1,721,003 research outputs found
Chiralisation of Euclidean polygonal tessellations for the design of new auxetic metamaterials
No full textChiral honeycombs are one of the main classes of mechanical metamaterials with the potential to exhibit auxetic behaviour. In this work, we propose a new class of chiral metamaterials based on uniform Euclidean tessellations and their dual counterparts. In total, ten new structures were designed and analysed using Finite Element analysis under periodic boundary conditions, with eight of these systems showing the capability of possessing a negative Poisson's ratio. The relationship between the various geometric parameters defining the systems and the resultant mechanical properties was also studied. We show that ‘chiralisation’, i.e. introduction of chirality and rotational elements within the system, has the ability to transform even complex geometries, which in their original state possess a high positive Poisson's ratio, into auxetic metamaterials and hope that this work can act as a blueprint for the design of auxetic structures with novel topologies
Novel chiral honeycombs based on octahedral and dodecahedral Euclidean polygonal tessellations
No full textIn this work, we explored the effect of ‘chiralisation’, i.e. the introduction of geometric chiral characteristics, on the mechanical properties of Euclidean polygonal tessellations containing octahedral or dodecahedral elements. This geometric transformation resulted in the design of three novel auxetic metamaterials which have the potential to exhibit large negative Poisson's ratios (ca. −1) coupled with high levels of in-plane isotropy. We have also examined the influence of the introduction of chiral nodes on the type of geometric arrangement (i.e. whether original or dual) of the base tessellation and also show how the extent of auxeticity may be controlled by tuning the geometric parameters of these systems. This work confirms the potential of Euclidean polygonal tessellations for the design of novel auxetic metamaterials and provides new insights into the deformation mechanisms and geometric conditions which impart this anomalous property
Tailoring the flexural response of sandwich structures with faces made from shape memory alloy composite actuators
No full textNon-porous grooved single-material auxetics
No full textOver the years, great advances have been made in the field of auxetic metamaterials where one of the main focuses was the production of systems which can be produced through simple and relatively inexpensive means. In this work, auxetic systems created through the introduction of elliptical grooves, meant to mimic the rotating units mechanism, are proposed and analyzed. These systems were found to have the potential to exhibit Poisson's ratios ranging from ca.-1 to the Poisson's ratio of the material of the system itself, with some systems also possessing the remarkable property of a zero Poisson's ratio. The final product is a non-porous metamaterial with potentially tailor-made Poisson's ratio properties
Self-expanding Nitinol stents for endovascular peripheral applications: A review
Full text linkPeripheral arterial diseases affect a significant portion of the global population, fostering research to find innovative and effective solutions to improve people's life. A primary focus for researchers and manufacturers is the continuous improvement of the most important, non-surgical treatment for this pathology, the endovascular stent. This device is the main feature enabling a lifesaving technique: the percutaneous vascular interventions. Stents are vital for restoring blood flow and enhancing long-term vessel patency, they are available in various materials, shapes and typologies. Recent advancements in stent design, particularly through additive manufacturing, create new opportunities for optimizing the device performance and possibly opening new areas of intervention. This review provides a detailed quantitative analysis on the most widely used category of devices: self-expanding stents made of Nitinol, a nickel-titanium alloy that shows a superelastic behavior. A set of figures of merit related to stent design are described and analyzed, with a focus on the influence of geometry on mechanical performance. Additionally, a comprehensive comparative analysis of the commercial stents evaluates the geometry and performance of many commercial solutions, including both arterial and venous types. This analysis offers quantitative tools to assist surgeons and designers in selecting the most important features of a stent with respect to its main application. To conclude this work, an overview of future manufacturing possibilities is provided mainly focusing on the additive manufacturing technology. The freedom of shape given by this method opens up new paths in terms of global shapes, strut geometry and sizes, revealing new avenues which point strongly towards ad-hoc and specifically patient-customized stent design
Comparative sequence analysis of a recA gene fragment brings new evidence for a change in the taxonomy of the Lactobacillus casei group.
No full textThe taxonomic positions of species of the Lactobacillus casei group have been evaluated by sequencing and phylogenetic analysis of a 277 bp recA gene fragment. High sequence similarity between strain ATCC 393T, currently designated as the type strain of L. casei, and the type strain of Lactobacillus zeae, LMG 17315T, has been established, while L. casei ATCC 334 and Lactobacillus paracasei NCDO 151T form a single phylogenetic group. The taxonomic status of species and strains at issue is discussed
Development and prototyping of SMA-metamaterial biaxial composite actuators
Full text linkShape memory alloys (SMA) are excellent candidates for implementation in actuator systems due to their ability to recover their original shape after high-strain loading through a thermally-induced phase transition. In this work, we propose and develop a novel SMA-metamaterial actuator which is capable of exhibiting a reversible, global elongation in multiple directions induced by the unidirectional contraction upon heating of a single SMA component. This actuator consists of (a) an SMA component, (b) a bias component and (c) the metamaterial geometry, with each component having a distinct function: (a) actuation activation, (b) reversibility of actuation upon deactivation and (c) amplifying and re-directing the uni-directional SMA actuation globally throughout the actuator, respectively. A prototype actuator was designed and tested in various configurations over multiple activation/deactivation cycles in order to demonstrate the functionality and reusability of this system. Furthermore, a theoretical model which predicts the actuation stroke of the system on the basis of the material properties of the SMA and bias components as well as the geometry of the metamaterial system was developed and validated. The findings of this work demonstrate the considerable potential of SMA-metamaterial actuators for implementation in systems requiring a multi-axial actuation output
Analysis of the Deformation Behavior and Mechanical Properties of Slit-Perforated Auxetic Metamaterials
No full textPerforated systems constitute one of the most important classes of mechanical metamaterials. In this work, two types of “I”-shaped slit perforation patterns are proposed which may be used to design perforated systems that mimic the deformation mechanisms of a variety of re-entrant and anti-tetrachiral honeycomb systems. Using finite element analysis, it is shown how these systems have the potential to exhibit a large spectrum of negative Poisson's ratios, ranging from extremely negative to zero, which are retained over a wide tensile strain range. A detailed analysis of the deformation behavior of these systems is also presented along with a comparison of the changes in overall expansion and Poisson's ratios of both systems observed upon loading with those predicted by previously formulated theoretical models of re-entrant and anti-tetrachiral systems. It is hoped that this work will be of considerable aid in the efforts of scientists to understand the underlying principles governing the production of auxetic mechanical metamaterials through the use of perforations and also stimulate further research on how these and similar mechanisms may be implemented in perforated systems to design other metamaterials with anomalous mechanical properties
Auxetic Perforated Mechanical Metamaterials with Randomly Oriented Cuts
No full textPerforated systems with quasi-disordered arrays of slits are found to exhibit auxetic characteristics almost as much as their traditional ordered "rotating-squares" counterparts. This provides a highly robust methodology for constructing auxetics that may be used for various practical applications such as skin grafting, where a high degree of precision may not always be achievable
A rosette approach for the determination of the compliance matrix
No full textThe computation of the Young's moduli and Poisson's ratio using finite element analysis is comparatively straight forward in the case of systems with non-orthogonal lattice vectors. However, the corresponding calculation of the shear coupling coefficients and the shear modulus is more demanding and less readily attainable. Yet these are needed in order to determine the compliance matrix, which is essential in order to obtain a complete description of the mechanical properties of a structure in any direction. Based on these considerations, this work aims at providing a general methodology for the computation of the complete compliance matrix for systems with non-orthogonal lattice vectors from measurements of the Young's modulus and Poisson's ratio obtained using a suitable rosette of unit cells. The theoretical framework will be outlined and then applied on an accordion-like honeycomb that does not possess a rectangular unit cell and has potential applications in cellular scaffolding, particularly of heart muscle tissue. For the computation, the values of the Young's moduli and Poisson's ratio from three differently-orientated non-rectangular unit cells were obtained using finite element simulations allowing the determination of the complete compliance matrix. The results were validated by comparison with numerical simulations carried out on another unit cell having a different orientation from the other three
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