An Auxetic Metamaterial with Tunable Positive to Negative Hygrothermal Expansion by means of Counter-Rotating Crosses

A type of mechanical metamaterial is proposed, whereby the effective coefficients of thermal and moisture expansions (CTE and CME, respectively) can be tuned from positive to negative values while maintaining a consistently auxetic (negative Poisson's ratio) behavior for the overall structure. Each unit consists of a pair of equal-armed crosses pin jointed at their centers, whereas their ends are connected via bridging beams using pin joints to facilitate free rotation. The bridging beams are of lower modulus and higher hygrothermal expansivity to permit negativity in metamaterial's overall Poisson's ratio and hygrothermal expansion. Two sets of effective Young's modulus, CME and CTE models are developed-one set for large deformation and the other for infinitesimal deformation. Results indicate that the effective Young's modulus is reduced while the expansion coefficients become more negative as the inclination angle of the equal-armed crosses increases, and that the application of tensile and compressive loads increases and decreases the effective Young's modulus, respectively. The establishment of effective zero thermal and moisture expansion conditions facilitates the application of the metamaterial structure that requires auxetic property within highly fluctuating environmental condition.

Automated summary

A mechanical metamaterial has been proposed that can tune the coefficients of thermal and moisture expansions, while maintaining a consistently auxetic behavior. By adjusting the inclination angle of the equal-armed crosses within the structure, it is possible to change the material's thermal and moisture expansion properties.