Browsing by Subject "structural"
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Item Electrochemical characterization and time-variant structural reliability assessment of post-tensioned, segmental concrete bridges(2010-07-14) Pillai Gopalakrishnan, RadhakrisIn post-tensioned (PT) bridges, prestressing steel tendons are the major load carrying components. These tendons consist of strands, ducts, and cementitious grout that fill the interstitial space between the strands and ducts. However, inspections on PT bridges have reported the presence of voids, moisture, and chlorides inside grouted ducts as the major cause of accelerated corrosion of strands. Corrosion of the strands has resulted in PT bridge failures in Europe and tendon failures in the United States. As most of the PT bridges have high importance measures and the consequences of failure are significant, it is important to maintain high levels of safety and serviceability for these bridges. To meet this goal, bridge management authorities are in dire need of tools to quantify the long-term performance of these bridges. Time-variant structural reliability models can be useful tools to quantify the long-term performance of PT bridges. This doctoral dissertation presents the following results obtained from a comprehensive experimental and analytical program on the performance of PT bridges. 1) Electrochemical characteristics of PT systems 2) Probabilistic models for tension capacity of PT strands and wires exposed to various void and environmental conditions 3) Time-variant structural reliability models (based on bending moment and stress limit states) for PT bridges 4) Time-variant strength and service reliabilities of a typical PT bridge experiencing HS20 and HL93 loading conditions and different exposure conditions for a period of 75 years The experimental program included exposure of strand specimens to wet-dry and continuous-atmospheric conditions. These strand specimens were fabricated to mimic void and/or grout-air-strand (GAS) conditions inside the tendons. It was found that the GAS interface plays a major role in strand corrosion. The GAS interfaces that are typically located in the anchorage zones of harped PT girders or vertical PT columns can cause aggressive strand corrosion. At these locations, if voids are present and the environment is relatively dry, then limited corrosion of the strands occurs. However, if the presence of high relative humidity or uncontaminated and chloride-contaminated water exists at these interfaces, then corrosion activity can be high. The strands were exposed for a period of 12, 16, and 21 months, after which the remaining tension capacity was determined. The analytical program included the development of probabilistic strand capacity models (based on the experimental data) and the structural reliability models. The timevariant tension capacity predicted using the developed probabilistic models were reasonably consistent with the tendon failures observed in PT bridges in Florida and Virginia. The strength reliability model was developed based on the moment capacity and demand at midspan. Service reliability model was developed based on the allowable and applied stresses at midspan. Using these models, the time-variant strength and service reliabilities of a typical PT bridge were determined based on a set of pre-defined constant and random parameters representing void, material, exposure, prestress, structural loading, and other conditions. The strength and service reliabilities of PT bridges exposed to aggressive environmental conditions can drop below the recommended values at relatively young ages. In addition, under similar conditions the service reliability drops at a faster rate than the strength reliability.Item Structural and Aerodynamic Interaction Computational Tool for Highly Reconfigurable Wings(2011-10-21) Eisenbeis, Brian JosephMorphing air vehicles enable more efficient and capable multi-role aircraft by adapting their shape to reach an ideal configuration in an ever-changing environment. Morphing capability is envisioned to have a profound impact on the future of the aerospace industry, and a reconfigurable wing is a significant element of a morphing aircraft. This thesis develops two tools for analyzing wing configurations with multiple geometric degrees-of-freedom: the structural tool and the aerodynamic and structural interaction tool. Linear Space Frame Finite Element Analysis with Euler-Bernoulli beam theory is used to develop the structural analysis morphing tool for modeling a given wing structure with variable geometric parameters including wing span, aspect ratio, sweep angle, dihedral angle, chord length, thickness, incidence angle, and twist angle. The structural tool is validated with linear Euler-Bernoulli beam models using a commercial finite element software program, and the tool is shown to match within 1% compared to all test cases. The verification of the structural tool uses linear and nonlinear Timoshenko beam models, 3D brick element wing models at various sweep angles, and a complex wing structural model of an existing aircraft. The beam model verification demonstrated the tool matches the Timoshenko models within 3%, but the comparisons to complex wing models show the limitations of modeling a wing structure using beam elements. The aerodynamic and structural interaction tool is developed to integrate a constant strength source doublet panel method aerodynamic tool, developed externally to this work, with the structural tool. The load results provided by the aerodynamic tool are used as inputs to the structural tool, giving a quasi-static aeroelastically deflected wing shape. An iterative version of the interaction tool uses the deflected wing shape results from the structural tool as new inputs for the aerodynamic tool in order to investigate the geometric convergence of an aeroelastically deflected wing shape. The findings presented in this thesis show that geometric convergence of the deflected wing shape is not attained using the chosen iterative method, but other potential methods are proposed for future work. The tools presented in the thesis are capable of modeling a wide range of wing configurations, and they may ultimately be utilized by Machine Learning algorithms to learn the ideal wing configuration for given flight conditions and develop control laws for a flyable morphing air vehicle.Item Structural Thermomechanical Models for Shape Memory Alloy Components(2014-04-18) Rao, AshwinThermally responsive shape memory alloys (SMA) demonstrate interesting properties like shape memory effect (SME) and superelasticity (SE). SMA components in the form of wires, springs and beams typically exhibit complex, nonlinear hysteretic responses and are subjected to tension, torsion or bending loading conditions. Traditionally, simple strength of materials based models/tools have driven engineering designs for centuries, even as more sophisticated models existed for design with conventional materials. In light of this, an effort to develop strength of materials type modeling approach that can capture complex hysteretic SMA responses under different loading conditions is undertaken. The key idea here is of separating the thermoelastic and the dissipative part of the hysteretic response by using a Gibbs potential and thermodynamic principles. The dissipative part of the response is later accounted for by a discrete Preisach model. The models are constructed using experimentally measurable quantities (like torque?twist, bending moment?curvature etc.), since the SMA components subjected to torsion and bending experience an in-homogeneous non-linear stress distribution across the specimen cross-section. Such an approach enables simulation of complex temperature dependent superelastic responses including those with multiple internal loops. The second aspect of this work deals with the durability of the material which is of critical importance with increasing use of SMA components in different engineering applications. Conventional S-N curves, Goodman diagrams etc. that capture only the mechanical loading aspects are not adequate to capture complex thermomechanical coupling seen in SMAs. Hence, a novel concept of driving force amplitude v/s number of cycles equivalent to thermodynamical driving force for onset of phase transformations is proposed which simultaneously captures both mechanical and thermal loading in a single framework. Recognizing the paucity of experimental data on functional degradation of SMAs (especially SMA springs), a custom designed thermomechanical fatigue test rig is used to perform user defined repeated thermomechanical tests on SMA springs. The data from these tests serve both to calibrate the model and establish thermodynamic driving force and extent of phase transformation relationships for SMA springs. A drop in driving force amplitude would suggest material losing its ability to undergo phase transformations which directly corresponds to a loss in the functionality/smartness of SMA component. This would allow designers to set appropriate driving force thresholds as a guideline for analyzing functional life of SMA components.