Our aim was to aid the viscoelasticity into the model for thermal convection by developing our own code in Fortran 90 and to study the role of viscoelasticity in this model. We should have included the viscoelasticity by Maxwell model; however, due to numerical instability we changed it for Oldroyd-B model. We were adding the terms of objective derivative into our code step by step and we were observing how they influence the behaviour of thermal convection. Partial time derivative and advective terms were included in whole complexity, the corrotational terms need more numerical testing. Our work suggest that the influence of viscoelasticity on thermal convection is noticeable. Powered by TCPDF (www.tcpdf.org)

POSSIBILITIES OF THE USE OF INDENTATION TESTS WHILE STUDYING THE MECHANICAL PROPERTIES OF SOFT TISSUES OF THE HUMAN LOCOMOTOR SYSTEM IN VIVO, IN SITU ABSTRACT: The aim of this work was to analyze possibilities of the use of indentation tests, while studying mechanical properties of human locomotor system soft tissues in vivo, in situ. The synthesis of published findings and the results of our own surveys were used. A mathematical model of the indentation test was created as a part of theory. The geometry of the task was formulated as a solid cylinder penetrating into an infinite half-space made of viscoelastic material. The solution is based on the traditional analytical approach concerning stationary loading of flexible material. This procedure is generalized for Neo-Hooke's hyperelastic material and large deformations. The quantification of dissipative processes is based on the assumption that the stress tensor of a viscoelastic material can be decomposed into a purely elastic constituent and a purely dissipative constituent. The model enables to determine both elastic and viscous properties of the material. Experimental part of the work includes validation experiments. Based on the findings, it can be stated that myotonometry is a valid diagnostic method that allows an objective assessment of muscle tone...

In der Aufbau und Verbindungstechnik von mikroelektronischen Komponenten finden vermehrt polymere Werkstoffe Einzug. Zum Beispiel wird klassisches Metalllot durch elektrisch leitfähige Klebstoffe ersetzt, beziehungsweise werden zur Fixierung von oberflächenkontaktierten elektronischen Bauelementen schnell härtende Epoxidharzklebstoffe eingesetzt. Insbesondere im automobilen Einsatzbereich werden hohe Anforderungen an die Funktionszuverlässigkeit an die elektronischen Komponenten gesetzt. Große Temperaturschwankungen sowie unterschiedlichste mechanische Lastfälle wirken auf die Elektronik ein. Die vorliegende Arbeit behandelt die Beschreibung des viskoelastischen Materialverhaltens am Beispiel eines Epoxidharzklebstoffs zur Fixierung mikroelektronischer Bauelemente in einem ausgedehnten Temperatur- und Zeitbereich. Es werden unterschiedliche experimentelle Vorgehensweisen zur Ermittlung des Relaxationsverhaltens diskutiert und eine für den großen Temperaturbereich optimierte Zeit-Temperaturverschiebung präsentiert. Die experimentellen Ergebnisse des Relaxationsverhaltens werden mittels einer Vielparameteranpassung unter Zuhilfenahme genetischer Algorithmen in ein Materialmodell übertragen. Zur Beschreibung des dreidimensionalen viskoelastischen Stoffverhaltens wird neben dem E-Modul auch die Poissonzahl mit unterschiedlichen Messmethoden in Abhängigkeit von Temperatur und Zeit untersucht. Insbesondere die berührungslose Messung von Längs- und Querdehnung im Zugrelaxationsversuch und anschließender Auswertung mittels Grauwertkorrelation wird als zielführend bewertet. Es wird ein Funktionsansatz vorgestellt, mit dem sich die Poissonzahl in Abhängigkeit von Temperatur und Zeit beschreiben lässt. Damit lässt sich das Relaxationsverhalten für sowohl den Schub- als auch dem Kompressionsmodul darstellen. Dies erhöht die Aussagegenauigkeit von numerischen Beanspruchungsanalysen und verbessert somit auch die Präzision der darauf basierenden von Lebensdauerprognosen.

Instrumented indentation testing was used to determine viscoelastic properties of bitumen, i.e. shear relaxation modulus. Review of previous studied reveals that indentation testing technique has been drawing increasing attempts in investigations of binder material mechanical properties. Various properties of bitumen, e.g. elastic, viscoelastic, have been successfully determined by indentation testing at different scales and test conditions. The response of bitumen to indentation was studied extensively under a wide range of test parameters such as temperature, loading rate, indenter geometry, etc. This experimental tool was also applied to asphalt cements grading and microstructure study. However, there have been limited numbers of studies at macro levels with the use of spherical indenters to characterize bitumen properties. This motivated the present study. Spherical indentations have been performed with balls of different curvature radii at -5 oC. Load – displacement curves have been measured and used to determine the relaxation moduli of the studied bitumen. Repeatability of the measurements has been evaluated. Obtained results were compared with the observations from DSR relaxation test. The influence of the measurement scale and load level on the results of instrumented indentation testing has been investigated. It has been found that instrumented indentation was able to characterize accurately the viscoelastic behavior of bitumen which can be described by Prony series and agrees well with the results from DSR tests. Nevertheless, the elastic solution failed to produce a proper description of bitumen response during loading phase.

No description available.

Cette étude porte sur l'élaboration de matériaux polymères nano-cellulaires via un procédé batch de moussage au CO2. Pour obtenir de tels matériaux, le moussage est provoqué dans des matériaux polymères nano-structurés afin de favoriser la nucléation hétérogène et d'obtenir des taux de nucléation et des densités de cellules élevées. Le moussage de terpolymères ABS et de PMMAs nanostructurés a été étudié, dans le but de produire des mousses nano-cellulaires avec une faible densité (inférieure à 0.3 g.cm-3) et une taille moyenne de cellules inférieure à 100 nm, correspondant aux morphologies de mousses requises pour obtenir des matériaux super isolants thermiquement. Le phénomène de nucléation, et donc la densité de cellules, sont apparus comme dépendant directement de la concentration et de la morphologie des agents nucléants, qui correspondent dans cette étude à des phases polymères immiscibles dispersées. L'élaboration de matériaux nano cellulaires a nécessité la compréhension des mécanismes de croissances de cellules, du rôle du CO2 en tant qu'agent gonflant et plastifiant ainsi que l'optimisation du procédé de moussage. Plus précisément, l'influence du comportement viscoélastique des matériaux polymères ainsi que celle des forces de surface sur la formation de cellules a également été étudié. Il a été montré que la formation de mousse se produisait dans un milieu viscoélastique, avec un comportement variant entre celui d'un solide et d'un liquide viscoélastique, et ce en fonction de la température et de la masses molaire des polymères / This work focuses on the fabrication of nano-cellular polymer materials by means of a CO2 batch foaming process. To produce such materials, the foaming has to be induced in nano-structured polymer materials in order to favour heterogeneous nucleation and thus to obtain high nucleation rate and high cell density. The foaming of ABS terpolymers and nanostructured PMMAs was investigated, with the aim of producing nano-cellular foams with low density (lower than 0.3 g.cm-3) and an average cell size of 100 nm, which corresponds to required foam morphologies for super thermal insulating applications. It has been shown that nucleation, and thus cell density, directly depends on the content and morphology of nucleating agents, corresponding to dispersed polymer immiscible phases. The production of nano-cellular materials required the understanding of cell growth mechanisms, the role of CO2 as blowing agent and plasticiser and process optimisation. Specifically, the influence of viscoelastic behaviour of polymer materials and surface forces on cell formation was also investigated. It was found that the foaming occurred in viscoelastic media, with transitional behaviour between solid and liquid, depending on foaming temperature and molar mass of polymers

Indiana University-Purdue University Indianapolis (IUPUI) / The interplay between biomechanics and blood perfusion in the optic nerve head (ONH) has a critical role in ocular pathologies, especially glaucomatous optic neuropathy. Elucidating the complex interactions of ONH perfusion and tissue structure in health and disease using current imaging methodologies is difficult, and mathematical modeling provides an approach to address these limitations. The biophysical phenomena governing the ONH physiology occur at different scales in time and space and porous media theory provides an ideal framework to model them. We critically review fundamentals of porous media theory, paying particular attention to the assumptions leading to a continuum biphasic model for the phenomenological description of fluid flow through biological tissues exhibiting viscoelastic behavior. The resulting system of equations is solved via a numerical method based on a novel hybridizable discontinuous Galerkin finite element discretization that allows accurate approximations of stresses and discharge velocities, in addition to solid displacement and fluid pressure. The model is used to theoretically investigate the influence of tissue viscoelasticity on the blood perfusion of the lamina cribrosa in the ONH. Our results suggest that changes in viscoelastic properties of the lamina may compromise tissue perfusion in response to sudden variations of intraocular pressure, possibly leading to optic disc hemorrhages.

This diploma thesis is focused on two parts. The first one deals with rheological behavior of sodium carboxymethylcellulose (CMC-Na) solutions in the presence of sodium and calcium ions. The solutions of various concentrations were prepared by solubilization in aqueous solutions of NaCl or CaCl2. The effect of monovalent and bivalent ions on apparent viscosity and viscoelastic properties of these solutions was studied. It was found that in the case of low concentrations of CMC-Na in the sample (up to 0.5% w/w) the elastic character increases. At the same time the stiffness of the sample is higher with higher concentration of calcium ion in the solution. This increase might indicate the formation of gel network. On the other hand the viscosity of these solutions decreases with higher concentration of calcium ions. In case of solutions with concentration higher than 1.5% w/w changes of viscosity did not happen by influence of addition ions. Rheological behavior was compared also with solutions prepared without adding an electrolyte as well as with acid form of carboxymethylcellulose (CMC-H). In terms of second part the molecular dynamic simulations of hyaluronan tetramer both sodium and calcium in aqueous medium were performed. Structure and arrangement of water molecules in the surrounding of hyaluronan chain were studied. Structure was described by formalism of radial distribution functions. In the case of system containing sodium ions only, the obtained results are in accordance with earlier published data. Presence of calcium ions does not cause qualitative changes during the radial distribution functions in comparison with the system with sodium ions.

Diploma thesis was aimed to deal with evaluation of microrheology method in the research of biopolymer-protein. Used biopolymer was sodium hyaluronate and proteins were trypsin and chymotrypsin. For measuring of microrheology were used particles with different radius (0,5 m and 1 m). It was found, that both substances have viscous charakter. Passive microrheology is suitable for measuring the viscoelastic properties of biopolymers.

No description available.