Design and mechanics of stackable auxetic S-shaped structures

The emerging auxetic lattices, driven by contemporary advancements in additive manufacturing, have garnered significant attention, with an emphasis on improving structural performance and energy absorption efficiencies while mitigating the contributions of material nonlinearities and instabilities. This research advances the state of the art of thin-strut auxetic structures by introducing a stacking schema to realize 3D S-shaped polystructures with rigid-body motion-dominated deformation, spatial symmetry, and persistent auxeticity. Several generations of these polystructures were 3D printed using vat photopolymerization with varying strut thickness ranging from 1 mm to 3 mm. The additively manufactured structures were mechanically tested under quasi-static and impact loading scenarios, providing physical evidence for the foreseen structural behaviors discussed above. The polystructures yielded load-bearing-to-mass ratios of 4.6 kN/kg, 7.6 kN/kg, and 12.3 kN/kg for t1, t2, and t3, respectively, with less than 10 g of weight, a result of the rigid body motion and the asymmetric rotation of the polystructure beams. Additionally, the impact response highlighted a structure-to-material transition, resulting in similar force maxima for t1 and t2, with 9 and 10 N (structure-dominated), whereas t3 reported a force of up to 49 N (material-dominated). However, irrespective of the strut thickness, the negative Poisson's ratio remained nearly unchanged (−0.22 ± 0.02), exemplifying the importance of the structural mechanics substantiated by the specific topological design of S-shaped unit cells. The research findings paved the way for the translational potential of S-shaped polystructures in practical applications for mitigating impact, including civilian and military loading conditions.