A Novel Re-Entrant Honeycomb with Tunable Auxetic Response and Enhanced In-Plane Mechanical Performance

Re-entrant honeycombs with tunable auxetic response have attracted increasing attention for energy absorption, yet their in-plane stiffness and load-bearing capacity are often limited by bending-dominated deformation and local instability. In this study, a re-entrant honeycomb with cosine-curved walls and X-shaped ligaments, denoted as X-CRS, was proposed to enhance the in-plane mechanical response and energy absorption performance. Three X-CRS configurations with different X-ligament angles were fabricated by fused deposition modeling using PLA and investigated through quasi-static compression tests and finite element simulations. The results show that the X-CRS-30 achieved the highest load-bearing performance, with approximate increases of 148%, 133%, and 102% in elastic modulus, collapse stress, and plateau stress, respectively, and the highest mean specific energy absorption (SEA) of 1.225 ± 0.015 kJ kg^-1. At a nominal compressive strain ε_c = 0.20, the numerical effective Poisson's ratios ν_eff were -0.676, -0.609, -0.376, and +0.156 for CRS, X-CRS-10, X-CRS-20, and X-CRS-30, respectively. Thus, X-CRS-20 retained auxeticity with more stable progressive collapse, whereas X-CRS-30 prioritized stiffness and energy absorption at the expense of auxetic response.