Composite metamaterial with sign-switchable coefficients of hygroscopic, thermal and pressure expansions

This paper proposes a truss microstructure that exhibits negative properties during increase in temperature or moisture and decrease in pressure but reverses to conventional properties during decrease in temperature or moisture and increase in pressure so that the material contracts regardless of the direction by which the environment condition changes. Both primary and secondary cells are bounded by 4 side rods to form squares at original state, with each primary cell containing a central rod connected diagonally. Sign-switchability of material properties is observed when the central rod is more responsive than the side rods during environmental fluctuations. The proposed microstructure exhibits zero environmental expansion when the environmental change is insignificant, and can be further designed to exhibit zero environmental expansion for large change in environmental fluctuation. This metamaterial exhibits expansion coefficients that switch between positive and negative values such that it always contracts with environmental changes.