Research Papers

Age Does Not Affect the Material Properties of Expanded Polystyrene Liners in Field-Used Bicycle Helmets

[+] Author and Article Information
Shannon G. Kroeker, Alyssa L. DeMarco

MEA Forensic Engineers & Scientists,
11-11151 Horseshoe Way,
Richmond, BC V7A 4S5, Canada

Stephanie J. Bonin

MEA Forensic Engineers & Scientists,
23281 Vista Grande Drive,
Laguna Hills, CA 92653

Craig A. Good

Collision Analysis,
43 Skyline Crescent NE,
Calgary, AB T2K 5X2, Canada;
Schulich School of Engineering,
University of Calgary,
2500 University Drive NW,
Calgary, AB T2N 1N4, Canada

Gunter P. Siegmund

MEA Forensic Engineers & Scientists,
11-11151 Horseshoe Way,
Richmond, BC V7A 4S5, Canada;
School of Kinesiology,
University of British Columbia,
210-6081 University Boulevard,
Vancouver, BC V6T 1Z1, Canada
e-mail: gunter.siegmund@meaforensic.com

1Corresponding author.

Manuscript received November 8, 2015; final manuscript received February 4, 2016; published online March 3, 2016. Assoc. Editor: Barclay Morrison.

J Biomech Eng 138(4), 041005 (Mar 03, 2016) (9 pages) Paper No: BIO-15-1567; doi: 10.1115/1.4032804 History: Received November 08, 2015; Revised February 04, 2016

Bicycle helmet foam liners absorb energy during impacts. Our goal was to determine if the impact attenuation properties of expanded polystyrene (EPS) foam used in bicycle helmets change with age. Foam cores were extracted from 63 used and unused bicycle helmets from ten different models spanning an age range of 2–20 yrs. All cores were impact tested at a bulk strain rate of 195 s−1. Six dependent variables were determined from the stress–strain curve derived from each impact (yield strain, yield stress, elastic modulus, plateau slope, energy at 65% compression, and stress at 65% compression), and a general linear model was used to assess the effect of age on each dependent variable with density as a covariate. Age did not affect any of the dependent variables; however, greater foam density, which varied from 58 to 100 kg/m3, generated significant increases in all of the dependent variables except for yield strain. Higher density foam cores also exhibited lower strains at which densification began to occur, tended to stay within the plateau region of the stress–strain curve, and were not compressed as much compared with the lower density cores. Based on these data, the impact attenuation properties of EPS foam in field-used bicycle helmets do not degrade with the age.

Copyright © 2016 by ASME
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Fig. 1

Helmet types tested in this study (left to right: Traditional, Kids Visor, and BMX)

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Fig. 2

Exemplar foam cores before and after testing (from Giro Torrent helmet)

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Fig. 3

Test equipment. The foam core rested on top of a platform mounted on a load cell. The impactor was dropped from 749 mm above the platform onto the top of the foam core. An accelerometer was attached to the impactor.

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Fig. 4

Exemplar data. The photographs show the sequence of foam compression at initial contact, at maximum compression, and after impact to a foam core. Representative plots for acceleration, force and crush versus time, and stress versus strain show that the foam cores exhibit typical impact responses for EPS foam. Initial contact occurred at time = 0 ms.

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Fig. 5

Stress versus strain plots for all 63 foam cores. The response consists of three phases: elastic, plateau, and densification. Foam properties were calculated from the elastic (dashed black line on inset) and plateau (solid black line on inset) regions of a foam core's stress versus strain plot (thick gray line on inset).

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Fig. 6

The variation in each of the six dependent variables with the year of manufacture. Helmet age was not significantly associated with any of the dependent variables.

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Fig. 7

The variation in each dependent variable with foam core density. All dependent variables, except for yield strain (top left panel), increased significantly with density.

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Fig. 8

As foam core density increased, peak foam core stress and strain decreased. Lower density cores exhibited all three phases of the typical stress–strain response (elastic, plateau, and densification). The lower density cores also displayed higher degrees of densification and wider plateaus.

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Fig. 9

The peak acceleration (+) during impact decreased as the density of the foam core increased (shown by the exponential curve fit—thick solid line). As the foam core absorbed increasing amounts of energy, the more dense specimens had smaller increases in acceleration. This is depicted by the instantaneous acceleration corresponding to different levels of energy absorption: 1J (□), 3J (○), 5J (⋄), 6J (♦), 7J (∇), and 8J (▴).



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