Dual arrowhead-shaped re-entrant auxetic hybrid metamaterial with adjustable thermal expansion

The lightweight metamaterial with the adjustable coefficient of thermal expansion (CTE) and Poisson's ratio (PR) has aroused great interest in potential engineering applications. However, the bifunctional metamaterials with dual negative properties exhibit relatively narrow ranges of CTE and PR, only considering one direction. In this work, a dual arrowhead-shaped re-entrant auxetic (DARA) hybrid metamaterial with controllable CTE is designed by combining a star-shaped re-entrant structure and a dual arrowhead inclined triangle structure. A theoretical model of the DARA unit cell is established to obtain CTE and PR solutions using the stiffness matrix method, which is verified by the finite element method (FEM). The theoretical model is used to further study the effect of geometric parameters, configuration, and base material on the thermal and mechanical properties. The DARA metamaterial can simultaneously realize the adjustable CTE and PR, especially a wide range of CTE under two directions, from negative to positive values. Furthermore, the 3D-printed DARA samples are fabricated to validate the CTE and PR values of the unit cell, which are in line with the FEM and theoretical values. The bimetamaterial unit cell exhibits positive, quasi-zero, or negative CTE and PR owing to the synergistic effect of reentrant and dual arrowhead structure. The deformation mechanism is proposed based on the structure's stretchbending coupling and stretch-dominated design.