Browsing by Subject "Viscoplastic"
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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 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 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 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.