Continuum models for the dynamic behavior of 1D nonlinear structured solids

Mención Internacional en el título de doctorThe main objective of this thesis is to propose and validate generalized continuum models for analyzing and predicting the free vibrations of 1D linear elastic structured solids subjected to finite deformations. With this aim, two different generalized continuum models that account for geometric nonlinearities have been proposed: a nonlocal nonlinear model and a gradient velocity nonlinear one, here referred to as the inertia gradient nonlinear model. These models contain one or more microstructural parameters. The formulation of the cited theories has been devised for general 3D solids, but in this thesis they have been applied to the study of nonlinear axial and nonlinear axial-transverse coupled vibrations of a kind of 1D structured solids. The 1D structured solids chosen as reference are such that, under certain conditions, the classical nonlinear continuum model adequately reproduces its main features. However, it is well known that the classical theory is not able to capture the size effect presented in this kind of solids. Moreover, a non-standard continualization technique has been applied to the discrete system in order to get appropriate continuous equations of motion. Unlike the postulated generalized continuum models, this continualization technique permits to derive, from the characteristics of the solid structure, the additional parameters appearing in the formulation. The results for the two nonlinear analyzed problems (axial vibrations and axialtransverse coupled vibrations) obtained from the classical, generalized nonlinear continuum, and continualized models have been compared to those derived from the discrete solution taken as reference. From this critical comparison, the advantages and shortcomings of the different theories have been pointed out clearly showing the ability of the generalized continuum models to adequately address the size effects in structured solids, observed in many practical applications.El objetivo principal de esta tesis es proponer y validar modelos del continuo generalizado para analizar y predecir el comportamiento de vibraciones libres de sólidos estructurados unidimensionales elásticos y lineales sometidos a deformaciones finitas. Con este objetivo, se han propuesto dos modelos del continuo generalizado diferentes que consideran las no linealidades geométricas: un modelo no lineal no local y otro no lineal de gradiente de la velocidad, aquí referido como modelo con gradiente de inercia. Estos modelos contienen uno o más parámetros microestructurales. Las formulaciones de dichas teorías se han planteado para sólidos tridimensionales, pero en esta tesis se han aplicado al estudio de las vibraciones no lineales longitudinales y las vibraciones no lineales acopladas axiales-transversales en un tipo de sólidos 1D estructurados. El sólido 1D estructurado elegido como referencia es tal que, bajo ciertas condiciones, el modelo continuo no lineal clásico reproduce adecuadamente sus características principales. Sin embargo, es bien sabido que la teoría clásica no es capaz de capturar los efectos de tamaño que presentan este tipo de sólidos. Por otra parte, se ha aplicado una técnica de continualización no estándar al sistema discreto con el fin de obtener las ecuaciones continuas del movimiento. A diferencia de los modelos del continuo generalizado postulados, esta técnica de continualización permite derivar, a partir de las características del sólido estructurado, los parámetros adicionales que aparecen en la formulación del problema. Los resultados para los dos problemas no lineales analizados (vibraciones axiales y axiales-transversales acopladas) obtenidos mediante los modelos continuo clásico, continuos no lineales generalizados, y los continualizados se han comparado con los derivados de la solución discreta que se toma como referencia. A partir de esta comparación crítica, se han señalado las ventajas y los inconvenientes de las diferentes teorías, mostrando claramente la capacidad de los modelos del continuo generalizado para tratar adecuadamente los efectos de tamaño en los sólidos estructurados, observados ´estos en muchas aplicaciones prácticas.The financial support of the Ministerio de Ciencia e Innovación de España (Project DPI/2014-57989-P) is greatly acknowledged. The subject of this Thesis is a core part of the research project.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: Enrique Barbero Pozuelo.- Secretario: Eugenio Giner Maravilla.- Vocal: Julian J. Rimol

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901

Fabrication and Characterisation of an Adaptable Plasmonic Nanorod Array

This thesis focused on the fabrication and characterisation of an adaptable plasmonic platform consisting of a regular array of vertically aligned gold nanorods, supported on a gold substrate. The research aimed to understand the optical properties of the array, with particular emphasis on determining the coupling effects between the plasmonic nanorods. Characterisation was performed, both in the near and far-field, by means of optical spectroscopy, finite element modelling (FEM) and electron energy loss spectroscopy (EELS). The work subsequently aimed to use the knowledge gained from characterisation to optimise the absorption of visible light and enhancement of the electric-fields surrounding the plasmonic nanoparticles by controlling the geometrical factors of the array. Lastly, the nanorod arrays were then utilised for photocatalytic applications following their coating in a semiconductor material. During this research, gold nanorod arrays were successfully fabricated by means of electrodeposition in anodic aluminium oxide (AAO) membranes, with accurate control over the geometrical factors. UV-Vis measurements revealed that coupling within the array resulted in the higher wavelength longitudinal mode of the nanorods blue-shifting considerably to occur within the visible spectrum. For short aspect ratio (AR) nanorods, this mode overlapped with that of the transverse mode, however for longer ARs the mode could be tuned throughout the visible spectrum. This was in agreement with FEM results, however, it was additionally revealed that strongly coupled nanorod arrays undergo a redistribution in their electric field from localisation at the end of the nanorods, to one within the middle or base of the nanorods for unsupported and supported arrays respectively. It was further found that the presence of the substrate led to the nanorods coupling with their substrate image leading to greatly red-shifted resonances. Through experimental EELS measurements it was confirmed that this red-shift is due to coupling with the substrate, in addition to further analysing the particle coupling effects for both dimers, and larger arrays of nanorods. It was found that for dimers with strong coupling the longitudinal mode splits into both a bright mode with symmetrically aligned dipoles, and a dark mode with anti-symmetrically aligned dipoles, thereby agreeing with plasmon hybridisation theory. Furthermore, as the number of particles within the array increases, the modes split to form hybridised bright and dark modes consisting of elements of each. An initial attempt at photocatalysis was also performed based on the degradation of methylene blue by injection of hot-electrons into TiO2. No significant increase in activity was found, attributed to the semiconductor layer completely covering the nanorods thereby not allowing available sites for the oxidation reaction to replenish lost electrons in the metal

Recent Advances in Linear and Nonlinear Optics

Sight is the dominant sense of mankind to apprehend the world at the earth scale and beyond the frontiers of the infinite, from the nanometer to the incommensurable. Primarily based on sunlight and natural and artificial light sources, optics has been the major companion of spectroscopy since scientific observation began. The invention of the laser in the early sixties has boosted optical spectroscopy through the intrinsic or specific symmetry electronic properties of materials at the multiscale (birefringence, nonlinear and photonic crystals), revealed by the ability to monitor light polarization inside or on the surface of designed objects. This Special Issue of Symmetry features articles and reviews that are of tremendous interest to scientists who study linear and nonlinear optics, all oriented around the common axis of symmetry. Contributions transverse the entire breadth of this field, including those concerning polarization and anisotropy within colloids of chromophores and metal/semiconducting nanoparticles probed by UV-visible and fluorescence spectroscopies; microscopic structures of liquid–liquid, liquid–gas, and liquid–solid interfaces; surface- and symmetry-specific optical techniques and simulations, including second-harmonic and sum-frequency generations, and surface-enhanced and coherent anti-Stokes Raman spectroscopies; orientation and chirality of bio-molecular interfaces; symmetry breaking in photochemistry; symmetric multipolar molecules; reversible electronic energy transfer within supramolecular systems; plasmonics; and light polarization effects in materials

Transport and coupling of phonons, electrons, and magnons in complex materials

In nanoscale systems, in which the relevant length scales can be comparable to the mean free paths and wavelengths of the energy, charge and spin carriers, it is necessary to examine the microscopic transport of heat, spin and charge at the atomic scale and the quantization of the associated quasiparticles. The intricacies of the transport dynamics can be even more complicated in materials with atomic scale complexities, such as incommensurate crystals, magnetic materials, and quasi-one-dimensional systems. Meanwhile, the transport properties and coupling between these quasiparticles is important in determining the strength of various thermoelectric and spincaloritronic phenomena, as well as the reliability of nanoscale electronics. This work seeks to further the understanding of the complicated transport dynamics in complex structured materials at nanometer and micrometer length scales, and to address some of the fundamental questions about the interactions between energy, charge and spin carriers in the conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT), the incommensurate higher manganese silicide (HMS) thermoelectric material, and the magnetic insulator yttrium iron garnet (YIG). These questions are addressed through a number of combined experimental approaches through the use of thermal conductance and thermoelectric property measurements of suspended nanostructures, inelastic neutron scattering, Brillouin light scattering, and electron microscopy. According to in-plane thermal and thermoelectric transport measurements of PEDOT thin films, the electronic thermal conductivity of this conducting polymer is found to be significant and exceeds that predicted by the Wiedemann-Franz law for metals. Furthermore, thermoelectric transport measurements of suspended HMS nanoribbons show a reduction in the lattice thermal conductivity by approximately a factor of two compared to bulk HMS, which is qualitatively consistent with that predicted from a diffuson model for thermal conductivity derived from the phonon dispersion of HMS. Lastly, pressure dependent Brillouin light scattering spectroscopy is used to determine the influence of hydrostatic stress on the dispersions of magnons and phonons in YIG, in order to determine the magnon and phonon peak frequency shift associated with localized laser heating induced strain.Mechanical Engineerin

Physics-Based Modeling of Material Behavior and Damage Initiation in Nanoengineered Composites

abstract: Materials with unprecedented properties are necessary to make dramatic changes in current and future aerospace platforms. Hybrid materials and composites are increasingly being used in aircraft and spacecraft frames; however, future platforms will require an optimal design of novel materials that enable operation in a variety of environments and produce known/predicted damage mechanisms. Nanocomposites and nanoengineered composites with CNTs have the potential to make significant improvements in strength, stiffness, fracture toughness, flame retardancy and resistance to corrosion. Therefore, these materials have generated tremendous scientific and technical interest over the past decade and various architectures are being explored for applications to light-weight airframe structures. However, the success of such materials with significantly improved performance metrics requires careful control of the parameters during synthesis and processing. Their implementation is also limited due to the lack of complete understanding of the effects the nanoparticles impart to the bulk properties of composites. It is common for computational methods to be applied to explain phenomena measured or observed experimentally. Frequently, a given phenomenon or material property is only considered to be fully understood when the associated physics has been identified through accompanying calculations or simulations. The computationally and experimentally integrated research presented in this dissertation provides improved understanding of the mechanical behavior and response including damage and failure in CNT nanocomposites, enhancing confidence in their applications. The computations at the atomistic level helps to understand the underlying mechanochemistry and allow a systematic investigation of the complex CNT architectures and the material performance across a wide range of parameters. Simulation of the bond breakage phenomena and development of the interface to continuum scale damage captures the effects of applied loading and damage precursor and provides insight into the safety of nanoengineered composites under service loads. The validated modeling methodology is expected to be a step in the direction of computationally-assisted design and certification of novel materials, thus liberating the pace of their implementation in future applications.Dissertation/ThesisDoctoral Dissertation Aerospace Engineering 201

NIR-emissive Alkynylplatinum(II) Terpyridyl Complex as a turn-on selective probe for heparin quantification by induced helical self-assembly behaviour

The extent of self-assembly viametal–metal and π-π stacking interactions, induced by the polyanionic biopolymers, enables the class of alkynylplatinum(II) terpyridyl complexes to be applicable for the sensing of important biomacromolecules through the monitoring of spectral changes. Strong demand arises for the design of selective and practical detection techniques for the quantification of heparin, a highly negative-charged polysaccharidethat can function as anticoagulant, due to the prevention of hemorrhagic complications upon overdose usage.Aconvenient sensing protocol for the detection of UFH and LMWH, two common forms of heparins in clinical use, in buffer and biological medium has been demonstrated with the spectral changes associated with the induced self-assembly of a NIR-emissive platinum(II) complex. The detection range has been demonstrated to cover clinical dosage levels and the structurally similar analogues can be effectively differentiated based on their anionic charge density and the formation of supramolecular helical assembly of the platinum(II) complex with them ...postprin