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Procedia Engineering 60 ( 2013 ) 143 – 150

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University doi: 10.1016/j.proeng.2013.07.013

Available online at www.sciencedirect.com

6

th

Asia-Pacific Congress on Sports Technology (APCST)

Impact energy attenuation performance of cricket helmets:

standard 2-wire drop test vs. pitching machine impact test

Toh Yen Pang

*

, Aleksandar Subic, Monir Takla

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, VIC 3083, Australia

Received 20 March 2013; revised 6 May 2013; accepted 9 June 2013

Abstract

Many researches have debated that the current 2-wire drop test method (AS/NZS 4499.1:1997 and AS/NZS 2512.3.2:1997) for cricket helmets does not adequately represent the dynamics of a real ball-helmet impact for fast, elite and express bowlers. This paper presents a new pitching machine test method that was developed to measure the impact performance of cricket helmets. A series of accelerometers were positioned at the centre of gravity (CG) of the test headform to measure the linear accelerations in x-, y- and z-directions due to impact by a cricket ball. The Head Injury Criterion (HIC) was calculated based on the resulting head acceleration (Hres), and impact duration.

When compared with the bare headform impacts, the tested helmets were able to reduce the Hres by 25% and 60% for

the 2-wire drop test and pitching machine respectively. Similarly, the tested helmets reduced the HIC values by about 82% and 83% for the 2-wire drop test and pitching machine respectively. However, the Hres and HIC values measured

using the 2-wire drop tests were significantly higher (p<0.05) than those measured using the pitching machine test. These differences were attributed largely to the differences in the masses of the test apparatus and the resultant impact energy, which was calculated based on the measured impact velocities and impactor mass. The repeatability of results obtained from the pitching machine may, however, be degraded at high velocity impacts due to the test set-up. For the time being, the developed pitching machine test is intended to supplement, but not to replace, the currently accepted cricket helmet standard test method.

© 2013 Published by Elsevier Ltd. Selection and peer-review under responsibility of RMIT University

Keywords: Cricket helmet; drop test; pitching machine; impact

* Corresponding author. Tel.: +61 3 9952 6128; fax: +61 3 9925 6108.

E-mail address: tohyen.pang@rmit.edu.au

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University

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There has been on-going debate about current routine cricket helmet test methods (AS/NZS 4499.1:1997 and AS/NZS 2512.3.2:1997) [1, 2], which are conducted by placing a helmet on a solid headform, striking it from height of 2m above the point of impact of a helmet with a falling weight of 1.5 kg and measuring the deceleration. It may be argued that the standard 2-wire drop test addresses a low velocity impact. The impact velocity normal to the impact surface is only 6.3m/s, which is equivalent to 30 J of impact energy. It is apparent that such apparatus does not adequately represent the correct dynamics of a real ball-helmet impact situation [3]. A typical high range ball-to-head impact while playing the games of cricket can reach velocities of more than 32-45 m/s for fast, elite and express bowlers [3-5].

Substituting the current methods of testing cricket helmets by an air cannon test for may offer a more realistic test method. Several researchers have undertaken the complex task of air cannon-like test methods with varied success. For instance, McIntosh and Janda [3] used an air cannon to fire a Kookaburra Comet cricket ball, at preset velocities of 19, 27, 36 and 45 m/s, impacting on different test helmets. The authors measured maximum resultant headform acceleration and found that all the helmets tested were able to reduce the headform acceleration by about 80% at the least severe impact. When the speed of impact was doubled, the reduction was only 40%.

To our knowledge, very little research has been done to quantitatively investigate the Head Injury Criterion (HIC), where its value is determined by considering magnitude of the acceleration obtained from the acceleration-time profile and duration of exposure. Such method has been widely used in automotive research to predict degree and likelihood of brain injury as well as to assess the relative effectiveness of protective devices such as a helmet [6-8].

The aim of this study has been to develop a new pitching machine test method, which incorporates an instrumented dummy head, for investigating the impact performance on various helmet designs. The helmets were tested using the new pitching machine test in order to compare their protective capabilities with the similar forms of impact as the standard 2-wire drop test.

2. Experimental procedure

In order to compare their impact attenuation properties, the helmets (size small to medium) were fitted onto a magnesium alloy headform. Three different helmets were selected for the study and they are N1 M2 and P3. Each helmet model was tested three times, without a faceguard, at each of the following impact sites (Fig. 1):

1. forehead 2. temple and 3. rear

A series of accelerometers were positioned within the cavity of the headform at its center of gravity (CG) to measure the accelerations in x-, y- and z-directions due to impact from the cricket ball. All accelerometers (PiezoStar Accelerometer, Type 8715A) had a range of ±5000g and were manufactured by Kistler Group. A coupler (Type 5134B) was used to provide excitation power and signal conditioning for the accelerometers. The accelerometer signals were acquired at a frequency of 10 kHz, which follows the recommendations of the Society of Automotive Engineers standard SAE J211 [9]. Data were collected using a Measurement-Computing USB-1608HS DAQ board housed in a steel chassis. All data obtained from the accelerometers were stored in a computer.

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N1 M2 P3

Front Temple Rear

Fig. 1. (a) Helmets used in the study (b) impact sites

Impact energy for each test was calculated using the moving assembly mass and impact velocity:

E = ½ mbv

2

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where mb = mass of the ball and v = velocity of the ball.

The entire ball-helmet impact sequence was captured using a high-velocity camera sampled at 5000 frames per second. The camera images enabled the analysis of the deformations of the impacted helmets to be carried out.

After each test, the impacted helmet was completed removed from the headform and inspected for any damages. For each impact, the Head Injury Criterion (HIC), which is most widely used as the quantitative index of safety in head protective devices, was calculated based on the resulting head acceleration and impact duration [10, 11]:

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where t1 and t2 are any two arbitrary times during the acceleration pulse, a is the resultant linear acceleration measured at the CG of the headform.

2.1. 2-wire drop test

A guided free-fall rig, which complies with the requirement of AS/NZS2512.3.2: 1997 [2], was used for the 2-wire drop test. The method of testing was to drop a striker with a cricket ball attached onto a helmet fitted on a standard headform. The total mass of the moving assembly with the Kookaburra Regulation ball was 1.56 Kg. The headform is firmly attached to a base plate with fixing holes at 15° centers for correct positioning of headform. The base plate was rigidly bolted to the concrete floor. A linear accelerometer was mounted at the cricket ball holder to measure the linear deceleration of the striker during and after the impact. The exact impact velocities were recorded using a light gate (Fig. 2(a)). The performance criterion of a test helmet in accordance with the standard (AS/NZS 2512.3.2:1997) is calculated as follows: (3) HIC= max t2,t1 t2− t1

(

)

t 1 2− t1 a t

( )

dt t1 t2

³

ª ¬ « « º ¼ » » 2.5 ­ ® ° ¯° ½ ¾ ° ¿°

Percent different = Mean deceleration (bare headform) - Maximum deceleration (test sites) Mean deceleration (bare headform) ×100%

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2.2. Pitching machine impact test

A pitching machine impact rig was co and helmet (Fig. 2(b)). A standard mag fixing holes at 15° centers. The base p Regulation cricket balls were fired from 6.26m/s and (b) 44.4m/s. The impact vel were spaced 100mm apart. The base pla The helmet is fitted on the headform instructions. The pitching machine was cricket helmets according to AS/NZS 44 adjusted and pre-set to achieve the target

Fig. 2. (a) Experimental apparatus for standard 2-wi

2.3. Statistical analysis

Differences between sample means w unequal variances model (p=0.05). Th experimental parameter for all test co significance in the statistical tests.

3. Results

3.1. Helmet impact energy attenuation wi

The mean decelerations for helmeted a 3. All the helmets were able to reduce m sites. The maximum reduction found at th

onstructed in order to simulate the impact between a c gnesium headform is mounted to an adjustable base

plate was rigidly bolted to the steel table. The K m the pitching machine at two different impact vel

ocities were measured using three photoelectric switc ate assembly was adjusted to align with the desired i m and the chinstrap is fastened according to man s raised to an appropriate height to align with the te

99.1:1997 [1]. The velocity knobs of the pitching ma impact velocity.

ire drop test (b) Experimental apparatus for pitching machine impac

were assessed using a two-tailed t-test with pair sa his t-test method allowed for the direct compariso onditions. A confidence level of 95% was the cr

ith 2-wire drop test

and bare headform for the three impact sites are compa more than 25% of the maximum deceleration recorded

he forehead areas. cricket ball -plate with Kookaburra ocities: (a) ches, which impact site. nufacturer’s

est line for achine were ct test amples and on of each riterion for ared in Fig. d at all test

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Fig 3. Mean linear deceleration of the striker impact

3.2. 2-wire drop test vs. pitching machine

The striker assembly of the 2-wire dr impact. The 2-meter drop height produce the pitching machine were adjusted to p There are no statistically significant diffe pitching machine. The impact energy w impactor mass; it ranged from 29 J to 32 respectively.

Table 1 summarises the results for the the pitching machine test with similar im drop test were significantly higher than attributed largely to the differences in th impact. The calculated HIC values wer unhelmeted tests. On the average the hel wire drop test and pitching machine respe

4. Discussion

In this study, the impact energy atte helmets were evaluated using: (i) a standa pitching machine impact test. Within the values for the overall peak linear accelera The results demonstrated a reduction in in HIC values by up to 69%, when a he protection at the tested impact sites. All the AS/NZS testing standard, indicating wearer from head injuries in an impact.

The impact energy absorption capab differences in the structure, outer shell an

4.1. Test Apparatus: 2-wire drop vs. Pitch

Comparison of the results for the two The impact velocity of the low velocity test to ensure that the desired ball veloci photoelectric switches. The target veloc range and showed no significant differenc

ting on the helmet and bare headform

e

rop test was dropped from a height of 2 m above th ed an equivalent impact speed of 6.26 m/s. The velocit produce similar impact speed as the standard 2-wire erences in impact velocity between the 2-wire drop t was calculated based on the measured impact velo 2 J and 2.9 J to 3.1 J for the drop test and pitching m e test helmets on all the impact sites of the 2-wire dr mpact velocity. The resultant acceleration and HIC for n those for the pithing machine test. These differe he masses of the test apparatus and the subsequent dy

re significantly lower for the helmeted tests as co lmet reduced the HIC values by about 82% and 83% ectively.

enuation performance of three commercially availa ard 2-wire drop test (according to AS/NZS 4499.1:199 e current level of impact velocity of 6.26 m/s a reduc ation of over 25% is achievable (Fig. 3).

n head linear acceleration in forehead impacts by up to elmet is worn (Table 1)). All tested helmets offer the

test data are significantly below the 300 g passing th g that the helmets provide sufficient degree of pro bilities of the tested helmets showed some variatio nd protective liner composition of the helmets.

hing machine

o test methods is made on the basis of similar impac pitching machine test was achieved by conducting p ity was obtained. The impact speeds were measured u cities of pitching machine and 2-wire drop were in ce.

he point of ty knobs of e drop test. test and the ocities and machine test op test and r the 2-wire ences were ynamics of ompared to % for the 2-able cricket 97) and, (ii) ction of the o 50%, and e necessary hreshold for otection for ons due to ct velocity. preliminary using three the similar

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machine impacts at the similar impact velocity (Table 1). Although there are caveats to comparing peak accelerations between 2-wire drop and pitching machine impacts, results reflect that there was a significant difference in the resultant head CG accelerations for the two different test apparatus. These differences were attributed largely to the differences in mass of the test apparatus. For the standard 2-wire drop test, its striker with a total mass of 1.5 kg delivered more severe impact even though its impact velocity were similar to the pitching machine test. Without the mass of the striker in pitching machine test, the amount of energy transferred to the head, which is calculated from the mass of the ball and its velocity at impact, is reduced.

It is possible to simulate realistic ball to head impacts in cricket using the cricket pitching machine method. However, it is acknowledged that the repeatability of the pitching machine may be degraded at the higher velocity impact. Some difficulties have been experienced attempting to achieve exactly the same impact speed in the repeated test due to variations, such as: the friction between the ball and the pneumatic rubber tyres and how the cricket ball being inserted into the enclosed chute to ensure accurate deliveries of ball between the tyres. The guide-wire rig, however, has been proven reliable, efficient and highly repeatable.

Table 1: Comparison of mean resultant headform acceleration maxima and HIC for drop test and pitching machine test rest for similar impact speed

Impact sites Test Method Helmet Models N1 M2 P3 Energy (J) Res. Accl. (g) HIC Energy (J) Res. Accl. (g)

HIC Energy (J) Res. Accl. (g) HIC Temple 2-wire 28.6 (0.6) 52 (4) 77 (40) 26.6 (4.8) 56 (6) 97 (55) 30.6 (2.6) 53 (8) 75 (16) Pitching 3.14 (0.17) 36 (3) 29 (12) 2.88 (0.34) 29 (13) 19 (12) 2.94 (0.29) 29 (13) 11 (6) Rear 2-wire 30.7 (0.9) 59 (5) 160 (42) 31.2 (1.6) 64 (9) 153 (28) 32.7 (1.7) 50 (11) 99 (59) Pitching 3.11 (0.32) 30 (4) 13 (11) 2.97 (0.06) 14 (3) 4 (3) 2.97 (0.10) 14 (3) 15 (3) Forehead 2-wire 31.0 (1.5) 29 (6) 13 (9) 31.0 (2.5) 129 (6) 615 (118) 29.8 (1.4) 55 (7) 99 (16) Pitching 3.07 (0.36) 14 (13) 6 (10) 3.04 (0.30) 16 (2) 4 (4) 2.96 (0.18) 16 (2) 4 (3)

4.2. Increased impact velocity

The 2-wire drop impact test procedures specified in the current standard for evaluating helmet performance might not be effective in assessing head protection. A typical high range ball-to-head impact velocity of 32-45 m/s has been investigated for fast, elite and express bowlers [3, 5]. A rough estimation

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of typical impact energies at such impact velocities is within the range of 80 -150J. In order to achieve such impact energy, the current drop height needed to be raised to 9.6m, which is quite unachievable. The pitching machine is intended to simulate realistic cricket ball traveling speeds and impact scenarios as observed in the cricket field.

If the threshold impact energy of the standard increased, one would expect that helmets would provide greater protection, however, the typical design change would be an increase in either liner density or liner thickness. These changes could provide the greater impact attenuation but may increase headform accelerations for impacts at velocities less than the standard test [12, 13]. There is also a limit to how thick a helmet can be before it is no longer practical or appealing to the user. Therefore, from a biomechanical perspective, it is important to assess, via further investigation, how changes in test height, impact velocity would affect helmet design and safety performance in an impact.

4.3. Head injury

Comparison of the resultant acceleration maxima from the experimental test values for human injury tolerance limits allows, to some extent, an assessment of the likelihood of sustaining certain degree of injury in the situation simulated [10, 14]. For instance, Head injury criterion (HIC) is a wide acceptance tool to predict degree and likelihood of brain injury. It is an integrated measure of exposure to linear acceleration, i.e., its value is determined by considering magnitude of the resultant acceleration from the acceleration-time profile and its duration of exposure. Using the threshold for concussion as a HIC of 1000, it was found that all test helmets did not offer adequate protection once the impact energy was greater than 140J.

It is acknowledged that the impact attenuation test results may be overestimated, more damage would be observed for the test helmets, especially during the high velocity impact. Due to the unyielding characteristic of the magnesium headform used in the test, the outer hard shell of the helmet and the liner are more easily compressed and deformed.

In light of these results, it may be pertinent to question whether current helmets provide adequate protection in high-energy impacts. This raises the question whether helmets should be assessed across

various levels of impact severity.

5. Conclusions

While it is recognised that helmets conforming to the Australian Standard have a guaranteed level of impact attenuation and provide sufficient protection to a wearer from head injury, the present experimental study has indicated considerable scope for improvements or compliment to current test methods to include high velocity impact.

It has become necessary to improve current helmet-testing regulations, as the current standard test procedures do not adequately represent real ball to head impacts in cricket. The pitching machine impact testing method for cricket helmet presented in this work simulates the impact of the ball to the head more realistically. The method and apparatus for testing cricket helmet of the present study, when compared to the standard test method and apparatus for testing helmet, has the advantage of being capable of (1) capturing a blow similar to those seen in ball-to-head impact, and (ii) delivering and measuring resultant linear acceleration—the vector sum of the individual linear X (forward), Y (lateral) and Z (Vertical) headform accelerations and HIC. The proposed testing method is intended to supplement, but not replace, currently accepted cricket helmet standards and testing method.

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Provision of helmets by Albion Sports is acknowledged. The authors would like to thank Mr. Peter Tkatchyk from the School of Aerospace, Mechanical and Manufacturing Engineering who helped in setting up the experiment.

References

[1] Standards Australia Standards New Zealand. Protective headgear for cricket Part 1: Helmets. AS/NZS 44991:1997: Standards Australia and Standards New Zealand; 1997.

[2] Standards Australia Standards New Zealand. Methods of testing protective helmets Methods 3.2: Determination of impact energy attenuation-Striker drop test. AS/NZS 251232:1997: Standards Australia Standards New Zealand,; 1997.

[3] McIntosh AS, Janda D. Evaluation of cricket helmet performance and comparison with baseball and ice hockey helmets. British Journal of Sports Medicine 2003;37:325-30.

[4] Stretch RA, Bartlett R, Davids K. A review of batting in men’s cricket. Journal of Sports Sciences 2000;18:931 - 49. [5] Stretch RA. The impact absorption characteristics of cricket batting helmets. Journal of Sports Sciences 2000;18:959 - 64. [6] Pellman EJMD, Viano DCDmPD, Tucker AMMD, Casson IRMD, Waeckerle JFMD. Concussion in Professional Football: Reconstruction of Game Impacts and Injuries. Neurosurgery 2003;53:799-814.

[7] Hering AM, Derler S. Motorcycle Helmet Drop Tests Using a Hybrid III Dummy. IRCOBI Conf 2000:307-20.

[8] Krishnamoorthy R, Takla M, Subic A, Scott D. Design Optimisation of Passenger Car Hood Panels for Improved Pedestrian Protection. Advanced Materials Research 2013;633:62-76.

[9] SAE. SAE J211-1 Instrumentation for Impact Test—Part 1—Electronic Instrumentation. SAE International 2007.

[10] Hutchinson J, Kaiser MJ, Lankarani HM. The Head Injury Criterion (HIC) functional. Applied Mathematics and Computation 1998;96:1-16.

[11] Kleinberger M, Sun E, Eppinger R, Kuppa S, Saul R. Development of Improved Injury Criteria for the Assessment of Advanced Automotive Restraint Systems. NHTSA Report; 1998.

[12] Thom DR, Hurt HH, Smith TA, Ouellet JV. Feasibility Study of Upgrading FMVSS No. 218, Motorcycle Helmets. Final Report 1997.

[13] Mills NJ, Gilchrist A. The effectiveness of foams in bicycle and motorcycle helmet. Accident Analysis & Prevention 1991;23:153-63.

[14] Eppinger R, Sun E, Bandak F, Haffner M, Khaewpong N, Maltese M, et al. Development of Improved Injury Criteria for the Assessment of Advanced Automotive Restraint Systems-II.: National Highway Traffic Safety Administration, US Department of Transportation.; 1999.

References

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