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

Abstract

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.

Adjustable positive and negative hygrothermal expansion metamaterial inspired by the Maltese cross

“…In comparison to a related but simpler metamaterial geometry consisting of counter-rotating crosses without the hinge rods and spiral springs [49], the magnitudes of the current effective CTE and CME are greater by three orders. This is attributed to the spiral bimaterial spring and greater extent of hinge rod rotation in comparison to the cross rotation, which effectively accentuates the hygrothermal strain and, thereby, enhances the coefficients of expansion.…”

“…Development of metamaterials with such linkage microstructures are now possible with recent advancement in three-dimensional printing technology by incorporating hard and soft materials to mimic pin joints. Although the capability to manifest in a counterintuitive manner-such as negative properties-permits materials and structures to perform in ways that are different from conventional ones, it is of greater practical importance if materials can be tuned to possess both positive and negative properties [49]. In this paper, a tuneable positive hygrothermal expansion (PHTE) and NHTE metamaterial is proposed, in which each unit consists of a pair of equal-armed crosses that are pin-jointed at their centres while their vertices are connected in such a manner that the shape of a Maltese cross is manifested.…”

“…As such, materials display negative hygrothermal expansion (NHTE) when they demonstrate negative moisture expansion (NME) and/or negative thermal expansion (NTE). Some examples of designed micro-architectures that facilitate NME, NTE and/or negative compressibility (NC) have been made possible by combining materials of non-negative but contrasting expansion coefficients in open-cell micro-truss systems with pin joints or rigid joints [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], bimaterial strips [40][41][42][43][44][45][46] and counterrotating parts [47][48][49]. The category of metamaterials with counter-rotating parts refers to those with scissor-like actions and is a subset of the pin-jointed microstructures.…”

Negative thermal expansion mechanical metamaterial using the swastika shape

“…A more comprehensive review can be obtained from a related monograph [9]. Of late, the design of unit cell geometries of metamaterials have drawn inspiration from cultural and religious geometrical patterns, such as Islamic geometrical patterns [10,11], Egyptian axe dipole [12], Jerusalem cross [13], Iron cross [14], Celtic cross [15], Maltese cross [16,17], Aztec geometrical patterns [18,19] and tangram puzzle [20,21]. We consider herein a mechanical metamaterial that exhibits inplane isotropic negative thermal expansion (NTE), which are constructed from rigid swastikas, as shown in figure 1(a).…”

Investigating the Effect of Relative Humidity on the Mechanics and Dynamics of Open‐Cell Polyurethane Auxetic Foams

“…The effect of moisture on the mechanical behavior of other auxetic metamaterials has also been evaluated analytically in refs. [47][48][49]. In those works, the moisture affects the mechanical properties of the metamaterials because of the embedding within the architectures of moisturesensitive components.…”