Cyclic temperature-dependent response of unit cell auxetic structures in an additively manufactured shape memory alloy.

This study investigates the mechanical response of additively manufactured NiTi sinusoidal hexagonal re-entrant auxetic structures, focusing on the combined influence of temperature and local geometry. The NiTi alloy fabricated by laser powder bed fusion (L-PBF) was thermomechanically characterised to identify conditions promoting enhanced superelastic response, while X-ray micro-computed tomography was used to assess internal porosity. Monotonic and strain-controlled cyclic tensile tests were performed on auxetic cells with three different fillet radii, using a dedicated setup combining digital image correlation and closed-loop temperature control. The results reveal a clear separation between material- and geometry-driven effects. Temperature primarily governs the load levels through the superelastic response of the alloy, whereas auxetic deformation behaviour and failure mechanisms are mainly controlled by the structural geometry, particularly the fillet radius. Increasing the fillet radius significantly affects the deformation and failure response of the auxetic structures, altering the relationship between failure strain and absolute displacement through the corresponding change in cell geometry. The experimentally measured Poisson's ratio was compared with predictions from analytical models. Although these overestimate the auxetic response, they provide a reasonable first-order estimate. These findings provide insight into the design of additively manufactured NiTi auxetic architectures.