Browsing by Subject "Viscoelastic"
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Item A time integration scheme for stress - temperature dependent viscoelastic behaviors of isotropic materials(2009-05-15) Khan, Kamran-AhmedA recursive-iterative algorithm is developed for predicting nonlinear viscoelastic behaviors of isotropic materials that belong to the thermorheologically complex material (TCM). The algorithm is derived based on implicit stress integration solutions within a general displacement based FE structural analyses for small deformations and uncoupled thermo-mechanical problems. A previously developed recursive-iterative algorithm for a stress-dependent hereditary integral model which was developed by Haj-Ali and Muliana is modified to include time-temperature effects. The recursive formula allows bypassing the need to store entire strain histories at each Gaussian integration point. Two types of iterative procedures, which are fixed point and Newton-Raphson methods, are examined within the recursive algorithm. Furthermore, a consistent tangent stiffness matrix is formulated to accelerate convergence and avoid divergence. The efficiency and accuracy of the proposed algorithm are evaluated using available experimental data and several structural analyses. The performance of the proposed algorithm under multi-axial conditions is verified with analytical solutions of creep responses of a plate with a hole. Next, the recursive-iterative algorithm is used to predict the overall response of single lap-joint. Numerical simulations of time-dependent crack propagations of adhesive bonded joints are also presented. For this purpose, the recursive algorithm is implemented in cohesive elements. The numerical assessment of the TCM and thermorheologically simple material (TSM) behaviors has also been performed. The result showed that TCM are able to describe thermo-viscoelastic behavior under general loading histories, while TSM behaviors are limited to isothermal conditions. The proposed numerical algorithm can be easily used in a micromechanical model for predicting the overall composite responses. Examples are shown for solid spherical particle reinforced composites. Detailed unit-cell FE models of the composite systems are generated to verify the capability of the above micromechanical model for predicting the overall nonlinear viscoelastic behaviors.Item A Viscoelastic-Viscoplastic Analysis of Fiber Reinforced Polymer Composites Undergoing Mechanical Loading and Temperature Changes(2013-08-09) Jeon, JaehyeukThis study presents a combined viscoelastic (VE)-viscoplastic (VP) analysis for Fiber Reinforced Polymer (FRP) composites subject to simultaneous mechanical load and conduction of heat. The studied FRP composites consist of unidirectional fibers, which are considered as linearly elastic with regards to their mechanical response, and isotropic polymeric matrix, which shows viscoelastic-viscoplastic response under various stresses and temperatures. Due to the viscoelastic and viscoplastic behavior of the polymeric matrix, the overall FRP composites exhibit a combined time-dependent and inelastic behavior. A simplified micromechanical model, consisting of a unit-cell with four fiber and matrix subcells, is formulated to homogenize the overall heat conduction and viscoelastic-viscoplastic responses of the FRP composites. The micromechanical model is compatible with a displacement based finite element (FE) and is implemented at the Gaussian integration points within the continuum finite elements, which is useful for analyzing the overall time-dependent response of FRP composite structures under various boundary conditions. The Schapery nonlinear integral model combined with the Perzyna viscoplastic model is used to describe the viscoelastic-viscoplastic response of the polymer constituents. An integrated time integration algorithm is formulated at the micromechanics level in order to solve the nonlinear viscoelastic-viscoplastic constitutive model at the matrix subcells and obtain the overall nonlinear response of the FRP. The viscoelastic-viscoplastic micromechanical model is validated usingexperimental data on off-axis glass/epoxy FRP composites available in literature. The overall response of the FRP composites determined from the simplified micromechanical model is also compared with the ones generated from microstructures of FRP with various fiber arrangements dispersed in homogeneous polymer matrix. The microstructural models of the FRP with detailed fiber arrangements are generated using FE. The effects of thermal stresses, due to the mismatches in the coefficient of thermal expansions of the fibers and polymeric matrix, and stress concentrations/discontinuities near the fiber and matrix interfaces on the overall thermo-mechanical deformation of FRP composites are studied using the two micromechanical models discussed above. Finally, an example of structural analysis is performed on a polymeric smart sandwich composite beam, having FRP skins and polymeric foam core with piezoelectric sensors integrated to the FRP skins, undergoing three point bending at an elevated temperature. The creep displacement is compared to experimental data available in literature.Item Characterization of asphalt concrete using anisotropic damage viscoelastic-viscoplastic model(Texas A&M University, 2007-04-25) Abdel-Rahman Saadeh, ShadiThis dissertation presents the integration of a damage viscoelastic constitutive relationship with a viscoplastic relationship in order to develop a comprehensive anisotropic damage viscoelastic-viscoplastic model that is capable of capturing hot mix asphalt (HMA) response and performance under a wide range of temperatures, loading rates, and stress states. The damage viscoelasticity model developed by Schapery (1969) is employed to present the recoverable response, and the viscoplasticity model developed at the Texas Transportation Institute (TTI) is improved and used to model the irrecoverable strain component. The influence of the anisotropic aggregate distribution is accounted for in both the viscoelastic and viscoplastic responses. A comprehensive material identification experimental program is developed in this study. The experimental program is designed such that the quantification and decomposition of the response into viscoelastic and viscoplastic components can be achieved. The developed experimental program and theoretical framework are used to analyze repeated creep tests conducted on three mixes that include aggregates with different characteristics. An experiment was conducted to capture and characterize the three-dimensional distribution of aggregate orientation and air voids in HMA specimens. X-ray computed tomography (CT) and image analysis techniques were used to analyze the microstructure in specimens before and after being subjected to triaxial repeated creep and recovery tests as well as monotonic constant strain rate tests. The results indicate that the different loading conditions and stress states induce different microstructure distributions at the same macroscopic strain level. Also, stress-induced anisotropy is shown to develop in HMA specimens.Item Characterization of thermo-mechanical and long-term behaviors of multi-layered composite materials(2009-06-02) Nair, Aravind R.This study presents characterization of thermo-mechanical viscoelastic and long-term behaviors of thick-section multi-layered fiber reinforced polymer composite materials. The studied multi-layered systems belong to a class of thermo-rheologically complex materials, in which both stress and temperature affect the time-dependent material response. The multi-layered composites consist of alternating layers of unidirectional fiber (roving) and randomly oriented continuous filament mat. Isothermal creep-recovery tests at various stresses and temperatures are performed on E-glass/vinylester and Eglass/ polyester off-axis specimens. Analytical representation of a nonlinear single integral equation is applied to model the thermo-mechanical viscoelastic responses for each off-axis specimen. Long-term material behaviors are then obtained through vertical and horizontal time shifting using analytical and graphical shifting procedures. Linear extrapolation of transient creep compliance is used to extend the material responses for longer times. The extended long-term creep strains of the uniaxial E-glass/vinylester specimens are verified with the long-term experimental data of Scott and Zureick (1998). A sensitivity analyses is then conducted to examine the impact of error in material parameter characterizations to the overall long-term material behaviors. Finally, the calibrated long-term material parameters are used to study the long-term behavior of multi-layered composite structures. For this purpose, an integrated micromechanical material and finite element structural analyses is employed. Previously developed viscoelastic micromodels of multi-layered composites are used to generate the effective nonlinear viscoelastic responses of the studied composite systems and then implemented as a material subroutine in Abaqus finite element code. Several long-term composite structures are analyzed, that is; I-shaped columns and flat panels under axial compression, and a sandwich beam under the point bending and transmission tower under lateral forces. It is shown that the integrated micromechanical-finite element model is capable of predicting the long-term behavior of the multilayered composite structures.Item Constitutive modeling of viscoelastic behavior of bituminous materials(2012-12) Motamed, Arash; Bhasin, AmitAsphalt mixtures are complex composites that comprise aggregate, asphalt binder, and air. Several research studies have shown that the mechanical behavior of the asphalt mixture is strongly influenced by the matrix, i.e. the asphalt binder. Therefore, accurate constitutive models for the asphalt binders are critical to ensure accurate performance predictions at a material and structural level. However, researchers who use computational methods to model the micromechanics of asphalt mixtures typically assume that (i) asphalt binders behave linearly in shear, and (ii) either bulk modulus or Poisson’s ratio of asphalt binders is not time dependent. This research develops an approach to measure and model the shear and bulk behavior of asphalt binders at intermediate temperatures. First, this research presents the findings from a systematic investigation into the nature of the linear and nonlinear response of asphalt binders subjected to shear using a Dynamic Shear Rheometer (DSR). The DSR test results showed that under certain conditions a compressive normal force was generated in an axially constrained specimen subjected to cyclic torque histories. This normal force could not be solely attributed to the Poynting effect and was also related to the tendency of the asphalt binder to dilate when subjected to shear loads. The generated normal force changed the state of stress and interacted with the shear behavior of asphalt binder. This effect was considered to be an “interaction nonlinearity” or “three dimensional effect”. A constitutive model was identified to accommodate this effect. The model was successfully validated for several different loading histories. Finally, this study investigated the time-dependence of the bulk modulus of asphalt binders. To this end, poker-chip geometries with high aspect ratios were used. The boundary value problem for the poker-chip geometry under step displacement loading was solved to determine the bulk modulus and Poisson’s ratio of asphalt binders as a function of time. The findings from this research not only improve the understanding of asphaltic materials behavior, but also provide tools required to accurately predict pavement performance.Item A fracture mechanics approach to accelerated life testing for cathodic delamination at polymer/metal interfaces(2013-05) Mauchien, Thomas Kevin; Liechti, K. M.This work presents a fracture mechanics analysis of the cathodic delamination problem for the polyurethane/titanium and polyurea/steel interfaces. The nonlinear behavior of both polymers was investigated. The recent Marlow model was used to define the strain energy function of the polymers. Viscoelastic effects of the polyurea were also studied. The Marlow model was associated with a nine-term Prony series. This model was seen to represent experimental data relatively well for a wide range of strain rates both in tension and compression. The driving force for delamination, the strain energy release rate G, is presented for both interfaces. Cathodic delamination data for several temperatures are presented as crack growth rate as a function of crack driving force. The approach recognizes that both temperature and stress can be used as accelerated life testing parameters.Item High frequency microrheology with optical tweezers(2015-08) Riegler, David; Raizen, Mark G.; Fink, ManfredThis thesis presents a method to measure the linear viscoelastic response of fluids by tracking and analyzing the thermal, Brownian motion of suspended tracer particles, known as passive microrheology. The particle is confined in a harmonic optical trap and its one dimensional trajectory is obtained by a home-built split beam detection system, which works similar but responds faster than position detection with commercial quadrant photodiodes. The theory which is necessary to convert the particle trajectory into the complex shear modulus is derived in detail, pointing out that the commonly used Mason-Weitz method needs to be modified in order to obtain correct results at high frequencies due to hydrodynamic effects of the fluid. It follows a detailed explanation of the data analysis procedure which is verified for water up to angular frequencies of 10⁷ rad/s in very good agreement with the theory. Finally, there is an outlook how to apply the method to actual complex fluids.Item Impact of viscoelastic polymer flooding on residual oil saturation in sandstones(2013-12) Ehrenfried, Daniel Howard; Balhoff, Matthew T.The objective of this research was to determine whether the use of polymer compounds with elastic properties can reduce residual oil saturation in porous media below that of brine or inelastic polymerized solutions. One hypothesis is that long-chain polymer molecules experience stress and a resulting strain when they flow through pore throat constrictions. If the fluid residence time in larger pore spaces is insufficient to allow full relaxation, then strain can accumulate. Sufficient strain results in normal forces which can impinge on oil interfaces and potentially mobilize them. A second hypothesis suggests that polymerized solutions can temporarily protect flowing oil filaments from snap off, allowing them to flow longer and de-saturate further than they would otherwise. The approach taken in this thesis was to conduct a series of core floods in several different sandstones using displacement fluids with elasticity ranging from none to those with extremely high relaxation times. Accelerated flow rate was also employed to reduce residence time and maximize the accumulation of elastic strain and normal force potential. Experiments were designed to provide direct comparisons between both non-elastic and elastic floods but also multiple floods with increasing elasticity. The results were inconclusive with some experiments showing additional oil recovery that could be attributed to elastic mechanisms. Most experiments, however, showed no significant difference between elastic and non-elastic floods when experimental parameters were controlled within narrow limits. This research did refine the experimental context in which elastic effects are most likely to be observed. As such, it can serve as a precursor to additional core flooding in oil-wet systems, experiments conducted at reservoir temperature, and those where the pressure gradient of the flood is held constant and the flow rate allowed to vary. Computer aided tomography could also be employed to visualize the mobilization of oil with different displacement fluids, identify where bypassed oil occurs with unstable floods, and determine how oil is subsequently mobilized with better conformance and or elasticity.Item Microstructure-based Computational Modeling of the Mechanical Behavior of Polymer Micro/Nano-composites(2013-08-26) Heydarkhan Tehrani, ArdeshirThis dissertation is devoted to the virtual investigation of the mechanical behavior of micro/nano polymer composites (MNPCs). Advanced composite materials are favored by the automotive industry and army departments for their customizable tailored properties, especially for strength and ductility compared to pure polymer matrices. Their light weight and low finished cost are additional advantages of these composite materials. Many experimental and numerical studies have been performed to achieve the optimized behavior of MNPCs by controlling the microstructure. Experiments are costly and time consuming for micro scale. Hence, recently numerical tools are utilized to help the material scientists to customize and optimize their experiments. Most of such numerical studies are based on characterizing the MNPCs through simple microstructures, as circular particles or straight fibers embedded in a specific polymer matrix. Although these geometries are effective in virtual modeling some types of composite material behavior, they fail to address some critical key micro-structural features, which are important for our goals. Firstly, they fail to properly address the randomness of particles. Secondly, 2D analyses have limitations and they can provide qualitative insight, rather than evaluate the quantitative response of the material behavior. Thus, in order to fill this gap, a user friendly software program, REV_Maker, is developed in this project for generating 2D and 3D RVEs (representative volume elements) to precisely represent the morphology of material in microstructural level. In models, polymers are usually considered as viscoelastic-viscoplastic or hyperelastic-viscoplastic materials without taking into account viscodamage models. Therefore, in this work rate- and time-dependent damage (viscodamage) is separately considered to fully investigate the initiation and growth of damage inside polymer composites. Besides, most of the common viscoelastic and viscoplastic models assumes small deformation; therefore, in this dissertation a procedure is established, which incorporates all required modifications to generalize a small strain constitutive model to its identical large deformation range. Thus, here a straightforward generalization and implementation method based on classical continuum mechanics is proposed, which due to its simplicity, can be applied to a wide range of elastoplastic constitutive models. Then, the available viscoelastic and viscoplastic models are extended to large strain framework. By applying the generalized viscous models, one may address and measure the large deformation response of MNPCs. Numerous simulations were conducted to predict the overall responses of micro/nano composites with different morphologies (particles volume fractions, orientations, and combinations). The effect of each particle, and the combination of particles on the composite responses are compared and presented.Item Modeling of the Aging Viscoelastic Properties of Cement Paste Using Computational Methods(2012-07-16) Li, XiaodanModeling of the time-dependent behavior of cement paste has always been a difficulty. In the past, viscoelastic behavior of cementitious materials has been primarily attributed to the viscoelastic properties of C-S-H components. Recent experimental results show that C-S-H may not exhibit as much creep and relaxation as previously thought. This requires new consideration of different mechanisms leading to the viscoelastic behavior of cement paste. Thus the objective of this thesis is to build a computational model using finite element method to predict the viscoelastic behavior of cement paste, and using this model, virtual tests can be carried out to improve understanding of the mechanisms of viscoelastic behavior. The primary finding from this thesis is that the apparent viscoelastic behavior due to dissolution of load bearing phases is substantial. The dissolution process occurring during the hydration reaction can change the stress distribution inside cementitious materials, resulting in an apparent viscoelastic behavior of the whole cementitious materials. This finding requires new consideration of mechanisms of time-dependent behavior of cementitious materials regarding the dissolution process of cement paste.Item Pore-scale modeling of viscoelastic flow and the effect of polymer elasticity on residual oil saturation(2014-12) Afsharpoor, Ali; Balhoff, Matthew T.Polymers used in enhanced oil recovery (EOR) help to control the mobility ratio between oil and aqueous phases and as a result, polymer flooding improves sweep efficiency in reservoirs. However, the conventional wisdom is that polymer flooding does not have considerable effect on pore-level displacement because pressure forces would not be enough to overcome trapping caused by capillary forces. Recently, both coreflood experiments and field data suggest that injecting viscoelastic polymers, such as hydrolyzed polyacrylamide (HPAM), can result in lower residual oil saturation. The hypothesis is that the polymer elasticity provides several pore-level mechanisms for oil mobilization that are generally not significant for purely-viscous fluids. Both experiments and modeling need to be performed to investigate the effect of polymer elasticity on residual oil saturation. Pore-scale modeling and micro-fluidic experiments can be used to investigate pore-level physics, and then used to upscale to the macro-scale. The objective of this work is to understand the effect of polymer elasticity on apparent viscosity and residual oil saturation in porous media. Single- and multi-phase pore-level computational fluid dynamics (CFD) modeling for viscoelastic polymer flow is performed to investigate the dominant mechanisms at the pore level to mobilize trapped oil. Several interesting results are found from the CFD results. First, the elasticity of the polymer results in an increase in normal stress at the pore-level; therefore, the normal stresses exerted on a static oil droplet are significant and not negligible as for a purely-viscous fluid. The CFD results show that viscoelastic fluid exerts additional forces on the oil-phase which may help mobilize trapped oil out of the porous medium. Second, due to the elasticity of polymer, the viscoelastic polymer has some level of pulling effect; while passing above a dead-end pore it can pull out the trapped oil phase and then mobilize it. However, both CFD modeling and micro-fluidic experiments show the pulling-effect is not likely the main mechanism to reduce oil saturation at pore-level. Third, dynamic CFD simulations show less deformation of the oil phase while viscoelastic polymer is displacing fluid compared to purely viscous fluid. It may justify the hypothesis that polymer elasticity resists against snap-off mechanism. As a result, when viscoelastic polymer displaces the oil ganglia, the oil phase does not snap off, and the oil phase remains connected, and therefore easier to move in porous media compared to disconnected oil. For single phase flow, a closed-form flow equation has been developed based on CFD modeling in converging/diverging ducts representative of pore throats. The pore-level equations were substituted into a pore-network model and validated against experimental data. Good agreement is observed. This study reveals important findings about the effect of polymer elasticity to reduce the residual oil saturation; however, more experiments and simulations are recommended to fully-understand the mobilization mechanisms and take advantage of them to optimize the polymer-flooding process in the field.Item Rheological Responses of Polymer Materials with Different Architectures(2011-08) Hu, Miao; McKenna, Gregory B.; Quitevis, Edward L.; Hedden, Ronald C.; Weeks, Brandon L.Polymer rheology is a sensitive method to determine the change in the molecular chemical structure, size, architecture, and composition for polymer materials. In this dissertation, we focus on the influence of the molecular architecture and composition to the rheological responses of some polymer with unique architecture. The first part of the work is to characterize the macrocyclic polymer materials prepared by the Ring Expansion Metathesis Polymerization (REMP) method. In this work, we tried different methods to get pure ring samples, examine their viscoelastic properties and compare the results with theoretical predictions. Four kinds of polymer rings (polyoctenamer, polyammonium, dendronized and wedge ring) were synthesized using two catalysts UC5 and UC6. However, limited by the reaction mechanism of the catalyst, we still can’t get 100% pure ring polymer with current reaction system. It is found that for monomers with simple structures, the samples prepared by the two catalysts contain at least 20% of linear chain contamination. Even we change the structure of the monomer and the use the macromonomer to increase the steric hindrance to help generate ring samples. The samples still contain certain amount of linear chain contamination. The relaxation behaviors of these ring/linear mixtures and some literature reported ring samples were analyzed by the retardation spectrum. It is found that polymer rings may reptate like the linear chains. Their relaxation time scales following what is predicted by the reptation model. Linear chain contamination will increase the viscosity, radius of gyration and plateau modulus of the polymer ring samples. With linear chain contamination, the “threading effect” will slow down the relaxation of polymer rings. But when the amount of linear chain contamination is high, the mixtures still relax following the reptation model. In the second part, the rheological responses of a new kind of branched polymer with well defined side chain structure and branching space were studied. The branched polymer was prepared by the Ring Opening Metathesis Polymerization (ROMP) of norbornane based macromonomers linked with different length or composition of side groups. Due to the spacing effect of the side chains, these branched polymers are hard to get entangled. For PLA brush polymers, with increasing side chain length, similar double relaxation behaviors were found as those long chain branched polymers. The brush polymers relax following the sequence of the segmental relaxation first, then the side chain (arm) relaxation and the terminal (whole chain) relaxation. With such high molecular weight, the samples are still not well entangled. The plateau found for the relaxation of the backbone is not the rubbery plateau and is actually the inverse value of the steady state recoverable compliance. For wedge polymer with enough high backbone DP, the samples can get entangled and reptate as a common linear chain. But influenced by the huge side chain structure, the polymer exhibits a much lower rubbery and glassy modulus. The large volume fraction of side chain strongly influenced the glassy behaviors of the brush polymer. The PLA brush, wedge and dendronized polymer have the same backbone, but their glass transition behaviors are strongly affected by the steric hindrance introduced by the side chain structures.Item Stretch-induced wrinkling of thin sheets(2013-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Ravi-Chandar, K.Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles.Item Viscoelastic{Viscoplastic Damage Model for Asphalt Concrete(2010-10-12) Graham, Michael A.This thesis presents a continuum model for asphalt concrete incorporating non- linear viscoelasticity, viscoplasticity, mechanically-induced damage and moisture- induced damage. The Schapery single-integral viscoelastic model describes the nonlinear viscoelastic response. The viscoplastic model of Perzyna models the time- dependent permanent deformations, using a Drucker-Prager yield surface which is modified to depend on the third deviatoric stress invariant to include more complex dependence on state of stress. Mechanically-induced damage is modeled using continuum damage mechanics, using the same modified Drucker-Prager law to determine damage onset and growth. A novel moisture damage model is proposed, modeling moisture-induced damage using continuum damage mechanics; adhesive moisture- induced damage to the asphalt mastic-aggregate bond and moisture-induced cohesive damage to the asphalt mastic itself are treated separately. The analytical model is implemented numerically for three-dimensional and plane strain finite element analyses, and a series of simulations is presented to show the performance of the model and its implementation. Sensitivity studies are conducted for all model parameters and results due to various simulations corresponding to laboratory tests are presented. In addition to the continuum model, results are presented for a micromechanical model using the nonlinear-viscoelastic-viscoplastic-damage model for asphalt mastic and a linear elastic model for aggregates. Initial results are encouraging, showing the strength and stiffness of the mix as well as the failure mode varying with moisture loading. These initial results are provided as a an example of the model's robustness and suitability for modeling asphalt concrete at the mix scale.