Materials like asphalt, asphalt concrete and polyimides that are used in the transportation and aerospace industry show viscoelastic behavior. These materials in the working environment are subject to degradation due to temperature, diffusion of moisture and chemical reactions (for instance, oxidation) and there is need for a good understanding of the various degradation mechanisms. This work focuses on: 1) some topics related to development of viscoelastic fluid models that can be used to predict the response of materials like asphalt, asphalt concrete, and other geomaterials, and 2) developing a framework to model degradation due to the various mechanisms (such as temperature, diffusion of moisture and oxidation) on polyimides that show nonlinear viscoelastic solid-like response. Such a framework can be extended to model similar degradation phenomena in the area of asphalt mechanics and biomechanics. The thermodynamic framework that is used in this work is based on the notion that the 'natural configuration' of a body evolves as the body undergoes a process and the evolution is determined by maximizing the rate of entropy production. The Burgers' fluid model is known to predict the non-linear viscoelastic fluid-like response of asphalt, asphalt concrete and other geomaterials. We first show that different choices for the manner in which the body stores energy and dissipates energy and satisfies the requirement of maximization of the rate of entropy production that leads to many three dimensional models. All of these models, in one dimension, reduce to the model proposed by Burgers. A thermodynamic framework to develop rate-type models for viscoelastic fluids which do not possess instantaneous elasticity (certain types of asphalt show such a behavior) is developed next. To illustrate the capabilities of such models we make a specific choice for the specific Helmholtz potential and the rate of dissipation and consider the creep and stress relaxation response associated with the model. We then study the effect of degradation and healing due to the diffusion of a fluid on the response of a solid which prior to the diffusion can be described by the generalized neo-Hookean model. We show that a generalized neo-Hookean solid - which behaves like an elastic body (i.e., it does not produce entropy) within a purely mechanical context - creeps and stress relaxes when infused with a fluid and behaves like a body whose material properties are time dependent. A framework is then developed to predict the viscoelastic response of polyimide resins under different temperature conditions. The developed framework is further extended to model the phenomena of swelling due to diffusion of a fluid through a viscoelastic solid using the theory of mixtures. Finally, degradation due to oxidation is incorporated into such a framework by introducing a variable that represents the extent of oxidation. The data from the resulting models are shown to be in good agreement with the experiments for polyimide resins.

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This work investigates the use of shape memory alloys (SMAs) for vibration isolation and damping of mechanical systems. The first part of this work evaluates the nonlinear dynamics of a passive vibration isolation and damping (PVID) device through numerical simulations and experimental correlations. The device, a mass connected to a frame through two SMA wires, is subjected to a series of continuous acceleration functions in the form of a sine sweep. Frequency responses and transmissibility of the device as well as temperature variations of the SMA wires are analyzed for the case where the SMA wires are pre-strained at 4.0% of their original length. Numerical simulations of a one-degree of freedom (1-DOF) SMA oscillator are also conducted to corroborate the experimental results. The configuration of the SMA oscillator is based on the PVID device. A modified version of the constitutive model proposed by Boyd and Lagoudas, which considers the thermomechanical coupling, is used to predict the behavior of the SMA elements of the oscillator. The second part of this work numerically investigates chaotic responses of a 1- DOF SMA oscillator composed of a mass and a SMA element. The restitution force of the oscillator is provided by an SMA element described by a rate-independent, hysteretic, thermomechanical constitutive model. This model, which is a new version of the model presented in the first part of this work, allows smooth transitions between the austenitic and the martensitic phases. Chaotic responses of the SMA oscillator are evaluated through the estimation of the Lyapunov exponents. The Lyapunov exponent estimation of the SMA system is done by adapting the algorithm by Wolf and co-workers. The main issue of using this algorithm for nonlinear, rateindependent, hysteretic systems is related to the procedure of linearization of the equations of motion. The present work establishes a procedure of linearization that allows the use of the classical algorithm. Two different modeling cases are considered for isothermal and non-isothermal heat transfer conditions. The evaluation of the Lyapunov exponents shows that the proposed procedure is capable of quantifying chaos in rate-independent, hysteretic dynamical systems.

Characterization of multiaxial mechanical properties of cardiovascular soft tissue is essential in order to better understand their growth and remodeling in homeostatic conditions and in response to injury or pathological conditions. Though numerous phenomenological models have been proposed to characterize such multiaxial mechanical behavior, the approach has certain drawbacks regarding experimental determination of the model coefficients. We propose a method that aims to overcome these drawbacks. The approach makes use of orthogonal polynomials to fit the biaxial test data and suggests a way to derive the strain energy function from these analytical fits by way of minimizing the deviation of the behavior from hyperelastic ideal. Using the proposed method, a strain energy function for a lymphatic vessel is derived and the method is compared with traditional ones that used non-orthogonal polynomials as independent variables in the functional form for strain energy. The unique coefficient values obtained using the proposed method, for the first time gives us an opportunity to attribute a physical characteristic of the material to the coefficient values. The method also provides a way to assess two different material behaviors by way of comparing their deviation from the hyperelastic behavior when a similar test protocol is used to collect the data, over a similar deformation range and the order of polynomial function is chosen so as to give a similar error of fit. The behavior of mesenteric lymph vessels from normal cows, cows subjected to sham surgery and those subjected to 3 days of edematous conditions by venous occlusion are compared using this method. To be able to better understand the changes in mechanical behavior, morphological analysis of the vessels was carried out and the geometric and structural changes in these vessels were studied. We found that the behavior of bovine mesenteric lymph vessels subjected to a high flow condition shows a small difference in their mechanical behavior as compared to the vessels from normal a cow and a cow subjected to sham surgery. The geometry and structure of these vessels also showed marked differences from the other two. The thickness to radius ratio increased and a rise in percentage of area occupied by smooth muscle cells and medial collagen was observed. Though not all the differences were statistically significant, we conclude that the behavior and the morphology are suggestive of the remodeling of the vessel in response to altered hemodynamic conditions and require further investigation.

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<p>Cemented carbides based on tungsten carbide and cobalt arecommonly produced by a powder metallurgy route including liquidphase sintering. The pressed compact densifies to almost halfits volume during sintering due to pore elimination. Thesintering behaviour changes with material composition, such ascarbide grain size, binder fraction, carbon content andaddition of cubic carbides.</p><p>This thesis is devoted to the study of constitutivebehaviour, in particular densification, and the microstructuralevolution during cemented carbide sintering. Dimensionalchanges are monitored using dilatometry with and withoutapplied external load. The microstructural evolution isinvestigated with light optical microscopy and scanningelectron microscopy. Thermodynamic calculations are used asreference.</p><p>Constitutive relations are derived for uniaxial viscosity,viscous equivalent of Poisson’s ratio and sintering stressbased on relative density and temperature. The relations areextended to a model describing sintering shrinkage withexplicit dependencies on carbide grain size and binder content.The model is divided in three stages of which two pertain tothe solid state and the third to liquid phase sintering. Solidstate shrinkage is suppressed in a material with coarsecarbides and in the stage of liquid phase sintering grain sizestrongly influences the uniaxial viscosity. The binder contentaffects primarily the later densification.</p><p>The effects of carbon content and grain size distribution onshrinkage have been studied. High carbon content enhancesshrinkage rate, but the effect of grain size distribution israther small. The mean carbide grain size is insufficient todescribe densification for very broad distributions only.</p><p>Shrinkage occurs through rearrangement andsolution-reprecipitation. Rearrangement is studied through theevolution of the pore size distribution and simulatedgenerically using a discrete element method.</p><p><b>Keywords:</b>Cemented carbides, Sintering, Constitutiverelations, Microstructure, Densification, Modelling</p>

The mechanical responses of the comminuted ceramic under impact is important in understanding penetration resistance of the target, modeling the penetration process, developing ceramic models and designing better armor systems. To determine the dynamic compressive responses of ceramic rubbles, a novel loading/reloading feature in SHPB experiments was developed to produce two consecutive loading pulses in a single dynamic experiment with two strikers and two shapers. The first pulse pulverizes the intact specimen into rubble after characterizing the intact material. After unloading of the first pulse, a second pulse loads the comminuted specimen and gives the dynamic constitutive behavior of the rubble.With this new experimental technique, several series of experiments were conducted on an oxide ceramic -- alumina AD995 and a non-oxide ceramic--hot pressed silicon carbide, SiC-N, with different strain rates, various volume dilatations and damaged levels under 26 MPa, 56 MPa and 104 MPa confinement. The results show that the strength of the damaged ceramic is not very sensitive to strain rates within this research range and the pulse separation once the damage attains a critical level. When slightly damaged far below a critical level, the specimen remains nearly elastic; when transitionally damaged, the specimen strength gradually decrease from the slight damage level to the heavy damage level. Increasing confinement increases the strength of the ceramics. The crack patterns were dominantly axial splitting for the slight damage, axial splitting and fragmentation for the intermediate damage, and fragmentation and comminution for the heavy damage. For SiC-N, the volume dilatation history shows a delayed failure. SEM observations indicated that microstructural failure mechanism is intergranular fracture for alumina and transgranular fracture for SiC-N.Mohr-Coulomb criterion was successfully employed to describe the damaged ceramic and the parameters were determined. JH-1 model was employed to describe the failed SiC-N in the linearly segmentation description of the strength and the parameters were also determined. Through the analysis of JH-1 model for SiC-N, the critical damage level can be taken as D = 1.0. JH-2 model was used to describe analytically the damaged AD995 and the parameters were obtained. The critical damage value is 0.88 for alumina determined directly from JH-2 model. The description of JH-1 model is equivalent to Mohr-Coulomb criterion while it is unsuitable for JH-2 model due to the non-linear description. Based on the analysis of existing models and current experimental data, an empirical constitutive material model was developed for the damaged ceramic, which well described the completely damaged ceramic, but was unable to model the partially damaged ceramic.

There are many causes of cholestasis, which results when the flow of bile acids is slowed or stopped. Bile acids are hydrophobic molecules synthesized from cholesterol in the liver, and when present in excess, are cytotoxic to cell membranes. Treatment options for cholestasis are limited, and if left untreated or inadequately treated, many patients will require a liver transplant; thus, underscoring the importance of successfully managing this disease. Activation of nuclear receptors in animal models has been shown to be hepatoprotective during bile acid-induced cholestasis; however, the mechanisms underlying the hepatoprotective effects are poorly understood. Therefore, the over-arching goal of this project is to glean an improved comprehension of the mechanisms of hepatoprotection during bile acid-induced cholestasis. All of the studies involve administration of CAR activators phenobarbital (PB), oltipraz (OPZ), 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene [TCPOBOP (TC)] or corn oil (CO) to C57BL/6 wild type (WT), or WT and CAR knockout (CAR-/-) mice prior to induction of intrahepatic cholestasis using the secondary bile acid lithocholic acid (LCA). Efflux transport proteins such as Mrps 3 and 4 are known to be up-regulated during cholestasis, and this was the first topic of exploration. Unexpectedly, the expression of efflux transporters was not consistently up-regulated in protected mice. However, a decrease in total liver bile acid concentrations was observed. These changes in hepatic bile acids indicated that bile acid biosynthesis may be relevant to hepatoprotection. Indeed decreases in total and individual bile acids correlated with hepatoprotection, and Cyp8b1 expression was also increased which could be suggestive of a shift in the bile acid biosynthesis pathway towards the formation of less toxic bile acid species. CAR may also have a role in cell death via apoptosis by altering Bcl-2 protein expression. Although apoptosis was decreased in hepatoprotected mice, an increase in the expression of Mcl-1 and Bcl-xL was not observed, suggesting hepatoprotection is not a direct result of CAR-induced Mcl-1 expression. These findings add significantly to the body of knowledge surrounding cholestatic liver disease and suggest that studies aimed toward manipulation of nuclear receptors are worthy of further exploration.

Understanding the behavior of geomaterials under explosive loading is essential for several applications in the mining and oil industry. To date, the design of these applications is based almost solely on empirical equations and tabulated data. Optimal designs require accurate and complete knowledge of rock behavior under various loading conditions. The vast majority of the properties available in the literature have been gathered by deforming the specimen slowly. These properties have been used to establish constitutive models which describe the behavior of rocks under static and quasi-static loading conditions. However, the dynamic properties and material constitutive models describing the behavior of geomaterials under high strain-rate loading conditions are essential for a better understanding and enhanced designs of dynamic applications. Some attempts have been made to measure dynamic properties of rocks. Also, some trials have been made to devise material models which describe the behavior of rocks and the evolution of damage in the rock under dynamic loading. Published models were successful in predicting tensile damage and spalling in rocks. However, there are no established models capable of predicting compressional damage in rocks due to dynamic loading. A recently-developed model, the RHT model, was formulated to describe the behavior of concrete over the static and dynamic ranges. The model was also formulated to predict compressional damage based on the strain rate at which the material is subjected to. The RHT model has been used successfully in several applications. The purpose of this research was to characterize one rock type as an example of a hard brittle rock. The physical properties of the rock as well as the static and dynamic mechanical properties were investigated. These properties were used to calibrate the RHT model and investigate its potentials to predict compressional damage in brittle materials. The calibrated model showed good precision reproducing the amplitude of the strain signals generated by explosive loading. It was also capable of predicting compressional damage with acceptable accuracy. Unfortunately, due to implementation restrictions, tensile and spall damage could not be captured by the model. The duration and shape of the strain pulse were also poorly modeled. / Thesis (Ph.D, Mining Engineering) -- Queen's University, 2010-12-22 17:54:05.887

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