Browsing by Author "G. Mahinthakumar, Committee Member"

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    Evolutionary Algorithms to Aid Watershed Management
    (2005-03-31) Dorn, Jason Liam; S. Liehr, Committee Member; S. Ranji Ranjithan, Committee Chair; E. D. Brill, Jr., Committee Member; G. Mahinthakumar, Committee Member
    Watershed management is a complex process involving multiple uses, diverse stakeholders, and a variety of computer-based hydrologic and hydraulic simulation models. Exploring for efficient solutions and making decisions about the best integrated management strategies to implement can be improved through the use of quantitative systems analytic techniques. In addition to identifying mathematically optimal solutions, these techniques should also be able to consider issues that may not be properly represented in the models or may be in conflict with one another. As the complexities of the system models grow, contemporary heuristic search methods, including evolutionary algorithms (EAs), are becoming increasingly common in quantitative analysis of such challenging decision-making problems. More research is needed to enhance and extend the capabilities of these newer search methods to meet the growing challenges. Further, these new systems analytic capabilities are best made accessible to practitioners through a generic computational framework that integrates the system simulation models with the suite of search techniques. Therefore, the purpose of this research is to develop new EA-based system analytic methods for addressing integrated watershed management problems and a computational framework within which their capabilities are enabled for watershed management applications. EA-based methods to generate good alternative solutions and for multiobjective optimization have been developed and tested, and their performances compare well with those of other procedures. These new methods were also demonstrated through successful applications to realistic problems in watershed management. These techniques were integrated into and implemented within a new computer-based decision support framework that supports the integration of the user's preferred watershed models, methods to perform uncertainty and/or sensitivity analyses thereon, and multiple state-of-the-art optimization heuristic search procedures to identify good management strategies that meet the problem-specific (e.g., fiscal or environmental) objectives and constraints. The design of the software framework is described with a demonstration of its capabilities via a case study involving several scenarios of a watershed management problem.
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    Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints
    (2009-01-05) Cheng, Pei-Yuan; G. Mahinthakumar, Committee Member; Murthy N. Guddati , Committee Member; Tasnim Hassan, Committee Chair; K. Linga Murty, Committee Member
    ABSTRACT CHENG, PEI-YUAN. Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints. (Under the direction of Dr. Tasnim Hassan.) In recent decades, some unexpected fatigue failure occurred in welded joint of metal structures under cyclic loading. In many cases, the cause for the failure could not be detected. A study at NC State University revealed that welding procedure could be one of the factors that was not appropriately considered in current design methodologies. The welding procedure can influence the strain response near weld toe in two ways: one is by generating residual stress, and the other is changing material properties in the heat affected zone (HAZ). It was the primary objective in this investigation to determine the influence of these two factors on strain response by conducting experiments and performing numerical simulations of welded piping joints. On experimental study a series of residual stress data were measured, using x-ray and neutron diffraction techniques, for welded piping joints. The measured results revealed that the initial maximum compressive residual stress of stainless steel piping joints is higher than the yield stress of base metal. Moreover, the axial residual stresses of stainless steel piping joints are mostly relaxed after 5 cycles. The change of mechanical material properties due to high temperature exposure was studied by conducting experiments on tubular specimens. It was obtained that some mechanical material properties changed after subjecting to high temperature cycles, and the changed material was correlated to the peak temperatures. A modified thermo-mechanical material heterogeneity model was then developed to improve the initial residual stress simulation at the weld toe. The heterogeneous material properties coupled with the Chaboche model were used for subsequent fatigue response simulation. Quarter-point elements were applied at the stress concentration locations. The analysis results showed that fatigue response and residual stress relaxation can be simulated well. Final two simulations in this research showed that the presence of initial residual stress influences strain amplitude and strain mean, both of which could influence the fatigue life of welded joints.