This thesis considers the modelling of two and three dimensional molecular networks with a view to being able to predict how the geometry of a network will affect the elastic constants and specifically the Poisson's ratios of the network. Materials with negative Poisson's ratios have much better engineering properties then those with positive Poisson's ratios. Theory states that a network polymer, with negative Poisson's ratios at a molecular level, would have much better properties than most materials with negative Poisson's ratios made to date. Molecular modelling has been used to examine the elastic constants of those two and three dimensional network polymers which are most likely to be synthesised in the near future. Such networks have been predicted to have either large positive or large negative Poisson's ratios depending on the molecular arrangement of the network. Poisson's ratios varying between 0.96 and -0.86 for the three dimensional cases and between -0.9 and 1.26 for the two dimensional cases have been calculated. Young's moduli in the order of 1 GPa have been observed for the three dimensional networks as compared to Young's moduli in the order of 20 - 400 kPa which have been experimentally measured for foam materials. Comparison with local density functional calculations for two 2-D networks with the molecular modelling have confirmed the negative Poisson's ratio in these networks and shown that it is not a function of the molecular modelling packages or force field used. The off-axis properties for both the two and three dimensional networks have been calculated. These show that whilst the networks with a positive Poisson's ratio in the principal axis directions always have a positive Poisson's ratio, those networks with a negative Poisson's ratio in the principal axis directions have off-axis Poisson's ratios that vary between large and positive and large and negative. In general the networks with positive Poisson's ratios are much more isotropic than those with negative Poisson's ratios. Analytical models which model the networks using simple beam theory have been produced for various two and three dimensional networks. These models can be used to predict the elastic constants of a network without the need to do time consumingmolecular modelling calculations to a first approximation. Comparison of the molecular models and analytical models has led to the development a library of force constants for two dimensional networks which can be used to more accurately predict the elastic constants of a network based on a knowledge of the geometry of the network and the constituent `sub-units' from which it is made

No description available.

Negative Linear Compressibility (NLC), where a material expands in a given direction when subjected to hydrostatic compression, is a rare elastic property that has received much attention recently, but has yet to be used in practical applications. What are the mechanisms responsible for this property in crystals and man-made structures? Are all mechanisms somehow related to the wine-rack model? Can we find an even simpler and more fundamental elucidation of NLC? Following this mechanistic approach, can we then identify “engineering” materials with NLC? To answer these questions, I have used a combination of analytical modelling based on beam theory and finite element analysis, to investigate several structures. At first, I examine in great detail the standard wine-rack in 2D and equivalents in 3D and identify the aspect ratio (close to two) at which NLC is maximum. By adding spacers I demonstrate that a cross is not a necessary condition, and that simpler angle changes in chains are sufficient to generate NLC. Looking for materials with intersecting straight chains, “zig-zag” chains or quasi-helical structures, I find that carbon fibre mats, some extruded polymers and some woods exhibit NLC. Finally, I show that elliptical voids in 2D sheets can also generate NLC in a way related to the wine-rack. This thesis improves the understanding of the mechanism(s) responsible for NLC by proving that a wine-rack is not necessary. Perhaps more importantly it suggests that the property can be exploited in several relatively common materials.

<p>  </p> <p>With the advent of femtosecond lasers in the early 1990s, ultrafast laser processing has proven to be an imperative tool for micro/nanomachining. Two-photon lithography (TPL) is one such unique microfabrication technique exploiting the nonlinear dependency of the polymerization rate on the irradiating light intensity to produce true three-dimensional structures with feature sizes beyond the diffraction limit. This characteristic has revolutionized laser material processing for the fabrication of micro and nanostructures. This research first gives a general overview of TPL, including its operating principle, experimental setup, compatible materials, and techniques for improving the final resolution of the fabricated structure. Insights to improve throughput and speed of fabrication to pave a way for the industrialization of this technique are provided.</p> <p>Following that, the report delves into the process of fabricating two true three-dimensional mechanical metamaterials via the stereolithography technique. This chapter encompasses the design, fabrication, and experimental analysis of a three-dimensional axisymmetric structure with elliptical perforations distributed periodically on the walls of the structure with varying thicknesses. Furthermore, this study discusses the significance of the elliptical perforations in terms of auxetic behavior and load-bearing capacity against a so-called plain structure under quasistatic compression loading.</p> <p>Finally, the report explores the technique of fabricating a true three-dimensional cylindrical auxetic structure via two-photon polymerization. This section of the report examines the optical setup utilized, the sample preparation procedure, and calibration experiments performed in order to fabricate a three-dimensional thin-walled right cylinder with bowtie like perforations arranged on the walls to promote the exhibition of symmetric negative Poisson’s ratio under uniaxial quasistatic compression loading.</p>

Les matériaux architecturés font émerger de nouvelles possibilités en termes de propriétés structurales et fonctionnelles, repoussant ainsi les limites des cartes d'Ashby. Le terme "matériaux architecturés" inclus toute microstructure conçue de façon astucieuse, de sorte que certaines de ses propriétés soient optimisées. Les exemples sont nombreux : composites fibreux et particulaires, matériaux cellulaires, structures sandwiches, matériaux tissés, structures treillis, etc. Un enjeu de taille pour l'emploi de tels matériaux est la prédiction de leurs propriétés effectives. Dans ce travail, deux types de microstructures sont considérées : des structures auxétiques périodiques et des milieux fibreux aléatoires. Les auxétiques sont des matériaux apparus au milieu des années 1980, présentant un coefficient de Poisson négatif. On attend des auxétiques qu'ils présentent des propriétés mécaniques améliorées, comme le module de cisaillement ou la résistance à l'indentation. Les milieux fibreux aléatoires considérés dans ce travail sont constitués de fibres 3D infinies interpénétrantes aléatoirement distribuées et orientées. Ce type de structure aléatoire est très défavorable à la détermination d'une taille de volume élémentaire statistiquement représentatif. Pour les deux types de matériaux, l'homogénéisation numérique à l'aide de la méthode des éléments finis est implémentée dans le but d'estimer les propriétés thermiques et mécaniques effectives. / Architectured materials bring new possibilities in terms of structural and functional properties, filling gaps and pushing the boundaries of Ashby's materials maps. The term "architectured materials" encompasses any microstructure designed in a thoughtful fashion, so that some of its materials properties have been improved. There are many examples: particulate and fibrous composites, foams, sandwich structures, woven materials, lattice structures, etc. One engineering challenge is to predict the effective properties of such materials. In this work, two types of microstructures are considered: periodic auxetic lattices and stochastic fibrous networks. Auxetics are materials with negative Poisson's ratio that have been engineered since the mid-1980s. Such materials have been expected to present enhanced mechanical properties such as shear modulus or indentation resistance. The stochastic fibrous networks considered in this work is made of 3D infinite interpenetrating fibres that are randomly distributed and oriented. This case of random structure is challenging regarding the determination of a volume element size that is statistically representative. For both materials, computational homogenization using finite element analysis is implemented in order to estimate the effective thermal and mechanical properties.

<p>Metamaterials are artificially structured materials which attain their unconventional macroscopic properties from their cellular configuration rather than their constituent chemical composition. The judicious design of this cellular structure opens the possibility to program and control the optical, mechanical, acoustic, or thermal responses of metamaterials. This Ph.D. dissertation focuses on scalable design and manufacturing strategies for optical and mechanical metamaterials.<br> <br> </p> <p>The fabrication of optical metamaterials still relies heavily on low-throughput process such as electron beam lithography, which is a serial technique. Thus, there is a growing need for the development of high-throughput, parallel processes to make the fabrication of optical metamaterials more accessible and cost-effective. The first part of this dissertation presents a scalable manufacturing method, termed “roll-to-roll laser induced superplasticity” (R2RLIS), for the production of flexible optical metamaterials, specifically metallic near-perfect absorbers. R2RLIS enables the rapid and inexpensive fabrication of ultra-smooth metallic nanostructures over large areas using conventional CO₂ engravers or inexpensive diode lasers. Using low-cost metal/epoxy nanomolds, the minimum feature size obtained by R2RLIS was <40 nm, facilitating the rapid fabrication of flexible near-perfect absorbers at visible frequencies with the capability to wrap around non-planar surfaces.</p> <p> </p> <p>The existing approaches for designing mechanical metamaterials are mostly <i>ad hoc</i>, and rely heavily on intuition and trial-and-error. A rational and systematic approach to create functional and programmable mechanical metamaterials is therefore desirable to unlock the vast design space of mechanical properties. The second part of this dissertation introduces a systematic, algorithmic design strategy based on Voronoi tessellation to create architected soft machines (ASMs) and twisting mechanical metamaterials (TMMs) with programmable motion and properties. ASMs are a new class of soft machines that benefit from their 3D-architected structure to expand the range of mechanical properties and behaviors achievable by 3D printed soft robots. On tendon-based actuation, ASMs deform according to the topologically encoded buckling of their structure to produce a wide range of motions such as contraction, twisting, bending, and cyclic motion. TMMs are a new class of chiral mechanical metamaterials which exhibit compression-twist coupling, a property absent in isotropic materials. This property manifests macroscopically and is independent of the flexible material chosen to fabricate the TMM. The nature of this compression-twist coupling can be programmed by simply tuning two design parameters, giving access to distinct twisting regimes and tunable onset of auxetic (negative Poisson’s ratio) behavior. Taking a metamaterial approach toward the design of soft machines substantially increases their number of degrees of freedom in deformation, thus blurring the boundary between materials and machines.</p>