# Browsing by Subject "viscoelasticity"

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Item A Micromechanical Model for Viscoelastic-Viscoplastic Analysis of Particle Reinforced Composite(2011-02-22) Kim, Jeong SikShow more This study introduces a time-dependent micromechanical model for a viscoelastic-viscoplastic analysis of particle-reinforced composite and hybrid composite. The studied particle-reinforced composite consists of solid spherical particle and polymer matrix as constituents. Polymer constituent exhibits time-dependent or inelastic responses, while particle constituent is linear elastic. Schapery's viscoelastic integral model is additively combined with a viscoplastic constitutive model. Two viscoplastic models are considered: Perzyna's model and Valanis's endochronic model. A unit-cell model with four particle and polymer sub-cells is generated to obtain homogenized responses of the particle-reinforced composites. A time-integration algorithm is formulated for solving the time-dependent and inelastic constitutive model for the isotropic polymers and nested to the unit-cell model of the particle composites. Available micromechanical models and experimental data in the literature are used to verify the proposed micromechanical model in predicting effective viscoelasticviscoplastic responses of particle-reinforced composites. Filler particles are added to enhance properties of the matrix in the fiber reinforced polymer (FRP) composites. The combined fiber and particle reinforced matrix forms a hybrid composite. The proposed micromechanical model of particle-reinforced composites is used to provide homogenized properties of the matrix systems, having filler particles, in the hybrid composites. Three-dimensional (3D) finite element (FE) models of composite's microstructures are generated for two hybrid systems having unidirectional long fiber and short fiber embedded in cubic matrix. The micromechanical model is implemented at the material (Gaussian) points of the matrix elements in the 3D FE models. The integrated micromechanical-FE framework is used to examine time-dependent and inelastic behaviors of the hybrid composites.Show more Item A multiscale model for predicting damage evolution in heterogeneous viscoelastic media(Texas A&M University, 2004-11-15) Searcy, Chad RandallShow more A multiple scale theory is developed for the prediction of damage evolution in heterogeneous viscoelastic media. Asymptotic expansions of the field variables are used to derive a global scale viscoelastic constitutive equation that includes the effects of local scale damage. Damage, in the form discrete cracks, is allowed to grow according to a micromechanically-based viscoelastic traction-displacement law. Finite element formulations have been developed for both the global and local scale problems. These formulations have been implemented into a two-scale computational model Numerical results are given for several example problems in order to demonstrate the effectiveness of the technique.Show more Item Constitutive Behavior of a Twaron? Fabric/Natural Rubber Composite: Experiments and Modeling(2011-02-22) Natarajan, Valliyappan D.Show more Ballistic fabrics made from high performance polymeric fibers such as Kevlar?, Twaron? and Spectra? fibers and composites utilizing these fabrics are among the leading materials for modern body armor systems. Polymeric fibers used to produce ballistic fabrics often behave viscoelastically and exhibit time- and rate-dependent stress-strain relations. This necessitates the study of the constitutive behavior of composites filled by ballistic fabrics. Rheological models based on discrete rheological components (including spring and dashpot) have been widely used to study the viscoelastic behavior of polymeric fabrics. Such rheological (or viscoelasticity) models are qualitatively useful in understanding the effects of various micro-mechanisms and molecular features on the macroscopic responses of ballistic fabrics. In the present work, the constitutive behavior of Twaron CT709? fabric/natural rubber (Twaron?/NR) composite is studied using three viscoelasticity models (i.e., a four-parameter Burgers model, a three-parameter generalized Maxwell (GMn=1) model, a five-parameter generalized Maxwell (GMn=2) model) and a newly developed para-rheological model. The new model utilizes a three-parameter element to represent the Twaron? fabric and the affine network based molecular theory of rubber elasticity to account for the deformation mechanisms of the NR constituent. The uniaxial stress-strain relation of the Twaron?/NR composite at two constant strain rates is experimentally determined. The values of the parameters involved in all the models are extracted from the experimental data obtained in this study. The stress-relaxation response (under a uniaxial constant strain) and the creep deformation (under a uniaxial constant stress) of the composite are also experimentally measured. The three viscoelasticity models considered here are capable of predicting the viscoelastic constitutive behavior of the composite with different levels of accuracy. The stress-strain relation at each strain rate predicted by the newly developed para-rheological model is seen to be in good agreement with the measured stress-strain curve over the entire strain range studied. It is shown that the new model also predicts the elastic moduli and ultimate stress of the Twaron?/NR composite well. All the four models are found to predict the initial relaxation response of the composite fairly well, while the long-term stress relaxation is more accurately represented by the para-rheological model. An implicit solution provided by the para-rheological model is shown to predict the creep response of the composite more accurately than all the other models at both the primary and secondary stages. The mathematical complexity that arises from including an additional Maxwell element to the GMn=1 model to obtain the GMn=2 model with enhanced predictability is traded with the use of simple characteristic time functions in the para-rheological model. These functions are found to greatly improve the predictability of the newly developed model for the stress relaxation modulus and creep compliance. This study also explores the utility of the para-rheological model as a tool to probe the micromechanisms and molecular features that are causally related to the macroscopically observed viscoelastic behavior of the composite. The relaxation and creep trends predicted by the para-rheological model indicate that the long time viscoelastic response of the composite lies between that of a crosslinked polymer and a semi-crystalline thermoplastic.Show more Item Finite Element Analyses of a Cyclically Loaded Linear Viscoelastic Biodegradable Stent(2014-12-03) Murphy, Jason KyleShow more Biodegradable polymers have been in use for biomedical applications such as sutures and various implants for many years. In the recent decade, research into the development of biodegradable cardiac stents has expanded. This is due to a need for a stent to perform its job and then be removed from the site of implantation, as up to 20% of all cases require re-intervention after 6 ? 12 months due to in-stent restenosis, as reported in 2010. In this study, the effect of viscoelastic and mechanical degradation behaviors on the performance of cylindrical annuli that mimic stents under cyclic loadings is examined. Two polymers are considered: poly-L-lactic acid (PLLA) and polyoxymethylene (POM). A numerical algorithm for an isotropic, linear, viscoelastic material with inclusion of degradation is developed and incorporated into the finite element software ABAQUS/CAE via a user-defined material subroutine (UMAT). A constant pressure meant to mimic the arterial wall?s resistance to expansion is coupled with a cyclic pressure equivalent to an ideal resting blood pressure. The degradation considered is defined as strain-induced. Loading is applied under two cases. A linearly ramped loading is first studied, followed by a creep-cyclic study. Circumferential stresses and strains, along with the degradation, are presented for both a simplified cylindrical annulus, as well as two typical real-world stent geometries. It is seen that not only do material properties affect deformation, but geometrical properties have a large effect as well. The uniform stresses and strains developed in the cylindrical annulus are far less than the non-uniform stresses and strains observed in the realistic stent geometries. This is due to localized stress and strains at the junctions in the mesh of the realistic geometries, where stress concentrations are a maximum. As predicted by the time-dependent material properties given for each material from previous experimental studies, the stresses, strains, and degradation observed are strongly dependent on the time-dependent material behaviors.Show more Item Micromechanics Modeling of Nonlinear and Time-dependent Responses of Piezoelectric 1-3, 0-3, and Hybrid Composites(2014-04-18) Lin, Chien-HongShow more Nonlinear electromechanical and polarization switching behaviors of piezoelectric materials and viscoelastic nature of polymers result in the overall nonlinear and hysteretic responses of active polymeric composites. Understanding the nonlinear behavior of the active polymeric composites is crucial in designing structures comprising of these active materials. This study presents three micromechanical models, i.e., fiber-, particle-, and hybrid-unit-cell models, to study the effective nonlinear and hysteretic electro-mechanical responses of 1-3, 0-3, and hybrid piezocomposites, respectively. The microstructures of the active composites are idealized with periodically distributed arrays of cubic representative unit cells. A unit cell is divided into several subcells. The fiber- and particle-unit-cell models consist of four and eight subcells, respectively. The hybrid-unit-cell model is derived based on the fiber-unit-cell model of 1-3 active composites consisting of fiber and matrix subcells, in which the matrix subcells are comprised of a particle-unit-cell model of 0-3 active composites. In order to obtain the overall nonlinear responses of the active composites linearized micromechanical relations are first used to provide trial solutions followed by iterative schemes in order to correct errors from linearizing the nonlinear responses. The micromechanical predictions are capable in predicting the overall nonlinear electromechanical, time-dependent, and polarization switching responses of active composites available in literature. Parametric studies are also performed to illustrate the effects of microstructural geometry and volume content of the piezoelectric inhomogeneities as well as loading history on the overall nonlinear and hysteretic responses of active composites. Finally, a multi-scale analysis of a functionally graded piezoelectric bimorph actuator using the developed particle-unit-cell model is given as an example of practical applications.Show more Item Thermomechanical Constitutive Modeling of Viscoelastic Materials undergoing Degradation(2012-07-16) Karra, SatishShow more Materials like asphalt, asphalt concrete and polyimides that are used in the transportation and aerospace industry show viscoelastic behavior. These materials in the working environment are subject to degradation due to temperature, diffusion of moisture and chemical reactions (for instance, oxidation) and there is need for a good understanding of the various degradation mechanisms. This work focuses on: 1) some topics related to development of viscoelastic fluid models that can be used to predict the response of materials like asphalt, asphalt concrete, and other geomaterials, and 2) developing a framework to model degradation due to the various mechanisms (such as temperature, diffusion of moisture and oxidation) on polyimides that show nonlinear viscoelastic solid-like response. Such a framework can be extended to model similar degradation phenomena in the area of asphalt mechanics and biomechanics. The thermodynamic framework that is used in this work is based on the notion that the 'natural configuration' of a body evolves as the body undergoes a process and the evolution is determined by maximizing the rate of entropy production. The Burgers' fluid model is known to predict the non-linear viscoelastic fluid-like response of asphalt, asphalt concrete and other geomaterials. We first show that different choices for the manner in which the body stores energy and dissipates energy and satisfies the requirement of maximization of the rate of entropy production that leads to many three dimensional models. All of these models, in one dimension, reduce to the model proposed by Burgers. A thermodynamic framework to develop rate-type models for viscoelastic fluids which do not possess instantaneous elasticity (certain types of asphalt show such a behavior) is developed next. To illustrate the capabilities of such models we make a specific choice for the specific Helmholtz potential and the rate of dissipation and consider the creep and stress relaxation response associated with the model. We then study the effect of degradation and healing due to the diffusion of a fluid on the response of a solid which prior to the diffusion can be described by the generalized neo-Hookean model. We show that a generalized neo-Hookean solid - which behaves like an elastic body (i.e., it does not produce entropy) within a purely mechanical context - creeps and stress relaxes when infused with a fluid and behaves like a body whose material properties are time dependent. A framework is then developed to predict the viscoelastic response of polyimide resins under different temperature conditions. The developed framework is further extended to model the phenomena of swelling due to diffusion of a fluid through a viscoelastic solid using the theory of mixtures. Finally, degradation due to oxidation is incorporated into such a framework by introducing a variable that represents the extent of oxidation. The data from the resulting models are shown to be in good agreement with the experiments for polyimide resins.Show more Item Viscoelastic properties of seed cotton and their effect on module shape and density(Texas A&M University, 2004-11-15) Hardin, Robert GlenShow more Modules for cotton storage and transport should be constructed with a shape that will resist collecting water to maintain the quality of seed cotton during storage. Meeting this specification requires knowledge of the relationship between the applied compressive force, deformation, and time for seed cotton. Several factors were tested to determine their effects on the height and density of seed cotton during compression, creep loading, and recovery. Models were used to describe these processes. These results were used to develop an algorithm capable of providing information on module shape to the module builder operator. The initial loading density did not affect the compressed density, but a slight effect was observed in the recovered density, due to the weight of the seed cotton. Picker harvested cotton was compressed to a greater density than stripper harvested cotton, but expanded more during recovery, resulting in similar final densities. Multiple compressions increased the density, but this increase was not physically significant after the third compression. Higher moisture content increased the density seed cotton could be compressed to slightly. Viscoelastic behavior was observed; however, the effect on density was small. Both the compression and creep curves were described using mathematical models. A compression model using an asymptotic true strain measure yielded high R2 values; however, some aspect of this process remained unexplained and the equation was limited in its predictive ability. Creep behavior was described using a modified Burgers model. This model was more accurate than the creep model, although a definite trend existed in the creep model residuals. A feedback algorithm was developed based on the observation that the compressed density was primarily dependent on the mass of seed cotton and not the initial density. By measuring the compressed depth of cotton in a module and the hydraulic pressure of the tramper foot cylinder, the resulting shape of the module can be predicted. Improved loading of the module builder is necessary to produce a desirably shaped module. More seed cotton needs to be placed in the center of the module, resulting in a surface that slopes down towards the outer edges.Show more