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A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson’s ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics. Significance Carbon has many faces––from diamond and graphite to graphene, nanotube, and fullerenes. Whereas hexagons are the primary building blocks of many of these materials, except for C ₂₀ fullerene, carbon structures made exclusively of pentagons are not known. Because many of the exotic properties of carbon are associated with their unique structures, some fundamental questions arise: Is it possible to have materials made exclusively of carbon pentagons and if so will they be stable and have unusual properties? Based on extensive analyses and simulations we show that penta-graphene, composed of only carbon pentagons and resembling Cairo pentagonal tiling, is dynamically, thermally, and mechanically stable. It exhibits negative Poisson's ratio, a large band gap, and an ultrahigh mechanical strength.

This paper describes two folded metamaterials based on the Miura-ori fold pattern. The structural mechanics of these metamaterials are dominated by the kinematics of the folding, which only depends on the geometry and therefore is scale-independent. First, a folded shell structure is introduced, where the fold pattern provides a negative Poisson’s ratio for in-plane deformations and a positive Poisson’s ratio for out-of-plane bending. Second, a cellular metamaterial is described based on a stacking of individual folded layers, where the folding kinematics are compatible between layers. Additional freedom in the design of the metamaterial can be achieved by varying the fold pattern within each layer.

Poisson effect is a phenomenon in the mechanics of geotechnical materials, which means that when a material is subjected to tension (or compression) in one direction, it will shrink (or expand) in the direction perpendicular to the tension (or compression) direction. This phenomenon of the correlation between lateral deformation and longitudinal deformation is called Poisson effect. Firstly, this paper analyzes the basic definition and range of Poisson’s ratio, including the homogeneous range at small strains. Secondly, it extends to the concept of negative Poisson’s ratio and the material structure of negative Poisson’s ratio. Then the determination method and value range of Poisson’s ratio in rock and soil were analyzed. For loose sand, Poisson’s ratio is usually between 0.2-0.4. For cohesive soils, the Poisson’s ratio may be higher, approximately between 0.3-0.5. The Poisson’s ratio of granite is usually between 0.2-0.3, while the Poisson’s ratio of shale may reach 0.3-0.4. This analysis is of great significance and guidance for a deeper understanding of the physical and mechanical properties of rock and soil masses, as well as for the stability analysis of geotechnical engineering.

We consider disordered solids in which the microscopic elements can deform plastically in response to stresses on them. We show that by driving the system periodically, this plasticity can be exploited to train in desired elastic properties, both in the global moduli and in local “allosteric” interactions. Periodic driving can couple an applied “source” strain to a “target” strain over a path in the energy landscape. This coupling allows control of the system’s response, even at large strains well into the nonlinear regime, where it can be difficult to achieve control simply by design.

In comparing a material's resistance to distort under mechanical load rather than to alter in volume, Poisson's ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and −1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poisson's ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poisson's ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt. Poisson's ratio describes the resistance of a material to distort under mechanical load rather than to alter in volume. On the bicentenary of the publication of Poisson's Traité de Mécanique , the continuing relevance of Poisson's ratio in the understanding of modern materials is reviewed.

Auxetic materials are a kind of non-conventional materials having negative Poisson’s ratio. They laterally expand when stretched or laterally shrink when compressed. Compared to conventional materials, auxetic materials have a number of enhanced properties that could be very interesting for some special applications. This paper reviews the latest achievements in auxetic materials, including their properties, structures and applications. A special discussion on their potential applications in textiles is also made. It is expected that this review could provide some useful information for the future development of auxetic textile materials.

Background Today, energy harvesting is a hot topic in the scientific community because of the scarcity and insufficiency of energy resources. Piezoelectric systems have been proven by many studies to be very efficient in energy harvesting. In addition, an increase in efficiency has been observed by using auxetic materials in piezoelectric systems due to their extraordinary properties. Because of its capability to sustainably provide wireless sensors and portable electronic devices, a worldwide effort is being made to capture energy using the mechanical vibrations of the environment. Purpose The main purpose was to understand auxetic materials and piezoelectric systems more clearly by making a comprehensive compilation. Methods This review article investigated an auxetic piezoelectric energy harvester (APEH) system focusing on the structure of auxetic materials along with their behavior and efficiency in the system. Results As a result of general examinations, it was seen that structures with negative Poisson ratios were much more efficient in energy harvesting in this type of system compared to conventional structures. Conclusion Due to APEH’s advantages such as simplicity, scalability, and high-power production, piezoelectric energy harvesting has gained more popularity than other vibration-based energy harvesting methods.

The development of auxetic materials, known for their unique negative Poisson’s ratio, is transforming various industries by introducing new mechanical properties and functionalities. These materials offer groundbreaking applications and improved performance in engineering and other areas. Initially found in natural materials, auxetic behaviors have been developed in synthetic materials. Auxetic materials boast improved mechanical properties, including synclastic behavior, variable permeability, indentation resistance, enhanced fracture toughness, superior energy absorption, and fatigue properties. This article provides a thorough review of auxetic materials, including classification and applications. It emphasizes the importance of cellular structure topology in enhancing mechanical performance and explores various auxetic configurations, including re-entrant honeycombs, chiral models, and rotating polygonal units in both two-dimensional and three-dimensional forms. The unique deformation mechanisms of these materials enable innovative applications in energy absorption, medicine, protective gear, textiles, sensors, actuating devices, and more. It also addresses challenges in research, such as practical implementation and durability assessment of auxetic structures, while showcasing their considerable promise for significant advancements in different engineering disciplines.

Following high profile, life changing long term mental illnesses and fatalities in sports such as skiing, cricket and American football—sports injuries feature regularly in national and international news. A mismatch between equipment certification tests, user expectations and infield falls and collisions is thought to affect risk perception, increasing the prevalence and severity of injuries. Auxetic foams, structures and textiles have been suggested for application to sporting goods, particularly protective equipment, due to their unique form-fitting deformation and curvature, high energy absorption and high indentation resistance. The purpose of this critical review is to communicate how auxetics could be useful to sports equipment (with a focus on injury prevention), and clearly lay out the steps required to realise their expected benefits. Initial overviews of auxetic materials and sporting protective equipment are followed by a description of common auxetic materials and structures, and how to produce them in foams, textiles and Additively Manufactured structures. Beneficial characteristics, limitations and commercial prospects are discussed, leading to a consideration of possible further work required to realise potential uses (such as in personal protective equipment and highly conformable garments).

By analyzing phonon dispersion, we have evaluated the average Young’s modulus and Poisson’s ratio in graphite and in graphene grown on Ru(0001), Pt(111), Ir(111), Ni(111), and BC 3 /NbB 2 (0001). In both flat and corrugated graphene sheets and in graphite, we find a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. The unique exception is graphene/Ni(111), for which we find different values because of the stretching of C-C bonds occurring in the commensurate overstructure (0.36 and 310 N/m for the Poisson’s ratio and Young’s modulus, respectively). Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. The high crystalline quality of graphene grown on metal substrates leads to macroscopic samples with high tensile strength and bending flexibility for use in technological applications such as electromechanical devices and carbon-fiber reinforcements.