# Browsing by Subject "Viscoelasticity"

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Item A Model for the Nonlinear Mechanical Behavior of Asphalt Binders and its Application in Prediction of Rutting Susceptibility(2013-04-30) Srinivasa Parthasarathy, AtulShow more The mechanical behavior of asphalt binders is nonlinear. The binders exhibit shear thinning/thickening behavior in steady shear tests and non-proportational behavior in other standard viscoelastic tests such as creep-recovery or stress relaxation tests. Moreover, they develop normal stress differences even in simple shear flows - a characteristic feature of nonlinear viscoelastic behavior. Many researchers have asserted the importance of considering the nonlinearity of the mechanical behavior of asphalt binders for accurately estimating their performance under field conditions, and for comparing and ranking them accordingly. In order to do so, it is necessary to have a robust and reliable nonlinear viscoelastic model. However, most of the models available in the literature do not capture the various features of the nonlinear response of asphalt binders accurately. Those that could are too complicated and still possess other shortcomings. Considering these issues, a new nonlinear viscoelastic model is developed here using a new Gibbs-potential based thermodynamic framework. The model is then corraborated with data from experiments in which the shear-thinning behavior and the nonproportional creep-recovery behavior were observed together. Finally, the model is used to evaluate the various criteria available for predicting rutting susceptibility of asphalt binders. Results of the analysis of the rutting prediction criteria show that each criterion characterizes the resistance to permanent strain shown by asphalt binders over a different range of applied stress - the zero-shear viscosity at very low stress levels, the Superpave criterion at very high stress levels and the MSCR test in the intermediate range of stresses.Show more Item A Multi-scale Framework for Thermo-viscoelastic Analysis of Fiber Metal Laminates(2010-01-14) Sawant, Sourabh P.Show more Fiber Metal Laminates (FML) are hybrid composites with alternate layers of orthotropic fiber reinforced polymers (FRP) and isotropic metal alloys. FML can exhibit a nonlinear thermo-viscoelastic behavior under the influence of external mechanical and non-mechanical stimuli. Such a behavior can be due to the stress and temperature dependent viscoelastic response in one or all of its constituents, namely, the fiber and matrix (within the FRP layers) or the metal layers. To predict the overall thermoviscoelastic response of FML, it is necessary to incorporate different responses of the individual constituents through a suitable multi-scale framework. A multi-scale framework is developed to relate the constituent material responses to the structural response of FML. The multi-scale framework consists of a micromechanical model of unidirectional FRP for ply level homogenization. The upper (structural) level uses a layered composite finite element (FE) with multiple integration points through the thickness. The micromechanical model is implemented at these integration points. Another approach (alternative to use of layered composite element) uses a sublaminate model to homogenize responses of the FRP and metal layers and integrate it to continuum 3D or shell elements within the FE code. Thermo-viscoelastic constitutive models of homogenous orthotropic materials are used at the lowest constituent level, i.e., fiber, matrix, and metal in the framework. The nonlinear and time dependent response of the constituents requires the use of suitable correction algorithms (iterations) at various levels in the multi-scale framework. The multi-scale framework can be efficiently used to analyze nonlinear thermo-viscoelastic responses of FML structural components. The multi-scale framework is also beneficial for designing FML materials and structures since different FML performances can be first simulated by varying constituent properties and microstructural arrangements.Show more Item AFM-based microrheology of biological cells : correlation of local viscoelasticity and motility(2003-08) Park, Soyeun, 1970-; Shih, Chih-Kang; Käs, Josef A.Show more Local viscoelasticity of a cell is important in understanding the extension of the lamellipodium, which contributes to the cell’s motility. It has been a challenge to accurately measure viscoelastic properties of a thin sample such as the lamellipodium of a cell (<1000 nm) due to the strong substrate effects and high stresses (>1 kPa). We account for the substrate effects by applying the two advanced models – the Chen and Tu models. The tightly regulated elastic moduli shown in the lamellipodium of fibroblasts manifestly display the successful adoption of these two models. In addition, these models provide the local Poisson ratio and adhesive state of a cell: the regions near the lamellipodium are well adhered while the regions further back to the main body are non-adhered. Our AFM technique successfully illuminates the heterogeneous nature of the cytoskeleton over the entire regions of the cell. By extending these models to the frequency-dependent microrheology technique, we decompose the elastic moduli into the loss and the storage moduli. Our AFM microrheology technique distinctly differentiates the malignantly transformed fibroblasts from the normal fibroblasts: the malignantly transformed fibroblasts display a decrease in viscoelastic moduli of the lamellipodium. Considering that motilities as well as viscoelastic properties of cells are induced by cytoskeletal changes, we focus our attention to illuminating on the cell’s protrusive mechanism correlated with the viscoelastic properties. To do this, we quantify the parameters of cell’s motility by analyzing time-lapsed phase contrast images. The resulting data show an increase in the motile activity caused by malignant transformation. In conclusion, these results are combined to suggest the correlation between the enhanced motility and the decrease in viscoelastic moduli. This conclusion is successfully explained by considering the microscopic model of the cell motility, i.e ‘Elastic Brownian Ratchet’ (Mogilner et al., 1996). It is understood that the lack of actin cross-linking proteins observed in malignantly transformed fibroblasts causes a cell to be softer and more motile. An increase in thermal fluctuations of softer cells can expedite the intercalation of G-actin that leads the cell’s protrusive motility.Show more Item An abstract model for flow of a two-phase viscoelastic fluid in a tube(Texas Tech University, 1985-12) Deshpande, Vasanti AnantShow more A phenomenological model has been developed to predict the time-dependent flow behavior of a suspension of solid particles in viscoelastic fluid. The model consists of a Maxwell model in parallel with a plastic model, which has a yield stress parameter Y. The effect of the solid particles is studied by introducing a function f(Ï†), where Ï† is the volume fraction of the solid in the viscoelastic fluid. Einstein's, Mooney's and Simha's models, which are good for suspensions of rigid spheres in Newtonian fluids, are used for f(t) to examine the effect of <> on the velocity profiles in tube flow. The volume fraction was varied from 0 to 307. An explicit finite difference method is used to solve the integrodifferential equation for viscosities up to 5.0 poise, and relaxation times between 1 and 5 seconds. Pressure drops are calculated using the average velocity obtained from the computed velocity profiles. Limitations of the numerical method and the simple rheological model are discussed and recommendations for the future have been made.Show more Item Anisotropic Characterization of Asphalt Mixtures in Compression(2012-10-08) Zhang, Yuqing 1983-Show more Rutting is one of the major distresses in asphalt pavements and it increases road roughness and traps water, which leads to wet-weather accidents due to the loss of tire-pavement friction and hydroplaning. The fundamental mechanisms of rutting have not been well addressed because of the complexity of asphalt mixtures. A comprehensive characterization of the asphalt mixtures in compression was accomplished by mechanistically modeling the inherent anisotropy, viscoelasticity, viscoplasticity and viscofracture of the material. The inherent anisotropy due to preferentially oriented aggregates was characterized by a microstructural parameter (i.e., modified vector magnitudes) which could be rapidly and accurately measured by lateral surface scanning tests and physically related to anisotropic modulus ratio. The anisotropic viscoelasticity was represented by complex moduli and Poisson's ratios in separate orthogonal directions that were determined by an efficient testing protocol. Master curve models were proposed for the magnitude and phase angle of these complex variables. The viscoplasticity were intensively modeled by an anisotropic viscoplastic model which incorporated 1) modified effective stresses to account for the inherent and stress-induced anisotropy; 2) a new model to provide a smooth and convex yield surface and address the material cohesion and internal friction; 3) a non-associated flow rule to consider the volumetric dilation; and 4) a temperature and strain rate dependent strain hardening function. The viscofracture resulting from the crack growth in compression led to the stress-induced anisotropy and was characterized by anisotropic damage densities, the evolution of which was modeled by the anisotropic pseudo J-integral Paris' laws. Results indicated that the undamaged asphalt mixtures were inherently anisotropic and had vertical to horizontal modulus ratios from 1.2 to 2.0 corresponding to the modified vector magnitudes from 0.2 and 0.5. The rutting would be underestimated without including the inherent anisotropy in the constitutive modeling. Viscoelastic and viscoplastic deformation developed simultaneously while the viscofracture deformation occurred only during the tertiary flow, which was signaled by the increase of phase angle. Axial and radial strain decomposition methods were proposed to efficiently separate the viscoplasticity and viscofracture from the viscoelasticity. Rutting was accelerated by the occurrence of cracks in tertiary flow. The asphalt mixture had a brittle (splitting cracks) or ductile (diagonal cracks) fracture when the air void content was 4% and 7%, respecitvely. The testing protocol that produced the material properties is efficient and can be completed in one day with simple and affordable testing equipment. The developed constitutive models can be effectively implemented for the prediction of the rutting in asphalt pavements under varieties of traffic, structural, and environmental conditions.Show more Item Computation of synthetic seismograms by the method of characteristics(Texas Tech University, 1979-08) Castro, Louis ReyesShow more Not availableShow more Item Development of a computational method for inverting dynamic moduli of multilayer systems with applications to flexible pavements(2014-08) Xu, Qinwu; Prozzi, Jorge AlbertoShow more Most existing computational methods for inverting material properties of multilayer systems have focused primarily on elastic properties of materials or a static approach. Typically, they are based on a two-stage approach: (I) modeling structural responses with a computer program, and (II) estimating layer properties mathematically using the response outputs determined in stage I without interactions with the governing state partial-differential-equation (PDE) of stage I. This two-stage approach may not be accurate and efficient enough for inverting larger scale model parameters. The objective of this research was to develop a computational method to invert dynamic moduli of multilayer systems with applications to flexible pavements under falling weight deflectometer (FWD) tests, thereby advancing existing methods and fostering understanding of material behaviors. This research first developed a finite-element and Newton-Raphson method to invert layer elastic moduli using FWD data. The model improved the moduli seeds estimation and achieved a satisfactory accuracy based on Monte Carlo simulations, addressing the common back-calculation issue of no unique solutions. Consequently, a time-domain finite-element method was developed to simulate dynamic-viscoelastic responses of the multilayer systems under loading pulses. Simulation results demonstrated that the dynamic-viscoelastic-damping-coupled model could emulate structural responses more accurately, thereby advancing existing simulation approaches. By using the dynamic-viscoelastic-response model as one computation module, this research led to the development of a PDE-constrained Lagrangian optimization method to invert dynamic moduli and viscoelastic properties of multilayer systems. The Lagrangian function was used as an objective function, with a regularization term and governing-state PDE constraint. Both the first-order (gradient) and second-order variation (Hessian matrix) of the Lagrangian were computed to satisfy necessary and sufficient optimality conditions, and Armijo rule was modified to determine a stable step length. The developed method improved computation speed significantly, and it is superior for large-scale inverse problems. The model was implemented for evaluating flexible pavements under FWD tests and for inverting the master curve of dynamic moduli of the asphalt layer. Independent computer coding was developed for all numerical methods. The computational methods developed may also be applied to other multilayer systems, such as tissues and sandwich structures at different time and length scales.Show more Item Durability of adhesive joints between concrete and FRP reinforcement in aggressive environments(2004) Park, Soojae; Liechti, K. M.Show more The durability of the bondline between concrete and its fiber-reinforced polymer reinforcement was characterized at various temperature and humidity levels. The bondline consisted of an epoxy primer, an epoxy putty and an epoxy saturant. In principle, fracture could occur anywhere in this bondline, but attention was focused on the concrete/primer interface in this study because preliminary experiments indicated that this was the dominant failure mechanism. The initial part of the constitutive modeling of the epoxy primer was conducted using linear viscoelastic experiments. Confined compression experiments determined two linear material functions simultaneously. Because this was a relatively new experiment, the results were validated by conducting bulk compliance experiments. The viscoelastic region of the bulk modulus was as wide as that of the tensile and shear relaxation moduli. This result contradicts previous conceptions but is agreement with some other recent observations. Thermal and hygral expansions were also measured and used in a hybrid nonlinear viscoelastic constitutive model. The hybrid model captured the hygrothermal nonlinear viscoelastic deformation of the epoxy primer. This model is a combination of Schapery’s model and Popelar’s shear modified free volume model. Torsion tests were conducted and used to calibrate the distortional parameters in the free volume model. Tension experiments were performed at four different temperature and humidity levels and were used to calibrate the dilatational, thermal and hygral parameters in the hybrid model. The linear and hygrothermal nonlinear viscoelastic constitutive models were used in the analysis of time-dependent interfacial fracture between concrete and epoxy primer. A generalized time-dependent J integral was used as a fracture parameter for characterizing the time-dependent interfacial fracture. This was used instead of the strain energy release rate and the stress intensity factor because of the nonlinear viscoelastic deformation of the primer. Schapery’s pseudo stress model was calibrated using tension data at various temperature and humidity levels because it is required for the generalized J integral. An instrumented wedge test was conducted in order to determine the interfacial fracture energy at several loading rates and various temperature and humidity levels. The crack length was measured as a function of wedge speeds during steady state crack growth. The generalized J integral and cohesive zone size or failure zone size were computed using finite element analyses that incorporated the pseudo stress model. The pseudo stress model, cohesive zone size and the generalized J integral were all used to compute the work input into the failure zone, which was then equated to the fracture energy. The loading rate, temperature and humidity level all affected the fracture energy, which decreased with increasing temperature and humidity levels.Show more Item Effects of moisture on the dimensional and viscoelastic properties of glassy polymers(Texas Tech University, 2003-12) Zheng, YongShow more One cause of long-term dimensional changes in glassy polymers is the gradual evolution of the glassy structure and viscoelastic properties through aging processes in the glassy state. Many applications of polymers involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. In this work, results from a study of a glassy epoxy subjected to isothermal RH-jumps are reported. Similar to the temperature jump experiments of Kovacs, the volume recovery responses in different histories, intrinsic isopiestic (constant RH), memory effect and the asymmetry of approach, are obtained. In addition, the effects of structural recovery on sorption and desorption of water and the physical aging responses of this epoxy resin are also measured. The experimental results qualitatively support the hypothesis that water has an effect similar to temperature on structure (volume, enthalpy) of the glass-forming material. However, quantitatively, at the same RH-temperature states, the glasses formed by RH jumps show anomalous differences from those formed by temperature jumps. The conventional TNM-KAHR model is modified to fit the experimental results of isothermal RH jump experiments. Furthermore, application of the potential energy landscape theory to describe the glasses formed by relative humidity jumps, and especially to the observed differences between temperature glass and concentration glass, is discussed.Show more Item Efficient frequency response analysis of structures with viscoelastic materials(2006) Swenson, Eric Dexter; Bennighof, Jeffrey K.Show more Item Evaluation of viscoelastic materials: The study of nanosphere embedment into polymer surfaces and rheology of simple glass formers using a compliant rheometer(Texas Tech University, 2008-08) Hutcheson, Stephen Anthony; McKenna, Gregory B.; Rasty, Jahan; Simon, Sindee L.; Weeks, Brandon L.Show more Viscoelasticity is a fundamental property of many materials such as polymers, inorganic glasses, biological materials, small molecule glass formers, and composites. This fundamental property is what links the research presented here. There are two focuses that will be presented: 1. A background of nanoparticle is presented and a viscoelastic model is applied to determined the actual rheological behavior of materials. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. 2. Instrument compliance effects caused by both the transducer and entire instrument itself can induce large errors on shear measurements of viscoelastic properties of materials. Examples of instrument compliance effects on the measurement of the material properties of small molecular glass formers and a commercially available polydimethysiloxane (PDMS) rubber using a commercial rheometer are presented. A technique is presented and applied to correct for compliance effects in stress relaxation experiments and dynamic frequency sweep experiments. Recommendations are made for both experimental and instrument design to avoid and/or minimize compliance effects.Show more Item Extreme energy absorption : the design, modeling, and testing of negative stiffness metamaterial inclusions(2013-08) Klatt, Timothy Daniel; Seepersad, Carolyn C.; Haberman, Michael R.Show more A persistent challenge in the design of composite materials is the ability to fabricate materials that simultaneously display high stiffness and high loss factors for the creation of structural elements capable of passively suppressing vibro-acoustic energy. Relevant recent research has shown that it is possible to produce composite materials whose macroscopic mechanical stiffness and loss properties surpass those of conventional composites through the addition of trace amounts of materials displaying negative stiffness (NS) induced by phase transformation [R. S. Lakes, et al., Nature, 410, pp. 565-567, (2001)]. The present work investigates the ability to elicit NS behavior without employing physical phenomena such as inherent nonlinear material behavior (e.g., phase change or plastic deformation) or dynamic effects, but rather the controlled buckling of small-scale structural elements, metamaterials, embedded in a continuous viscoelastic matrix. To illustrate the effect of these buckled elements, a nonlinear hierarchical multiscale material model is derived which estimates the macroscopic stiffness and loss of a composite material containing pre-strained microscale structured inclusions. The nonlinear multiscale model is then utilized in a set-based hierarchical design approach to explore the design space over a wide range of inclusion geometries. Finally, prototype NS inclusions are fabricated using an additive manufacturing technique and tested to determine quasi-static inclusion stiffness which is compared with analytical predictions.Show more Item Finite Element Analysis of Indentation in Fiber-Reinforced Polymer Composites(2012-07-16) Ravishankar, ArunShow more This thesis employs a finite element (FE) method for numerically simulating the mechanical response of constituents in a fiber-reinforced polymer (FRP) composite to indentation. Indentation refers to a procedure that subsumes a rigid indenter of specific geometry to impress the surface of a relatively softer material, with a view of estimating its mechanical properties. FE analyses are performed on a two-dimensional simplified microstructure of the FRP composite comprising perfectly bonded fiber, interphase and matrix sections. Indentation response of the constituents is first examined within the context of linearized elasticity. Time-dependent response of the polymer matrix is invoked by modeling the respective constituent section as a linear isotropic viscoelastic material. Furthermore, indentation responses to non-mechanical stimulus, like moisture absorption, is also simulated through a sequentially coupled analysis. A linear relationship describing the degradation of elastic moduli of the individual constituents with increasing moisture content has been assumed. The simulations subsume a point load idealization for the indentation load eventually substituted by indenter tips with conical and spherical profiles. Results from FE analyses in the form of load-displacement curves, displacement contours and stress contours are presented and discussed. With the application of concentrated load on linearly elastic constituents for a given/known degree of heterogenity in the FRP, simulations indicated the potential of indentation technique for determining interphase properties in addition to estimating the matrix-fiber interphase bond strength. Even with stiffer surrounding constituents, matrix characterization was rendered difficult. However, fiber properties were found to be determinable using the FE load-displacement data, when the load-displacement data from experimentation is made available. In the presence of a polymer (viscoelastic) matrix, the surrounding elastic constituents could be characterized for faster loading rates when viscoelastic effects are insignificant. Displacements were found to be greater in the presence of a polymer matrix and moisture content in comparison with a linearly elastic matrix and dry state. As one would expect, the use of different indenter tips resulted in varying responses. Conical tips resulted in greater displacements while concentrated load produced greater stresses. Further it was found that, despite the insignificant effects due to surrounding constituents, analytical (Flamant) solution for concentrated, normal force on a homogeneous, elastic half-plane becomes inapplicable in back calculating the elastic moduli of individual FRP constituents. This can be attributed to the finite domain and the associated boundary conditions in the problem of interest.Show more Item Integrated biomechanical model of cells embedded in extracellular matrix(2009-05-15) Muddana, Hari ShankarShow more Nature encourages diversity in life forms (morphologies). The study of morphogenesis deals with understanding those processes that arise during the embryonic development of an organism. These processes control the organized spatial distribution of cells, which in turn gives rise to the characteristic form for the organism. Morphogenesis is a multi-scale modeling problem that can be studied at the molecular, cellular, and tissue levels. Here, we study the problem of morphogenesis at the cellular level by introducing an integrated biomechanical model of cells embedded in the extracellular matrix. The fundamental aspects of mechanobiology essential for studying morphogenesis at the cellular level are the cytoskeleton, extracellular matrix (ECM), and cell adhesion. Cells are modeled using tensegrity architecture. Our simulations demonstrate cellular events, such as differentiation, migration, and division using an extended tensegrity architecture that supports dynamic polymerization of the micro-filaments of the cell. Thus, our simulations add further support to the cellular tensegrity model. Viscoelastic behavior of extracellular matrix is modeled by extending one-dimensional mechanical models (by Maxwell and by Voigt) to three dimensions using finite element methods. The cell adhesion is modeled as a general Velcro-type model. We integrated the mechanics and dynamics of cell, ECM, and cell adhesion with a geometric model to create an integrated biomechanical model. In addition, the thesis discusses various computational issues, including generating the finite element mesh, mesh refinement, re-meshing, and solution mapping. As is known from a molecular level perspective, the genetic regulatory network of the organism controls this spatial distribution of cells along with some environmental factors modulating the process. The integrated biomechanical model presented here, besides generating interesting morphologies, can serve as a mesoscopic-scale platform upon which future work can correlate with the underlying genetic network.Show more Item MECHANICAL PROPERTIES OF ULTRATHIN POLYMER FILMS INVESTIGATED BY A NANOBUBBLE INFLATION TECHNIQUE: SURFACE TENSION, GEOMETRY AND MOLECULAR ARCHITECTURE EFFECTS(2011-08) Xu, Shanhong; McKenna, Gregory B.; Simon, Sindee L.; Weeks, Brandon L.; Quitevis, Edward L.Show more It is very important to understand the mechanical properties of polymers at the nanoscale with the continuing demand of decreasing the size of circuit in the electronic industry. There has been considerable research on the confinement effect in thin films with a variety of techniques, often with conflicting results. Most of the previous work has been done in a pseudo-thermodynamic mode, where the glass transition temperature (Tg) is taken to be the break in the temperature dependence of a property [e.g. ellipsometry, Brillouin scattering]. A method based on the determination of a dynamic property, the absolute biaxial creep compliance, has been developed by O’Connell and McKenna. The method is a scaled down version of the classic bubble inflation technique. They found that the Tg of polystyrene (PS) decreases with film thickness, while it doesn’t change for poly(vinyl acetate) (PVAc). The most surprising finding is that the rubbery plateau compliance decreases dramatically for both materials. These results are unexplained though it has been suggested that the observed stiffening at the nanometer size scale could be attributed to surface tension. In this thesis, we investigated a new material (poly (n-butyl methacrylate) (PBMA)) that shows significantly different behavior from PVAc or PS and that provides new evidence that the stiffening of the rubbery plateau region in ultrathin polymer films is a nanoconfinement effect. We developed the stress-strain analysis and energy balance approach to separate the surface tension contribution to the observed rubbery stiffening. We found that the surface tension contribution for PBMA is much larger than that of PVAc. The rubbery stiffening of PBMA is much less than PS and PVAc. The surface tension of PBMA doesn’t change with decreasing film thickness. Further, the geometry effect in the nanobubble inflation technique was investigated by comparing the creep behavior of circular bubbles with that of rectangular bubbles. The accuracy of the analytical approximate solutions was evaluated by comparing with the finite element (FE) analysis for simulation of the inflation of rectangular bubbles. We found that the shape of the bubble obtained from the experiment is consistent with that of FE. We also found that the reduction of Tg and the rubbery plateau compliance for rectangular bubbles are consistent with those of circular bubbles. So geometry is not the reason for the observed stiffening effect. Next, we investigated the molecular architecture effect in the nanobubble inflation technique by comparing the creep behavior of linear PS with that of the three-arm star PS. Both the reduction of Tg and the stiffening in the rubbery region for star PS is consistent with those of linear PS. In the last part of this thesis, the capability of the nanobubble inflation technique to investigate the yield and fracture behavior of ultrathin films was demonstrated. Stepped pressure is applied to ultrathin films until it broke. We found that for 33nm film, it transit from brittle failure to yield with increasing temperature. The yield stress decreases with increasing temperature. For 22nm film, it always failed without yield.Show more Item Non-Linear Drying Diffusion and Viscoelastic Drying Shrinkage Modeling in Hardened Cement Pastes(2010-07-14) Leung, Chin K.Show more The present research seeks to study the decrease in diffusivity rate as relative humidity (RH) decreases and modeling drying shrinkage of hardened cement paste as a poroviscoelastic respose. Thin cement paste strips of 0.4 and 0.5 w/c at age 3 and 7 days were measured for mass loss and shrinkage at small RH steps in an environmental chamber at constant temperature. Non-linear drying diffusion rate of hardened cement was modeled with the use of Fick's second law of diffusion by assuming linearity of diffusion rate over short drops of ambient relative humidity. Techniques to determine drying isotherms prior to full equilibration of mass loss, as well as converting mass loss into concentration of water vapor were developed. Using the measured water vapor diffusivity, drying shrinkage strain was modeled by the theory of poroviscoelasticity. This approach was validated by determining viscoelastic properties from uniaxial creep tests considering the effect of aging by the solidification theory. A change in drying diffusion rate at different RH was observed in the 0.4 and 0.5 w/c pastes at different ages. Drying diffusion rate decreases as RH drops. This can be attributed to a change in diffusion mechanisms in the porous media at smaller pore radius. Shrinkage modeling with an average diffusion coefficient and with determined viscoelastic parameters from creep tests agreed well compared to the shrinkage data from experiments, indicating that drying shrinkage of cement paste may be considered as a poroviscoelastic reponse.Show more Item Short timescale Brownian motion and applications(2015-08) Mo, Jianyong; Raizen, Mark G.; Downer, Mike; Fiete, Gregory A; Bengtson, Roger D; Kumar, PawanShow more This dissertation details our experiments and numerical calculations on short timescale Brownian motion and its applications. We test the Maxwell-Boltzmann distribution using micrometer-sized spheres in liquids at room temperature. In addition to that, we use Brownian particles as probes to study boundary effects imposed by a solid wall, viscoelasticities of complex fluids, slippage at solid-fluid interfaces, and fluid compressibility. The experiments presented in this dissertation relies on the use of tightly focused laser beams to both contain and probe the Brownian motion of microspheres in fluids. A dielectric sphere near the focus of a laser beam scatters some of the incident photons in a direction which depends on the particle's position. Changes in the particle's position are encoded in the spatial distribution of the scattered beam, which can be measured with high sensitivity. It is important to emphasize that the Brownian motion in this dissertation is exclusive for translational Brownian motion. We have reported shot-noise limited measurements of the instantaneous velocity distribution of a Brownian particle. Our system consists of a single micron-sized glass sphere held in an optical tweezer in a liquid in equilibrium at room temperature. We provide a direct verification of a modified Maxwell-Boltzmann velocity distribution and a modified energy equipartition theorem that account for the kinetic energy of the liquid displaced by the particle. Our measurements con rm the distribution over a dynamic range of more than six orders of magnitude in count-rate and five standard deviations in velocity. We have reported high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately at wall. At short distances, we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of particle in a bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes at at low frequencies. We have numerically studied Brownian motion of a microsphere in complex fluids. We show that Brownian motion of immersed particles can be dramatically affected by the viscoelastic properties of the host fluids. Thus, this fact can be used to extract the properties of complex fluids via observing the motion of the embedded particles. This will be followed by two experimental demonstrations of obtaining the viscosities of water and acetone. We also study Brownian motion with partial and full slip boundary conditions both on the surface of a sphere and a boundary. We show that the motion of particles can be significantly altered by the boundary condition of fluid flow on a solid surface. We suggest that this fact can be used to measure the slippage, namely the slip length. Lastly, I will discuss the efforts to study fluid compressibility and nonequilibrium physics using a short duration pulsed laser. We expect to increase the postion sensitivity from current 10⁻¹⁵ m/[square root of Hz] to about 10⁻¹⁹ m/[ square root of Hz] by using a pulsed laser with a peak power of 10^8 W. With such a high position sensitivity, we expect to be able to resolve the compressibility of fluids. We will also discuss a few future experiments studying non-equilibrium physics.Show more Item Spectral/hp Finite Element Models for Fluids and Structures(2012-07-16) Payette, GregoryShow more We consider the application of high-order spectral/hp finite element technology to the numerical solution of boundary-value problems arising in the fields of fluid and solid mechanics. For many problems in these areas, high-order finite element procedures offer many theoretical and practical computational advantages over the low-order finite element technologies that have come to dominate much of the academic research and commercial software of the last several decades. Most notably, we may avoid various forms of locking which, without suitable stabilization, often plague low-order least-squares finite element models of incompressible viscous fluids as well as weak-form Galerkin finite element models of elastic and inelastic structures. The research documented in this dissertation includes applications of spectral/hp finite element technology to an analysis of the roles played by the linearization and minimization operators in least-squares finite element models of nonlinear boundary value problems, a novel least-squares finite element model of the incompressible Navier-Stokes equations with improved local mass conservation, weak-form Galerkin finite element models of viscoelastic beams and a high-order seven parameter continuum shell element for the numerical simulation of the fully geometrically nonlinear mechanical response of isotropic, laminated composite and functionally graded elastic shell structures. In addition, we also present a simple and efficient sparse global finite element coefficient matrix assembly operator that may be readily parallelized for use on shared memory systems. We demonstrate, through the numerical simulation of carefully chosen benchmark problems, that the finite element formulations proposed in this study are efficient, reliable and insensitive to all forms of numerical locking and element geometric distortions.Show more Item Thermo-Viscoelastic-Viscoplastic-Viscodamage-Healing Modeling of Bituminous Materials: Theory and Computation(2012-10-19) Darabi Konartakhteh, MasoudShow more 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.Show more