Browsing by Subject "Viscoplasticity"
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Item Anisotropic Characterization of Asphalt Mixtures in Compression(2012-10-08) Zhang, Yuqing 1983-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.Item Damage initiation, progression and failure of polymer matrix composites due to manufacturing induced defects(Texas A&M University, 2007-09-17) Chowdhury, Khairul AlamIn polymer matrix composites (PMCs) manufacturing processes can induce de- fects, e.g., voids, fiber misalignment, irregular fiber distribution in the cross-section and broken fibers. The effects of such defects can be beneficial or deleterious de- pending on whether they cause failure suppression or enhancement by localized de- formation processes e.g., crazing, shear yielding and fiber-matrix debonding. In this study, a computational approach is formulated and implemented to develop solu- tions for general boundary-value problems for PMC microstructures that accounts for micromechanics-based constitutive relations including fine scale mechanisms of material failure. The defects considered are voids, and the microstructure is explic- itly represented by a distribution of fibers and voids embedded in a polymer matrix. Fiber is modeled as a linearly elastic material while the polymer matrix is mod- eled as an elastic-viscoplastic material. Two distinct models for the matrix behavior are implemented: (i) Drucker??????Prager type Bodner model that accounts for rate and pressure-sensitivity, and (ii) improved macromolecular constitutive model that also accounts for temperature dependence, small-strain softening and large-strain harden- ing. Damage is simulated by the Gearing-Anand craze model as a reference model and by a new micromechanical craze model, developed to account for craze initiation, growth and breakdown. Critical dilatational energy density criterion is utilized to predict fiber-matrix debonding through cavitation induced matrix cracking. An extensive parametric study is conducted in which the roles of void shape, size and distribution relative to fiber in determining damage initiation and evolution are investigated under imposed temperature and strain rate conditions. Results show there are significant effects of voids on microstructural damage as well as on the overall deformational and failure response of composites.Item Effect of electro-mechanical loading in metallic conductors(2010-12) Gallo, Federico Guido; Ravi-Chandar, K.; Mear, Mark E.; Satapathy, Sikhanda S.; Liechti, Kenneth M.; Landis, Chad M.The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with viii mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor. In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results.Item Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory Alloys(2010-07-14) Kumar, Parikshith K.Most e orts on High Temperature Shape Memory Alloys (HTSMAs), have focused on improving their work characteristics by thermomechanical treatment methods. However, the in uence of transformation induced plasticity (TRIP) and viscoplasticity during actuation has not been studied. The objective of this dissertation work was to study the in uence of plasticity and viscoplasticity on the transformation characteristics that occur during two common actuation-loading paths in TiPdNi HTSMAs. Thermomechanical tests were conducted along di erent loading paths. The changes in the transformation temperature, actuation strain and irrecoverable strain during the tests were monitored. Transmission Electron Microscopy (TEM) studies were also conducted on select test specimens to understand the underlying microstructural changes. The study revealed that plasticity, which occurs during certain actuation load paths, alters the transformation temperatures and/or the actuation strain depending on the loading path chosen. The increase in the transformation temperature and the irrecoverable strain at the end of the loading path indicated that the rate independent irrecoverable strain results in the generation of localized internal stresses. The increased transformation temperatures were mapped with an equivalent stress (which corresponds to an internal stress) using the as-received material's transformation phase diagram. A trend for the equivalent internal stress as a function of the applied stress and accumulated plastic strain was established. Such a function can be implemented into thermomechanical models to more accurately capture the behavior of HTSMAs during cyclic actuation. On the contrary, although the viscoplastic strain generated during the course of constant stress thermal actuation could signi cantly reduce actuation strain depending on the heating/cooling rate. Additional thermomechanical and microstructural tests revealed no signi cant change in the transformation behavior after creep tests on HTSMAs. Comparing the thermomechanical test results and TEM micrographs from di erent cases, it was concluded that creep does not alter the transformation behavior in the HTSMAs, and any change in the transformation behavior can be attributed to the retained martensite which together with TRIP contributes to the rate independent irrecoverable strain. As a consequence, a decrease in the volume fraction of the martensite contributing towards the transformation must be considered in the modeling.Item Thermo-Viscoelastic-Viscoplastic-Viscodamage-Healing Modeling of Bituminous Materials: Theory and Computation(2012-10-19) Darabi Konartakhteh, MasoudTime- 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.