Auxetic mechanical metamaterials with symmetry-broken Re-entrant units

Three innovative auxetic metamaterials with symmetry-broken unit cells and possible applicability in the construction and automotive industries are presented by breaking the mirror symmetries of the conventional reentrant unit cell. The single symmetry-broken re-entrant (SBR), double symmetry-broken re-entrant (DBR), and hybrid symmetry-broken re-entrant (HBR) structures are fabricated via 3D printing and examined by experimental compression tests and finite element analysis (FEA). The proposed metamaterials exhibit remarkable specific energy absorption (SEA) properties in comparison to the benchmark symmetric-unit honeycomb, with the SBR model showing 103.9 % higher SEA. The novel structures also show superior stability under large compressive deformation, exhibiting excellent auxeticity in comparison to the symmetric-unit model. Parametric investigations reveal that the level of asymmetricity of the unit cells plays a key role in the stability and superior mechanical properties of the proposed metamaterials under compression. One set of parametric studies discovers an HBR structure with a special combination of SBR and DBR units which outperforms the parent structures under compression. Finally, parametric investigation of transverse and inclined compression of the structures indicated superiority of the DBR metamaterial to other structures due to having a deformation mechanism with organized self-contact regions. The DBR metamaterial owns 314 % and 86 % higher SEA rather than the benchmark honeycomb under transverse and inclined compression, respectively.