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• 3D bi-material triangle re-entrant honeycomb with adjustable Poisson's ratio is proposed. • The intrinsic mechanisms of tunable CTE and PR are revealed by theoretical models. • The finite element analysis is used to calculate the band structures and width of the total effective bandgap. • This work provides a design idea for 3D metamaterials to regulate CTE, PR and the bandgap. In the aerospace industry, precision components need to serve in a comprehensive environment of severe temperature changes, vibrations, and noise. Therefore, it is necessary to design a multifunctional mechanical metamaterial that integrates a tunable coefficient of thermal expansion (CTE), Poisson's ratio (PR), and bandgap. Here, a 3D bi-material triangle re-entrant honeycomb (3D-BTRH) with adjustable Poisson's ratio is proposed. Due to the geometric symmetry, the 3D-BTRH realizes the isotropy of mechanical properties in the three coordinate axes. The intrinsic mechanisms of tunable CTE and PR of the 3D-BTRH are revealed by building theoretical models. The accuracy of the theoretical analysis of PR and Young's modulus is verified by simulation and experiment. The finite element analysis is used to calculate the band structures and width of the total effective bandgap utilizing Bloch's theorem and Bloch's periodic boundary conditions. The accuracy of the band structure calculations is verified by calculating the transmission characteristic curve. The parametric analysis shows that the 3D-BTRH can achieve tunable CTE and PR in the directions of the three axes, and has a large width of the total effective bandgap. Moreover, this work provides a design idea for 3D metamaterials to simultaneously regulate CTE, PR, and the bandgap, which offers opportunities for better engineering practice. [Display omitted] [ABSTRACT FROM AUTHOR]

· A bi-material triangle re-entrant honeycomb (BTRH) with the tunable coefficient of thermal expansion (CTE), poisson's ratio (PR), and bandgap is proposed. · Analytical analysis and numerical simulations are implemented to quantify the evolution of the effective CTE, PR, and young modulus. · The band structures of the BTRH are calculated by finite element analysis. · The effects of geometric parameters and material combinations on the mechanical properties are investigated. The future mechanical metamaterials should achieve multifunctional integration to meet the requirements of structures and materials in the aviation field for service in complex environments. In this paper, a bi-material triangle re-entrant honeycomb (BTRH) with the tunable coefficient of thermal expansion (CTE), Poisson's ratio (PR), and bandgap is proposed. Analytical analysis and numerical simulations are implemented to quantify the evolution of the effective CTE, PR, and Young's modulus. Experiments are conducted to verify the correctness of the theory of the effective PR and Young's modulus. Based on Bloch's theorem, the band structures of the BTRH are calculated by finite element analysis, and the correctness of the band structure calculation is verified by calculating the transmission characteristic curve. The effects of geometric parameters and material combinations on the mechanical properties are investigated systematically. The results of analytical and numerical simulations exhibit that the CTE and PR can be coupled with regulation from positive to negative through changing the geometry configuration and material combinations of the BTRH. Meanwhile, the function of tunable bandgap and large total effective bandgap width can be obtained. This work provides an important reference for the rational design of multifunctional metamaterials to maintain shape stability and efficient sound insulation. [Display omitted] [ABSTRACT FROM AUTHOR]