Abstract
A metamaterial that can manifest both positive and negative coefficients of moisture and thermal expansion is presented herein, based on inspiration from the Maltese cross. Each unit of the metamaterial consists of a pair of equal-armed crosses pin-joined at their junctions to permit rotation, but elastically restrained by a bimaterial spiral spring, and four pairs of hinge rods to translate the relative rotational motion of the pair of equal-armed crosses into translational motion of the connecting rods. The effective coefficients of moisture and thermal expansion models were developed for small and large changes in the hygrothermal conditions using infinitesimal (approximate) and finite (exact) motion analyses, respectively, with the former giving constant effective coefficients with respect to environmental changes. Results indicate that the approximate method underestimates the magnitude of both the effective expansion coefficients under cooling and drying but overestimates magnitudes of both coefficients during heating and moistening, and that the change in both expansion coefficients is more drastic during cooling and drying than during heating and moistening. In addition to providing another micro-lattice geometry for effecting expansion coefficients of either signs, this metamaterial exhibits auxetic property.
Highlights
There are times when breakthroughs in science are impelled by necessity, at other times by accidental discovery, and still other times advancement is made by drawing inspiration from nature
A tuneable positive hygrothermal expansion (PHTE) and negative hygrothermal expansion (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
Perusal to the analyses indicates that the effective on-axes coefficient of moisture expansion (CME) and coefficient of thermal expansion (CTE) models are proportional to the number of coils n and the mean radius rm but inversely proportional to the thickness h of the bimaterial spiral springs
Summary
There are times when breakthroughs in science are impelled by necessity, at other times by accidental discovery, and still other times advancement is made by drawing inspiration from nature. Of interest is a class of metamaterial that manifests counterintuitive behaviour; these materials possess negative values for mechanical properties that are normally positive. The Hoberman circle consists of rigid pairs of rods that are pin-jointed at their mid-span, and these pairs of rods are arranged in a circle; the metamaterial can be made to exhibit negative thermal expansivity by the inclusion of beam elements of a different material that possesses high CTE [47]. 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
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