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Development of an impact fatigue device for the evaluation of open-cell polyurethane foam
Turner, Caroline Rose
Turner, Caroline Rose
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2023
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This study investigates low cycle impact fatigue behavior of open-cell polyurethane foams directly relevant to protective combat helmet liner materials. While combat helmet liners undergo a wide range of strain rates in their lifetime, here we focus on the intermediate regime of 1 to 10^2 s^-1 to examine common non-catastrophic dynamic field occurrences to better understand both the reduction in energy absorption, as well as explore performance degradation metrics for liner replacement. A unique piece of equipment that leverages a modified Hopkinsonbar design was specifically designed and built to provide autonomous and consistent intermediate fatigue loading via a controlled projectile. Impact cycles from 1 to 10^3 were explored, compressing to 80% strain in the densification region of the foam on initial impact, with an impact energy of 2.5 J per cycle and frequency of 0.1 Hz. Two commercially developed foam materials were considered in the study, one that is currently used in the Advanced Combat Helmet and the other which is considered as a potential next-generation liner material. The former foam, dubbed Hard A, had pore densities on the order of 500 microns and an average density of 48 kg/m^3, whereas the latter, dubbed High Density B, had pore diameters closer to 70 microns and an average density of 224 kg/m^3. After impact fatigue loading, the samples were then loaded using a drop tower with a 7.5 J impact, where a combination of full-field imaging leveraging Digital Image Correlation (DIC) combined with a force transducer provided the resulting stress-strain response and were compared with non-fatigued samples. The area under the stress-strain curve served as a first order estimate of energy absorption evolution per cycle magnitude and microcomputed tomography was used to examine qualitative damage in the form of bent or broken cell walls. The results show distinct differences between the foams, where Hard A exhibited significant degradation in energy absorption per cycle magnitude, but transitions to the densification regime at strains over 20% greater than High Density B, which remains relatively cycle insensitive. These findings suggests that (at least within the loading conditions and metrics considered here) the potential next-generation liner material may be more well suited for intermediate stress-wave driven fatigue events while not compromising protective capability.
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