The impact performance of density-graded polyurea elastomeric foams

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The overarching goal in sports gears to achieve higher levels of impact protection without an increased weight penalty motivated the pursuit of density gradation in cellular solids. The research reported herein studied the impact performance of density-graded polyurea elastomeric foams, including different gradation and interfacing strategies. The latter leveraged the natural adhesion properties of polyurea foam slurry to sequentially assemble graded structures with seamless interfaces. Moreover, conventional discretely graded polyurea foam structures were manufactured using bulk polyurea adhesive by individually bonding pre-fabricated sheets. Six density-graded foam configurations were characterized, including bi- and tri-layered foams with naturally bonded or adhered interfaces. Additionally, standard mono-density polyurea and another benchmark foams were tested as controls. Extracted samples were submitted to impact loading using an instrumented drop weight tower. The impact efficacy of the foams was analyzed using three dynamic performance metrics (DPMs), including the specific energy absorptivity (SEA), absorption efficiency, and local buckling-induced undulation, based on the averaged stress-strain response. The mono-density polyurea foam reported the largest SEA, outperforming all density-graded foam configurations. The limited density gradation is attributed to the inferior performance of graded structures. A brief biomechanics case study is presented to assess the potential interrelationship between the investigated foam structures and the head injury criterion (HIC), showing the superior performance of adhered trilayer polyurea foams. Finally, the average stress-strain responses were fitted using an empirical model, elucidating the effect of strain rates and base materials on their overall impact behavior.