Impact resistance of additively manufactured 3D double-U auxetic structures

Metallic cellular materials and structures offer outstanding combinations of mechanical and physical properties with lightweight, are essential in multifunctional applications. Auxetic lattice architectures with negative Poisson's ratios are potential templates for this purpose owing to their unprecedented deformation mechanism contrary to traditional periodic structures and fully dense metals. In this study, the fabrication of intensive 3D double-U hierarchical structures by selective laser melting with additively-manufactured stainless steel is presented and their mechanical responses are characterized by theoretical, numerical and experimental methods. Compression experiments and systematical finite element simulations are carried out to investigate the influence of strain hardening of constitute material, impact velocity, initial curvature (concave configuration with negative Poisson's ratio and convex configuration with positive Poisson's ratio) and functional gradient (graded cell wall thickness and density) on crushing properties of 3D double-U hierarchical lattices. Transitions of different deformation modes (low-velocity, medium-velocity and high-velocity crushing modes) owing to the inertial effects are observed with the increase of impact velocity. The metallic 3D double-U auxetic structures show high stiffness, strength, toughness and impact resistance at low relative density. The energy absorption and impact protection capacities of concave and convex 3D double-U hierarchical structures are compared and discussed based on the compressive results. This work provides valuable guidance in the design, fabrication, characterization and application of lightweight auxetic lattice structures.