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Thermal expansion and stress of solid materials can significantly affect structural safety and instrument accuracy in many engineering fields. An alternative way to overcome these drawbacks is to use materials with controllable coefficients of thermal expansion (CTEs). This work presents a new three-dimensional (3D) material with controllable CTEs that is composed of chiral honeycomb layers connected by a set of inclined struts. Although each single chiral layer has positive CTEs, by carefully designing geometric parameters and material constituents, the negative thermal expansion (NTE) can be established. Analytical solutions of the equivalent CTEs are proposed using the classical beam theory, which were verified by finite element simulations. The results show that the proposed material can achieve a large range of CTEs (from positive to negative), and has negative Poisson's ratios in the chiral honeycomb planes. In addition, isotropic negative and zero thermal expansion can be obtained with appropriate design parameters.

Thermal stress impacts various engineering fields significantly, such as aerospace and precision instruments. This adverse effect can be greatly reduced, if not eliminated, by the application of micro-thermal expansion materials, and bi-material beams are widely utilized in the design of micro-thermal expansion structures, thereby exhibiting great application potentials. The elasticity solution of bi-material curved beam under free thermal expansion has been proposed by scholars. Based on this solution, the simplified form is proposed in this paper, and extended to the case where the rotation angles at both ends of the circular arc are constrained under thermal loads. Besides, the geometric parameters and the nonlinear problems of the thermal expansion of bi-material curved beam are analyzed. In addition, a novel type of negative thermal expansion material has been designed by applying the bi-material curved beam to the tetra chiral and anti-tetra chiral materials. The proposed material has greater negative thermal expansion effect than the traditional tetra and anti-tetra chiral materials that are with straight beams.

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