Behaviour of 3D printed re-entrant chiral auxetic (RCA) geometries under in-plane and out-of-plane loadings

Three-dimensional printing of auxetic materials is an emerging research field due to their intriguing negative Poisson's ratio (NPR) behaviour which enables improved load resistance than conventional materials for various applications. In this research, re-entrant chiral auxetic (RCA) structures of three different geometries were printed using polylactic acid (PLA) filament. The PLA material was also characterised in compression and tension. In total, 27 RCA specimens of varied cell sizes were designed, printed and tested under in-plane tension and compression and out-of-plane bending loads. Digital image correlation method was employed to study the failure modes, displacement profiles, load-displacement curves and ductility characteristics. Results showed that the samples with the smallest cell sizes exhibited the highest resistance against compression, tension and out-of-plane loading. The elastic modulus and Poisson's ratio was also higher for these geometries with the maximum NPR of -9 in compression and -5 in tension. In comparison, the larger cell geometries due to wider gaps between the cells showed higher deformability which resulted in high energy absorption characteristics in compression and tension. Whereas in out-of-plane bending, all designed geometries, irrespective of cell sizes exhibited similar deformability with the maximum ductility obtained for the RCA structures made of smaller cells.