Exponential stress mitigation in structured granular composites

Granular media has been used throughout history as rudimentary yet effective impact mitigation. The unique response of natural granular media is associated with the existence of a network of stress propagation pathways, i.e. a force chain network, which spatially and temporally redirects and moderates the impulse. A variety of structured materials have been proposed to improve the impact mitigating properties compared to natural systems. However, these engineered materials use permanent deformation or viscoelastic properties to dissipate energy, generally limiting their lifetime or effective frequency and temperature range. Here, we take inspiration from natural granular media to engineer a structured composite that exhibits an exponentially fast decay of the leading transmitted pulses. The ordered network geometry allows for an analytical description of the transmitted pulses, which we validate though experiments and numerical simulations. In contrast to other structured materials used for impact mitigation, these networks exhibit reversible deformation, function over all frequencies, and possess a low relative density. Our results open new possibilities for the design and realization of increasingly complex material systems with engineered stress wave transmission pathways.