Foam structures with a negative poisson's ratio
27 Feb 1987-Science (American Association for the Advancement of Science)-Vol. 235, Iss: 4792, pp 1038-1040
TL;DR: A novel foam structure is presented, which exhibits a negative Poisson's ratio, and such a material expands laterally when stretched, in contrast to ordinary materials.
Abstract: A novel foam structure is presented, which exhibits a negative Poisson's ratio. Such a material expands laterally when stretched, in contrast to ordinary materials.
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TL;DR: In this paper, the authors presented a new mechanism to achieve a negative Poisson's ratio in a two-dimensional arrangement of rigid squares connected together at their vertices by hinges as illustrated in Fig. 1.
Abstract: Auxetic materials exhibit the very unusual properties of becoming wider when stretched and narrower when squashed [1], that is they have negative Poisson’s ratios. Apart from the pure scientific interest of having materials showing such an unconventional property, a negative Poisson’s ratio gives a material several other beneficial effects such as an increased shear stiffness, an increased plane strain fracture toughness and an increased indentation resistance. These properties make auxetics superior to conventional materials for many practical applications [1, 2]. In recent years several auxetics have been fabricated by modifying the microstructure of existing materials, including foams [2] and microporous polymers [3]. A number of molecular auxetics have also been proposed [4–9] one example being α-cristobalite [7]. The auxetic behavior in these materials can be explained in terms of their geometry and deformation mechanism. Thus, the hunt for new auxetic materials is frequently approached through searching for geometric features which may give such behavior [9, 10]. In this letter we present a new mechanism to achieve a negative Poisson’s ratio. This is based on an arrangement involving rigid squares connected together at their vertices by hinges as illustrated in Fig. 1. This may be viewed as a two dimensional arrangement of squares or as a projection of a particular plane of a three dimensional structure. This latter type of geometry is commonly found in inorganic crystalline materials [8, 9, 11]. Referring to Fig. 1, for squares of side length “l” at an angle θ to each other, the dimensions of the unit cell in the Oxi directions are given by:
604 citations
TL;DR: In this paper, the relationship among structures, materials, properties and applications of auxetic metamaterials and structures is discussed. And the challenges and future work on the topic of auxetics are also presented to inspire prospective research work.
Abstract: Materials and structures with negative Poisson's ratio exhibit a counter-intuitive behaviour. Under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also termed as 'auxetics'. Many desirable properties resulting from this uncommon behaviour are reported. These superior properties offer auxetics broad potential applications in the fields of smart filters, sensors, medical devices and protective equipment. However, there are still challenging problems which impede a wider application of auxetic materials. This review paper mainly focuses on the relationships among structures, materials, properties and applications of auxetic metamaterials and structures. The previous works of auxetics are extensively reviewed, including different auxetic cellular models, naturally observed auxetic behaviour, different desirable properties of auxetics, and potential applications. In particular, metallic auxetic materials and a methodology for generating 3D metallic auxetic materials are reviewed in details. Although most of the literature mentions that auxetic materials possess superior properties, very few types of auxetic materials have been fabricated and implemented for practical applications. Here, the challenges and future work on the topic of auxetics are also presented to inspire prospective research work. This review article covers the most recent progress of auxetic metamaterials and auxetic structures. More importantly, several drawbacks of auxetics are also presented to caution researchers in the future study.
603 citations
Journal Article•
591 citations
TL;DR: In this article, a clear classification of mechanical metamaterials have been established based on the fundamental material mechanics, which can be divided into strong-lightweight (E/ρ), pattern transformation with tunable stiffness, negative compressibility (−4G/3), and strong light-weight (S/ρ).
554 citations
Cites background from "Foam structures with a negative poi..."
...Typically, these mechanical metamaterials carry a variety of significantly enhanced mechanical properties, such as zero or negative Poisson’s ratios [26,28], vanishing shear modulus [20,33,34], negative stiffness [36], negative compressibility [37], singularly nonlinear behaviour [38,39], and customised topological microstructures [40–42], all of which distinguishes them from conventional natural materials....
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...For instance, plastically deformed foams and honeycombs are known [28,29,311] to provide either negative Poisson’s ratios or a Poisson’s ratios whose sum exceeds unity along a stretch direction....
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...These auxetic metamaterials include re-entrant or hinged honeycombs and foams [28,30,76,339,347], similar to another directional semi-auxetic solid based on combining or rotating units....
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...auxetic metamaterials [28,62,309,310], in that they respond differently to different external stimuli....
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TL;DR: In this article, a numerical topology optimization method is used to design and fabricate compliant micromechanisms and material structures with negative Poisson's ratio (NPR) using a laser micromachining setup.
Abstract: This paper describes a new way to design and fabricate compliant micromechanisms and material structures with negative Poisson's ratio (NPR). The design of compliant mechanisms and material structures is accomplished in an automated way using a numerical topology optimization method, The procedure allows the user to specify the elastic properties of materials or the mechanical advantages (MA's) or geometrical advantages (GA's) of compliant mechanisms and returns the optimal structures. The topologies obtained by the numerical procedure require practically no interaction by the engineer before they can be transferred to the fabrication unit. Fabrication is carried out by patterning a sputtered silicon on a plasma-enhanced chemical vapor deposition (PECVD) glass with a laser micromachining setup. Subsequently, the structures are etched into the underlying PECVD glass, and the glass is underetched, all in one two-step reactive ion etching (RIE) process. The components are tested using a probe placed on an x-y stage. This fast prototyping allows newly developed topologies to be fabricated and tested within the same day.
508 citations
Cites methods from "Foam structures with a negative poi..."
...A practical material with NPR was first developed by Lakes [ 16 ], who treated an open-walled foam material with heat and pressure to obtain the NPR effect....
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References
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TL;DR: The mechanical properties (elastic, plastic, creep, and fracture) of cellular solids or foams are related to the properties of the cell wall material and to the cell geometry.
Abstract: The mechanical properties (elastic, plastic, creep, and fracture) of cellular solids or foams are related to the properties of the cell wall material and to the cell geometry The properties are well described by simple formulae Such materials occur widely in nature and have many potential engineering applications
909 citations
TL;DR: In these studies, encystation of Giardia lamblia in vitro was demonstrated by morphologic, immunologic, and biochemical criteria and will aid in understanding the differentiation of an important protozoan pathogen.
Abstract: The cyst form of Giardia lamblia is responsible for transmission of giardiasis, a common waterborne intestinal disease. In these studies, encystation of Giardia lamblia in vitro was demonstrated by morphologic, immunologic, and biochemical criteria. In the suckling mouse model, the jejunum was shown to be a major site of encystation of the parasite. Small intestinal factors were therefore tested as stimuli of encystation. An antiserum that reacted with cysts, but not with cultured trophozoites was raised in rabbits and used as a sensitive probe for differentiation in vitro. Cultured trophozoites that were exposed to bile salts showed a more than 20-fold increase in the number of oval, refractile cells that reacted strongly with anticyst antibodies, and in the expression of major cyst antigens. Exposure to primary bile salts resulted in higher levels of encystation than exposure to secondary bile salts. These studies will aid in understanding the differentiation of an important protozoan pathogen.
157 citations
TL;DR: In this paper, a method of calculating the anisotropy of Poisson's ratio is applied to trigonal and tetragonal crystals in the literature, and the method of calculation is extended to hexagonal crystals.
Abstract: A method of calculating the anisotropy of Poisson's ratio is applied to trigonal and tetragonal crystals in the literature. The method of calculation is extended to hexagonal crystals. The results are illustrated by reference to Tl and Cd.
96 citations