Composite tree-like re-entrant structure with high stiffness and controllable elastic anisotropy

Critical drawback of poor stiffness in traditional auxetic materials limits them to be applied where negative Poisson's ratio behavior and excellent load-bearing capacity are simultaneously desired. In this work, a general prescription for designing auxetic lattice structures with high stiffness and controllable isotropic/anisotropic elastic properties is proposed. Based on it, we rationally design a class of novel auxetic lattice structures whose deformation is mainly governed by the stretching mechanisms and conducted an investigation on a tree-like re-entrant structure. A combination of theoretical predictions, numerical simulations and tensile test experiments have been carried out to gain a comprehensive understanding of the structural in-plane elastic mechanical properties along both the principal axes as well as in the off-axis directions. Besides, theoretical model for designing tree-like re-entrant structure with isotropic elastic properties is established and experimentally verified with specimens fabricated with CFRP composite. The results of this research has proven the tree-like re-entrant structure exhibits excellent effective Young's modulus, shear modulus and obvious negative Poisson's ratio effect along omni-direction. Investigation on the directional dependence of structural effective elastic properties has further indicated the proposed structure presents a great potential for designing auxetic material with controllable anisotropic elastic properties. In particular, the proposed structure is capable in achieving isotropic design with any negative Poisson's ratio between −1 and 0. The design philosophy presented in this paper provides a general prescription for solving the challenge of poor stiffness in traditional auxetic materials and paves a new way for isotropic auxetic design.