This doctoral thesis deals with the study of electro-hydrodynamics in the area of numerical modelling of biomechanical systems, concretely in the method of bioimpedance plethysmography. Solving tasks of pulsatile blood flow in the elastic vessel wall is currently one of the most complicated problem in mechanics and biomechanics due to the interaction of two continua on the common boundary. The whole system is additionally loaded by diagnostic electric current. This doctoral thesis was created in cooperation with the Institute of Scientific Instruments of the CAS, v. v. i. Brno with the team engaged in medical signals (the leader Ing. Pavel Jurák, CSc.). Experimental measurements were made independently in the St. Anne's University Hospital Brno in the International Clinical Research Center ICRC and in the Mayo Clinic USA.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第18459号 / 医博第3914号 / 新制||医||1005(附属図書館) / 31337 / 京都大学大学院医学研究科医学専攻 / (主査)教授 木村 剛, 教授 坂田 隆造, 教授 戸口田 淳也 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DGAM

This thesis utilizes the Immersed Interface Finite Element Method (IIFEM) for shape optimization and material design, while also investigating the modeling and parameterization of lung tissue for Diver Underwater Explosion (UNDEX) simulations. In the first part, a shape optimization scheme utilizing a four-noded rectangular immersed-interface element is presented. This method eliminates the need for interface fitted mesh or mesh morphing, reducing computational costs while maintaining solution accuracy. Analytical design sensitivity analysis is performed to obtain gradients for the optimization formulation, and various parametrization techniques are explored. The effectiveness of the approach is demonstrated through verification and case studies. For material design, the study combines topological shape optimization with IIFEM, providing a computational approach for architecting materials with desired effective properties. Numerical homogenization evaluates effective properties, and level set-based topology optimization evolves boundaries within the unit cell to generate optimal periodic microstructures. The design space is parameterized using radial basis functions, facilitating a gradient-based optimization algorithm for optimal coefficients. The method produces geometries with smooth boundaries and distinct interfaces, demonstrated through numerical examples. The thesis then delves into modeling the mechanical response of lung tissues, particularly focusing on hyperelastic and hyperviscoelastic models. The research adopts a phased approach, emphasizing hyperelastic model parametrization while reserving hyperviscoelastic model parametrization for future studies. Alternative methods are used for parametrization, circumventing direct experimental tests on biological materials. Representative material properties are sourced from literature or refit from existing experimental data, incorporating both empirically derived data and practical values suitable for simulations. Damage parameter quantification relies on asserted quantitative relationships between injury levels and the regions or percentages of affected lung tissue. / Doctor of Philosophy / This research explores the following themes: optimizing shapes, designing materials using repetitive identical building blocks, and understanding how divers' lungs respond to underwater explosions. When computationally analyzing structures with multiple materials, the conventional method involves creating meshes that align with material interfaces, which can be intricate and time-consuming. The Immersed Interface Finite Element Method (IIFEM) is introduced as a computational approach that simplifies this process, utilizing a uniform grid for analysis regardless of interface shape. Consider a plate with a hole or other inclusions. Shape optimization seeks the optimal hole/inclusion shape for withstanding specific loading. Traditional optimization processes necessitate iterative mesh recreation, a step circumvented by employing IIFEM. This technique also extends to creating micro-building blocks of materials, enabling the architectural design of materials with desired qualities. Materials with specific properties, like strength or flexibility can be achieved. This thesis also addresses the challenge of understanding how divers' lungs respond to underwater explosions, a crucial aspect of safety. Advanced computer models are used to mimic the behavior of lung tissue under shock loads. Directly testing materials and tissues can be difficult and restricted. Techniques like gathering data from scientific papers and refitting existing experimental data are utilized to obtain the information needed. Also, it is hard to directly measure how much damage an underwater explosion does to a diver's lungs. Thus, the level of damage was quantified based on assertions about the relationship between different injury severities and how much lung tissue is affected.

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Compression of elastomer elements are nonlinear due to the cross-linked molecular structure owing to a property known as hyperelasticity. Hyperelasticity is defined as the nonlinear stress-strain behavior shown by rubber like materials which can be strained up to a range of 700% in tension and up to 40% in compression. The stress-strain behavior is modeled by using different material models which predict the behavior very precisely.  Linear actuators from Cascade Drives AB uses a patented load sharing mechanism using elastically deformable elements to distribute the torque evenly between all the gears interacting with a common gear rack. An accurate model predicting the response of elastomer under compression has been developed in this thesis project. The elastomers were loaded in compression to provide flexibility for the system. First a static model was developed, where both a rectangular and a cylindrical roller model were analyzed.  The two models were derived using a continuum mechanics approach and the stiffness of the elastomers along with the torque output of the gearbox was calculated. A MATLAB model and an FEA model using ANSYS was created, and the results were compared. An error estimate between the MATLAB and FEA results for the rectangular and roller model was plotted for a certain β° of rotation of the gear. The models were also checked for different materials and the output torque for the different materials was plotted and analyzed. Finally, the experimental results were compared with the MATLAB results for the rectangular and roller models. The rectangle and roller model can be both used to predict the behaviour of using elastomers as the load sharing elements in applications. / Kompression av elastomerelement är olinjär till följd av den tvärbundna molekylära strukturen, en egenskap som kallas hyperelasticitet. Hyperelasticitet definieras som det icke-linjära spännings-töjningsbeteendet som uppvisas av gummiliknande material vilka kan töjas upp till av 700% och upp till komprimeras upp till 40%. Spänningsbelastningsbeteendet modelleras med hjälp av olika materialmodeller som förutsäger beteendet.  Cascade Drives linjäraktuatorer använder elastiskt komprimerbara element i sin lastfördelningsmekanism för att använda multipla plinjonger på ett kuggrack utan att få ett överbestämt system. Lastfördelningsmekanismen ger även en viss flexibilitet för systemet.  En modell som förutsäger responsen hos elastomerer under kompression har utvecklats i denna avhandling. Två geometriska former undersöktes modeller togs fram för både en rektangulär och cylindrisk rulle. De två modellerna härleddes med en kontinuummekanisk metod och elastomerernas styvhet beräknades. En MATLAB-modell och en FEM-modell i ANSYS skapades och resultaten jämfördes och en feluppskattning modellerna gjordes. Modellerna undersöktes också för olika material och utmattningsegenskaperna för de olika materialen analyserades.  Rektangel- eller rullmodellen kan båda användas för att förutsäga hur en elastomer skulle bete sig i en växellådsapplikation.

Styrene-Butadiene Rubber (SBR) is a form of rubber compound that is widely used in the tire industry. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. In this study, a material aging model that can predict the hyperplastic response of styrene-butadiene rubber (SBR) was mathematically developed using an integrated testing and continuum damage model framework. Coupling between the mechanical changes of SBR to the change in the chemical properties, specifically crosslink density (CLD) was also investigated. SBR dogbone shaped samples were accelerated aged in an aging oven at various temperatures and aging periods. Subsequently, hyperelastic tests were conducted to obtain the high strain response taking the 'Mullin's effect' into consideration. These responses were calibrated to different hyperelastic material models and the Arruda-Boyce model was chosen, due to its stable behavior and optimal fit. An aging evolution function was developed based on the variation in the model coefficients. This damage model is able to predict the hyperelastic response of SBR as it ages. A user material subroutine (UMAT) was also implemented in Abaqus based on the obtained aging evolution function to predict the stress response of SBR for varied applications. Additionally, to couple the chemical variations with the hyperelastic response, the rubber structure and composition was probed using Fourier-transform infrared spectroscopy (FTIR). The degradation of additives and SBR polymer chains were analyzed microscopically to explain the impact on the macroscopic properties. This study helps to correlate the change in crosslink density to ameliorate mechanical properties, such as strain at break, modulus, and stiffness. The effects of aging on the viscoelastic properties of SBR were also studied. Dynamic Mechanical Analysis (DMA) was used to characterize the viscoelastic response. Master curves of storage and loss modulus were generated using the time-temperature superposition principle (TTSP). The friction coefficient was estimated from the storage and loss modulus using a simplified form of the Persson equation [1]. CLD was also estimated from DMA data. Wear experiments were conducted on the Dynamic Friction Tester (DFT) for various aging conditions. The estimated friction coefficient was compared to the one from the experiments. Archard's law was used to correlate the frictional energy to the volume loss during wear experiments. Correlation between the wear and the viscoelastic properties of SBR is also studied. Finally, the lifetime of SBR for various aging temperatures is predicted using various models. [1] M. Ciavarella, "A Simplified Version of Persson's Multiscale Theory for Rubber Friction Due to Viscoelastic Losses," J. Tribol., vol. 140, no. 1, 2018, doi: 10.1115/1.4036917. / Master of Science / Elastomers or rubbers are they are generally referred to are an indispensable part of human life. They are made up of long-chain polymer units linked to one other through crosslinks. This peculiar morphology of rubbers is what gives them their unique characteristics. There are as many as 40,000 known products that use some form of rubber as the primary raw material. Apart, from this, they are also widely used in aviation and aerospace, automobiles, dampers and absorbers, civil engineering, electronics, medical, toys, clothing, sports, footwear, and so on. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. Over the last couple of years, it has also played a pivotal role in personal protective equipment (PPE) and masks worn by billions of people and frontline workers all over the globe. The fact that rubber is included in the EU's list of critical raw materials highlights its global importance. However, over the past several years, the rubber supply has dwindled. COVID-19 also caused disruptions in the supply chain of rubber. As the effects of COVID-19 are fading, there has been a spike in the demand for rubber; the primary reason being automotive tires! Even though substantial research is being conducted to try and replace rubber as a raw material with synthetic alternatives such as polyurethane, the excellent blend of damping, friction and wear characteristics, heat dissipation provided by natural rubber cannot be replicated by any of these laboratory compounds. Hence, at this time, there is an increased need to conserve and improve the longevity of rubber compounds. Styrene-Butadiene Rubber (SBR) is a form of a rubber compound that is widely used in the tire industry. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. The present study aims to model changes in the hyperelastic (large stretching) behavior of SBR using a Continuum Damage Mechanics (CDM) approach. This mathematical model is translated into ABAQUS, a finite element analysis software to study the mechanical response of components with various geometries and loading conditions. Secondly, the effects of aging on the viscoelastic behavior of SBR is studied. This helps us to estimate the cross-link density (CLD) as well as the friction coefficient of SBR as it is aged. The impact of aging on the wear and friction properties of SBR is studied experimentally. Finally, using various mechanical and chemical models the lifetime of SBR is estimated for various aging temperatures. Thus, the end goal of the study is to drive the development of new rubber compounds that will help improve the service life of rubbers and also have a positive impact on the environment.

Rubber-like materials are submitted to cyclic loading conditions in various applications. Fillers are always incorporated within rubber compounds. They improve the mechanical properties but induce a significant stress-softening under cyclic loadings. The physical source of the softening is not yet established and its modeling remains a challenge. For a better understanding of the softening, filled rubbers are submitted to cyclic loadings. In order to quantify the effects of the loading intensity and the number of cycles, original methods are proposed to characterize the softening. To study the influence of the material microstructure on the softening, compounds with various compositions are considered.Non proportional tensile tests including uniaxial and biaxial loading paths are applied in order to highlight the softening induced anisotropy. Such unconventional experimental data are used to provide a general criterion for the softening activation. A constitutive modeling grounded on a thorough analysis of experimental data is proposed. The model is based on a directional approach. The Mullins softening is accounted for by the strain amplification concept and is activated by a directional criterion. The model ability to predict non proportional softened material responses is demonstrated

>Les maladies cardiovasculaires sont la première cause de mortalité dans le monde et font actuellement l’objet de nombreuses recherches. Dans le cas d’études des artères, un des objectifs est d’améliorer la compréhension des mécanismes biologiques impliqués dans des maladies comme l’hypertension, l’athérosclérose ou l’anévrisme. Les études mécaniques qui ont été menées s’appuient essentiellement sur des approches expérimentales in vitro, ce qui en limite leur intérêt et application dans le diagnostic clinique. Dans ce travail, un modèle théorique de comportement mécanique 3D de l’artère carotide prenant en compte le caractère hyperélastique, anisotrope, actif, précontraint et dynamique de la structure est proposé. Les mesures expérimentales sont obtenues in vivo sur des carotides communes de rats d’une part, et humaines de manière non invasive, d’autre part. Le problème mécanique aux limites est résolu semi-analytiquement sur un cycle cardiaque, considérant le tissu environnant. Les valeurs optimales des paramètres du modèle, en particulier de ceux décrivant les caractéristiques mécaniques de microconstituants pariétaux (élastine, collagène, muscle lisse), sont évaluées par régression non linéaire. Le modèle proposé permet (i) de reproduire les évolutions de pression luminale mesurées in vivo et (ii) de donner une évaluation des distributions de contraintes pariétales cohérentes avec la physiologie artérielle. Une corrélation entre l’âge des patients et les paramètres décrivant les contraintes résiduelles et les fibres de collagène, montre l’intérêt du modèle théorique et l’originalité de cette approche qui pourra donc être utilisée dans l’étude de pathologies artérielles. / Cardiovascular diseases are the number one cause of death globally and they are currently the subject of many researches. In studies of arteries, one of the aims is to improve understanding in biological mechanisms involved in diseases such as hypertension, atherosclerosis or aneurysm. The mechanical studies that were carried out predominantly rely on in vitro experimental testing, which limit their interest and application in clinical diagnosis. In this work, a theoretical modelling of the 3D carotid artery mechanical behaviour is proposed by assuming a hyperelastic, anisotropic, active, pre-stretched and dynamic wall structure. The experimental measurements were obtained in vivo from rat and human common carotid arteries, with non invasive recordings in the human case. The mechanical boundary value problem is solved semi-analytically over a cardiac cycle by also assuming the surrounding perivascular tissue. The best-fit values of the model parameters are estimated by nonlinear least-squares method, in particular those describing the mechanical characteristics of wall microconstituents (elastin, collagen, smooth muscle). The proposed modelling is able (i) to reconstruct the in vivo dynamic measured intraluminal pressures and (ii) to compute the wall stress fields which seem to be consistent with the arterial physiology. A correlation between patient age and the parameters related to residual stresses and collagen fibres shows the relevance of the theoretical modelling and the originality of the approach which, thereby, would be able to be used in studies of arterial pathological cases.