Towards advanced bicycle helmet test methods

Abstract

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.

Full text

Proceedings,InternationalCyclingSafetyConference2014
18‐19November2014,Göteborg,Sweden
ABSTRACT
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head
formitselfwithitsinstrumentation,thegeometryoftheimpactedsurface,theheadimpactlo‐
cation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.There‐
fore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con‐
ditions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evalua‐
tionandoptimizationofbicyclehelmetsagainstbiomechanicalcriteriaandunderrealisticim‐
pactconditions.
Keywords:bicyclehelmet,testmethod,impactconditions,headinjurycriteria
Towardsadvancedbicyclehelmettestmethods

R.Willinger1,C.Deck1P.Halldin2,D.Otte3

1ICUBE,CNRS‐Biomechanics
UniversityStrasbourg
2rBoussingault,Strasbourg,France
e‐mail:remy.willinger@unistra.fr
2KTHandMIPS
UniversityStockholm
Stockholm,Sweden
e‐mail:peter.halldin@kth.se
3MUH
UniversityofHannover
Hannover,Germany
e‐mail:dietmar.otte@MUH.de

2
1INTRODUCTION
Itiswellknowninthescientificcommunitythatheadrotationalaccelerationisacritical
headloadingwhichcanleadtobraininjury.Concerningneurologicalinjuries,Holbourn(1943)
[1]suggestedthattherotationalaccelerationinducedbyagivenimpactcauseshighshear
strainsinthebrain,thusrupturingthetetheringcerebralbloodvessels,neoandsubcortical
tissue.Thisauthorwasthefirstwhosuggeststheimportanceofrotationalaccelerationinthe
appearanceofcerebralconcussion.In1967,Ommayaetal.[2]proposedamethodinorderto
extendtheresultsofexperimentsonconcussionproducingheadrotationsonlowerprimate
subjectstopredicttherotationsrequiredtoproduceconcussionsinman.Achartofangular
accelerationrequiredtoreproduceconcussionintherhesusmonkeyindicatesthatanacceler‐
ationof40000rad/s²willhavea99%probabilityofproducingconcussionwhichwasexpectto
correspondstoanangularaccelerationof7500rad/s²forhuman.
Ommayaetal.(1968)[3]studiedtheeffectofwhiplashinjuryonrhesusmonkeysand
showedthatiftheheadwassubjectedtoarotationalaccelerationaboveathresholdvalue,
subduralandsubarachnoidinjurieswereobtained.Inastudybasedonprimates,Gennarelliet
al.(1982)[4]proposedthatarotationalaccelerationexceeding175000rad/s²wouldproduce
SDHintherhesusmonkey.Withtheobjectivetoinvestigatetheinfluenceoftheheadrota‐
tionalaccelerationontheintra‐cerebralmechanicalparametersunderaccidentalheadimpact,
atotalof69realworldheadtraumaweresimulatedwithandwithoutconsideringtheangular
rotationbyDecketal.2007[5].Thenumericalsimulationoftheseheadtraumabyconsidering
linearandrotationalaccelerationontheonehandandlinearheadaccelerationonlyonthe
otherhandpermittedittodemonstrateandtoexpressquantitativelythedramaticinfluence
oftherotationalaccelerationonbothintra‐cerebralloadingandbrain‐skullrelativemotion,
supposedtoleadrespectivelytoneurologicalinjuriesandsubduralhaematomarespectively.In
thisstudytheeffectofangularaccelerationwasfoundtoincreasetheintracerebralshearing
stressforallaccidentcasesconsideredofabout50%whatevertheimpactseveritywas.Kleiv‐
enetal.2007[6]aswellasZhangetal.2001[7]demonstratedthattheangularkinematicsof
theheadwasthemostimportantfactorindeterminingthebrainstrain,basedonnumerical
simulationofrealworldheadtrauma.MorerecentlyTakhountsetal[8],[9]usedaheadFE
modelinordertoestablishaheadinjurycriteriaforrotationalaccelerationcalledBRIC.
Inparallelwiththedemonstrationofthecriticalroleofheadangularaccelerationinbrain
injury,anumberofstudiesfocussedontheheadkinematicsinrealworldaccidentinorderto
demonstratethatatangentialloadingoftheheaddoesexistinadditiontothenormalimpact
velocity.Millsetal.(1996)[10]showedthatobliqueimpactsarethemostcommonsituations
inmotorcyclecrashes.MorerecentlyBourdetetal.(2011,2012)[11],[12]quantifiedthehead
rotationalaccelerationduetothetangentialcomponentoftheheadimpact,byreconstructing
realworldandvirtualmotorcycleaswellasbicycleaccidents.
Despitethiswidelyrecognizedunderstandingofheadrotationalloadingandeffectofthe
inducedrotationalaccelerationtothebrain,noheadprotectionstandardiscurrentlyconsider‐
ingheadrotationalacceleration.OnlyECER2205EU[13]motorcyclehelmetstandardconsid‐
ersatangentialimpactconditionbuthelmetevaluationislimitedtotherecordingofthetan‐
gentialforcewhichinformsaboutfrictioncharacteristicsofthehelmetbutnotaboutits
protectioncapabilityagainsttangentialimpacts.Onepossiblereasonforthecurrentsituation
besidetheincreasedcomplexityandcostofthetestdeviceitselfisthatnoacceptedheadro‐
tationthresholdhasbeenestablishedyet.Anumberofmaximumheadrotationalaccelera‐
tionshavebeenproposedintheliterature[10],[14]–[16]butnoneofthemconsiderthetime
evolutionofthisparameter.Moreover,itisobviousthatthemaximumheadrotationalaccel‐
erationisafunctionofrotationaxisandcombinedlinearacceleration,twoaspectswhichare
nottakenintoaccountinexistingproposals.Totheauthor’sopinion,theonlywaytointegrate
thecomplexityofbraingeometryandbrainmaterialpropertiesistoprogresstowardstissue
levelbraininjurycriteriaasproposedinexistingFEmodelbasedheadinjurycriteria[6],[7],
[17].In2004Deckdemonstratesthathelmetoptimizationstronglydependsontheheadsub‐
stituteandinjurycriteriatakenintoaccount.Morerecentlyfirstattemptsweremade(Tinard
etal.2012[18])inordertooptimizenewhelmetsagainstbiomechanicalcriteriabycoupling
thehumanheadmodeltoahelmetFEmodel.Advancedmodelbasedheadinjurycriteriahave

3
alsobeensuggestedinrecentattemptstoimprovebicyclehelmettestmethods(Decketal
2012)[19].
InthedomainofbicyclehelmetevaluationMilneetal2012and2013[20]suggestedanew
helmetassessmentmethodusingmodelbasedheadinjurycriteriaunderbothlinearandtan‐
gentialimpactconditions,exactlyasHansenetal2013[21]inthecontextofthedevelopment
ofanadvanced'honeycomb"bicyclehelmet.Inasimilarway,butinthecontextofhockey
helmetevaluationPostetal.2013[22]suggestedtoimpactahelmetedHybridIIIheadneck
systemandtointroducethelinearandrotationalaccelerationintoanexistingheadFEmodel
inordertoassesstheinjuryrisk.
Inordertoprogressinthefieldofhelmetprotectionagainsttangentialimpactsanumberof
attemptswereproposedintheliterature.Aldmanetal.1970[14]droppedahelmetedhead‐
formfixedtoadummyneckagainstarotatingsteeldisc.In2001Halldinetal.[15]designeda
newobliqueimpacttestformotorcyclehelmetsbasedonaninstrumentedfreeHybridIII
dummyheaddroppedverticallyagainstahorizontallymovingplate.MorerecentlyPangetal.
2011[16]publishedanovellaboratorytestinordertoinvestigateheadandneckresponses
underobliquemotorcyclehelmetimpactsusingamobileanvil.Thisproposalisbasedonatest
rigconsideringahelmetedHybridIIIheadfittedtotheHybridIIIneckitselffixedtoa20kg
masswhichdropsagainstaslidingplate.Thistestpermitstherecordingoftheheadkinematics
aswellasneckloading.Reportedtestconditionsarecharacterizedbya5.4to7.7m/simpact
velocityandaplatespeedadaptedtoprovidea45to90°impactangle.Impactdirectionsare
eitherfrontalorlateral.
ConcerningHokeyhelmets,Gerberichetal.(1987)[23]andFlicketal.2005[24]investigat‐
edhokeyheadtraumaandreconstructedexperimentallytypicalimpactconditionsappliedto
theHybridIIIheadandnecksystem.Itwasshownthatbothlinearandrotationalheadaccel‐
erationsaresignificantandcanpotentiallyleadtobraininjuryaslongasheadinjurycriteria
proposedbyZhangetal.2001[7]areconcerned.Morerecently,Rousseauetal.2009[25],
[26]developedahokeyhelmettestbenchwherethehelmetedHybridIIIheadwasfixedtoa
HybridIIIneckandimpactedfrontallyorlaterallywithanimpactor.Linearandrotationalhead
accelerationintherangeof100to120gand3to6krad/s²wererecordedrespectively.This
methodwasappliedtohelmetmaterialevaluationandshowedthatspecifichelmetstructure
canhaveverydifferentoutcomeintermsoflinearversusrotationalheadacceleration.Inaddi‐
tion,Rousseauetal.2009[27],investigatedtheinfluenceoftheimpactpointdeviationinre‐
gardstothecenterofmassaswellastheneckrigidityonthelinearandrotationalheadaccel‐
eration.Thisstudydemonstratedthatheadrotationalaccelerationisverysensitivetoimpact
positionrelativetothecenterofmassandthattheincreaseofneckrigidityleadstoaheadro‐
tationdecreasing.Moreover,Walsh2010[28]investigatedtheeffectofimpactdirectionina
5‐ 15°rangeonHybridIIIheadkinematic,anditappearedthatthisparameterinfluences
muchmoretherotationalresponseasthelinearone.Inordertofurtherinvestigatetheseas‐
pectsWalshetal.2009[29],investigatedhelmetedHybridIIIheadkinematicwhenfixedon
HybridIIIneckandimpactedatanumberofpointsaroundtheheadandbyconsideringseveral
impactangles.Firstresultisthatevenwhendirectedalongthecenterofmass,rotationalac‐
celerationcanbeashighas10krad/s².Moregenerally,itwasshownthatimpactangleinflu‐
encessignificantlybothlinearandrotationalacceleration.FinallyWalshetal.2011[30]consol‐
idatedtheseresultswith20furtherimpactsanddemonstratedthathighestlinearacceleration
wasobtainedforradialdirectedimpactforallimpactpoints.Inthisstudytheauthorsplotted
linearversusrotationalaccelerationforallimpacts.Purecorrelationwasfund(R²=0.4)
demonstratingthatbothinjuryparametersmustberecordedduringtestasonecannotbees‐
timatedmeanstheother.
Aslongasbicyclehelmetsareconcernednoimprovementofcurrentstandardtestshas
beenproposedtotheauthor’sknowledge.Thereforethepresentpaper’objectiveistopresent
aproposalontheimprovementofthedifferentaspectsofbicyclehelmettestmethod.
PresentedisaglobalcriticalanalysisoftheexistingEN1078bicyclehelmettestmethodand
theproposalofitsevolution.Atotaloffourseparateaspectswillbediscussed,i.e.headimpact
conditions,headsubstitute,headimpactlocationandheadinjurycriteria.



4
2HEADIMPACTCONDITIONS
Inthecurrentstandardtest,impactconditionsarecharacterizedbyalinearvelocityof5.42
m/sand4.57m/s,twotemperatures(‐20°C,+50°C)andonewet(+20°C)condition.Thehel‐
metedheadformimpactstwoanvils,flatat5.42m/sandcurbstoneat4.57m/sanditiswell
knownthattheheadisfreeatnecklevelforEN1078standard.
Iftheheadinitialvelocityseemstobereasonableinregardofrealworldaccidentsituation,
andfallalonesimulations,thefactthatthisvelocityhasonlyanormalcomponentisnotac‐
ceptable.Ithasbeendemonstratedby(Bourdetetal2012[10])thatasignificanttangential
velocityexistswhichleadstoheadrotationalaccelerationinadditiontothelinearaccelera‐
tion.Comingtothetemperatureitisimportanttomentionthatveryfewbicyclist’saccidents
occurattemperaturesaslowas‐20°Corat+50°C.WithinCOSTactionTU1101,ithasbeen
shown,basedon7180realworldbicycleaccidentsfromGIDAS,that92.6%oftheaccidentoc‐
curredunderdryweatherconditionsandthat96.8%happenedattemperaturesbetween1
and30°C(figure1).Itisthereforenolongeracceptablethathelmetoptimizationincludesma‐
terialbehavioratextremelowandhightemperatures.Inasimilarwayaccidentanalysisshows
thatbicyclistheadsonlyrarelyimpactcurbstoneandthatthemostfrequentimpactedsurfaces
areclearlyflatrigidsurfaces.WithinCOSTT1101andbasedontheGIDASdatabaseonly4.4%
ofthecasesrevealedaheadimpactagainstanangularsurface.
Followingtothepreviouscriticalanalysisofheadimpactcondition,thehereafterproposalis
made.First,justtwotemperatures(0°Cand+30°C)andthewetconditionat+20°Ccouldbe
considered.Inadditionitissuggestedtocanceltheimpactsagainstthecurbstoneandto
maintainonlyimpactsagainstflatanvils.Nochangeissuggestedforthelinearimpactvelocity
whichcouldremainat5.42m/s.However,basedonrealworldaccidentanalysisandsimulated
accidentsitisproposedtoincludethreetangentialimpacts,characterizedbyavelocityof6.5
m/sagainsta45°inclinedanvil.Theimpactpointswillbefurtherdefinedinthefollowingsec‐
tions.

Figure1:ResultsbasedonGIDASandillustratingthat96.8%oftheaccidentsoccurbetween1
and30°C.



5
3HEADFORMCHARACTERISTICS
Thecurrentheadformsarecharacterizedbyanon‐deformable“head‐shaped”masse.The
valueofthemassincludes“some”neckeffectsanditsinertiaisnotcontrolled.Itshouldbe
mentionedthatexistingsimpleheadformssuchasthePedestrianISOheadformpresentsin‐
ertiaquitefarfromhumanheadinertiaasshownintable1.Importantimprovementwouldbe
possiblebyreplacingtheseISOheadformsbyHybridIIIhead.Mainargumentstodosoare:
 Morerealisticheadmass
 Morerealisticheadinertia,anessentialaspectifrotationisconsidered
 Deformableskin,anessentialaspectforhelmetoptimisation
 EasylinktoHybridIIIneck
 Possibilitytofixrotationaltransducers
Acriticalpointwhichhastobemanagedisthesizingaspect,ascurrentlyanumberofsizes
areavailablefortheISOcyclistormotorcyclisthelmetheadform.Thiscanbesolvedthrough
thedifferentversionoftheHybridIIIdummyheadsasillustratedintable2.Asitiswellknown
thatsizesA,C,E,J,MandOrepresent95%ofsizesusedinstandard,onlythesesizeswould
showinterest.Itappearsintable2thatthesesizeswouldadequatelybecoveredbyfivesizes
oftheHybridIIIheadsfamily.

Mass[kg]Ixx [kg.m²] I
yy
[kg.m²] Izz[kg.m²]
ISOPedestri‐
an4.511.10‐311.10‐3110.5.10‐3
HybridIII50t
h
4.517.088.10‐318.872.10‐322.685.10‐3
HumanHead4.517.996.10‐318.360.10‐321.902.10‐3
ISOHelmet5.7Notcontrolled
Table1.Synthesisofheadforminertialpropertiesandcomparisonwithhumanheadcharac‐
teristics.


EN960headform
size
Headcircum‐
ference[mm]DummymodelHeadcircum‐
ference[mm]
A500HybridIII3YearOld508
B510
C520HybridIII6YearOld520.7
D530
E540HIII5thFemale(or10years)538.5
F550HIII5thFemale(or10years)538.5
G560
J570HybridIII95thLargeMale584
K580
L590
M600HybridIII50thMale597
N610
O620HybridIII50thMale597
P630
Q640
Table2.ComparisonbetweentheEN960headformscircumferencesandHybridIIIdummy
heads.TheA,C,E,J,MandOsizesrepresent95%ofsizeusedinglobalstandardsandarecov‐
eredbytheHybridIIIheadsfamily


6
Afurtherquestionconcerningtheheadformisrelatedtoitsboundaryconditionatnecklevel.
AsforthetangentialimpactitisrecommendedtousetheHybridIIIheadanumericalsimula‐
tionhasbeenperformedwiththreehelmetedheadformmodels,i.e.ISOheadform,HybridIII
headandHybridIIIheadconnectedtoHybridIIIneckinframeworkofCostTU0111.Impacts
weresimulatedunderoccipitalimpactagainsta45°inclinedanvil.Resultsshowninfigure2
demonstratethatISOheadformhasaverynon‐realisticresponseduetoitsnon‐controlledin‐
ertia.OntheotherhandbothHybridIIIandcoupledHybridIIIheadandneckpresentsimilar
resultsintermsofrotationalaccelerationduringthe10firstmilliseconds.Obviouslytheneck
influencestheheadkinematiclongeraftertheimpactitself.


Figure2:Rotationalaccelerationcomputedwiththreedifferenthelmetedheadformsand
headboundaryconditionsunderoccipitalimpactagainsta45°inclinedanvil.

4IMPACTPOINTS
InthecurrentstandardimpactpointsarechosenanywhereabovetheR‐R’testlineasillus‐
tratedinfigure3.Accidentinvestigations(Bourdetetal2012[10])clearlyshowthatacritical
areaisthetemporalzoneandthisaspectshouldbeimproved.Thereforethepresentproposal
suggeststolowerthetestlineinasimilarwayassuggestedbyOtteetal2013[31]inframe‐
workofCostactionTU0111(figure3).TheselinearimpactsshouldbeconductedwithHybrid
IIIheadaloneandstrictlyinlinewithG(centreofmassofthehelmetedhead).Gyrometer
shouldcontrolthisaspectandaverylowacceptedrotationalaccelerationshouldbeimposed.


7

Figure3:Illustrationofcurrenttestlinea),impactpointstothehead(Bourdetetal2012b)
andproposalofnewtestlineasfromOtteetal2013c)

Comingtothetangentialimpacttests,theISOheadformshasbeenreplacedbytheHybrid
IIIhead,asthisdummyheadhasmuchmorerealisticinertiaproperties.Forthesetestsitis
suggestedtofreelydropthehelmetedheadwitha6.5m/sinitialvelocityagainsta45°inclined
anvilandtorecordrotationalaccelerationinadditiontothelinearacceleration.Thefirstpro‐
posedimpactisatangentialimpactinthesagittalplan(pointA)leadingtorotationaroundthe
Yaxis(lateralrighttoleft)direction.Thetwonexttangentialimpactsarelocatedatparietal
level(pointsBandC)andwillbeappliedinthefrontalplane,oneintroducingrotationaround
theX(postero‐anterior)directionandoneintroducingarotationaroundtheZ(verticalas‐
cendant)direction(figure4).


Front_xLat_xLat_z
Figure4:Illustrationofthethreetangentialimpactconditions.Fromlefttoright,impactintro‐
ducingangularaccelerationaroundYaxes(calledFront_x),Xaxes(calledLat_x)andfinallyZ
axes(calledLat_z)



8
5HEADINJURYCRITERIA
Currentlythresholdsconcerninghelmetperformancearesetintermsofmaximumhead‐
formacceleration(fixedat250G)accordingtotheWSUtolerancecurvereportedinfigure5
andproposedinthe1950’s.Toprotecttheheadinanautomotiveenvironment,HIChasbeen
introducedinthe1970’sasrecalledinfigure5.Thiscriteria,isbasedonthelinearheadaccel‐
erationevolutionovertimeandhasbeensetataround1000forlinearfrontaloroccipitalim‐
pact.Formotorcyclehelmets,thiscriterionhasbeensetatHIC2400whichhasnosenseina
biomechanicalpointofview.ForbicyclehelmetsHICisnotconsidered.Inadditionitmustbe
recalledthatHICdoesnotintegratelateraldirectionorrotationalaccelerationandisunableto
predictskullfracture.Itisthereforeaverylimitedheadinjurycriterion.

Figure5: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how‐
ever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,im‐
provedheadinjurycriteriatospecificinjurymechanismshavebeendefinedtakingintoconsid‐
erationthetimeevolutionofbothlinearandrotationalheadaccelerationasrecalledhereaf‐
ter.
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pre‐
dictiontoolsthatHIC,sothepresentproposalistoimplementimproved,modelbasedheadin‐
jury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.
Intheproposedapproachtheexperimentalheadlinearandrotationalheadaccelerationwill
constitutetheinputswhichwilldrivetheheadFEmodel,inchargeofthelattertocomputethe
injuryparametersrelatedto,subduralhaematomaandneurologicalinjury.Bythismethodolo‐
gyitwillbepossibletopredictheadinjuryriskmeansacoupledexperimentalversusvirtual
testingprocedureasillustratedinfigure6.

2
1
2.5
21 21
1
()
t
t
HIC t t adt
tt








9
Figure6:Illustrationofthecoupledexperimentalversusnumericalheadimpacttestmethod
basedonnovelmodelbasedheadinjurycriteria.

6CONCLUSION
ThispaperpresentsaproposalforapossibleevolutionofcurrentEN1078bicyclehelmet
standard.Atotaloffourkeyaspectshavebeenreviewedinacriticalway,i.e.headimpact
conditions,headsurrogate,headimpactlocationandheadinjurycriteria.Foreachoftheseis‐
suesaconcreteimprovementproposalhasbeenmadeinordertoopenadiscussiononfurther
researchneeded.Atheadimpactconditionslevelitisproposedtoimplementattangential
headimpacttestswithahelmetedHybridIIIhead.Thisimprovedheadhasalsotheadvantage
ofmorerealisticinertialpropertiesandinterfacecharacteristicsbetweenheadformandhel‐
met.Itisalsosuggestedtoremovetestsunderextremetemperatureoragainstthecurbstone
anvil.Furtherimprovementconcernstheimpactlocationasthecurrenttestlineexcludesany
impacttothetemporalregionwhichhasbeenshowntobeacriticalarea.Afinalkeyevolution
whichisproposedconcernstheassessmentoftheheadinjuryriskforwhichacoupledexperi‐
mentalversusnumericmethodisproposedinordertointroducemodelbasedheadinjurycri‐
teria.Itisexpectedthattheevolutionofhelmetstandardtestmethodwillenableadvanced
helmetevaluationandoptimizationagainstbiomechanicalcriteria.


Acknowledgement:TheauthorswishtoacknowledgetheFrenchDepartmentofTransport
fortheirsupportinframeworkofPREDIT‐BICTETEproject,theECforCostactionTU1011and
FondationMAIFfortheirsupport.

REFERENCES
[1]H.A.H.S.,“MECHANICSOFHEADINJURIES,”TheLancet,vol.242,no.6267,pp.438–441,
Oct.1943.
[2]A.K.Ommaya,P.Yarnell,A.E.Hirsch,andE.H.Harris,“ScalingofExperimentalDataon
CerebralConcussioninSub‐HumanPrimatestoConcussionThresholdforMan,”SAEInter‐
national,Warrendale,PA,670906,Feb.1967.
[3]A.K.Ommaya,F.Faas,andP.Yarnell,“Whiplashinjuryandbraindamage:anexperi‐
mentalstudy,”JAMA,vol.204,no.4,pp.285–289,Apr.1968.

10
[4]T.A.GennarelliandL.E.Thibault,“Biomechanicsofacutesubduralhematoma,”JTrau‐
ma,vol.22,no.8,pp.680–686,Aug.1982.
[5]C.Deck,D.Baumgartner,andR.Willinger,“Influenceofrotationalaccelerationonintra‐
cranialmechanicalparametersunderaccidentalcircumstances,”inProceedingofIRCOBI
Conference,Maastricht,TheNetherlands,2007.
[6]S.Kleiven,“Predictorsfortraumaticbraininjuriesevaluatedthroughaccidentreconstruc‐
tions,”StappCarCrashJ,vol.51,pp.81–114,Oct.2007.
[7]L.Zhang,K.H.Yang,andA.I.King,“Comparisonofbrainresponsesbetweenfrontaland
lateralimpactsbyfiniteelementmodeling,”J.Neurotrauma,vol.18,no.1,pp.21–30,Jan.
2001.
[8]E.G.Takhounts,V.Hasija,S.A.Ridella,S.Rowson,andS.M.Duma,“Kinematicrotational
braininjurycriterion(bric),”EnhancedSafetyofVehicles,pp.11–0263,2011.
[9]E.G.Takhounts,M.J.Craig,K.Moorhouse,J.McFadden,andV.Hasija,“Developmentof
BrainInjuryCriteria(BrIC),”StappCarCrashJ,vol.57,pp.243–266,Nov.2013.
[10]N.J.MillsandA.Gilchrist,“Responseofhelmetsindirectandobliqueimpacts,”Interna‐
tionalJournalofCrashworthiness,vol.2,no.1,pp.7–24,1996.
[11]N.Bourdet,C.Deck,V.Tinard,andR.Willinger,“Behaviourofhelmetsduringheadim‐
pactinrealaccidentcasesofmotorcyclists,”InternationalJournalofCrashworthiness,vol.
17,no.1,pp.51–61,2011.
[12]N.Bourdet,C.Deck,R.P.Carreira,andR.Willinger,“HeadImpactConditionsintheCase
ofCyclistFalls,”ProceedingsoftheInstitutionofMechanicalEngineers,PartP:Journalof
SportsEngineeringandTechnology,Apr.2012.
[13]“ECE22.05,Uniformprovisionconcerningtheapprovalofprotectivehelmetsandtheirvi‐
sorsfordriverandpassengersofmotorcyclesandmopeds.”.
[14]B.Aldman,B.Lundell,andL.Thorngren,“NON‐PERPENDICULARIMPACTS‐ANEXPERI‐
MENTALSTUDYONCRASHHELMETS,”Publicationof:IRCOBISECRETARIAT,Sep.1976.
[15]P.Halldin,A.Gilchrist,andN.J.Mills,“Anewobliqueimpacttestformotorcyclehel‐
mets,”InternationalJournalofCrashworthiness,vol.6,no.1,pp.53–64,Jan.2001.
[16]T.Y.Pang,K.T.Thai,A.S.McIntosh,R.Grzebieta,E.Schilter,R.DalNevo,andG.Rech‐
nitzer,“Headandneckresponsesinobliquemotorcyclehelmetimpacts:anovellaboratory
testmethod,”InternationalJournalofCrashworthiness,vol.16,no.3,pp.297–307,2011.
[17]C.DeckandR.Willinger,“ImprovedheadinjurycriteriabasedonheadFEmodel,”Inter‐
nationalJournalofCrashworthiness,vol.13,no.6,pp.667–678,2008.
[18]V.Tinard,C.Deck,andR.Willinger,“Newmethodologyforimprovementofhelmetper‐
formancesduringimpactswithregardstobiomechanicalcriteria,”Materials&Design,vol.
37,no.0,pp.79–88,May2012.
[19]C.Deck,N.Bourdet,A.Calleguo,P.R.Carreira,andR.Willinger,“Proposalofanimproved
bicyclehelmetstandard,”innternationalCrashworthinessConferenceProceedings,Milan,
Italy,2012.

11
[20]C.Deck,N.Bourdet,A.Calleguo,P.R.Carreira,andR.Willinger,“Proposalofanimproved
bicyclehelmetstandard,”innternationalCrashworthinessConferenceProceedings,Milan,
Italy,2012

[21]K.Hansen,N.Dau,F.Feist,C.Deck,R.Willinger,S.M.Madey,andM.Bottlang,“Angular
ImpactMitigationsystemforbicyclehelmetstoreduceheadaccelerationandriskoftrau‐
maticbraininjury,”AccidentAnalysis&Prevention,vol.59,no.0,pp.109–117,Oct.2013.
[22]A.Post,A.Oeur,E.Walsh,B.Hoshizaki,andM.D.Gilchrist,“Acentric/non‐centricimpact
protocolandfiniteelementmodelmethodologyfortheevaluationofAmericanfootball
helmetstoevaluateriskofconcussion,”ComputerMethodsinBiomechanicsandBiomedical
Engineering,pp.1–16,Mar.2013.
[23]S.G.Gerberich,R.Finke,M.Madden,J.D.Priest,G.Aamoth,andK.Murray,“Anepide‐
miologicalstudyofhighschoolicehockeyinjuries,”ChildsNervSyst,vol.3,no.2,pp.59–64,
1987.
[24]K.Flik,S.Lyman,andR.G.Marx,“AmericanCollegiateMen’sIceHockeyAnAnalysisofIn‐
juries,”AmJSportsMed,vol.33,no.2,pp.183–187,Feb.2005.
[25]P.Rousseau,A.Post,T.B.Hoshizaki,R.Greenwald,A.Ashare,andS.W.Dean,“ACom‐
parisonofPeakLinearandAngularHeadformAccelerationsUsingIceHockeyHelmets,”
JournalofASTMInternational,vol.6,no.1,p.101877,2009.
[26]P.RousseauandT.B.Hoshizaki,“Theinfluenceofdeflectionandneckcomplianceonthe
impactdynamicsofaHybridIIIheadform,”ProceedingsoftheInstitutionofMechanicalEn‐
gineers,PartP:JournalofSportsEngineeringandTechnology,vol.223,no.3,pp.89–97,
Sep.2009.
[27]P.Rousseau,A.Post,andT.B.Hoshizaki,“Theeffectsofimpactmanagementmaterialsin
icehockeyhelmetsonheadinjurycriteria,”ProceedingsoftheInstitutionofMechanicalEn‐
gineers,PartP:JournalofSportsEngineeringandTechnology,vol.223,no.4,pp.159–165,
Dec.2009.
[28]E.S.WalshandT.B.Hoshizaki,“SensitivityanalysisofaHybridIIIheadandneckformto
impactanglevariations,”presentedatthe8thConferenceoftheInternationalSportsEngi‐
neeringAssociation,Vienna,Austria,July12thto16th.
[29]E.S.Walsh,P.Rousseau,S.Foreman,andT.B.Hoshizaki,“THEDETERMINATIONOFNOV‐
ELIMPACTCONDITIONSFORTHEASSESMENTOFLINEARANDANGULARHEADFORMAC‐
CELERATIONS,”inISBSconference,2009.
[30]E.S.Walsh,P.Rousseau,andT.B.Hoshizaki,“Theinfluenceofimpactlocationandangle
onthedynamicimpactresponseofaHybridIIIheadform,”SportsEngineering,vol.13,no.3,
pp.135–143,Feb.2011.
[31]OtteD,WieseB.:Influencesontheriskofinjuryofbicyclists‘headandbenefitofbicycle
helmetsintermsofinjuryavoidanceandreductionofinjuryseverity.SAEpaper2014‐01‐
0517.
