Smart Honeycomb “Mechanical Metamaterials” with Tunable Poisson's Ratios

Abstract

Mechanical metamaterials represent a class of deformable systems, which exhibit macroscopic deformations, mechanical, and/or thermal properties. These emerge due to the structure of their subunits rather than their materials composition and typically exhibit anomalous (normally negative) macroscopic structural, mechanical, or thermal property/properties caused by a change in shape/size of the system. Herein, a class of honeycombs is discussed, which push to the extreme the classical definition of “mechanical metamaterials,” exhibiting temperature‐tunable Poisson's ratio properties. More specifically, centrosymmetric honeycombs with T‐shaped joints constructed from different materials are shown to exhibit temperature‐dependent Poisson's ratio values, which can be either positive or negative (auxetic) depending on the external stimulus the system is subjected to. The sign and magnitudes of the Poisson's ratio values are explained in terms of particular geometries that these composite honeycomb systems attain at different temperature conditions. In particular, auxeticity is attributed to the transformation of the T‐shaped units to re‐entrant units. Practical aspects on how these properties may be achieved are discussed, including the possibility that the changes in shape, and hence Poisson's ratios, are induced via different extents of dryness on opposite surfaces of ligaments made from absorbent materials.

On the Design of Multimaterial Honeycombs and Structures with T‐Shaped Joints Having Tunable Thermal and Compressibility Properties

“…Having established that the concepts presented previously can be used to generate deformations which convert T‐shaped junctions to arrow (↑) or Y‐shaped junctions upon the application of temperature and/or pressure variations, it would be useful to discuss some of the potential applications that this concept may have. These include the construction of alternative designs, possibly even more versatile systems with tunable properties, particularly when bearing in mind that the re‐entrant version of these honeycombs (obtained through a change in temperature/pressure) is expected to be auxetic, whereas the non‐re‐entrant versions are expected to exhibit conventional Poisson's ratio behavior …”

“…These include the construction of alternative designs, possibly even more versatile systems with tunable properties, particularly when bearing in mind that the re-entrant version of these honeycombs (obtained through a change in temperature/pressure) is expected to be auxetic, whereas the non-re-entrant versions are expected to exhibit conventional Poisson's ratio behavior. [59] For example, it is possible to construct other space filling models based on this concept of bimaterial strips. In particular, one may build systems which upon deformation resemble the starshaped space filling systems as exemplified by Grima et al [55] some of which are shown in Figure 14.…”

“…More complex designs based on this concept, where the vertical ligaments are in the form of I-shaped units, are also investigated. Finally, the potential of these systems to exhibit tunable Poisson's ratio properties (discussed in more detail elsewhere [59] ) as well as more complex 2D and 3D constructs are introduced. These include ones in the form of 3D tubular objects which can tighten upon heating, a feature which can make them particularly useful in real applications, for example, in the manufacture of adhesiveless grips that tighten when hand-held.…”

“…This led to various studies in the last decades which were focused on the design and investigation of honeycombs specifically engineered to exhibit anomalous or superior thermomechanical properties, such as a negative Poisson's ratio (auxetic behavior) . Given the advantages of honeycombs over nonporous solids, when put in the context of the developments which are being made on “negative thermomechanical properties,” it is not surprising that research in the field of “negative honeycombs” is still the subject of extensive research …”

Auxetics and Other Systems with “Negative” Characteristics

“…[25] In the last article of this section, 'Smart Honeycomb "Mechanical Metamaterials" with Tunable Poisson's Ratios', James N. Grima-Cornish et al show how the composite honeycombs having T-shaped junctions could be made to exhibit temperaturedependent Poisson's ratios. [26] This time the section other systems with "negative" characteristics is represented by three papers. The first of them, 'On the Design of Multimaterial Honeycombs and Structures with T-Shaped Joints Having Tunable Thermal and Compressibility Properties' by Reuben Cauchi et al, shows that the composite honeycombs having T-shaped junctions, which were also the subject of the last paper of the previous section, can exhibit negative thermal expansion and/or negative compressibility, the magnitude of which could be fine-tuned though design.…”