynamics approach to model a commercially available e-scooter and simulate 180 falls using human body models. We modelled different pothole sizes to test whether the pothole width and depth influences the onset of falls and head-ground impact velocity and force. We also tested whether the e-scooter travelling speed has an influence on the head-ground impact velocity and force. The simulations were carried out with three human body models to ensure that the results of the study are inclusive of a wide range of rider sizes. For our 10 in. diameter e-scooter wheels, we found a sudden increase in the occurrence of falls when the pothole depth was increased from 3 cm (no falls) to 6 cm (41 falls out of 60 cases). When the falls occurred, we found a head-ground impact force of 13.2 ± 3.4kN, which is larger than skull fracture thresholds. The head-ground impact speed was 6.3 ± 1.4 m/s, which is the same as the impact speed prescribed in bicycle helmet standards. All e-scooter falls resulted in oblique head impacts, with an impact angle of 65 ± 10° (measured from the ground). Decreasing the e-scooter speed reduced the head impact speed. For instance, reducing the e-scooter speed from 30 km/h to 20 km/h led to a 14% reduction in the mean impact speed and 12% reduction in the mean impact force, as predicted by the models. The models also showed that the median male riders were sustaining higher head-ground impact force and speed compared with the small female and large male riders. The findings of this study can assist authorities and e-scooter hiring companies to take more informed actions about road surface conditions and speed limits. These results can also help define representative impact test conditions for assessing the performance of helmets used by e-scooter riders in order to reduce head and brain injuries in e-scooter falls.
... The influence of the neck and body in the kinematics of the headform was not included in this study. Several studies suggest that the neck and body play only a small role during helmeted head impacts [35,36] whereas other studies suggest that the presence of the neck and body have a significant influence on head kinematics and that it is strongly dependent on the impact configuration [37,38]. Isolated head surrogate was used in this study due to its simplicity and because this is the condition used in the European helmet testing regulation [26]. ...
Article
Full-text available
Oblique impacts of the helmet against the ground are the most frequent scenarios in real-world motorcycle crashes. The combination of two factors that largely affect the results of oblique impact tests are discussed in this work. This study aims to quantify the effect of the friction at the interface between the headform and the interior of a motorcycle helmet at different magnitudes of tangential velocity. The helmeted headform, with low friction and high friction surface of the headform, was dropped against three oblique anvils at different impact velocities resulting in three different magnitudes of the tangential velocity (3.27 m/s, 5.66 m/s, 8.08 m/s) with the same normal component of the impact velocity (5.66 m/s). Three impact directions (front, left-side and right-side) and three repetitions per impact condition were tested resulting in 54 impacts. Tangential velocity variation showed little effect on the linear acceleration results. On the contrary, the rotational results showed that the effect of the headform’s surface depends on the magnitude of the tangential velocity and on the impact direction. These results indicate that a combination of low friction with low tangential velocities may result into underprediction of the rotational headform variables that would not be representative of real-world conditions.
... As demonstrated by Meaney and Smith [20] and Willinger et al. [21], there is still inadequate protection by traditional helmets. Therefore, it is necessary to call for in-depth scientific research to escalate the level of bicycle helmet safety [22]. ...
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In this study, a novel expandable bicycle helmet, which integrates an airbag system into the conventional helmet design, was proposed to explore the potential synergetic effect of an expandable airbag and a standard commuter-type EPS helmet. The traumatic brain injury mitigation performance of the proposed expandable helmet was evaluated against that of a typical traditional bicycle helmet. A series of dynamic impact simulations on both a helmeted headform and a representative human head with different configurations were carried out in accordance with the widely recognised international bicycle helmet test standards. The impact simulations were initially performed on a ballast headform for validation and benchmarking purposes, while the subsequent ones on a biofidelic human head model were used for assessing any potential intracranial injury. It was found that the proposed expandable helmet performed admirably better when compared to a conventional helmet design—showing improvements in impact energy attenuation, as well as kinematic and biometric injury risk reduction. More importantly, this expandable helmet concept, integrating the airbag system in the conventional design, offers adequate protection to the cyclist in the unlikely case of airbag deployment failure.
... However, cyclists often fall off their bicycles and impact their heads at angles that are not always direct and usually varies between 30°and 60° (Bourdet et al., 2012;Bourdet et al., 2014). These impacts not only can cause linear acceleration but can also result in rotational acceleration due to the tangential forces to the head (McIntosh et al., 2013;Willinger et al., 2019). Many studies have shown that the rotational acceleration or rotational velocity rather than the linear acceleration are responsible for causing large shear strains in the brain tissue, which could lead to strain concentration (Laksari et al., 2015;Laksari et al., 2018;Abderezaei et al., 2019;Laksari et al., 2020;Mojahed et al., 2020), and potentially result in mild TBI (Holbourn, 1943;Holbourn, 1944;Hardy et al., 2007;Post and Blaine Hoshizaki, 2015;Deck et al., 2019). ...
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Full-text available
Cycling accidents are the leading cause of sports-related head injuries in the US. Conventional bicycle helmets typically consist of polycarbonate shell over Expanded Polystyrene (EPS) foam and are tested with drop tests to evaluate a helmet’s ability to reduce head kinematics. Within the last decade, novel helmet technologies have been proposed to mitigate brain injuries during bicycle accidents, which necessitates the evaluation of their effectiveness in impact testing as compared to conventional helmets. In this paper, we reviewed the literature to collect and analyze the kinematic data of drop test experiments carried out on helmets with different technologies. In order to provide a fair comparison across different types of tests, we clustered the datasets with respect to their normal impact velocities, impact angular momentum, and the type of neck apparatus. When we analyzed the data based on impact velocity and angular momentum clusters, we found that the bicycle helmets that used rotation damping based technology, namely MIPS, had significantly lower peak rotational acceleration (PRA) and Generalized Acceleration Model for Brain Injury Threshold (GAMBIT) as compared to the conventional EPS liner helmets (p < 0.01). SPIN helmets had a superior performance in PRA compared to conventional helmets (p < 0.05) in the impact angular momentum clustered group, but not in the impact-velocity clustered comparisons. We also analyzed other recently developed helmets that primarily use collapsible structures in their liners, such as WaveCel and Koroyd. In both of the impact velocity and angular momentum groups, helmets based on the WaveCel technology had significantly lower peak linear acceleration (PLA), PRA, and GAMBIT at low impact velocities as compared to the conventional helmets, respectively (p < 0.05). The protective gear with the airbag technology, namely Hövding, also performed significantly better compared to the conventional helmets in the analyzed kinematic-based injury metrics (p < 0.001), possibly due to its advantage in helmet size and stiffness. We also observed that the differences in the kinematic datasets strongly depend on the type of neck apparatus. Our findings highlight the importance and benefits of developing new technologies and impact testing standards for bicycle helmet designs for better prevention of traumatic brain injury (TBI).
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The risk of brain trauma has been associated with the rotational kinematics leading to the development of helmets with a variety rotational management technologies. The purpose of this paper was to employ a rotation specific test protocol to evaluate the effectiveness of two of these technologies. Dynamic response of the head was measured to assess the performance of each technology. Three cycling helmets with identical construction were included in this study. One helmet with no rotational technology, an established, commercial technology and a novel helmet rotational technology designed and assembled by the authors were tested. A drop test onto a 45° anvil was used to measure the ability of each helmet to manage the dynamic response of the head form during a series of impacts. The results revealed both rotational helmet technologies resulted in lower peak rotational acceleration and brain strain, however each technology demonstrated unique performance characteristics depending on the impact condition.
Technical Report
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In the United States, all bicycle helmets must comply with the standard created by the Consumer Product Safety Commission (CPSC). In this standard, bike helmets are only required to by tested above an established test line. Unregulated helmet performance below the test line could pose an increased risk of head injury to riders. This study quantified the impact locations of damaged bike helmets from real-world accidents and tested the most commonly impacted locations under CPSC bike helmet testing protocol. Ninety-five real-world impact locations were quantified. The most common impact locations were side-middle (31.6%), rear boss-rim (13.7%), front boss-rim (9.5%), front boss-middle (9.5%), and rear boss-middle (9.5%). The side-middle, rear boss-rim, and front boss (front boss-middle and front boss-rim regions combined) were used for testing. Two of the most commonly impacted regions were below the test line (front boss-rim and rear boss-rim). Twelve purchased helmet models were tested under CPSC protocol at each location for a total of 36 impacts. An ANOVA test showed that impact location had a strong influence on the variance of peak linear acceleration (PLA) ( p = 0.002). A Tukey HSD post hoc test determined that PLA at the side-middle (214.9 ± 20.8 g) and front boss (228.0 ± 39.6 g) locations were significantly higher than the PLA at the rear boss-rim (191.5 ± 24.2 g) location. The highest recorded PLA (318.8 g) was at the front boss-rim region. This was the only test that exceeded the 300 g threshold. This study presented a method for quantifying real-world impact locations of damaged bike helmets. Higher variance in helmet performance was found at the regions on or below the test line than at the region above the test line.
Article
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Rotational motion of the head as a mechanism for brain injury was proposed back in the 1940s. Since then a multitude of research studies by various institutions were conducted to confirm/reject this hypothesis. Most of the studies were conducted on animals and concluded that rotational kinematics experienced by the animal's head may cause axonal deformations large enough to induce their functional deficit. Other studies utilized physical and mathematical models of human and animal heads to derive brain injury criteria based on deformation/pressure histories computed from their models. This study differs from the previous research in the following ways: first, it uses two different detailed mathematical models of human head (SIMon and GHBMC), each validated against various human brain response datasets; then establishes physical (strain and stress based) injury criteria for various types of brain injury based on scaled animal injury data; and finally, uses Anthropomorphic Test Devices (ATDs) (Hybrid III 50th Male, Hybrid III 5th Female, THOR 50th Male, ES-2re, SID-IIs, WorldSID 50th Male, and WorldSID 5th Female) test data (NCAP, pendulum, and frontal offset tests) to establish a kinematically based brain injury criterion (BrIC) for all ATDs. Similar procedures were applied to college football data where thousands of head impacts were recorded using a six degrees of freedom (6 DOF) instrumented helmet system. Since animal injury data used in derivation of BrIC were predominantly for diffuse axonal injury (DAI) type, which is currently an AIS 4+ injury, cumulative strain damage measure (CSDM) and maximum principal strain (MPS) were used to derive risk curves for AIS 4+ anatomic brain injuries. The AIS 1+, 2+, 3+, and 5+ risk curves for CSDM and MPS were then computed using the ratios between corresponding risk curves for head injury criterion (HIC) at a 50% risk. The risk curves for BrIC were then obtained from CSDM and MPS risk curves using the linear relationship between CSDM - BrIC and MPS - BrIC respectively. AIS 3+, 4+ and 5+ field risk of anatomic brain injuries was also estimated using the National Automotive Sampling System - Crashworthiness Data System (NASS-CDS) database for crash conditions similar to the frontal NCAP and side impact conditions that the ATDs were tested in. This was done to assess the risk curve ratios derived from HIC risk curves. The results of the study indicated that: (1) the two available human head models - SIMon and GHBMC - were found to be highly correlated when CSDMs and max principal strains were compared; (2) BrIC correlates best to both - CSDM and MPS, and rotational velocity (not rotational acceleration) is the mechanism for brain injuries; and (3) the critical values for angular velocity are directionally dependent, and are independent of the ATD used for measuring them. The newly developed brain injury criterion is a complement to the existing HIC, which is based on translational accelerations. Together, the two criteria may be able to capture most brain injuries and skull fractures occurring in automotive or any other impact environment. One of the main limitations for any brain injury criterion, including BrIC, is the lack of human injury data to validate the criteria against, although some approximation for AIS 2+ injury is given based on the angular velocities calculated at 50% probability of concussion in college football players instrumented with 5 DOF helmet system. Despite the limitations, a new kinematic rotational brain injury criterion - BrIC - may offer a way to capture brain injuries in situations when using translational accelerations based HIC alone may not be sufficient.
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The objective of this study was to determine the influence of impact deflection and neck compliance on the Gadd severity index (GSI), peak linear acceleration, and peak angular acceleration during a front impact to a Hybrid III head using a pneumatic linear impactor. Impact deflection was performed by translating the headform laterally and was shown to be effective at reducing the linear and angular accelerations as well as the GSI. Neck compliance was altered using one Hybrid III 50th percentile neck and two modified Hybrid III necks. A less compliant neck increased linear acceleration but decreased angular acceleration and GSI. When compared with estimated injury thresholds, the results demonstrated that an increase in the lateral translation or a decrease in the neck compliance resulted in a significant decrease in the risk of injury as reflected by peak linear and angular accelerations and the GSI.
Article
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Head injury remains one of the most frequent and severe injuries sustained by vehicle occupants, motorcyclists, pedestrians and cyclists in road accidents and account for approximately 40% of road fatalities in the European Union (EU). One essential requirement for reducing the incidence of fatal and severe head injuries is to develop head injury assessment methods that can accurately and comprehensively assess the potential head injury risk under a broad range of head impact conditions. At present, the most widely accepted method of assessing head injury risk in road safety research is the Head Injury Criterion (HIC). However, HIC only considers the injury risk to the head resulting from linear head accelerations. In an attempt to develop improved head injury criteria for specific mechanisms, 68 head impact conditions that occurred in motor sport, motorcyclist, American football and pedestrian accidents were re-constructed with a state of the art finite element (FE) human head model (ULP head model). Statistical regression analysis was then carried out on the head loading parameters from the accidents, such as the peak linear and rotational acceleration of the head, and predictions from the head model, such as the Von Mises stress or strain and pressure in the brain, in order to determine which of the investigated parameters provided the most accurate metrics for the injuries sustained in the real world head trauma under consideration. The results show that Von Mises shearing strain within the brain is much better correlated with moderate Diffuse Axonal Injuries (DAI) as HIC or acceleration peaks are. For severe DAI, however, this improvement is less important. Another significant improvement of injury prediction based on FE head model is the one related to skull fracture, for which the proposed criteria present a higher correlation factor than HIC. Finally, SubDural Haematomas (SDH) are also better predicted with the FE model than HIC even if improvement is still needed for this injury mechanism.
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This study deals with the risk of injury to the bicyclist's head and the benefits of wearing a bicycle helmet in terms of reduction of injury severity or even injury avoidance. The accident data of 4,245 injured bicyclists as a randomized sample, collected by a scientific research team within the GIDAS project (German In-Depth Accident Study) were analyzed. Given that head injuries result in approximately 40% of bicycle-related crashes, helmet usage provides a sensible first-level approach for improving incidence and severity of head injuries. The effectiveness of the bicycle helmet was examined using descriptive and multivariate analysis for 433 bicyclists with a helmet and 3,812 bicyclists without a helmet. Skull fractures, severe brain injuries and skull base fractures were up to 80% less frequent for bicyclists wearing a helmet. Among individuals 40 years of age and older, a significant increase of severe head injuries occurred if no helmet was used compared to younger persons with helmet.
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A new oblique impact test for motorcycle helmets is described, simulating a fall from a motorcycle on to the road surface or the windshield of a car. An instrumented headform falls vertically to impact a horizontally moving rigid rough or deformable surface. Both the impact site on the helmet, and the vertical and horizontal velocities, can be varied, while the headform linear and rotational accelerations are measured. The rig was used to compare a new helmet design with current helmets, which are designed to pass impact tests in which the impact force is perpendicular to the helmet surface. The new design, which has a low friction layer between the shell and the liner, reduced, by up to 50%, the rotational acceleration of the head compared with conventional designs.
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American football reports high incidences of head injuries, in particular, concussion. Research has described concussion as primarily a rotation dominant injury affecting the diffuse areas of brain tissue. Current standards do not measure how helmets manage rotational acceleration or how acceleration loading curves influence brain deformation from an impact and thus are missing important information in terms of how concussions occur. The purpose of this study was to investigate a proposed three-dimensional impact protocol for use in evaluating football helmets. The dynamic responses resulting from centric and non-centric impact conditions were examined to ascertain the influence they have on brain deformations in different functional regions of the brain that are linked to concussive symptoms. A centric and non-centric protocol was used to impact an American football helmet; the resulting dynamic response data was used in conjunction with a three-dimensional finite element analysis of the human brain to calculate brain tissue deformation. The direction of impact created unique loading conditions, resulting in peaks in different regions of the brain associated with concussive symptoms. The linear and rotational accelerations were not predictive of the brain deformation metrics used in this study. In conclusion, the test protocol used in this study revealed that impact conditions influences the region of loading in functional regions of brain tissue that are associated with the symptoms of concussion. The protocol also demonstrated that using brain deformation metrics may be more appropriate when evaluating risk of concussion than using dynamic response data alone.
Article
In this study a new method for enhancing helmet performances during an impact is considered. In a first step an approved composite helmet finite element model is coupled with an anatomical head finite element model and is evaluated in terms of injury risks (risks of neurological injuries or subdural haematoma) under normative impact conditions (ECE 22.05 standard). Results show that the risk of head injuries is very high even if the considered helmet passes the standard. Thus a second step consists in proposing a new method for improving the helmet behaviour in case of impact by focusing on the outer shell characteristics and by assessing the head injury risk with the human head finite element model. A modal analysis of the entire helmet model is performed and makes it possible to define areas of the outer shell to be modified. Under standard impact conditions this new virtual helmet model conduces to a huge decrease of the head injury risk, both in terms of neurological injuries as well as in terms of subdural haematoma.