Creep compliance of Hot Mix Asphalt (HMA) is an important property to characterize the material's viscoelatic behavior. It is used to predict HMA thermal cracking at low temperature and permanent deformation at high temperatures. There are several experimental methods to measure the creep compliance. Two of these methods were used in this thesis; uniaxial compressive and indirect tension (IDT) creep compliance. The tests were conducted at five temperatures (-15, 5, 20, 30, and 40°C) with a static loading for 1000-sec to characterize two typical HMA mixes used in Virginia, a base and a surface mix. Creep compliance master curves (CCMC) were developed by shifting the curves to a reference temperature using time-temperature superposition. Three mathematical functions, Prony series, power and sigmoidal, were fitted to the experimental data using regression analysis. Uniaxial CCMC were also predicted based on dynamic modulus measurements using method for interconversion of vicoelastic properties recommended in the literature. Finally, the susceptibility of the mixes to thermal cracking was evaluated based on the creep compliance measurements at low temperature. The regression analysis showed that the three mathematical models considered are appropriate to model the CCMC over a wide ranger of reduced times. The sigmoidal model provided the best fit over the entire range of reduced times investigated. This model also produced the best results when used in the interconversion procedures. However, there were noticeable differences between the CCMC predicted using interconversion and the experimental measurements, probably due to nonlinearity in the material behavior. The m-values for the base mix were higher using the creep results measured with both configurations. / Master of Science

Adhesives and adhesively bonded structures are being considered as a viable alternative to conventional fastening methods. In order to gain wider acceptance, it is essential to address the issue of the mechanical characterization of adhesive materials and its implementation in the design of bonded joints. While measuring the in-situ properties of the adhesive in a joint is a difficult task, characterizing its bulk properties is a relatively simpler undertaking. The objective of this study was to propose and verify an experimental procedure that would allow the analytical prediction of the viscoelastic behaviour of a bonded joint, using bulk adhesive properties. The Arcan joint geometry was chosen because of the simple state of stress within the adhesive. / Master of Science

Present stress analysis of adhesively bonded joints suffers from inadequate adhesive material characterization. The lack of correlation between bulk adhesive properties and the corresponding in-situ behavior has led to numerous adhesive test geometries. The current study was an attempt to predict the nonlinear viscoelastic response of an adhesive in-situ using properties obtained from a pure shear test geometry. Four candidate adhesive test geometries were studied both analytically and experimentally in terms of accurate shear property determination and realistic adhesive bond simulation. The thick adherend joint was chosen as the experimental reference of actual viscoelastic response in-situ; the Arcan specimen provided a pure shear stress state for material viscoelastic characterization. Results of finite element analysis and extensive experimental evidence suggest that residual thermal stresses alter the in-situ adhesive properties compared to the bulk adhesive: Furthermore, preliminary results indicate that the free volume nonlinear viscoelastic theory accounts for the effect of residual strains on the in-situ adhesive mechanical response. / M.S.

The numerical simulation of viscoelastic flows was studied in this work. In particular the effect of mesh refinement on the quality and the convergence of the finite element method was examined, as well as the differences that may be found by using several rheological models to describe the behaviour of the non-Newtonian fluids. The finite element simulation of viscoelastic fluid flows results in non-linear simultaneous equation systems that have to be solved iteratively. The iterations for all the viscoelastic models and most flow geometries have been found to diverge when the stress level or the elasticity of the flow increases above some certain level. The limit of convergence depends both on the mesh used for the discretization of the flow domain and on the rheological model. The limit usually decreases with mesh refinement. The effect of the mesh refinement on the convergence and the accuracy of the solution was studied here in two flow geometries: flow into an abrupt contraction (4/1 contraction ratio) and slit flow over a transverse slot. The penalty formulation of the finite element method (FEM) was used to numerically calculate the stress and the velocity fields in the flow domain using a number of coarse and fine meshes. Several rheological models were used, with their parameters chosen so that they would best fit a certain polystyrene melt. The solutions obtained were compared to results of g flow birefringence measurements and streamline photographs of the same material flowing under the same conditions that were simulated. The range of conditions that were covered by the calculations was shear stress at the die wall of 0-43 kPa, flow rates of 0-17 (mm³/ sec mm-width) and elasticity of 0-11 Deborah number. Even though oscillations in all numerical solutions were observed around the corners of the flow domain, it was found that the overall agreement of the numerical results with the experimental data was reasonable. The coarse meshes showed lower oscillations near the comers, but the accuracy of their predictions were poor. The limit of convergence for such meshes was the highest. Finer meshes on the other hand, showed higher oscillations near the comers and lower limit of convergence, but more accurate results away from the corner. It seems that the optimum mesh for an engineering calculation is an intermediate fine mesh that will give relatively high limits and reasonable accuracy. On the effect of the rheological model, it was found that the lower limit of convergence was given by the upper convected Maxwell model (UCM). The Leonov-like model also gave low limits. The Phan-Thien Tanner (P-T T) and the White-Metzner (W-M) models, on the other hand, showed quite higher limits in terms of the maximum stress levels and flow rates that they could handle. In terms of the quality of the solution inside the convergence range of each model, there is very little difference between the results of the models. In general, the Phan-Thien Tanner and the White-Metzner models show slightly better solutions. A possible reason for the better behaviour of these two models is believed to be the shear thinning viscosity and primary normal stress difference coefficient that these models are able to predict in simple flows. A few other characteristics of the two flows that were studied included the hole pressure, the entrance pressure loss and the presence of extensional fields around the contraction. It was found that the numerical method gave lower results for the hole pressure than the experimental data. Two models (W-M and UCM) gave a maximum in the entrance pressure loss and then a decrease towards negative values as the wall shear stress in the die increased. The P-T T and the Leonov-like models predicted a monotonic increase with the wall shear stress. Finally, there are two areas with strong elongational flow field in the contraction flow. One extends along the centerline of the die and the other lies along a line that starts from the reentrant comer and extends towards the upstream wall at an angle of around 45° (but depending on the flow rate). It is believed that this area is related to the natural entry angle, at which the viscoelastic fluid enters the contraction. / Ph. D. / incomplete_metadata

Previous decades of cartilage research have predominantly focused on decoupling the solid and fluid interactions of the mechanical response. The resulting biphasic and triphasic models are widely used in the biomechanics community. However, a simple viscoelastic model is able to account for the stress-relaxation behavior of cartilage, without the added complexity of solid and fluid interactions. Using a viscoelastic model, cartilage is considered a single material with elastic and dissipative properties. A mechanical characterization is made with fewer material parameters than are required by the conventional biphasic and triphasic models. This approach has tremendous utility when comparing cartilage of different species and varying healths. Additionally, the viscoelastic models can be easily extended in dynamic analysis and FEA programs. Cartilage primarily experiences compressive motion during exercise. Therefore, to mimic biological function, a mechanical test should also compress the cartilage. One such test is a viscoelastic stress-relaxation experiment. The Prony and fractional calculus viscoelastic models have shown promise in modeling stress-relaxation of equine articular cartilage. The elastic-viscoelastic correspondence principle is used to extend linear viscoelasticity to the frequency domain. This provides a comparison of articular cartilage based on stored and dissipated moduli. The storage and loss moduli metrics are hypothesized to serve as benchmarks for evaluating osteoarthritic cartilage, and provide guidelines for newly engineered bio-materials. The main goal of the current study is to test the applicability of modeling articular cartilage with viscoelastic models. A secondary goal is to establish a rigorous set of harvesting techniques that allows access to fresh explants with minimal environmental exposure. With a complex substance like cartilage, it is crucial to avoid unnecessary emph{in vitro} environmental exposure. Additional areas of study include: determining the strain-dependency of the mechanical response, exploring the response of cartilage in different fluid mediums such as saline, synovial fluid, and synthetic substitutes, and studying the time-dependent properties of cartilage during stress-relaxation experiments. Equine stifle joints, which are mechanically analogous to human knees, are harvested and used for analysis in this study. It is believed that the proposed viscoelastic models can model other articulating joints as well. If viscoelastic theory can be used to characterize cartilage, then comparisons can be drawn between real and artificial cartilage, leading to better joint replacement technology.

abstract: Asphalt binder is a complex viscoelastic hydrocarbon, whose performance depends upon interaction between its physical and chemical properties, both of which are equally important to the successful understanding of the material. Researchers have proposed various models linking linear viscoelastic (LVE) and microstructural parameters. However, none of these parameters provide insight into the relationship in the non- linear viscoelastic NLVE domain. The main goals of this dissertation are two fold. The first goal is to utilize the technique of Laser Desorption Mass Spectroscopy (LDMS) to relate the molecular structure of asphalt binders to its viscoelastic properties. The second goal of the study is to utilize different NLVE characterization tools and analysis procedures to get a clear understanding of the NLVE behavior of the asphalt binders. The goals of the study are divided into four objectives; 1) Performing the LDMS test on asphalt binder to develop at the molecular weight distributions for different asphalts, 2) Characterizing LVE properties of Arizona asphalt binders, 3) Development of relationship between molecular structure and linear viscoelasticity, 4) Understanding NLVE behavior of asphalt binders through three different characterization methods and analysis techniques. In this research effort, a promising physico-chemical relationship is developed between number average molecular weight and width of relaxation spectrum by utilizing the data from LVE characterization and the molecular weight distribution from LDMS. The relationship states that as the molecular weight of asphalt binders increase, they require more time to relax the developed stresses. Also, NLVE characterization was carried out at intermediate and high temperatures using three different tests, time sweep fatigue test, repeated stress/strain sweep test and Multiple Stress Creep and Recovery (MSCR) test. For the intermediate temperature fatigue tests, damage characterization was conducted by applying the S-VECD model and it was found that aged binders possess greater fatigue resistance than unaged binders. Using the high temperature LAOS tests, distortion was observed in the stress-strain relationships and the data was analyzed using a Fourier transform based tool called MITlaos, which deconvolves stress strain data into harmonic constituents and aids in identification of non-linearity by detecting higher order harmonics. Using the peak intensities observed at higher harmonic orders, non-linearity was quantified through a parameter termed as “Q”, which in future applications can be used to relate to asphalt chemical parameters. Finally, the last NLVE characterization carried out was the MSCR test, where the focus was on the scrutiny of the Jnrdiff parameter. It was found that Jnrdiff is not a capable parameter to represent the stress-sensitivity of asphalt binders. The developed alternative parameter Jnrslope does a better job of not only being a representative parameter of stress sensitivity but also for temperature sensitivity. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2018

Vita. / Digitized by Kansas Correctional Industries

The focus of this dissertation is on refining atomic force micrscopy (AFM) methods and data analysis routines to measure the viscoelastic mechanical properties of soft polymer and biological materials in relevant fluid environments and in vivo using a range of relevant temperatures, applied forces, and loading rates. These methods are directly applied here to a several interesting synthetic and biological materials. First, we probe poly(n-butyl methacrylate) (PnBMA), above, at and below its glass transition temperature in order to verify our experimental procedure. Next, we use AFM to study the viscoelastic properties of coating materials and additives of silicone-based soft contact lenses in a tear-like saline solution. Finally, a major focus in this dissertation is determining the fundamental mechanical properties that contribute to the excellent sensitivity of the strain sensing organs in a wandering spider (Cupiennius salei) by probing under in vivo conditions. These strain-sensing organs are known to have a significant viscoelastic component. Thus, the cuticle of living spiders is directly investigated in near-natural environments (high humidity, temperatures from 15-40 °C). The main achievements of these studies can be summarized through the following findings: We suggest that full time-temperature-modulus relationships are necessary for the understanding of soft materials systems, and present a practical method for obtaining such relationships. These studies will have a direct impact on both scientists in the metrology field by developing practical experimental procedures and data analysis routines to investigate viscoelastic mechanical properties at the nanoscale, and future materials scientists and engineers by showing via spider mechanosensory systems how viscoelasticity can be applied for functional use in sensing technology.

The thesis presents novel formulations of hereditary cohesive zone models able to capture rate-dependent crack propagation along a defined interface. The formulations rely on the assumption that the measured fracture energy is the sum of an intrinsic fracture energy, related to the rupture of primary bonds at the atomic or molecular level, and an additional dissipation caused by any irreversible mechanisms present in the material and occurring simultaneously to fracture. The first contribution can be accounted for by introducing damage-type internal variables, which are to be driven by a rateindependent evolution law in order to be coherent with the definition as intrinsic energy. It is then proposed that the additional dissipation can be satisfactorily characterised by the same continuum-type material constitutive law obeyed by the interface material considered as a continuum: it is postulated that the dimensional reduction whereby a three-dimensional thin layer is idealized as a surface does not qualitatively alter the functional description of the free energy. The specific application considered is mode-I crack propagation along a rubber interface. After focusing on viscoelasticity as a suitable candidate to reproduce rubber’s behaviour, firstly the most common relaxation function, namely a single exponential term, is considerd after which the attention is turned to the use of fractional calculus and the related fractional integral kernel. A comparison with experimental results is presented. A shortcoming of the proposed approach is then noted, in that certain features of experimentally measured responses (i.e.the non-monotonicity of the critical energy-release rate with respect to crack speed) will be shown to be out of reach for the described modelling paradigm. A novel micromechanical formulation is then sketched in an attempt to qualitatively understand the phenomenon. An additional interface damaging mode is introduced, physically inspired by the desire to reproduce the formation of fibrils in a neighbourhood of the crack tip. Fibril formation is then driven by a variational argument applied to the whole of the interface, yielding its non-local character. Upon the introduction of an anisotropic fracture energy, motivated by experimental considerations, it is noted how the model can predict a non-monotonic energy-release rate vs crack speed behaviour, at least for a simple loading mode.

Les matériaux viscoélastiques sont utilisés dans tous les domaines de l'ingénierie et des systèmes mécaniques, de l'électroménager, spatial, l'automobile, l'aéronautique ou le génie civil (ponts...) grâce à leur capacité d’amortir les chocs ou de filtrer les vibrations. Ce travail constitue une contribution à l’étude du comportement dynamique non linéaire des composants viscoélastiques notamment les élastomères. Dans ce mémoire, on introduit, d’abord, les propriétés mécaniques des élastomères, pour les aspects viscoélasticité et friction. Un rappel des différents phénomènes physiques et une liste non-exhaustive des modèles existants dans la littérature sont présentés. Ensuite, on propose des techniques expérimentales afin de décrire le comportement dynamique sous sollicitations uniaxiales d’un élastomère. Une description des bancs d’essais, des chaines d’analyse vibratoire, des méthodes de traitement des données des essais et d’analyse des mesures expérimentales est détaillée dans ce manuscrit. Une nouvelle approche du modèle de Maxwell généralisé a été proposée pour décrire le comportement dynamique du composant viscoélastique. Ce modèle permet une description précise et une bonne connaissance du comportement dynamique des composants viscoélastiques en fonction de l’amplitude, de la précharge et de la fréquence. La dissipation d'énergie identifiée sous forme d'amortissement peut être issue de l'amortissement intrinsèque des matériaux polymère comme de la friction aux interfaces dans le cas de composants caoutchoucs non adhérisés sur les pièces, dans ce travail, un nouveau modèle visco-tribologique a été développé en couplant les propriétés rhéologiques linéaires du modèle de Maxwell généralisé et le modèle de frottement de Dahl pour la description du comportement de frottement hystérétique des liaisons viscoélastiques non adhérisées. / Viscoelastic materials are used in all areas of engineering and mechanical systems, appliances, aerospace, automotive, aerospace and civil engineering (bridges...) through their ability to absorb shock and vibration filtering. This work is a contribution to the study of nonlinear dynamic behavior of viscoelastic components particularly elastomers. In this dissertation, we introduced the mechanical properties of elastomers, for both viscoelasticity and friction aspects. A review of the different physical phenomena and a non-exhaustive list of existing models in the literature are presented. Then, we propose experimental techniques to describe the dynamic behavior under uniaxial stress of an elastomer. A description of test benches, vibration analysis chains, methods of processing data and analysis of experimental measurements is detailed in this manuscript. A new approach of generalized Maxwell model was proposed to describe the dynamic behavior of viscoelastic component. This model allows an accurate description and a good knowledge of the dynamic behavior of viscoelastic components depending on amplitude, frequency and preload. Energy dissipation identified as damping can be from intrinsic damping of the polymer as friction at the interfaces in case of not bonded rubber component to mechanical part, a new viscoelastic model tribological was developed by combining the rheological properties of linear generalized Maxwell model and the Dahl friction model for describing the behavior of viscoelastic hysteretic friction of not bonded connections.