6,155 research outputs found

    Age Replacement and Service Rate Control of Stochastically Degrading Queues

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    This thesis considers the problem of optimally selecting a periodic replacement time for a multiserver queueing system in which each server is subject to degradation as a function of the mean service rate and a stochastic and dynamic environment. Also considered is the problem of optimal service rate selection for such a system. In both cases, the performance metric is the long-run average cost rate. Analytical expressions are obtained, in terms of Laplace transforms, for the nonlinear objective functions, necessitating the use of numerical Laplace transform inversion to evaluate candidate solutions in conjunction with standard numerical algorithms. Due to the convexity of the objective function, the optimal replacement time is computed using a hybrid bisection-secant method which yields globally optimal solutions. The optimal service rates are obtained via gradient search methods but are only guaranteed to provide locally optimal solutions. The analytical results are implemented on three notional examples that demonstrate the benefits of dynamically adjusting service rates under the described maintenance policy

    A FRAMEWORK FOR THE ECONOMIC EVALUATION OF ENVIRONMENTAL SCIENCE

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    Economists, especially agricultural economists, have undertaken extensive analysis of the gains of technological-based scientific research. This is in stark contrast to the efforts undertaken to understand the economic effects of environmental scientific research. Economic evaluation of environmental science is important because knowledge-based government agencies are regularly required to justify their research expenditure and set clear priorities for their research programmes. This paper addresses the gap in the literature by offering a general framework for evaluating environmental scientific research. The paper is structured around two themes central to appraisals of environmental research: (a) the non-market nature of environmental outcomes; and (b) the pathways to achieve these outcomes. Some of the more important and unique issues addressed include the links between the natural systems being researched, the benefits in terms of resulting goods and services, and their subsequent values, as well as the factors influencing the overall contribution research makes to environmental decision-making.Environmental Economics and Policy, Research and Development/Tech Change/Emerging Technologies,

    A Fuzzy Inference System Approach for Evaluating the Feasibility of Product Remanufacture

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    In the recent past, efforts have been made in enhancing sustainable manufacturing aimed at protecting the environment and saving natural resources. Among the efforts that have been explored include strategies to ensure responsible end-of-life product management so as reduce the impact on the environment and achieve effective use of resources. Towards this end, reduce, reuse and recycle product disposal strategies have found a lot of consideration in manufacturing. Of the product reuse strategies, remanufacturing has been widely applied owing to its unique feature of rendering the remanufactured product as good as new. For remanufacturers, this strategy leads to provision of quality products comparable to new their new counterparts at a reduced cost. Remanufacturing also leads to a sustainable environment through energy and material savings, as well as minimized solid wastes. Remanufacturing however, poses challenges related to collection of the returns or cores, manufacturing process planning, resource allocation, warranty estimation and redistribution. These challenges are due to product and process complexities, customer requirements, and uncertainties associated with product take back and the remanufactured products’ market-base. Key among these challenges is the remanufacturing process which is complicated, labor intensive with varying process times. In most cases the routing of these processes is stochastic in nature, based on the condition of the returned product. There is also the negative perception among consumers that remanufactured products are less superior to new ones, which calls for the need to allocate preferably longer warranty periods for the remanufactured product to induce confidence in the consumer while at the same time keeping the warranty costs low. The objectives of this study were informed by challenges faced by a local remanufacturing firm. They include: (1) a detailed study of the current remanufacturing process of the firm’s products; (2) identification of bottlenecks in the process to make recommendations for improvement; (3) develop a decision support system for assessing product remanufacture; (4) assess warranty allocation options for remanufactured product reuse. The study revealed that there are bottlenecks in the current remanufacturing process and suggested an improvement to enhance efficiency. This bottlenecks include overutilization of some of the process centers such as the diagnostic testing and the after-repair testing centers which lead to the product spending more time in the system than necessary. To improve the system performance the capacities of the bottleneck centers were increased which yielded significant reduction in the time the product spends in the system. The key contribution of this dissertation is the development of a decision support system based on a bi-level fuzzy linguistic computing approach. This model integrates qualitative and quantitative product attributes in determining the remanufacturability of a product. The fuzzy-based model established remanufacturability metric, herein referred to as an index, is applied to assess the feasibility of remanufacturing two products that were used as a case study. A number of warranty scenarios are considered to ascertain the impact of different warranty periods on the cost of warranty. The results show that the additional warranty cost for product reuse is a function of the period of first use and the residual life of the produc

    Remanufacturing and Advanced Machining Processes for New Materials and Components

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    "Remanufacturing and Advanced Machining Processes for Materials and Components presents current and emerging techniques for machining of new materials and restoration of components, as well as surface engineering methods aimed at prolonging the life of industrial systems. It examines contemporary machining processes for new materials, methods of protection and restoration of components, and smart machining processes. • Details a variety of advanced machining processes, new materials joining techniques, and methods to increase machining accuracy • Presents innovative methods for protection and restoration of components primarily from the perspective of remanufacturing and protective surface engineering • Discusses smart machining processes, including computer-integrated manufacturing and rapid prototyping, and smart materials • Provides a comprehensive summary of state-of-the-art in every section and a description of manufacturing methods • Describes the applications in recovery and enhancing purposes and identifies contemporary trends in industrial practice, emphasizing resource savings and performance prolongation for components and engineering systems The book is aimed at a range of readers, including graduate-level students, researchers, and engineers in mechanical, materials, and manufacturing engineering, especially those focused on resource savings, renovation, and failure prevention of components in engineering systems.

    Optimizing the Prioritization of Natural Disaster Recovery Projects

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    Prioritizing reconstruction projects to recover a base from a natural disaster is a complicated and arduous process that involves all levels of leadership. The project prioritization phase of base recovery has a direct affect on the allocation of funding, the utilization of human resources, the obligation of projects, and the overall speed and efficiency of the recovery process. The focus of this research is the development of an objective and repeatable process for optimizing the project prioritization phase of the recovery effort. This work will focus on promoting objectivity in the project prioritizing process, improving the communication of the overall base recovery requirement, increasing efficiency in utilizing human and monetary resources, and the creation of a usable and repeatable decision-making tool based on Value-Focused Thinking and integer programming methods

    Landscape Ecology

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    This book has been written to present major and efficient applications in landscape ecology, as well as to propose a solid action for this category of topics. The book aims to illustrate various treatment methods of the land-use models impact on landscape ecology creation. The book is divided into three parts: Part I: Ecological interpretation of land-use act - in this part, ecosystem and land use turn out to be a significant factor in the process of creating an ecological landscape. Part II: Landscape district in applied ecological analysis - this part attempts to illustrate the best possible model of analysis integrated with landscape in practical case studies. Part III: The anthropogenic impacts on landscape creation - this part discusses the human impact on landscape creation

    Optimal Preventive Maintenance Strategies for a Production System Subject to Random Shocks.

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    The failure rate of production equipment is usually increasing as wear accumulates with usage. An improved preventive maintenance approach is accomplished through predictive maintenance, where an indicator of wear, like vibration or heat, is measured and used to determine the optimal time of an adjustment (or maintenance) such as realignment, oil change or replacing seals. The state of the unit is restored into the same original as new state both after adjustment or failure (followed by a repair). This research will develop mathematical models for a single and two unit production system. In the development of the one unit model, a cost criterion and indicator variable will be used for deciding when adjustment should take place. The cost to be minimized is the long-run average cost of adjustments and failures. An optimal solution to this problem will be obtained via dynamic programming and compared to an approximate steady state solution based on renewal theory. This approximation (like other earlier works) disregards the fact that after failure (that has a small probability) the unit is restored to its original state. Both models provide an upper control limit (UCL) on the indicator variable which triggers an adjustment when exceeded. It will be shown that disregarding the restoration after failure in the cost approximation causes the UCL to be underestimated. The resulting cost penalty is considerable in most cases. For the two unit system, an optimizing mathematical model will be developed by monitoring an external variable for each unit and using this information collectively as a predictor for failure. The cost to be minimized is the long-run average cost of adjustments, system overhaul, and failures. In the general case, the system with n units, it is shown why the current approach becomes more intractable as n increases. An alternate methodology is suggested. Finally, using a simplified decision policy, a simulation model is offered as a safeguard that the mathematical model is realistic. Sensitivity and factor analysis results are also provided for both the single and two unit systems
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