Browsing by Subject "Creep"
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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 creep of single crystal superalloys(Texas A&M University, 2006-10-30) Prasad, Sharat ChandIn this work, a constitutive theory is developed, within the context of continuum mechanics, to describe the creep deformation of single crystal superalloys. The con- stitutive model that is developed here is based on the fact that as bodies deform the stress free state that corresponds to the current configuration (referred to as the "natural configuration", i.e., the configuration that the body would attain on the removal of the external stimuli) evolves. It is assumed that the material possesses an infinity of natural (or stress-free) configurations, the underlying natural configuration of the body changing during the deformation process, with the response of the body being elastic from these evolving natural configurations. It is also assumed that the evolution of the natural configurations is determined by the tendency of the body to undergo a process that maximizes the rate of dissipation. Central to the theory is the prescription of the forms for the stored energy and rate of dissipation functions. The stored energy reflects the fact that the elastic response exhibits cubic symmetry. Consistent with experiments, the elastic response from the natural configuration is assumed to be linearly elastic and the model also takes into account the fact that the symmetry of single crystals does not change with inelastic deformation. An ap- propriate form for the inelastic stored energy (the energy that is `trapped' within dislocation networks) is also utilized based on simple ideas of dislocation motion. In lieu of the absence of any experimental data to corroborate with, the form for the inelastic stored energy is assumed to be isotropic. The rate of dissipation function is chosen to be anisotropic, in that it reflects invariance to transformations that belong to the cubic symmetry group. The rate of dissipation is assumed to be proportional to the density of mobile dislocations and another term that takes into account the damage accumulation due to creep. The model developed herein is used to simulate uniaxial creep of <001>, <111> and <011> oriented single crystal nickel based su- peralloys for a range of temperatures. The predictions of the theory match well with the available experimental data for CMSX-4. The constitutive model is also imple- mented as a User Material (UMAT) in commercial finite element software ABAQUS to enable the analysis of more general problems. The UMAT is validated for simple problems and the numerical scheme based on an implicit backward difference formula works well in that the results match closely with those obtained using a semi-inverse approach.Item Creep and dynamic abnormal grain growth of commercial-purity molybdenum(2005-08) Ciulik, James R.; Taleff, Eric M.In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed.Item Energy Piles in Cooling Dominated Climates(2014-04-10) Akrouch, GhassanAir pollution is one of the main environmental problems mankind faces in the 21^(st) century caused by to the extensive use of fossil fuels. One of the opportunities to overcome this problem is to develop new technologies and methods to profit from the energy stored in the ground. A promising high-efficiency technology for the thermal control of buildings is the shallow geothermal energy. This technology is growing rapidly because it consumes less conventional energy for operation, which in turn results in fewer CO_(2) emissions. This technology harnesses constant and moderate ground temperature for thermal control of a building using foundation piles. Outside air temperature changes with the season, while ground temperature remains moderate and constant. In summer, ground temperature is lower than air temperature, and so the ground may be used as a heat sink. The opposite is true in winter; the ground becomes a heat source. This technology is used efficiently in cold, heating dominated climates. Could this be true in hot, cooling dominated climates? To achieve the ultimate goal and answer the above question, this study considered the different elements of a full SGES, namely: soil, climate, energy pile, and ground source heat pump. First, The need for a new, easy, and quick in-situ method to thermally characterize soils lead to the development of the Thermal Cone Test. Second, the soil-climate interaction and its effect on the thermodynamic efficiency of energy piles was an important factor to consider, where the decrease in soil saturation leads to a decrease in the heat exchange rate of energy piles. Third, the thermal use of foundation pile changes the pile and surrounding soil temperature where both materials are temperature dependent. This change in temperature leads to a change in the mechanical behavior of energy piles. Fourth, a full-scale test on installed and instrumented energy piles group was needed to understand the thermodynamics of a full system and to provide experimental data for a full economic study. Finally, this study was capped by an economic analysis to evaluate the cost, benefits, payback period, and feasibility of SGES in cooling dominated climates. The study presented in this dissertation found that integrating energy piles in heating and cooling systems in hot, cooling dominated climates could be economical and environmentally friendly solution, but attention should be paid to the thermodynamic efficiency of the system when unsaturated soil layer is encountered, and to the long term mechanical behavior of foundation piles in high plasticity clay where additional settlement could take place resulting from the increased creep rate caused by soil heating.Item In defense of the Slow Creep(2015-05) Hickcox, James Wesley Lloyd; Shea, Andrew Brendan; Lewis, Deborah; Glavan, James; Kelban, StuartMy thesis as an MFA candidate, "Slow Creep", is a narrative short film about teens in the 90s renting a film and accidentally summoning a monster. Through this paper I intend to both touch on the process of its creation and defend it as an artistic piece worthy of bestowing on me the title: "Master of Fine Arts". I will also briefly discuss the history of my work, and the philosophical concepts that shape my approach to creating (and ingesting) media.Item Mechanical and thermal properties of kenaf/polypropylene nonwoven composites(2013-05) Hao, Ayou; Chen, Jonathan Yan; Koo, Joseph H.; Kovar, Desiderio; Krifa , Mourad; Shi, Li; Xu, BugaoThe objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics.Item Model of strain-related prestress losses in pretensioned simply supported bridge girders(2014-05) Gallardo Méndez, José Manuel; Bayrak, Oguzhan, 1969-Prestressed concrete construction relies on the application of compressive stresses to concrete elements. The prestressing force is typically applied through the tensioning of strands that react against the concrete and induce compression in the concrete. Loss of prestress is the decrease of this pre-applied stress. The conservative estimation of the prestress losses is imperative to prevent undesired cracking of the prestressed element under service loads. A large fraction of the prestress losses is a consequence of concrete deformations. This fraction of the losses can be identified as strain-related losses, and these occur due to instantaneous elastic shortening, and time-dependent creep and shrinkage. Creep and shrinkage of concrete depend on many factors that are extremely variable within concrete structures. The time-dependent behavior of concrete is not well-understood, but recent findings in the topics of concrete creep and shrinkage provide a better understanding of the underlying mechanisms affecting the nature of these two phenomena. However, current design practices and prestress loss estimation methods do not reflect the state-of-the-art knowledge regarding creep and shrinkage. The main objective of this dissertation was the study and estimation of strain-related prestress losses in simply supported pretensioned bridge girders. Simply supported pretensioned girders are widely designed, produced and frequently used in bridge construction. Due to this common use, pretensioned concrete bridge girders has become fairly standardized elements, which results in a reduced variability in the behavior of pretensioned bridge girders, as compare to that of less standardized concrete structures. Hence, a simplified method was calibrated to estimate prestress losses within pretensioned girders to an adequate level of accuracy. To achieve an acceptable accuracy experimental data from the monitoring of pretensioned simply supported girders was used for the calibration of the method. The accuracy of this simplified method is comparable to that achievable using more elaborate methods developed for generic concrete structures.Item Short-term and time-dependent stresses in precast network arches(2015-08) Yousefpoursadatmahalleh, Hossein; Helwig, Todd Aaron, 1965-; Bayrak, Oguzhan, 1969-; Jirsa, James O; Williamson, Eric B; Mear, Mark EDue to their structural efficiency and architectural elegance, concrete arches have long been used in bridge applications. However, the construction of concrete arches requires significant temporary supporting structures, which prevent their widespread use in modern bridges. A relatively new form of arch bridges is the network arch, in which a dense arrangement of inclined hangers is used. Network arches are subjected to considerably smaller bending moments and deflections than traditional arches and are therefore suitable for modern, accelerated construction methods in which the arches are fabricated off-site and then transported to the bridge location. However, service-level stresses, which play a critical role in the performance of the structure, are relatively unknown for concrete network arches and have not been sufficiently investigated in the previous research on concrete arches. The primary objective of this dissertation is to improve the understanding of short-term and time-dependent stresses in concrete arches, and more specifically, concrete network arches. The research presented herein includes extensive field monitoring of the West 7th Street Bridge in Fort Worth, Texas, which is the first precast network arch bridge and probably the first concrete network arch bridge in the world. The bridge consists of twelve identically designed concrete network arches that were precast and post-tensioned before they were transported to the bridge site and erected. A series of vibrating wire gages were embedded in the arches and were monitored throughout the construction and for a few months after the bridge was opened to traffic. The obtained data were processed, and structural response parameters were evaluated to support the safe construction of the innovative arches, identify their short-term and time-dependent structural behavior, and verify the modeling assumptions. The variability of stresses among the arches was also used to assess the reliability of stress calculations. The results of this study provide valuable insight into the elastic, thermal, and time-dependent behavior of concrete arches in general and concrete network arches in particular. The knowledge gained in this investigation also has broader applications towards understanding the behavior of indeterminate prestressed concrete structures that are subjected to variable boundary conditions and thermal and time-dependent effects.Item Structural recovery and physical aging in polymer glasses in plasticizing environments(Texas Tech University, 2006-12) Banda, Lameck; McKenna, Gregory B.; Quitevis, Edward L.; Dai, Lenore L.; Simon, Sindee L.Small molecule plasticizers impart significant effects on the viscoelastic and mechanical responses of polymer glasses. The effects of plasticization on the structural recovery and mechanical responses of polymer glasses have been investigated using carbon dioxide as a small molecule plasticizer and an epoxy resin and polystyrene as model polymer glasses. This work reports the first physical aging results on a polymer glass subsequent to carbon dioxide pressure jumps. Also reported are the first structural recovery results of a polymer glass subsequent to small molecule plasticizer jumps. Consistent with the hypothesis that for a polymer glass, a change in plasticizer concentration is similar to a change in temperature, the three signatures of structural recovery; intrinsic isopiestics, asymmetry of approach, and the memory effect were constructed and shown to be qualitatively similar to those observed after temperature jumps. However, quantitative and anomalous differences were also observed. This work was modeled and illustrated the limitations of current phenomenological models. Subsequent work showed that, in fact, concentration glasses are fundamentally different from temperature-hyperquenched glasses. An attempt to investigate the real-time monitoring of mass change during the structural evolution of a polymer glass was made. The results report serious fundamental errors in the use of piezoelectric devices for the investigation of mass changes in compliant materials. Since these devices have been used as sensitive mass sensing devices, these results exposed their limitations. A semi-quantitative model of errors induced is also presented. The anomalous differences reported above led to the motivation to complete the thermodynamic surface of polymer glasses by studying the calorimetric response after carbon dioxide pressure jumps. To date there are no results reported in the literature on a direct measurement of the enthalpy recovery for glassy polymers subsequent to changes in temperature or small molecule concentration. This work reports the first measurements of the enthalpy recovery responses of polymer glasses subsequent to plasticizer concentration changes by measuring the heat flow under isothermal conditions as the structure evolves. The first intrinsic isopiestics, asymmetry of approach, and memory effects in enthalpy recovery after carbon dioxide jumps for polystyrene and the epoxy resin are reported.Item Temperature, stress, and strength development of early-age bridge deck concrete(2011-08) Pesek, Phillip Wayne; Folliard, Kevin J.; Drimalas, ThanosIn bridge deck concrete, early-age cracking can lead to substantial serviceability and structural integrity issues over the lifespan of the bridge. An understanding of the temperature, stress, and strength development of concrete can aid determining the early-age cracking susceptibility. This project, funded by the Texas Department of Transportation, evaluated these properties for various bridge deck materials and mixture proportions. The research presented in this thesis involved a laboratory testing program that used a combination of semi-adiabatic calorimetry, rigid cracking frame, free shrinkage frame, and match cured cylinder testing program that allowed the research team to simulate the performance of common bridge deck mixture designs under hot and cold weather conditions. In this program, the semi-adiabatic calorimetry was used, with previously generated models, to generate the temperature profile of the mixture. The rigid cracking frame and free shrinkage frame were used to evaluate the restrained stress development and the unrestrained volume changes, respectively, under the simulated temperatures. The match-cure cylinder testing program allowed the research team to generate a strength development profile for the concrete mixtures under the various simulated temperature profiles. Results from the laboratory program revealed that in hot weather simulations, ground granulated blast furnace slag mixtures developed the lowest stress / strength ratios, and in cold weather simulations, Class F fly ash mixtures developed the lowest stress / strength ratios. In general, use of SCMs and limestone coarse aggregate results in mixtures that generate less heat and lower stress / strength ratios. Isothermal testing showed that shrinkage reducing admixtures were effective in reducing early-age strains from chemical shrinkage. In addition to the laboratory testing program, a field testing program was completed to measure the temperature development of four bridge decks during the winter and summer months. The recorded concrete temperatures and the effects of the environmental conditions at the time of the pour will aid in the calibration and validation of the temperature prediction component of ConcreteWorks for bridge deck construction. In addition, experience gained through these field pours resulted in an optimized instrumentation procedure that will aid in the successful collection of data in future projects.