4D Printing of NiTi Auxetic Structure with Improved Ballistic Performance

Auxetic structures have attracted attention in energy absorption applications owning to their improved shear modulus and enhanced resistance to indentation. On the other hand, Four-dimensional (4D) printing is an emerging technology that is capable to 3D print smart materials with additional functionality. This paper introduces the development of NiTi negative Poisson's ratio structure with superelasticity/shape memory capabilities for improved ballistic applications. An analytical model was initially used to optimise the geometrical parameters of a re-entrant auxetic structure. It was found that re-entrant auxetic structure with a cell angle of -30° produced the highest Poisson's of -2.089. 4D printing process using powder bed fusion system was used to fabricate the optimised NiTi auxetic structure. The measured negative Poisson's ratio of the fabricated auxetic structure was found in agreement with both the analytical model and the finite element simulation. A finite element model was developed to simulate the dynamic response of optimised auxetic NiTi structure subjected to different projectile speeds. Three stages of the impact process describing the penetration of the top plate, auxetic structure, and bottom plate have been identified. The results show that the optimised auxetic structures affect the dynamic response of the projectile by getting denser towards the impact location. This helped to improve the energy absorbed per unit mass of the NiTi auxetic structure to about two times higher than the solid NiTi plate and five times higher than the solid conventional steel plate.