The ability of flexible metamaterials with different internal structures based on a hexagonal cell to mechanically resist penetration by a rigid spherical impactor was experimentally studied. Metamaterial samples with chiral and non-chiral internal structures were 3D-printed from flexible TPU 95A plastic (thermoplastic polyurethane). The metamaterial’s internal structure was based on cells shaped like a convex regular hexagon or a concave hexagon on one or both sides. Samples of similar masses were prepared for these three types of metamaterials and compared for their ability to reduce the kinetic energy of impactors at a velocity of approximately 190 m/s at temperatures of 16 and 25°C. For both chiral and non-chiral samples, it was found that auxetics (metamaterials with biconcave hexagonal cells) exhibited the highest resistance to impact penetration at both temperatures. Samples of metamaterials with a honeycomb structure (with a convex hexagonal cell) exhibited the lowest resistance. A metamaterial with a herringbone structure (with a hexagonal cell concave only on the right or left side) showed an intermediate result. For this structure, higher resistance was observed in samples with cell concavity in the direction of impact movement, and lower resistance was observed in samples with cell concavity against the direction of movement. At a temperature of 25°C, all samples exhibited higher resistance to impact penetration than at 16°C. It was noted that in all cases considered, chiral samples resisted impact better than those without chirality.
Ivanova et al. (2026) studied this question.