Tech Briefs

This liner can be used to mitigate shock waves that cause traumatic brain injuries such as blast-induced trauma.

An Advanced Combat Helmet liner design uses the novel idea of including filler materials inside channels in the liner. An energy-absorbing foam was selected for the main liner structure, and several filler material candidates of widely varying properties are being considered. To date, material has been evaluated both experimentally and numerically. Numerical studies will include coupled simulations with a detailed finite element head model, providing insight into the effect of the new liner on the brain’s response to a blast wave impact.

To design and test the liner, experimental and computational techniques were used. To isolate effects of material properties on the transmitted blast wave characteristics, experiments first were carried out on “flat plate” samples. Essentially, this amounts to a rectangular sandwich consisting of layers of foam and filler material. Filler materials being considered include aerogel, glycerin, water, glass beads, and Volcanic Tuff.

The physical modeling aspect of the study was accomplished by varying surrogate materials, geometric configuration, and blast loading conditions in order to determine the measurable behavior variations. Quantitative results were obtained through pressure, acceleration, strain, and displacement measurements. These results suggested large initial amplification of pressures at anterior locations near the shell/gel interface. Material property effects and geometric features effects were seen by larger responses with the material of lower stiffness and more severe responses with the facial-feature shell models over the solid shell models.

Extreme accelerations were experienced with oscillatory behavior over the duration of the blast. In addition, significant relative displacement was observed between the shell and the gel material suggesting large strain values. Further quantitative results were obtained through shadowgraph imaging of the blast scenarios. The shadowgraph imaging confirmed the approximation that global movement of the target was minimal during the blast on a different time scale. The complete results then provided a means of comparison to actual measured behavior from surrogate models to injury mechanisms in computational and clinical trials. Furthermore, the data obtained can be used in computational validation.

The blast mitigation aspect of the study was accomplished by applying blast loading conditions to the various materials. Composite structures were constructed using various filler materials that varied density, porosity, viscosity, and particle size. Quantitative results were then obtained by measuring transmitted wave profiles behind the respective samples and comparing to free-field loading conditions. Attenuation effectiveness was then determined by the reduction of blast profile characteristics (peak overpressure, pulse duration, and impulse). The results of these experiments showed that lower-density, porous materials caused blast profile resembling scaled air blasts. Specifically, shorter wave front rise times and negative overpressure values were observed. The higher-density materials exhibited the greatest attenuation by lowering the overall peak pressure, lengthening the duration, and slowing the rise to peak amplitude.

This resulted in lower overall impulse values. Furthermore, significant frequency distribution was observed, surpassing the effectiveness of the solid foam control sample and the lower-density materials.

This work was done by Laurence R. Young, Steven F. Son, George A. Christou, Matthew D. Alley, Rahul Goel, Andrew P. Vechart, and Benjamin R. Schimizze of the Massachusetts Institute of Technology for the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. ONR-0018

This Brief includes a Technical Support Package (TSP).

Fluid Helmet Liner for Protection Against Blast-Induced Traumatic Brain Injury (reference ONR-0018) is currently available for download from the TSP library.

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