MATERIALD SAN SCIENC SPORTN EI S

Editedby: F.H. (Sam) Froes andS.J. Haake

Dynamics

Shock Absorbing Effectivenesf so Hockey Liner Foams After Exposure to Repeated Impacts SpyrouE. T.B.and Hoshizaki

Pgs. 199-209 TIMS 184 Thorn Hill Road Warrendale 15086-751A P , 4 (724) 776-9000 SHOCK ABSORBING EFFECTIVENES HOCKEF SO Y HELMET LINER FOAMS AFTER EXPOSURE TO REPEATED IMPACTS

E. Spyrou and T.B. Hoshizaki

Sport Maska Inc. Department of Research and Development 600 Boul. Industriel St-Jean-sur-Richelieu, Quebec 4S7B J3 ,, Canada

Abstract hockee ic e yTh helme consideres i t multiplda e impac nature game tth d th devic o f et an e o e edu the prolonged use of by players. The liner foam is one of the many variables that influence impact absorption of a helmet. The purpose of this study was to investigate how inner liner foams, use hocken di y helmets, perfor mtermn i energf so y attenuation, following exposure repeateo t d impacts. Foam samples use thin di s experimen typesto wertw f :eo vinyl nitrile (VN) and expanded polypropylene (EPP) and measured 100 x 100 mm and were 12.5 mm thick. There were two phases to this experiment. In the first phase, which also served as the conditioning phase for the second, twenty impacts were performed repeatedly (1 minute between impacts) on thre level o e(C2J foamtw 0 f impacf energy1 e s)o o eacr th d sfo f d an ho t )an severitO (C J 5 y( liner type. In the second phase, a control set (Co) of foams (3 samples / liner type), not exposed to prior impact, along with the two sets of foams conditioned in phase I were tested once with an impact of 20 J. A monorail drop test was used to evaluate the foams. The type of liner foam and the severity of multiple impacts were both found to substantially affect liner impact absorption properties. Overal bese th l t performanc foamP observes EP e,e wa althoug th mors y db wa e N hV efficien absorbinn i t g impac t lowea t r energies. Such informatio e verb yn e nusefuca th n i l design of helmets and can possibly help prevent injuries.

Materials and Science in Sports Edited by F.H. (Sam) Froes (ThS IM e Minerals, Metal Materials& s Society), 2001

200 Introduction

With activities there exists a certain risk of injury - the game of is no exception. The environment of play presents many chances for collision with the boards (e.g. wood, meta glass)& l , metal goal posts surfacee ic , , hockey sticks pucke th , , skate othed an s r equipment, as well as with the players of the opposing team. Consequently, ice hockey has a high rate of injury, frequently to the head and face (1,2,3,4,5,6,7). Priority, therefore, has been causee studgivee th th f heayf o no t s o d injurie meand san protectionf so hockee ic e yTh . helmet has improved protection to the head since its use became mandatory. The insurgence though of concussions recently promptes ha , d renewed attentio helmeo t n t designt no t quicA .bu k- necessaril bese y th solutio - t problee th o nt m woul tougheo t e d b performanc e nth e requirements set in certification standards. Certification standards have been a determinant factor of helmet performance and thus have influenced design to a great extent. It has been suggested that helmets be designed specifically for the unique conditions of each sport and all conceivable conditions shoul e consideredb taked dan n into accoun helmen i t t design r (8)feasibilitFo . y purposes, standard limitee sar scopen di , therefore overlooking importane somth f eo t conditions under whic game hth s playedei suce On h. conditio helmety nwa usee e woular sy th db e db players and how helmet performance is affected by the frequency and severity of blows that it is subjected to, during its lifespan. majoo Tw r type heaf so d protective devices exist firse Th .t type singla , e impact device uses i , d in very high energy impacts. Such impacts generally occu racn i r e car, motorcycle bicycld an , e accidents wher helmee eth t provides protection agains singla t e considere s i cras d han d unfio t t further protect the wearer following the crash. The second type, a multiple impact device, usually withstands impacts of less energy but is more durable. It is more effective in handling multiple impact situations. The ice hockey helmet falls under the second type of protective devices. This is due to the likelihood of less severe repetitive impact situations in the game of hockey.

Initial research and development of ice hockey helmets was based on fundamental studies of human injury tolerance measures (9,10,11,12). In particular, linear acceleration levels have been shown to be a principal variable implicated in skull and cerebral injury. In general, the safety 2 threshol r headfo d impact s considerei s z (wheraroune 9.8= g b z 10 o g dt m/s1 de30 ), with minimal risk of skull fracture below 300 g; however, the relationship between linear

acceleration and cerebral damage such as concussioz n in not well understood. As a result, the ultimate goal of helmets has been to reduce the energy transferred to the head. The extent of energy reductio s boti n a hfunctio e magnitudth f o n f deformatioo e e resultinth d an ng force proportiona lineae th o rt l deceleration achievo T . e this, helmets typically consis shela f o tl (outer covering) cushionina , g material (also calle liner)chinstrape a dth d an ,maio Tw n. type f foaso m deformation exist: plastic and elastic. The plastic material will not recover to its original shape following impact whereas elastic material will recover firse th t n casI . kinetie eth c energa f yo striking object is completely absorbed when the material has been fully compressed. Under a plastic deformation materiae th , l sustain deformatioa s n whic s beyonhi e elastidth c limid an t permanent damage is done. When the load is released, the material will not regain its original shap wilt distortee bu lb mechanica s it d dan l properties deformatiowile differene th lb o t e du tf no the materia moleculae th t la r seconlevele th n .I d case importane ,th t featur thas ei t while maximum force develope t affectedno s d i time th pea,o e t k forc doubleds ei . Unde elastin ra c deformatioe nth material can return to its original shape without any permanent damage. The bulk of material display both elastic and plastic properties to a certain degree. Depending on the use of the helmet, the material it is composed of, should possess more or less of the two material properties discussed (elasti . plastic)cvs r exampleFo . hocken i , y wher possibilite eth repeatea f yo d impact scenario 201 exists, material with more elastic properties woul suitable db e since recover materiae th f yo s i l crucial in dealing with subsequent impacts. Bishop indicates that these material consist of medium density resilient foams (13).

Predicting impact absorption characteristics of helmets has proven to be difficult given the numerous material, geometric, and environmental factors and their interactions (14). The purpos f thio e so investigat t stud s wa y e effec th ef inne o t r liner typd environmentaan e l conditions (i.e. energy of impact and multiple impacts) on the impact attenuating characteristics of ice hockey helmets. To avoid confounding factors resulting from whole helmet testing, flat sectional samples representing typica hockee lic y helmet materials were evaluated (12,15).

Methods

Samples

Liner samples were constructed possessing common construction characteristic f hockeo s y helmets presentl ymarkete use th squar m n di m sampl A .0 e line10 ex rconsiste 0 tha10 ta f do was 12.thick typem o 5m innef Tw .so r liner material were used: expanded polypropylene (EPP) an d5 kg/m 9 viny d 3an l a densitlinernitrilN 5 6 d V f ha sd eo y an (VN P ) EP foam e Th . respectively.

Instrumentation

Monorail System. The impact testing apparatus used was a monorail or guided-fall system (Figure 1), which consisted of a cylindrical metal guide supported from a beam anchored to a cement block at the lower end. This arrangement provided stability and uniform movement of the aluminum spherical impactor (calibration , diameter = 14.605 mm, mass = 4005 ± 5 grams) used to impact the samples (16). A carriage assembly supported the spherical impactor, universae th f o y lb ybalwa l joint combinee Th . d impacto e masth f so carriagd an r e assembly was 5.15 kg. Rotational movement of the spherical impactor was prohibited during impact. Given that this is the standardized impactor required for system calibrations, it was chosen to permi resulte th t thif so s studappliee industriab e o yth t n di l setting.

An adjustable remote release mechanis uses m frecarriago wa e dt eth e assembly wit initiao hn l velocity. This release mechanis adjustee heighb y n an m achievt ca o t ta d desiree eth d impact energy level surface Th . e upon whic samplee hth s were positione r impacflaa dfo s t steewa t l anvil with a surface area of 0.09 m2. In turn this anvil was attached to a steel and concrete foundation weighin . Thikg s6 arrangemeng13 t reduced vibration obtaio t s a no ss reliabl e impact data.

Measurement d Datan sa Acquisition e dependenTh . t variable measuree peath ks wa d acceleration (gravitational units of g's = 9.81 m/s ) of the impactor. The uniaxial accelerometer (353B18, PCB™) was inserted at the center the universal ball joint (vertically collinear with the

impact location measuro t ) acceleratioe eth z durinunitn ng i f o s g impact transducee Th . s wa r capable of withstanding a shock of 500 gz without damage and had a frequency response that ranges fro10,00o t variation m% 0 5 witz 0± H ha light-emittinA . g diode (LED), supportea n do U-shaped metal basuses measureo wa dt e impact velocity metaA . l flag attache carriage th o dt e assembly interrupte lighe dth t beam just prio impaco rt t wit sample'e hth s highest point.

202 Figure 1: Two views of the monorail drop test apparatus: a. elevated position and . impacb t position.

Both the impact acceleration and the time interval (from carriage release to interruption of the light beam) were recordee analoguth a vi do digita t e l (A/D) converter board after being amplified (DX1010, Cadex ™) at a sampling rate of 10 kHz and an input voltage range of ± 5 V. Custom made software was used to process these signals and calculate impact velocity and

impac . Thesg t e data were store haro dt d dis subsequenr kfo t analysis. z

Sample Preparation

Prio testingo rt impace th , t markes sitewa eacn do h sampl consistentlo et y impac same th t e site for each of the drops per sample. An identification number was used to keep track of the particular material specification sample th f o se being samplee testedth l Al . s were tested under ambient conditions (20 ± 3 °C).

System Calibration

systee calibrateTh s mwa d prio initiatino rt gseriea testsf so sphericae Th . l impactor alons ewa impacte modulaA . velocita t d2% a r ± f 5.5elastomeriys o 2m/ c programmer (MEP ShorA , e Durometer hardness 58-6= ; diamete5 0± r =12) attache ; thicknes flaa mm 5 mm t5 n d2 o s= support base was used as the impact surface for the calibration. The MEP was conditioned with ten hits and then three drops were recorded and if peak acceleration did not record a mean value 5 (on ± e 5 standaro39 f d deviation), testing woul t commencno d e unles e systeth ss mwa adjusted.

203 Drop Procedure

Afte systee th rcalibrated s mwa sample th , placeflas e th etwa steen do l anvi thao poine s l th t f o t impact contacted the lowest point of the spherical impactor and secured in that position with double sided adhesive tape (3M™). There werphaseo etw thio t s s experiment (see TableI below) firse th t n phaseI . , which also serveconditionine th s da g second e phasth r efo , twenty impacts were performed repeatedl minut1 y( e between impacts thren o ) e foame eacr th fo sf ho levelo J (C f 0 energytw 2o f )1 impac s o d lined an an ]t r O secon e severittypeth (C n J I . d 5 y[ phase controa , t (Co se f lfoam o ) sample3 ( s s/ line r type) t exposeno , prioo dt r impact, along with the two sets of foams conditioned in phase I were tested once with an impact of 20 J. The three energy levels of impact used in this study (5, 10 and 20 J) corresponded to impact velocities of 1.4, 2.0, and 2.8 m/s ± 2% (theoretical drop heights of 0.1, 0.2 and 0.4 m) respectively. Impact velocities immediately prior to impact were measured using a LED optical coupler.

Table I The Two Phases of the Experiment and a Summary of Independent and Dependent ______Variables______Phase Independent Variables I II Inner Liner Foam Types EPP & VN EPP & VN

Energy Level / Condition (CJ 210 ) d an ) 5J(d CQ & Ci - €2 from phase i Twenty trial samplr spe e Impact Trial (repeated measures) (Ti, T2,..., T2o) Dependent Variables

Acceleration gmax §max

Design and Treatment of the Data e experimentaTh l desig n a 3-wa e stud uses th r phas f ywa dfo o ANOVAeI , fixed effects design witbetween-bloco htw k treatment within-bloce on d san k treatment phasr 2-waa Fo . en y ANOVA, fixed effects was used. The design notation for phases I and n respectively are as follows:

S3(E2xL2)xT2o (1) S3(C3xL2) (2)

where: S = sample inne= rL liner foam type E = energy level of conditioning C = conditioning type T = impact trial

From these designs, the main factors and interactions between them were examined. Statistica™ statistical softwar uses analyseo wa dt datae eth .

204 Results

The results obtained by this study are presented under the two phases. To evaluate sample performance, peak acceleration (gmax) was used as the discriminating variable. Figures 2 through summariz4 average eth e impac tindependenlevele e responsth th f l so al r efo t variables studied fo. r n phasd an eI

Phase I

Analysi f variancso e (repeated measures) reveale three th f ed o factor thao significand tw t ha s t main effects (p<0.05 impace th n )o t attenuating performancef o . o Thialss tw case r oswa th efo the three interactions between factors.

Main Effects. There were no significant differences in mean fogr the main effect liner foam x ma type. The two liners, EPP and VN, had mean gmax values of 59 and 56 gz, respectively. On the contrary, factors energy level of conditioning and impact trial revealed significant differences (p<0.05). Mean values of 34 and 83 g were observed for the 5 and 10 J energy levels of repeated impact conditioning. As for thz e repeated measures factor impact trial, most of the change was evidenced at the beginning with trial 1 showing a mean gmax value of 38 gz, trial 2:

46 gz, trial 3: 51 gz, trial 4: 53 gz, and trial 5: 55 gz. In subsequent trials, values were raised by

h

approximatel lasd 20g evertan e y trial1 trialth t 3 y 2- reac b .o st g 5 h6 z z

100

90

80

"| 70 ^9 .2 60 Z

30

20 20 impacts at 5J 20 impacts at 10J

Figure 2: Interaction between the variables foam type and energy level of conditioning in phase I of the experiment. This 2-way interaction was significant (F(l,8)=5.37; p<0.05).

Further, figur reveale3 s thaperformancee th t botf o , h foams, leveld impacr 3 f afte d e sof th ran t remains relatively flat unti e 20th lth impact. Furthermore demonstrateP EP , a mors e abrupt deterioration in performance at trials 1 through 3 (32, 37, and 39 gz compared to 24, 26, and 27 gzforVN).

205 Interactions three th f e O .2-wa y 3-wainteractionse on e y th interaction d an thin i , s phase th f eo study, the interaction F x E, between the liner foam and energy level of repeated impact conditioning, along wit e interactiohth , betweeT x nE n energy leve impacd an l t trial, were founsignificane b o dt t (p<0.05) f particulaO . r interes firse th t s i tinteractio ) whicE x s hi F n( displayed graphically in Figure 2. With the trial effect collapsed, it is evident that after repeated foaf energN impacto V mmors J i e 5 yth e t sa efficien absorbinn i t impace gth P t EP tha e nth foam (mean gmax value of 28 gz for VN vs. 40 gz for EPP). The trend is reversed after repeated impacts at the 10 J energy level (86 gz for VN vs. 79 gz for EPP).

1 3 5 7 9 11 13 15 17 19 1 3 5 7 9 11 13 15 17 19 0 impactJ 2 0 1 t a s 0 impact2 J 5 t a s

Figure 3: Impact attenuation performance of the EPP and VN liner foams afte consecutiv0 2 r Joul0 e1 energyimpactf d eo an 5 t .sa

same Inth e figure, when studyin performance gth foamo tw e s th f afte o f impact 0 eo J 2 r 0 1 t sa energy observe w , e that botg2 z)(6 h, growin2 foam d deae an ar s d) gz g eve 9 t trialna (4 1 s th further apart as the trials progress, reaching the maximum difference at the 20 trial (88 gz for EPP and 100 gz for VN). Phase n

Analysis of variance performed under the second phase of the experiment revealed that both factors - liner foam type and impact conditioning - had significant main effects (p<0.05) on the impact attenuating performance. A significant interaction was also found to exist between these maio tw n effects.

Main Effects. Significant differences (p< 0.05) were identified between levels of both main effects, foam type and condition. Mean gmax values for the two levels of foam type, EPP and , respectively thregz e th 9 r eVN36 impacfo d d , an weran , 1 t conditionine26 g levels (Co, d Cjan , maie th C2 nf ) o effec t condition. ,z g 133 2 , 47 341 d an ,

206 Interaction. Post hoc tests (Tukey HSD) revealed significant differences (p< 0.05) between the foaN V m d typean termP n si EP f measo n gstatisticao ma t conditionN a x . 2 d lan difference1 s s between mean gmax values were found for EPP and VN samples at condition 0 (control samples wit prioo hn r impacts) mea, e respectivelyTh . VN n d g3 maan 15 x P valued , EP an were r 4 sfo 11 : gz at C0, 247 and 434 gz at Q, and 423 and 521 gz at C2. These trends are shown in Figure 4. An interesting observation in this same figure is the narrowing of the gap between the two foams at condition level €2.

600 550 500 ,,,.-••4 450 400 350 300 250 200 150

100

50 Cj Condition

Figure 4: Interaction between the variables foam type and condition level e experimentinth phasf o n e . This 2-way interactio s significanwa n t (F(2,12)= 14.23; p<.05). The asterisk indicates significant differences amongst the two foams at the respective condition.

Discussion

Performanc e hockeic n a y f helmeo e t depend numbea n o s f factoro r theid an sr interactions. Althoug helmee hth comprises i t primarilf do impaco ytw t absorbing components (i.eshele th . l ane linerth d ) many variables within thes o componentstw e n significantlca , y influence th e impact absorption characteristic a helmete presen f th o s n I . t experimente mosth f to o tw , frequently used liner foam commercialln i s y availabl hockee eic werN V y e d helmetsan P EP , evaluated under three variations of impact conditioning: no prior impacts (control) and multiple impact . NumerouJ) levelo s0 1 (20tw f energd t o sa ) an s othe5 y( r liner foam variablea f o s helmet such as foam thickness and density, were not varied in this study.

e resultTh s revealed tha tfoaP overal EP s mmori e th el efficien absorbinn i t e energgth f yo impact. This surpris o findinn f o s egi sinc largea e numbe f commerciallo r y available helmets, hockee ic y included fore r anotheron m o n i , , have incorporate foaP theimEP n i e rdth designs. Further analysis of the results though, complicates matters with respect to helmet design. EPP foam was inferior to VN when the energy of impact was set at 5 J while the opposite was true foamo whetw e s nth wer energyf o eJ teste 0 additionn 1 I . t da , when studyin firse gth t couplf eo

207 impacts, out of the 20 in total during phase I of the experiment, we see that the performance is identical, indicating that 10 J is the level where both foams intersect before reversing the performance trend in favor of EPP. This is proof that foams with unique material characteristics, suc s resiliencya h r specifie morfo ar ,t fi e c impact conditions, addine importancth o gt f o e understanding the conditions in which the game is played. One of the problems that is contemplated in the design of ice hockey helmets is the lack of information on the relationship between the environment and the injury, in other words the mechanism of injury. It is possibly eas fino identifo e middlt yt th do s e enda spectru t th eyf o sno ground t m bu r instance Fo . , hocke gama s yi e where multiple impact heae realitya man th w s di o st Ho y. impacte th d san degre severitf eo thesf yo e impact anybody'e b n sca s guess multiplw Ho . e impacts influence eth protection capabilitie e hockeic e th y f helmeo s t depen thesn do e bit f informationo s e th n I . present experiment, f afteenerg o impact0 2 J r 5 y t bota s h foams showe deterioratioda n ni performance initially - more evident with EPP foam possibly due to a more vulnerable surface structure - with no significant deterioration subsequently to that. After 20 impacts at 10 J the deterioration of both foams was evident throughout. The above mentioned means that a certain number of impacts at 10 J of energy will influence performance of a helmet substantially. How many games and/or practices does it take before the helmet has reached, after a series of repeated impacts, that critical point that makes it inadequate to protect against an injury, even at an impact energy level much lower than the one used in present certification standards, is unknown. Informatio thif no s natur certainln eca y mak differencea establishinn ei g performance criteria that woul t onldno y benefi desige draftine th helmeta th f bettea no d f gan o t r standard but most importantl protectioe yth enjoymend nan wearere th f o t .

Conclusions e typ f lineTh eo e severitr th foa d f multiplman yo e impacts were both foun substantiallo dt y affect liner impact absorption properties. Overall the best performance was observed by the EPP foam, although VN was more efficient in absorbing impact at lower energies.

Further study of protective equipment design variables, such as foam density and thickness, and their influence on impact attenuation, under multiple impact conditions, can bring about a knowledge base that can be very useful in solving complex design problems. In addition, it can provide useful informatio when no helmena t shoul deemee db d unsaf preventinn ei g injuro yt the head and therefore determine the lifespan of a helmet. The discovery of ways to better simulat fatigue/agine eth g pattern f helmetso s withi laboratore nth y setting shoul addressee db d in greater detail. Finally, great strides in helmet development have been achieved but unless we better understand the environment of the game and the injury itself, improvements to the helmet design without compromising the essence of the game will be hard to achieve.

Acknowledgements

The authors would like to acknowledge the technical support offered from the laboratory staff at Sport Maska Inc.

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