Time- and rate-dependent materials such as polymers, bituminous materials, and soft materials clearly display all four fundamental responses (i.e. viscoelasticity, viscoplasticity, viscodamage, and healing) where contribution of each response strongly depends on the temperature and loading conditions. This study proposes a new general thermodynamic-based framework to specifically derive thermo-viscoelastic, thermo-viscoplastic, thermo-viscodamage, and micro-damage healing constitutive models for bituminous materials and asphalt mixes. The developed thermodynamic-based framework is general and can be applied for constitutive modeling of different materials such as bituminous materials, soft materials, polymers, and biomaterials. This framework is build on the basis of assuming a form for the Helmohelotz free energy function (i.e. knowing how the material stores energy) and a form for the rate of entropy production (i.e. knowing how the material dissipates energy). However, the focus in this work is placed on constitutive modeling of bituminous materials and asphalt mixes. A viscoplastic softening model is proposed to model the distinct viscoplastic softening response of asphalt mixes subjected to cyclic loading conditions. A systematic procedure for identification of the constitutive model parameters based on optimized experimental effort is proposed. It is shown that this procedure is simple and straightforward and yields unique values for the model material parameters. Subsequently, the proposed model is validated against an extensive experimental data including creep, creep-recovery, repeated creep-recovery, dynamic modulus, constant strain rate, cyclic stress controlled, and cyclic strain controlled tests in both tension and compression and over a wide range of temperatures, stress levels, strain rates, loading/unloading periods, loading frequencies, and confinement levels. It is shown that the model is capable of predicting time-, rate-, and temperature-dependent of asphalt mixes subjected to different loading conditions.

Techn. Univ., Habil.-Schr.--Braunschweig, 2001. / Literaturverz.S. [159] - 168.

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 26, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Mian Liu. Vita. Includes bibliographical references.

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Ο σκοπός της παρούσης διδακτορικής διατριβής ήταν η θεωρητική και πειραματική μελέτη της επίδρασης της αναπτυσσόμενης βλάβης που προέρχεται από διαφορετικές αιτίες στην μηχανική συμπεριφορά συνθέτων υλικών. Οι πηγές βλάβης που μελετήθηκαν ήταν: 1.Υγροθερμική Κόπωση 2. Θερμική Κόπωση 3. Ύπαρξη κεντρικής κυκλικής οπής διαφορετικών διαμέτρων ή εγκοπής διαφορετικών μηκών 4. Διαφορετικοί ρυθμοί παραμόρφωσης 5. Ερπυσμός και επανάταξη, και 6. Συνδυασμός των παραπάνω, όπως: ερπυσμός μετά από υγροθερμική ή θερμική κόπωση, ερπυσμό σε διαφορετικές θερμοκρασίες και χρονικές διάρκειες. Τα σύνθετα υλικά που μελετήθηκαν ήταν κοκκώδη, ινώδη και τύπου σάντουιτς σύνθετα υλικά. Για την καλύτερη μελέτη της μηχανικής και βισκοελαστικής συμπεριφοράς των συνθέτων υλικών, εφαρμόστηκε ένας σημαντικός αριθμός θεωρητικών μοντέλων, όπως είναι: 1. Το μοντέλο των τεσσάρων παραμέτρων. 2. Το μοντέλο των τριών παραμέτρων. 3. Το μοντέλο εναπομένουσων ιδιοτήτων, RPM. 4. Το θεωρητικό μοντέλο ανάλυσης των διεπιφανειακών ιδιοτήτων σε σάντουιτς υλικά. 5. Το μοντέλο πρόβλεψης βισκοελαστικής συμπεριφοράς στη μη γραμμική περιοχή. Τα συμπεράσματα που προέκυψαν από την παραπάνω μελέτη ήταν ιδιαίτερα ενδιαφέροντα. / The aim of the present investigation is to study not only experimentally but also theoretically the effect of damage due to different damage sources, which analytically are: a) hygrothermal fatigue, b) thermal fatigue, c) the existence of central hole (with different diameters) or notch (with different lengths), d) creep and relaxation, e) different stain rates and f) combination of the previous such as creep after hygrothermal or thermal fatigue and creep on different temperatures and time, on the mechanical and viscoelastic behavior of composites materials (reinforced with fibers and grains ), sandwich structures and different types of resin. For the better observation of the mechanical and viscoelastic behavior of composites materials, it was applied several theoretical models, which are: a) The four parameter model, b) the three parameter model, c) the RPM model (Residual Property Model) for the prediction of the residual mechanical properties of materials after damage from different sources, d) the theoretical model for the analysis of interfacial properties of sandwich structures and e) the viscoelastic model for the determination of the non elastic properties of composites material in function of the applied stress. From the present investigation useful conclusions were conducted for the determination of the mechanical behavior of composites materials (reinforced with fibres and grains or sandwich structures). In addition, the experimental results of this study were in great deal with those results of the application of the already mentioned theoretical models.

La métastase des cellules cancéreuses est un processus caractérisé par un comportement de la cellule anormal. Les propriétés mécaniques particulières des cellules cancéreuses est l'une des caractéristiques pathologiques principales. Ces propriétés sont liées à leurs capacités à envahir les tissus avoisinants, à transmigrer et à proliférer vers de nouveaux sites. La progression du cancer est caractérisée par la perturbation et la réorganisation du cytosquelette d'actine ainsi que par des changements des propriétés mécaniques des cellules, probablement liés à la capacité dupliquée des cellules cancéreuses à migrer et à s'adapter à l'environnement.Les propriétés mécaniques sont essentielles pour la régulation des fonctions cellulaires comme la migration, l'adhésion, la prolifération, et la différentiation, et les anomalies sont associées aux pathologies, en particulier le cancer. Les propriétés mécaniques sont aussi dépendantes du micro-environnement de la cellule, et la rigidité des substrats modifie les propriétés mécaniques internes des cellules, ainsi que la structure du cytosquelette. Ainsi, comprendre les processus impliqués dans les variations des propriétés viscoélastiques est essentiel pour l'étude de la progression des tumeurs.La microscopie à force atomique (AFM) a prouvé être un outil fiable pour sonder les propriétés mécaniques statiques et dynamiques (sur de grande gamme de fréquence) de matériaux mous, comme les spécimens biologiques, à de petites échelles et grande résolution.Dans cette étude, nous proposons de nouveaux marqueurs du cancer basé sur une approche mécanobiologique. Les propriétés viscoélastiques de cellules cancéreuses de la vessie sont mesurées par des expériences d'indentation dynamiques par AFM. Cette méthode est validée en utilisant des gels de polyacrylamide et un modèle à fractions dérivées est proposé pour décrire le comportement mécanique de ces gels. Ensuite, le module de cisaillement complexe de trois lignées cellulaires à potentiel métastatique différent est mesuré à trois positions différentes de la cellule: le noyau, le périnoyau et la périphérie de la cellule. En utilisant des drogues d'inhibition de l'actine, les propriétés mécaniques sont corrélées à la microstructure de l'actine obtenue par microscopie confocale. Nous proposons un modèle simplifié pour décrire le comportement des modules élastiques G' et visqueux G''. Une relation entre l'invasivité des cellules cancéreuses et leur propriétés mécaniques est aussi mis en avant. En particulier, nous trouvons que le plateau élastique et la fréquence de transition (quand G'=G'') peuvent être utilisés comme marqueurs d'invasivité. Enfin, nous mesurons le module de cisaillement complexe de cellules cancéreuses adhérentes sur des environnements mécaniques et biologiques différents, et des propriétés intrigantes de la périphérie des cellules cancéreuses sont reportées. / Cancer cell metastasis is a multi-stage process characterized by cell malfunctional behavior. Some of the major pathological characteristics of cancer cells are their particular mechanical properties which are linked to their ability to invade surrounding tissues, transmigrate and proliferate at new sites. There are evidences that cancer progression is characterized by disruption and reorganization of the actin cytoskeleton as well as changes in the mechanical properties. This change is probably associated with the enhanced capability of cancer cells to migrate and adapt to changing environments.The mechanical properties are essential for the regulation of cell functions like migration, adhesion, proliferation and differentiation, and abnormalities are connected with pathologies, in particular cancer metastasis. The mechanical properties are also dependent on the micro-environment of the cell, as substrate stiffness changes cell internal mechanical properties, as well as the cytoskeleton structure. Thus, the understanding of the mechanics involved in the variation of the viscoelastic properties is crucial for the study of tumor progression.Atomic force microscopy (AFM) has proved to be a reliable tool to probe static and frequency–dependent mechanical properties of soft materials, like biological specimens, at small scale with high resolution.In this study, we propose new markers of cancer metastasis based on a cell mechanics approach. We report on the viscoelastic properties of human bladder cancer cells measured by dynamic indentation experiments using AFM. This method is first calibrated using polyacrylamide gels and a fractional model is proposed to describe the behavior of such gels. We then investigate the complex shear modulus of three different cell lines with different metastatic potential. We probe the elastic G' and viscous G'' modulus at three different locations across the cell: nucleus, perinucleus and the cell periphery. With the use of actin inhibitory drugs, we correlate mechanical properties and the actin microstructure obtained by confocal microscopy imaging. We propose a simplified power-law model to describe the behavior of the elastic and viscous moduli. We also report a relationship between the malignancy of cancer cells and their viscoelastic properties. In particular, we find that the elastic plateau modulus and the transition frequency (frequency at which G' = G'' ) can be used as markers of invasiveness. Then, we probe the complex shear modulus of cancer cells on different mechanical and biological environments and we report intriguing properties of the periphery of cancer cells.

abstract: This dissertation features a compilation of studies concerning the biophysics of multicellular systems. I explore eukaryotic systems across length scales of the cell cytoskeleton to macroscopic scales of tissues. I begin with a general overview of the natural phenomena of life and a philosophy of investigating developmental systems in biology. The topics covered throughout this dissertation require a background in eukaryotic cell physiology, viscoelasticity, and processes of embryonic tissue morphogenesis. Following a brief background on these topics, I present an overview of the Subcellular Element Model (ScEM). This is a modeling framework which allows one to compute the dynamics of large numbers of three-dimensional deformable cells in multi-cellular systems. A primary focus of the work presented here is implementing cellular function within the framework of this model to produce biologically meaningful phenotypes. In this way, it is hoped that this modeling may inform biological understanding of the underlying mechanisms which manifest into a given cell or tissue scale phenomenon. Thus, all theoretical investigations presented here are motivated by and compared to experimental observations. With the ScEM modeling framework I first explore the passive properties of viscoelastic networks. Then as a direct extension of this work, I consider the active properties of cells, which result in biological behavior and the emergence of non-trivial biological phenotypes in cells and tissues. I then explore the possible role of chemotaxis as a mechanism of orchestrating large scale tissue morphogenesis in the early embryonic stages of amniotes. Finally I discuss the cross-sectional topology of proliferating epithelial tissues. I show how the Subcellular Element Model (ScEM) is a phenomenological model of finite elements whose interactions can be calibrated to describe the viscoelastic properties of biological materials. I further show that implementing mechanisms of cytoskeletal remodeling yields cellular and tissue phenotypes that are more and more biologically realistic. Particularly I show that structural remodeling of the cell cytoskeleton is crucial for large scale cell deformations. I provide supporting evidence that a chemotactic dipole mechanism is able to orchestrate the type of large scale collective cell movement observed in the chick epiblast during gastrulation and primitive streak formation. Finally, I show that cell neighbor histograms provide a potentially unique signature measurement of tissue topology; such measurements may find use in identifying cellular level phenotypes from a single snapshot micrograph. / Dissertation/Thesis / Ph.D. Physics 2011

[pt] Este trabalho apresenta um modelo computacional aplicável à análise de sistemas estruturais viscoelásticos submetidos a grandes deslocamentos, com particular atenção ao fenômeno da instabilidade. A discretização dos modelos é obtida através de elementos finitos isoparamétricos bidimensionais que podem ser empregados na análise de colunas, pórticos, arcos e cascas axissimétricas. A estabilidade elástica do sistema é verificada ao longo de trajetórias de equilíbrio definidas no espaço carga-deslocamentos, onde a ocorrência de pontos de bifurcação ou de pontos-limite é indicada através da troca de sinal do pivô da matriz de rigidez tangente. A inclusão de um modelo viscoelástico linear para o material possibilita a avaliação do efeito do tempo de carregamento sobre a carga de flambagem da estrutura. O mecanismo de instabilidade correspondente à flambagem viscoelástica envolve duas variáveis básicas: a magnitude da carga (carga crítica) e a duração da carga (tempo crítico). Os exemplos apresentados ilustram esses conceitos e fornecem resultados interessantes a respeito dos efeitos da viscoelasticidade sobre a flambagem em diferentes sistemas estruturais. / [en] This thesis presents a computational model for the analysis of viscoelastic structures undergoing large displacements, with particular attention to unstable phenomena. The discrete model utilizes two-dimensional isoparametric finite elements in the analysis of columns, frames, arches and axially symmetric shells. The elastic stability of the system is verified along the equilibrium paths in the multidimensional load-displacements space, with bifurcation or limit points indicated by sign changes of the pivot of the tangent stiffness at every incremental step. A linear viscoelastic model for the material is included, allowing for the consideration of the effect of loading time on the buckling load for the structure. Thus, the mechanism leading to loss of stability, corresponding to viscoelastic buckling, involves two basic variables: load magnitude and duration of the load, designated as critical load and critical time. The examples presented herein enlighten such concepts and provide interesting results about viscoelastic effects on buckling of different structural systems.

L’influence des propriétés microscopiques de l’interphase entre la matrice et les fibres sur le comportement mécanique macroscopique n’est pas suffisamment connue dans le domaine des polymères renforcés par fibres courtes. Dans le cadre de cette thèse, une étude systématique des propriétés géométriques et mécaniques de l’interphase est réalisée concernant la description des effets sur la réponse viscoélastique linéaire du composite. Dans ce contexte, les résultats présentés mettent l’accent sur l’interaction entre la modélisation micromécanique et la caractérisation expérimentale. D’une part, un nouveau modèle micromécanique en deux étapes est développé pour la description d’un composite anisotrope à trois phases avec interphases. D’autre part, les paramètres du matériau utilisés pour la modélisation micromécanique sont identifiés avec des méthodes expérimentales aux échelles micro- et macroscopiques. En comparaison des résultats expérimentaux avec les propriétés effectives calculées de matériau composite, une inférence peut être faite sur les propriétés mécaniques du composite à partir de celles de l’interphase. Par conséquent, une méthode inverse est proposée offrant un accès aux propriétés inconnues de l’interphase. Enfin, la combinaison de la modélisation micromécanique et des résultats expérimentaux permet une meilleure compréhension des propriétés mécaniques de l’interphase, qui n’étaient auparavant pas accessibles au moyen de seules approches expérimentales / In order to improve the mechanical properties of short fiber composites, the fiber-matrix adhesion is decisive and depends strongly on the intersection region between the fiber and the matrix material. However, no perspicuous information about the influence or mechanical properties of the fiber-matrix interphase in short fiber reinforced thermoplastic composites is available. Thus, the present thesis aims for a systematic identification of the geometrical and mechanical impacts of an interphase on the linear-viscoelastic behavior in short glass fiber reinforced thermoplastics. Thereby, the performed investigations are focused on the interaction between micromechanical material modeling and experimental testing. On the one hand, a two-step modeling approach is developed for the realistic description of an entire three phase composite with interphase including anisotropic and linear-viscoelastic effects. On the other hand, the input of this model is provided by different experimental testing methods ranging from the micro- to the macroscale characterization of the composite and matrix material. By comparing these experimental results with the linear-viscoelastic modeling output, the impact of the interphase on the mechanical properties of the composite is accessible. Thus, it is shown that a realistic material modeling and experimental investigations are closely interlinked

Les propriétés viscoélastiques des fluides déterminent leur écoulement. L’étude de ces propriétés a de nombreuses applications industrielles et académiques qui concernent la matière dite « molle » (polymères, colloïdes, tensioactifs, protéines, ...). L’approche proposée permet d'étudier ces propriétés sur une gamme de fréquence allant de 1 à 100 kHz. La méthode utilise la mesure de la vibration d’une microstructure actionnée électromagnétiquement et immergée dans le fluide à caractériser. La réponse en fréquence du système mécanique, mesurée optiquement ou électriquement, est caractéristique du milieu dans lequel la structure est immergée. Une méthode analytique dédiée aux micropoutres, pour l’extraction des propriétés rhéologiques du milieu, a été améliorée tout au long de la thèse.La méthode analytique développée, pour être appliquée, nécessite la précision d’un système optique complexe pour mesurer sans artefact les propriétés mécaniques de l’interaction micropoutre-liquide. Ainsi les liquides opaques ne peuvent être caractérisés avec cette approche. De plus la mesure peut difficilement être intégrée dans un dispositif portable tout-électronique. Afin de pallier ces difficultés et de proposer une mesure de la viscoélasticité en milieu opaque, la stratégie de mesure du capteur jusqu’au traitement des signaux ont été réévalués : (1) des microstructures en « U » ont été fabriquées, (2) une méthode de mesure intégrée a été mise en place et (3) une méthode de traitement à fréquence unique a été utilisée. Finalement, un liquide opaque viscoélastique, le yaourt, a pu être caractérisé in-situ tout au long de la fermentation lactique permettant de démontrer la validité et l’applicabilité de la méthode mise en œuvre pour le suivi en temps réel de la viscoélasticité. / The study of viscoelastic properties has many industrial and academic applications related to "soft matter" like polymers, colloids, surfactants or proteins. The present approach measures these properties in a frequency range from 1 to 100 kHz. The method uses the measurement of the vibration of a microstructure actuated electromagnetically and immersed in the fluid that has to be characterized. The frequency response of the mechanical system, which is measured optically or electrically, is characteristic of the environment in which the structure is immersed in. An analytical method dedicated to microcantilevers for the extraction of the rheological properties has been improved during this PhD thesis.The analytical method developed requires the accuracy of a complex optical system for measuring without artifact the mechanical properties of microcantilever-liquid interaction. Thus opaque liquids cannot be characterized with this sytem. In addition, the measurement cannot easily be integrated. To overcome these difficulties and provide the measurement of viscoelasticity into opaque medium, some strategy was reassessed: (1) “U” shaped microstructures were fabricated; (2) an integrated measurement method was developed and (3) a single frequency method was used to calculate the viscoelasticity.Finally, a viscoelastic and opaque liquid, such as yogurt, has been characterized in situ during the lactic fermentation to demonstrate the validity and the applicability of the method for the real-time monitoring of viscoelasticity.