Experimental realization of an optimized visco-elastic auxetic metamaterial for enhanced impact mitigation

Auxetic lattices have demonstrated potential for impact mitigation thanks to their unique configurations. Building upon a prior theoretical framework, this study presents the experimental realization of a theoretically optimized, impact-mitigating bilayer auxetic metamaterial. However, physically realizing the idealized material parameters assumed in theoretical models remains a significant challenge. A central innovation of this work is the development of precise, custom digital polymer formulations, created by tuning the blending ratios of base 3D-printing photopolymers to realize theoretically optimal visco-elastic properties. To support this, we first characterized the broadband visco-elastic behavior of these candidate digital materials formed from a series of 3D-printable photopolymers using dynamic mechanical analysis (DMA). The data informed the development of a combined nonlinear visco-elastic constitutive model—integrating a multi-term Prony series with a neo-Hookean formulation—that accurately captures the intricate mechanical behaviors of real-world polymers. This model guided the selection of optimal digital formulations for fabricating an optimized bilayer metamaterial; its performance was then evaluated via drop-tower impact tests. The finite element predictions, using the nonlinear constitutive model, demonstrated excellent agreement with experiments, particularly throughout the critical loading phase across a wide range of impact severities. This validated our design’s effectiveness in achieving simultaneous force and impulse reductions and highlighted a transition from a material-dominated to a hybrid deformation regime governed cooperatively by material and structural properties. By successfully bridging the gap between computational design and physical realization, this work demonstrates a reliable methodology for designing the next generation of high-performance protective metamaterials.