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A cohesive zone model approach is used in order to study the behaviour of cracks in elasto-plastic materials. The cohesive zone model being studied is time-dependent, unlike standard cohesive zone models in elasto-plasticity. The stress distribution over the cohesive zone is related to the normalised equivalent stress functional, and is expressed in the form of an Abel-type integral equation. During the stationary crack stage as well as the propagating crack stage, the aim is to study the behaviour of the cohesive zone length with respect to time as well as the crack tip opening. To aid accomplishing this aim, the stress intensity factor was set to zero at the cohesive zone tip. As well as other material parameters, the external applied load participates in the model equations. We will consider two cases for the external load, namely the case when this load is constant in time, and the case when this load behaves linearly with time. We will implement numerical schemes to obtain the crack growth as well as the cohesive zone growth with respect to time for both the elastic case and the visco-elastic case while considering different sets of parameters. The numerical convergence rates are obtained for each of the problems solved. This justifies the suitability of the numerical schemes used.

Progress in the burgeoning field of organic electronics is enabling the development of novel technologies such as low-cost, printable solar cells and flexible, high-resolution displays. One exciting avenue of research in this field is nanostructured hybrid organics such as quantum dot (QD)-polymer devices. The incorporation of QDs can greatly improve a device’s efficiency and gives one the ability to tune its electrical and optical characteristics. In order for such technologies to be commercially viable, it is important to classify their mechanical integrity and reliability. Surprisingly little is known about the mechanical properties of QD-polymer thin films (<100 nm). This is in part due to challenges of: (1) isolating the mechanical response of a thin film from the underlying substrate, (2) obtaining a homogeneous dispersion of QDs in the film, and (3) the sensitivity of mechanical properties to the inherent rate dependence of polymer deformation (i.e., viscoelasticity). All of these challenges can introduce significant errors in the measurement of mechanical properties. Furthermore, the deformation mechanisms in nanocomposites are not well understood, so it is difficult to predict the effect of adding QDs on the mechanical behavior of films. In this thesis, these challenges are addressed for characterizing the mechanical properties of thin films of CdSe QD-poly[2-methoxy-5-2(2΄-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) nanocomposites using quasi-static nanoindentation testing. Elastic modulus, hardness, and creep are measured as a function of QD concentration and loading and unloading rates. The QDs' ligands are removed by pyridine treatment prior to mixing with MEH-PPV to improve dispersion. The films are prepared via spin-coating onto glass substrates and subsequent annealing in air. Efforts are taken in the mechanical testing to minimize errors due to viscoelastic creep and interference from the substrate. Transmission electron microscopy reveals that the QDs are relatively well-dispersed in the polymer matrix. It is observed that adding QDs increases the elastic modulus (E) and hardness (H) of the films, while reducing the viscoelastic creep. Both E and H increase linearly with the volume percent of QDs. E ranges from 14.5 GPa to 52.7 GPa for pure MEH-PPV (0% QDs) and 100% QD films, respectively, while H ranges from 220 MPa to 1430 MPa for the same films, respectively. The films behave viscoelastically at lower QD loading, but assume a more granular character as the loading approaches 100%.

Mudanças nas características mecânicas de tecidos biológicos geralmente estão relacionadas com algum tipo de patologia. Técnicas de imagens elastográficas são métodos quantitativos de se estimar as propriedades mecânicas de tecidos. Em geral, o objetivo destas técnicas de imagem é medir o movimento do tecido provocado por uma força interna ou externa. Por meio desse movimento, parâmetros viscoelásticos do meio em análise são reconstruídos. A força de excitação pode ser tanto quasi-estática, como dinâmica. O trabalho apresentado nesta tese aborda as técnicas de elastografia dinâmica e quasi-estática. Na abordagem quasi-estática, a elasticidade não-linear é estudada através de phantoms com características que simulam as do tecido humano. Na abordagem dinâmica, o movimento dinâmico promovido por força de radicação acústica é avaliado através de técnicas ultrassônicas e magnéticas. O desenvolvimento de materiais para serem usados como phantoms para elastografia por ultrassom é descrito. O comportamento elástico não-linear desses materiais foi analisado através de resultados de ensaios mecânicos. Esses materiais foram desenvolvidos para apresentarem uma relação tensão/deformação que não dependesse do módulo de cisalhamento para pequenas deformações, e foram projetados para serem usados em phantoms em que configurações heterogêneas são empregadas (por exemplo, phantoms com inclusões esféricas). O efeito da não-linearidade elástica dos materiais sobre o contraste, a relação sinal ruído e a relação contraste ruído de imagens elastográficas de um phantom contendo inclusões esféricas, sofrendo altas deformações (até 20%) foi investigada. Foi demonstrada a viabilidade de se medir movimentos vibratórios induzidos por feixes acústicos confocais através de um ultrassom Doppler que utiliza ondas contínuas. A interferência de feixes de ultrassom com pequena diferença de frequências provoca o aparecimento de uma força dinâmica no alvo. Foi demonstrada a formação de imagens de uma esfera rígida imersa em um phantom viscoelástico, através da varredura de ambos os transdutores (confocal e Doppler) pelo plano focal do transdutor confocal. O comportamento dinâmico de uma esfera magnetizada induzido por força de radicação acústica foi investigado. A esfera foi suspensa em água em configuração de pêndulo. Forcas estática de longa (poucos segundos) e curta (poucos milisegundos) duração foram utilizadas. O movimento da esfera foi medido através de um sensor magnetoresistivo. A partir da nova posição de equilíbrio em resposta à força de radicação de longa duração, a amplitude dessa força foi estimada. Para se estimar a viscosidade da água, o movimento de relaxação da esfera após a força ter sido desligada foi ajustado a um modelo de movimento-harmônico amortecido. O movimento de uma esfera rígida imersa em um phantom feito de gelatina, deslocada por força de radicação acústica, foi avaliado por meio de ecos ultrassônicas obtidos com um sistema pulso/eco. A teoria utilizada para se estimar os parâmetros viscoelásticos do phantom, usando o movimento induzido na esfera, é uma extensão da teoria usada para se estimar a viscosidade da água. / Changes in the mechanical properties of soft tissues may be related with pathological disorders. Elasticity imaging is a quantitative method of estimating the mechanical properties of the tissue. In general, the aim of this technique is to measure tissue motion caused by external or internal forces and use it to reconstruct the viscoelastic parameters of the medium. The excitation stress used can be (quasi-) static or dynamic. Both elastographic approaches are explored in this thesis work. In the quasi-static approach, the nonlinear elasticity is studied through tissue-mimicking phantom experiments. In the dynamic approach, the dynamic motion provided through acoustic radiation force is evaluated using ultrasonic and magnetic techniques. The development of phantom materials for elasticity imaging is reported. These materials were specifically designed to provide nonlinear stress/strain relationship that can be controlled independently of the small strain shear modulus of the material, and were designed for use in phantoms where heterogeneous configurations (e.g, spherical targets in a uniform background) are employed. The effects of phantom materials nonlinearity over the strain contrast, signal-to-noise ratio and contrast-to-noise ratio of a phantom containing spherical inclusions undergoing large deformations (up to 20%) were investigated.The feasibility of measuring vibration movement, through a mono-channel continuous wave Doppler system, induced by focused confocal beams, is demonstrated. The interference of two ultrasonic beams promotes a dynamic force to the target. The ability to form images of a rigid spherical inhomogeneity embedded in viscoelastic phantom by scanning both ultrasonic transducers (confocal and Doppler) across the confocal transducer focal plane is presented. The dynamic behavior of a rigid magnetic sphere induced by an acoustic radiation force was investigated. The sphere was suspended in water in a simple pendulum configuration. Steady forces of long (few seconds) and short (few milliseconds) durations were used. The movement of the magnetic sphere was tracked using a magnetoresistive sensor. From the new equilibrium position of the sphere in response to the long-duration static radiation force, the amplitude of this force was estimated. To access the water viscosity, the relaxation movement after the acoustic force had stopped was fitted to a harmonic-motion model. The motion of a rigid sphere embedded in gelatin phantom, displaced by acoustic radiation force, was evaluated using the ultrasonic echoes from a pulse-echo system. The theory used to estimate the viscoelastic parameters of the phantom, from the oscillation of the rigid sphere is an extension of the relation used to estimate the water viscosity.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 381-382. / Sc.D.

Neste trabalho é analisada a medição de viscosidade de líquidos por ultrasom pelo método da reflexão de ondas de cisalhamento. O método baseiase na medição da magnitude e fase do coeficiente de reflexão quando as ondas de cisalhamento incidem na interface entre um sólido e o líquido testado. Foram analisados os conceitos teóricos do método da medição de viscosidade e desenvolvida a metodologia experimental a fim de evitar a influência indesejada de fatores externos, como a temperatura e a instabilidade da eletrônica, sobre a medição. Foram obtidos resultados com dois óleos alimentícios (azeite de oliva e óleo de milho) e quatro óleos automotivos do tipo SAE (40, 90, 140 e 250). Foi mostrado que para uma freqüência de operação baixa o suficiente para garantir comportamento Newtoniano do líquido, os valores de viscosidade têm excelente concordância numérica com os valores obtidos usando o viscosímetro rotacional, no caso dos óleos menos viscosos. Já no caso dos óleos mais viscosos, foram obtidos valores de viscosidade menores que os obtidos com viscosímetro rotacional, e o aumento no módulo elástico sugere um comportamento mais viscoelástico. Foi mostrado que o método de cálculo simplificado usado por alguns autores, que permite calcular a viscosidade a partir somente da magnitude do coeficiente de reflexão, não fornece os valores esperados de viscosidade. / This work deals with the ultrasonic measurement of liquids viscosity by means of the shear reflectance method. The method is based on the measurement of the magnitude and phase of the complex reflection coefficient of shear waves at a solidliquid sample interface. Basic concepts of the viscosity measurement method were analyzed and the experimental methodology was developed to avoid the undesired influence of external factors, such as temperature and electronic instability. Experimental results were obtained with two kinds of eatable oils (olive and corn oils) and four different automotive oils (SAE 40, 90, 140 e 250). For an operating frequency low enough to obtain Newtonian liquid behavior, as for less viscous liquids, it was shown that the viscosity results have good agreement with rotational viscometer measurements. In the case of more viscous liquids, the measured viscosity values were smaller than those obtained by the rotational viscometer, and the elastic modulus is increased, suggesting viscoelastic behavior. It is shown that a simplified model used by other authors, which obtains the viscosity without measuring the phase of the complex reflection coefficient, results in greater errors associated with the values obtained by the rotational viscometer.

Many microorganisms have the ability to propel themselves through their fluid environments by periodically actuating their body. The biological fluid environments surrounding these microswimmers are typically complex fluids containing many high-molecular weight protein molecules, which give the fluid non-Newtonian rheological properties. In this thesis, we investigate the effect that one such rheological property, viscoelasticity, has on microswimming. We consider a classical model of a microswimmer, the so-called Taylor's waving sheet and generalise it to arbitrary shapes. We employ the Oldroyd-B model to study its swimming analytically and numerically. We attempt to develop a mechanistic understanding of the swimmer's behaviour in viscoelastic fluids. It has recently been suggested that continuum models of complex biological fluids might not be appropriate for studying the swimming of flagellated microorganisms as the size of biological macromolecules is comparable to the typical width of a microorganism's flagellum. A part of this thesis is devoted to exploring this scenario. We propose an alternative method for modelling complex fluids using a two-fluid depletion region model and we have developed a numerical solver to find the swimming speed and rate of work for the generalised Taylor's waving sheet model swimmer using this alternate depletion region model. This thesis is organised as follows. In the first chapter, we outline a physical mechanism for the slowing down of Taylor's sheet in an Oldroyd-B fluid as the Deborah number increases. We demonstrate how a microswimmer can be designed to avoid this. In the second chapter, we investigate swimming in an Oldroyd-B fluid near a solid boundary and show that, at large amplitudes and low polymer concentrations, the swimming speed of Taylor's sheet increases with De. In the third chapter, we show how the Oldroyd-B model can be adapted using depletion regions. In the final chapter, we investigate optimal swimming in a Newtonian fluid. We show that while the organism's energetics are important, the kinematics of planar-wave microswimmers do not optimise the hydrodynamic 'efficiency' typically used for mathematical optimisation in the literature.

Neste trabalho é analisada a medição de viscosidade de líquidos por ultrasom pelo método da reflexão de ondas de cisalhamento. O método baseiase na medição da magnitude e fase do coeficiente de reflexão quando as ondas de cisalhamento incidem na interface entre um sólido e o líquido testado. Foram analisados os conceitos teóricos do método da medição de viscosidade e desenvolvida a metodologia experimental a fim de evitar a influência indesejada de fatores externos, como a temperatura e a instabilidade da eletrônica, sobre a medição. Foram obtidos resultados com dois óleos alimentícios (azeite de oliva e óleo de milho) e quatro óleos automotivos do tipo SAE (40, 90, 140 e 250). Foi mostrado que para uma freqüência de operação baixa o suficiente para garantir comportamento Newtoniano do líquido, os valores de viscosidade têm excelente concordância numérica com os valores obtidos usando o viscosímetro rotacional, no caso dos óleos menos viscosos. Já no caso dos óleos mais viscosos, foram obtidos valores de viscosidade menores que os obtidos com viscosímetro rotacional, e o aumento no módulo elástico sugere um comportamento mais viscoelástico. Foi mostrado que o método de cálculo simplificado usado por alguns autores, que permite calcular a viscosidade a partir somente da magnitude do coeficiente de reflexão, não fornece os valores esperados de viscosidade. / This work deals with the ultrasonic measurement of liquids viscosity by means of the shear reflectance method. The method is based on the measurement of the magnitude and phase of the complex reflection coefficient of shear waves at a solidliquid sample interface. Basic concepts of the viscosity measurement method were analyzed and the experimental methodology was developed to avoid the undesired influence of external factors, such as temperature and electronic instability. Experimental results were obtained with two kinds of eatable oils (olive and corn oils) and four different automotive oils (SAE 40, 90, 140 e 250). For an operating frequency low enough to obtain Newtonian liquid behavior, as for less viscous liquids, it was shown that the viscosity results have good agreement with rotational viscometer measurements. In the case of more viscous liquids, the measured viscosity values were smaller than those obtained by the rotational viscometer, and the elastic modulus is increased, suggesting viscoelastic behavior. It is shown that a simplified model used by other authors, which obtains the viscosity without measuring the phase of the complex reflection coefficient, results in greater errors associated with the values obtained by the rotational viscometer.

O presente trabalho relaciona-se à modelagem viscoelástica estrutural, objetivando-se a simulação numérica, com ênfase às estruturas de concreto. Faz-se, inicialmente, uma revisão dos modelos reológicos básicos unidimensionais para facilitar o entendimento das características intrínsecas da resposta viscoelástica das estruturas, tais como o fenômeno de fluência. Dentre as teorias que permitem levar em conta a idade do concreto no instante da aplicação do carregamento, são apresentadas a da hereditariedade e a do envelhecimento. Apresentam-se, em seguida, as funções de fluência e relaxação em notação apropriada à implementação numérica, combinada com o Método dos Elementos Finitos. Alguns aspectos da integração numérica das relações constitutivas ao longo do tempo são discutidos, com a apresentação, em particular, de um esquema de solução explícito fundamentado na Técnica dos \'Núcleos Degenerados\'. Finalmente, exemplos constituídos da análise de treliças planas ilustram a aplicabilidade dos modelos e procedimentos numéricos comentados. / The present work is related to structural viscoelastic modeling aiming numerical simulation, with emphasis on concrete structures. First of all, a revision of unidimentional basic reologic models is given, in order to enhance the inherent features of structural viscoelastic response, such as creep phenomenon. Among the theories able to consider the concrete age at the instant of loading, the hereditary and the aging theories are here presented. After that, the creep and relaxation functions are given in appropriate notation for numerical implementation, combined with the Finite Element Method. Some aspects of time numerical integration of constitutiva relations are discussed, mainly an explicit solution scheme based on the so called Degenerate Kernel Technic. Finally, examples consisting of plane trusses analysis illustrate the applicability of the models and numerical procedures commented.