HELMET ATTENUAT I ON OF THE HEAD RESPONSE I N OBL I QUE I MPACTS TO THE GROUND

by

B . Al dma n , B . Lund e l l , L . Thorngren

Department of Traff i c Safety , Cha l mers Uni vers i ty of Techno l ogy , Gothenburg

a nd

T . Turbe l l

Road User a nd Veh i c l e D i v i s i on , Nati ona l Swed i s h Road and Traffi c Research

I n st i tute , L i nköp i n g , Swede n

Background .

Reports of i nves t i gations i n to two-wh eel er acc i d ents i nd i cate that the head of the r i der often i mpacts another vehi c l e , a fi xed obstac l e , a nd/or the ground ( As h ton et a l 1 97 6 ) . A number of d i fferent acc ident s i tuati ons can be d i s­ti ng u i s hed i n these reports . What type of head i mpact the r i der exper i ences to s ome extent depend s u pon whether he comes to rest near the p l ace of the co l l i s i on or con t i nues , more or l es s i n h i s i n i t i a l d i recti on , some d i s tance from the po i nt of f i r s t contact wi th the other vehi c l e . In s i ng l e veh i c l e acc i d ents a s im i l a r pattern can be seen s i nce the dr i ver may i mpact fi xed obstac l es or j u s t s l i d e a l ong the road ( H i ght et a l 1 9 7 3 , Newma n 1 976 , Lang­wi eder 1 977 ) .

S i nce both veh i c l es i n a co l l i s i on u su a l ly are mov i ng i n d i fferent d i recti ons , a contac t between the head of the r i der of the two-whee l ed vehi c l e and some part of the other vehi c l e genera l ly wou ld be an ob l i qu e , rather than a perpen­d i cu l ar i mpac t . I f the head d oes not h i t the other veh i c l e the rERSon i s u su­a l ly that the two-whee l er has col l i d ed w i th a l ow part of a car , the front or the rea r , a nd then conti nues over that part and s tri kes the ground , or an obs tac l e , at some d i s ta nc e . The i mpac t to the ground wi l l then u s ua l ly be ob l i que i . e . ta ke p l ace at a n acute ang l e . Th i s ang l e w i l l be determi ned both by the r i d er� s hor i zonta l vel oc i ty , at the time of the col l i s i on , a nd by h i s verti ca l v e l oc i ty , d etermi ned by the he ight t o wh i ch he was d i verted a s a re­s u l t of the f i rs t c o l l i s i on . Dur i n g the t ime the body i s a i r-borne i t may be tumbe l i ng and there i s genera l l y no way of pred i ct i n g i ts atti tude at the. t i me of i mpac t to the ground .

I ntu i ti v e ly one wou l d th i n k , however , that i n mos t s i tuati ons where the head i mpac ts the ground a t a n acute ang l e , the b ody wou l d be mov i n g i n the d i rec­ti on of i ts l ong ax i s w i th the head as l ead i ng part and the face l oo k i ng down . Another frequent a tt i tude wou ld then perhaps be one w i th the feet as l ead ing part a nd the face turned u pwards . I n these s i tuat i ons a l arge rel a t i ve move­ment of the he lmet on the head at impact wou l d seem pos s i b l e because many helmets can eas i ly s l i de i n the forward or rearward d i rec t i on on the head .

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An i ntu i ti ve ly l es s probab l e acc i dent s i tuati on wou l d be one where the body , wh i l e a i r- born e , moves s i deways w i th i ts l on g ax i s perpend i cu l ar to the d i rec­ti on of travel a t impac t . Due to the s hape of the head a l arge r e l a t i v e move­ment between he lmet and head wou l d s eem to be l es s l i ke l y in thi s s i tuati on because a he lmet cannot eas i l y rotate around the head i n th i s p l an e .

He lmets , worn by accident v i ct i ms , often have marks and scratches wh ich i nd i ­cate con tac t wi th objects i n the envi ronment . Careful exami nati on of such he lmets w i l l often d i sc l ose v a l ua b l e i nformat i on about the acc i d ent sequenc e . Al though a perpend i cu l ar i mpact t o a smooth , f l a t surface may not v i s i b ly dama­ge an e l a s t i c outer s he l l , the l i ner usua l ly i s more or l es s permanently de­formed i n such contacts . Impacts on curved s urfaces w i th sma l l rad i i usua l ly res u l t in i ndenta t i ons in the shel l a s wel l a s in deforma t i on of the l i ner . Obl i que i mpac ts more often are i d ent i f i a b l e by s cratches on the surface of the s he l l and s ome deformati on of the l i ner . Stud i e s of such he l mets worn i n acc idents i nd i cate that obl i que i mpacts where the body moves s i deways at the t ime of i mpact are not unc ommo n , part i cu l ar l y not i n s i n g l e veh i c l e acci dents ( F i gure 7 ) . From th i s s hort ana lys i s i t wou l d s eem that the head impac t , i n the maj or i ty of a l l two-wheeler co l l i s i on , wou ld be ob l i que ra ther than perpend i cu l ar to the impac ted surface whether th i s surface i s s ome part of a car , a fi xed ob­s tac l e or the ground .

I n a prev i ous paper ( Al dman et a l 1 97 6 ) a method was descr i b ed for experimen­ta l s tud i e s of obl i que i mpacts agai n s t a number of s i mu l a ted road surfaces . I t was a l so s tated i n that paper that a va l i dati on of the tes ts wou l d be made us i ng an i ns trumented d ummy i n free fa l l drop tests . Th i s paper descri bes such va l i da t i o n tests wh i ch were performed at the National Swed i s h Road and Traff i c Research Labora tory at L i nköpi n g , w i th the aim to s i mu l ate as real i s ­t i c l y as pos s i b l e ob l i que head i mpacts to the ground i n two-whee l er accidents .

Method .

An anth oropometr i c 50 perc ent i l e ma l e tes t dummy , w i th the same i ns trumented head and rubber neck as i n the tests reported i n the prev i ou s paper , was u sed i n drop tests w i th obl i que ground impacts . The ground was covered w i th a new , unpo l i s hed ashpa l t concrete surface Ab8t a type common i n c i t ies . Before impac t the dummy was su spend ed i n an au tomat i c q u i c k rel ease mecha n i sm at the s i de of a test car t . Fi gure 1 . The h e i g h t and the pos i t i on of the dummy cou l d be s o a l tered that the dummy wou l d i mpact the road a t d i fferent atti tudes . The hor i z on ta l vel oc i ty component had four d i fferent d i recti ons re l at i ve to the dummy :

- Para l l el to the dummy� s sag i tta l p l ane and d i rected towa rd s the feet or the head , referred to bel ow as feet- f i rs t or head-f irst ( F or H ) .

- Perpend i c u l a r to the d ummy� s l eft or r i g h t s i d e , referred to a s l eft-- s ide-f irst or r i gh t- s i de-fi r s t ( L or R ) .

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Vert/c a l. vie w F i gure 1 . Tes t s i te .

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F i gure 2 .

Th i s a tti tude was u sed i n twe l ve tes ts , n i ne of these wi th the l eft-s ide-fi rs t ( L ) , two wi th head - f i r s t ( H ) and one w i th the feet-f irst ( F) .

Fi gure 3 .

Th i s atti tude wa s u sed i n four tests , three wi th the l eft- s i de-fi rst ( L ) and one wi th the head - f i r s t ( H ) .

F i gure 4 .

Th i s a tti tude was u sed i n one test with the l eft-s i de - f i r s t ( l ) .

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F i gure 5 .

Th i s a tti tude was used i n two tests , one wi th the head - f i rst ( H ) and one wi th the r i ght- s ide-fi rst ( R ) .

The hori zonta l ve l oc i ty component was approx i mate l y 8 . 3 m/s i n the maj or part of the tests . I n four tes ts i t was 4 . 0 , 4 . 2 , 1 1 . 4 and 1 4 . 0 m/s respect i v e l y .

The fal l i ng h e i g h t o f the l owest part o f the d ummy wa s approximate ly 1 . 4 m , g i v i ng a verti cal v e l oc i ty component of 5 . 2 m/s , i n a l l tests but one , i n wh i ch i t was 1 . o m ( 4 . 4 m/ s ) .

L i near and angu l ar head accel erati ons were recorded and the i mpact s equence wa s covered by s evera l h i g h s peed cameras . Al l tests were performed i ndoors a t amb i ent room tempera ture .

On ly one type of he l met of on ly one s i ze was used throu ghout the test seri es . Th i s type of h e l met had a po lycarbonate shel l and a l i ner of expanded poly­s tyr ene . Spec i a l efforts were mad e to press the he l met f i rm l y on to the dummy ' s h ead before each drop . The same procedure was used for each test and a rather h i gh force (� 500 N) wa s u sed for th i s purpos e . Th i s force was ma i nta i ned wh i l e the ch i n s trap was fas tened and p l aced i n i dent i ca l l y the same pos i t i on at each tes t by means of a ch i n c u p . Each he l met was u sed on ly for one tes t .

Resu l ts .

A summary of the resu l ts i s s hown i n tab l e 1 b e l ow . A total of 1 9 tes ts were mad e , two of these are not i nc l uded i n tab l e l because they were performed mere ly to test the dummy re l ease mec han i sm and no record i ng s were made . I n one of the tes ts , where the dummy was dropped i n the atti tud e shown i n f i gure 5 and wi th the r i gh t s i d e l ead i ng , the i mpact to the ground was d i rect­ly to the dummy ' s face and the chock was not a ttenuated by the helmet .

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Atti tude ac- Hor i zonta l moti on Verti ca l Peak l i near Peak angu- No cord i ng to a tti tude m/s v e l oc i ty accel era- l ar acce l e- of fi g . no m/s ti on rati o� tes ts

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2 L 8 . 1 -8 . 4 5 . 2 1 20- 1 50 5500- 1 1 400 4 3 II 8 . 2-8 . 3 5 . 2 1 38 - 1 62 6000-1 2300 3

. 4 II 8 . 4 5 . 2 1 50 8 700 2 H 7 . 8 5 . 2 1 40 7700 2 F 8 . 4 5 . 2 1 20 1 2400 5 H 8 . 3 5 . 2 1 05 4800 5 R 8 . 4 5 . 2 240 1 9000 2 L 8 . 4 4 . 4 1 20 7200 2 II 4 . 0-4 . 2 5 . 2 1 34 7000-7 400 2 II 1 1 . 4- 1 4 . 0 5 . 2 1 30- 1 35 1 0000- 1 0200

Tab l e l .

The res u l ts i n the f i r s t two l i nes of tab l e l a re qu i te s i mi l ar . For both i mpact atti tudes there i s a great var i at ion between the i nd i v i d u a l resu l ts . One poss i b l e expl a na t i on for thi s wou l d be that the dummy i n s ome of the tests , dur i ng i ts free fa l l , had rotated somewhat arou nd i ts l on g i tud i na l axi s . I n that case one shou l d er cou ld wel l i mpact the ground earl i er than i ntended , i . e . s l ig!rtly before or s i mu l taneou s ly wi th the head . Such a ro­tat i on wa s s een i n s ome of the f i l ms but the moment of shou l der i mpact i s obscured by the c l oth i ng used to protect the tors o at i mp ac t .

Even i f the i mpact atti tudes i l l u s trated i n fi gures 2 - 5 are not eas i ly d i s ­t i n g u i shed i n the resu l ts i t seems reasona b l e to as sume that the resu l ts re­corded refl ect the real acc i d ent s i tuati on qu i te we l l .

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Another fac tor contr i buti ng to the scatter i n the res u l ts cou l d be the ch ara c­teri s ti cs of the s i mu l ated road s urface . In the su rface l ayer of new a s pha l t concrete the s tone materi a l may not be qu i te even l y d i stri buted and th i s cou l d be a s s umed to have a notab l e effect on the fri cti on between the road su rface and the he lmet shel l s . In prev i ou s exper iments th i s c i rcumstance .had been found to i nfl uence the peak v a l ue of the angu l a r accel era t i on at i m-pact a nd a certa i n amount of the observed s catter i n these va l ues cou l d pro­bably be attr i bu ted to the fact that the head i mpacts to the ground were d i s tr i bu ted over a l a rge area of the aspha l t concrete surface .

Al so i n th i s respect i t i s a s s umed that the test s i tu a t i on refl ects q u i te wel l what happens i n real acci dents .

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The recorded l i near and angu l ar accel erati ons are qu i te s i mi l ar to those re­ported i n the prev i ous paper where the s ame h e l meted head was d ropped aga i ns t s i mu l a ted road s u rfaces moun ted on a rota t i n g d i s c .

S i n ce very few experi ments o f a s i mi l ar nature a s those reported h ere seem to have been performed , i t i s d i ff i cu l t to assess the real i sm of th i s k i nd of s i mu l a t i on . Th i s i n parti cu l ar i s the case for the ki nemati cs of the dummy dur i ng i mpac t . Theoret i ca l ly one wou l d assume that a human body wou l d have be­haved d i fferently i n some respects under s i m i l ar cond i t i on s .

I n c ompari son w i th some cadaver tes ts , where the body fa l l s to the ground w i t h a hori zonta l ve l oc i ty component , thi s dummy s eems t o be somewhat more el a s ti c wi th a tendency to bounce somewhat more eas i ly then the cadavers do at ground contac t . Th i s , i n comb i n ati on w i th a road s u rface w i th rather h i gh fr i c t i o n , a l so i mp l i es that the dummy was per haps more i nc l i ned to rol l after impact rather than to s l i de a l on g the road surfac e . Th i s was parti cu l ar ly the case when the body moved s i deways .

The i nf l uence of th i s dummy character i.s t i c on the response of the h e l meted head cou l d be ra ther comp l e x . In general one wou l d expect several , more d i s t i nctly separa ted and more v i o l e n t , second ary head impac ts to the ground than i n real l i fe acc i d ents .

I n the tes t ser i es reported here the acce l erometer s i gna l s were recorded on magneti c tape and the record i ngs were extended l on g enough to i nc l ude sub­s equent head i mpacts to the ground . In fi gure 6 one of these extended record s i s s h own .

m .

� u u < � 0 < Lu I

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0 550 MS

Fi gure 6 . Resu l tant l i near acce l erati on from f i rs t and second ground i mpacts .

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I t seems q u i te obv i ou s from these record i ngs that s econd ary head i mpacts are cons iderab ly l es s v i o l ent than the f i r s t ground contac t .

Upon i mpact the road s urface made very c l ear i mpri nts on the s urface o f the he lmet s he l l s a nd therefore i t was qu i te easy to i d ent i fy the l ocati on of the var i ous b l ows . The impr i nts con s i s ted of a number of s cratch mar k s s ta i ned wi th as pha l t a nd so the d i recti on o f movement cou l d a l so be i dent i f i ed for each contac t . I n s p i te of the fact that the s houl der part of the dummy was rather s t i ff compared to a human body , the i mpr i nts were often l oca ted i n the l ower parts of the he lmet she l l s parti c u l a r l y i n the front and i n the back de­pend i n g u pon the i mpact s i tua t i o n . As a matter of i nterest i t was observed that each one of the pri mary i mpacts was l oca ted or at l east reached be l ow the l i ne i nd i cated i n the ISO 1 5 1 1 s ta ndard as the l ower l i mi t for impact test i n g ( fi gure 8 ) .

In the tes ts where the body moved s i deways d u r i n g impact one wou l d anti c i pate l es s tors i ona l l oad i mposed on the neck beca u s e the dummy rotated more than expected . Th i s wou l d i mp ly a weaker i n f l uence from th i s l i n k between head and torso on the k i nema t i cs of the head . In th i s respect'.: the dummy cou l d be cons i dered to err on the s afe s i d e . However , i n the tests where the body moved i n i ts l on g i tud i na l d i rec t i on du r i ng i mpact a l arge rel a t i ve movement between the head a nd the he lmet cou l d occur and s ti l l the rubber neck cou l d to some extent b e -ei ther compressed o r extended and i n th i s s i tuati on one wou l d ass ume the s i mu l a ti on to be l es s real i s t i c . These probl ems are d ea l t wi th i n a subsequent paper where a compari son i s a l so made between accel era­ti ons recorded i n d i fferent types of ob l i que i mpact tests .

Conc l u s i ons .

- The resu l ts from these tests w i th a comp l ete anthropometr i c dummy are i n accordance wi th res u l ts from a test method where a ra i l - gu i ded dummy head i mpacts a rotati ng d i s c . However , when a comp l ete dummy i s u sed the i mpact s i tuat i on i s more comp l ex and the s catter between i nd i v i dua l tests greater .

- The l i near acce l erati ons were w i t h i n the expected range and the angu­l ar accel erati ons often reac hed h i gh l evel s .

- The pri mary i mpacts reached or were l ocated bel ow the l i ne i nd i ca ted i n the ISO 1 51 1 s tand ard as the l ower l i mi t for i mpact testi n g . I t was a l s o found tha t t h e secondary i mpacts gave con s i derably l ower acce l erat i ons than the f i r s t i mpact and that they rare l y were l ocated at the s ame s i te as the pri mary i mpac t .

- The f i nd i ngs i nd i cate that present s tandards for approval of crash hel mets i n some respects do not refl ect the course of events i n rea l acci dents .

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F i g u r e 7 . H e l mets worn i n rea l a c c i d e n ts .

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F i gure 8 . Hel mets u sed i n s i mu l a t i on tes ts .

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References .

ALDMAN ,B . , LUNDELL , B . , THORNGREN , L . ( 1 976 ) : Non-Perpend i c u l a r Impacts - An Exper imenta l Study on Cra s h He lmets . Proceed i ngs o f the IRCOB I -Meeti ng on B i omechani cs of I nj ury to Ped e s tri ans , Cyc l i s ts and Motorcyc l i s ts . Amsterdam, September 1 97 6 . pp 322-33 1

ASHTON , S . J . , PEDD ER , J . B . , MACKAY , G . M . ( 1 976 ) : A Rev i ew of R i ders and Pedest­r i ans i n Traff i c Col l i s i ons . Proceed i ngs of the I RCOB I -Meeti ng on B i omechan i c s o f Inj ury t o Ped e s tr i ans , Cyc l i sts and Motorcyc l i sts . Amsterd am , September 1 976 . PP 1 29- 1 7 1

H I GHT , P . V . , S I EGEL , A . W . 1 NAHUM , A . M . ( 1 973 ) : I nj ury Mechani sms i n Motorcyc l e Co l l i s i ons . Proceed i ngs of the 1 7 th Conference of AAAM , Ok l ahoma C i ty , Okl ahoma , USA. P P 1 66- 203

LANGW I EDER , K . ( 1 977 ) : Col l i s i on Characteri s ti cs and I n jur ies to Motorcyc l i sts and Moped Dri vers . Proceed i ngs of the 2 l s t Stapp Car Crash Conference , New Or l ean s , Loui s i ana , USA . pp 259-301

NEWMAN , J . A . ( 1 976 ) : Characteri st i cs of Motorcyc l e Acc i dents . Proceed i ngs of the I RCOB I - Mee t i n g on B i omecha n i cs of I nj ury to Pedes tr i ans , Cycl i s ts and Motorcyc l i sts . Ams terdam . September 1 97 6 . pp 250-259 .

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