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Research Papers

Optimal Material Properties for Mitigating Brain Injury During Head Impact

[+] Author and Article Information
Frank W. Zok

Materials Department,
University of California,
Santa Barbara, CA 93106

Manuscript received May 31, 2013; final manuscript received June 25, 2013; accepted manuscript posted July 12, 2013; published online October 16, 2013. Editor: Yonggang Huang.

J. Appl. Mech 81(3), 031014 (Oct 16, 2013) (5 pages) Paper No: JAM-13-1221; doi: 10.1115/1.4024992 History: Received May 31, 2013; Revised June 25, 2013; Accepted July 12, 2013

We present a methodology for identifying constitutive responses of crushable, linear-softening materials that would reduce the severity of brain injury caused by head impact in a typical automobile or sports collision. It is based on analysis of accelerations imparted to a spherical mass (representative of the human head) upon impact at prescribed velocity onto a flat padded structure. The resulting acceleration–time histories are used to calculate the corresponding Head Injury Criterion (HIC): a weighted product of acceleration and impact duration that has been found to correlate with the severity of brain injury. In the best-case scenario, the HIC is reduced by a factor of 1.84 relative to that obtained for a system optimized with a perfectly plastic foam. The optimal combinations of yield stress and crushing strain are not unique; that is, the optimum can be achieved with a range of strengths and crushing strains. The present solutions are expected to find utility in guiding the design of new polymer lattice materials for use in impact protection systems.

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Figures

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Fig. 1

Schematic of a spherical mass impacting a flat protective pad mounted on a flat rigid support

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Fig. 2

Schematic of potential compressive stress–strain curves for protective materials

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Fig. 3

Effects of HIC on probability of sustaining head injuries of varying severity (from 1 to 6 on the Abbreviated Injury Scale) and the current limit used by the Federal Motor Vehicle Safety Standards for adult vehicle occupants. The Abbreviated Injury Scale has been developed by the Association for the Advancement of Automotive Medicine. (Adapted from Refs. [2] and [8].)

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Fig. 6

HIC values for linear-softening and perfectly plastic materials optimized for two specific impact velocities (5 m/s and 7 m/s). Each dashed line represents the locus of minimum HIC values.

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Fig. 5

HIC contours comparing the full solution (solid lines) with contours based on the approximate solution (dashed lines). The shaded space corresponds to combinations of target response for which the spherical mass arrests prior to crushing under its center. Note the approximation for g(dc¯,σ¯Y) is inaccurate near this region. However, the predicted stopping distance is exact (i.e., not an approximation), and therefore, contours of equal stopping distance are identical to those shown in Fig. 4.

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Fig. 4

Contours of normalized HIC, g(dc¯,σ¯Y), as a function of dc¯ and σ¯Y, with contours of fixed stopping distance superimposed

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