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- Aspenberg, David, 1979-
(författare)
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Robust optimisation of structures : Evaluation and incorporation of variations in simulation based design
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the robustness of structures considering various uncertainties. The overall objective is to evaluate and develop simulation based design methods in order to find solutions that are optimal both in the sense of handling typical load cases and minimising the variability of the response, i.e. robust optimal designs. Conventionally optimised structures may show a tendency of being sensitive to small perturbations in the design or loading conditions. These variations are of course inevitable. To create robust designs, it is necessary to account for all conceivable variations (or at least the influencing ones) in the design process. The thesis is divided into two parts. The first part serves as a theoretical background for this work. It includes introductions to the concept of robust design, basic statistics, optimisation theory and metamodelling. The second part consists of five appended papers on the topic.The first and third papers focuse on the evaluation of robustness, given some dispersions in the input data. Established existing methods are applied, and for paper three, comparisons with experimentally evaluated dispersions on a larger system are made.The second and fourth paper introduce two new approaches to perform robust optimisation, i.e. optimisations where the mean performance and the robustness in the objectives are simultaneously optimised. These methods are demonstrated both on an analytical example and on a Finite Element model design example. The fifth paper studies the variations in mechanical properties between several different batches of the same steel grade. A material model is fitted to each batch of material, whereby dispersions seen in test specimens are transferred to material model parameter variations. The correlations between both test and material model parameters are studied.
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| 2. |
- Forsberg, Jimmy, 1974-
(författare)
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Structural optimization in vehicle crashworthiness design
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the optimization of structures subjected to impact loading. Major applications can be found in the crashworthiness design of vehicles. There is an industrial interest in using optimization methods in the crashworthiness design process. However, strongly nonlinear responses, including high frequency components, make the Finite Element (FE) simulations computationally demanding. This fact restricts the number of possible optimization methods that can be applied.In this work two optimization methods have been investigated: Response Surface Methodology (RSM) and Kriging. In both these methods the number of functional evaluations, i.e. here FE simulations, depends at least linearly on the number of design parameters. Thus, the number of design parameters is limited due to the computational effort. One objective of this work has been to reduce the total computational cost of the optimization process, or alternatively the possibility to use more design parameters at the same computational cost. When using RSM and Kriging, the number of functional evaluations in each iteration can be reduced, and/or the convergence of the optimization process can be improved. In many applications, RSM has been used with linear approximating response surfaces for robustness and efficiency reasons. Linear RSM may suffer from iterative oscillations, since the optimal design is often found on the boundary of the feasible design region. This oscillatory behaviour can be reduced if Kriging is used for the approximating response surfaces. However, it has been found that Kriging may have problems in fulfilling the constraints. In the initial design process of a structure very many alternative designs exist. At these early stages topology optimization is a mean of finding a structure that is optimal for the objective at hand. In this work a topology optimization approach is proposed for the design of crashworthiness structures. The resulting structure must still be subjected to an interpretation by engineers and also be improved by further optimization.
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| 3. |
- Redhe, Marcus, 1975-
(författare)
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On vehicle crashworthiness design using structural optimization
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation addresses the problems and possibilities of using structural optimization in vehicle crashworthiness design. The first part of the thesis gives an introduction to vehicle crashworthiness design. The optimization methods presented are also used to exemplify how structural optimization and robustness analysis can be used in vehicle crashworthiness design.In the second part of the thesis, five papers are appended, where different optimization methods are evaluated and improved for the usage in vehicle crashworthiness design. These papers concern the optimization methods Response Surface Methodology (RSM), Stochastic Optimization (SO) and Space Mapping (SM).Each method has its advantages and disadvantages. The Response Surface Methodology is the easiest method to use and the method that most often finds the best design of these three methods. Generally RSM is rather expensive, especially when many design variables are used. Then, SO is an effective alternative because in this method the number of evaluations is independent of the number of design variables, which is not the case for RSM. Space Mapping is the cheapest method, because it needs only one or two evaluations per iteration. However, SM is generally a method to fmd an improved design with fulfilled constraints and sometimes not the absolute optimum solution but to a low cost. Hence, both RSM and SO may produce better designs but at the price of more response evaluations.
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| 4. |
- Thellner, Mikael, 1972-
(författare)
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Multi-parameter topology optimization in continuum mechanics
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns structural optimization in general, and topology optimization in particular. The discipline structural optimization deals with the optimal design of load-carrying structures. In topology optimization the problem consists in finding the optimal shape of the external and internal boundaries of the structure and the number of holes. Topology optimization was originally developed as a subdiscipline of structural optimization and has been the subject of an intense research for some 15 years. The development has reached some level of maturity and conceptual consistency which have lead to a unified methodology and to new applications outside the original applications within structural mechanics. The general goal of this research has been to extend and apply the idea of topology optimization methods in new areas.Integrated topology and shape optimization is discussed. The goal is to use shape optimization to postprocess the often coarse layout which is the result of a topology optimization. Based on those ideas a new method for topology optimization with design-dependent loads is developed using simultaneous shape and topology variation. With a fixed design domain, on which the loads and constraints are specified, it is not possible to handle loads which depend on the design. Domain shape variation is therefore introduced, using techniques from shape optimization, simultaneously with topology optimization, to handle the design-dependent loads.The topology optimization method has been extended to convection-diffusion problems based on previously developed methodologies for topology optimization of fluids in Stokes flow. The problem considered is the optimal layout of heat-exchanging designs. For optimal performance two objectives are considered: the weighted integral average of the temperature and the average pressure drop in the domain.Engineering design often deals with multiple, possibly conflicting, objective functions or design criteria. The multidisciplinary aspect of engineering design has been introduced to topology optimization using a novel approach. Multidisciplinary topology optimization has been formulated in the game theory framework. The problem is solved as a noncooperative two player game and we determine a Nash equilibrium which is a computationally cheap alternative to Pareto optima. A coupled heat transfer-thermoelastic system has been studied for which a topology design approach is developed.
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