Auxetic cellular structures are modern metamaterials with a negative Poisson ratio [1,2]. They tend to expand in the lateral direction when subjected to tensile loading and vice versa in compression loading. This behaviour is often advantageous compared to other materials and benefits many applications, especially in engineering, crashworthiness, ballistic protection, medicine and fashion. However, it is necessary to reduce the cost of their production and improve their mechanical properties so they can be used in serial applications.

This work discusses three approaches to stiffer and cheaper modern auxetic cellular structures suitable for various deformation rates.

The recently developed Axisymmetrical chiral structure (ACS) was optimised using topology optimisation software and tested at different strain rates using experimental and computational techniques. This resulted in spatially graded ACS with a stiffer structure, providing 4.25 times higher energy absorption capability than regular ACS, with the specific energy absorption (SEA) the highest in the strut-based metamaterial class and thus ideal for future crash absorption applications [3].

The additively manufactured chiral auxetic cellular structures were embedded in silicone, resulting in hybrid auxetic cellula metamaterial. This enhances mechanical properties and postpones the brittle collapse of the auxetic structure at larger strains [4].

The macro-scale auxetic crash absorbers with inverted honeycomb structures were fabricated by bending and glueing the aluminium sheets. The open cellular space was then filled with polyurethane foam. The quasi-static and dynamic tests showed excellent mechanical responses and possibilities to adapt the mechanical response for application needs [5].

The three study cases prove that auxetic cellular structures with increased stiffness can be fabricated cheaply. Studies like this are necessary for future practical applications of novel metamaterials in the serial production of modern parts in engineering, medicine and fashion.

References
[1] K.E. Evans, M.A. Nkansah, I.J. Hutchinson, S.C. Rogers, Molecular Network Design, Nature. 353 (1991) 124. https://doi.org/10.1038/353124a0.
[2] N. Novak, M. Vesenjak, Z. Ren, Auxetic cellular materials - a Review, Strojniški Vestn. - J. Mech. Eng. 62 (2016) 485–493. https://doi.org/10.5545/sv-jme.2016.3656.
[3] N. Novak, M. Nowak, M. Vesenjak, Z. Ren, Structural Optimization of the Novel 3D Graded Axisymmetric Chiral Auxetic Structure, Phys. Status Solidi Basic Res. 2200409 (2022) 1–8. https://doi.org/10.1002/pssb.202200409.
[4] N. Novak, L. Krstulović-Opara, Z. Ren, M. Vesenjak, L. Krstulović, Z. Ren, M. Vesenjak, Mechanical properties of hybrid metamaterial with auxetic chiral cellular structure and silicon filler, Compos. Struct. 234 (2020). https://doi.org/10.1016/j.compstruct.2019.111718.
[5] N. Novak, H. Al-Rifaie, A. Airoldi, L. Krstulović-Opara, T. Łodygowski, Z. Ren, M. Vesenjak, Quasi-static and impact behaviour of foam-filled graded auxetic panel, Int. J. Impact Eng. 178 (2023) 104606. https://doi.org/10.1016/j.ijimpeng.2023.104606.