| 1. |
- Domeij Bäckryd, Rebecka, 1978-, et al.
(författare)
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Multidisciplinary design optimisation methods for automotive structures
- 2017
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Ingår i: International Journal of Automotive and Mechanical Engineering. - Pekan, Malaysia : Universiti Malaysia Pahang Publishing. - 2229-8649 .- 2180-1606. ; 14:1, s. 4050-4067
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Tidskriftsartikel (refereegranskat)abstract
- Multidisciplinary design optimisation (MDO) can be used as an effective tool to improve the design of automotive structures. Large-scale MDO problems typically involve several groups who must work concurrently and autonomously in order to make the solution process efficient. In this article, the formulations of existing MDO methods are compared and their suitability is assessed in relation to the characteristics of automotive structural applications. Both multi-level and single-level optimisation methods are considered. Multi-level optimisation methods distribute the design process but are complex. When optimising automotive structures, metamodels are often required to relieve the computational burden of detailed simulation models. The metamodels can be created by individual groups prior to the optimisation process, and thus offer a way of distributing work. Therefore, it is concluded that a single-level method in combination with meta-models is the most straightforward way of implementing MDO into the development of automotive structures. If the benefits of multi-level optimisation methods, in a special case, are considered to compensate for their drawbacks, analytical target cascading has a number of advantages over collaborative optimisation, but both methods are possible choices.
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| 2. |
- Golling, Stefan
(författare)
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A Study on Microstructure-Dependent Deformation and Failure Properties of Boron Alloyed Steel
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Developments in the automotive industry are driven by customer desires and legislative authorities. Legislation has restricted the emissions standards for vehicles, and has mandated the need for higher safety standards. The emission of carbon dioxide is directly related to fuel consumption, and the reduction in fuel consumption can be achieved by reducing the vehicle mass.A variety of methods have been used to reduce a vehicle’s mass while maintaining its crashworthiness. A technique using low-alloyed boron steel has been developed, and it enables the design of lighter body-in-white, while maintaining passenger safety. The technique is called press-hardening or hot stamping, and it involves the simultaneous forming and quenching of sheet metal. Press-hardened components have superior material properties compared to components made of mild steel. Another feature of compo-nents formed at elevated temperatures is the possibility of tailoring material properties in desired regions of the component. This is realized by using specially designed tools that allow differential in-die cooling rates and thus direct control of the formed microstructure. Using this technique, it is possible to manufacture a high-strength region next to a high-ductility section divided by a transition zone of mixed microstructure.The present work aims to determine the influence of mixed microstructures on the mechanical properties of low-alloyed boron steel. An experimental heat-treatment process is used to form multi-phase microstructures with a variety of phase volume fractions present in the composite. Digital image correlation is used to investigate the deformation of tensile specimens under loading. This full-field technique and a suitable constitutive model enables us to evaluate the flow and fracture properties of heat-treated samples. Microstructural characterization is used to determine the type of phases present and their average volume fraction in the composites.The findings from experimental studies are compared to results predicted by a constitutive model. A modeling strategy is employed to determine the effective material properties depending on the properties of single-phase characteristics. Failure of the material is indicated by stress-based fracture criteria. Numerical issues in finite-element modeling concerning the mesh-size sensitivity are addressed using a regularization method.The results of the experimental work aids the calibration and validation of the proposed microstructure-based modeling approach, and a knowledge of the processing history enables the prediction of the overall hardening behavior and fracture elongation. A comparison of experimental results, which are not used for calibration, with numerical results shows that there is good agreement.
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| 3. |
- Kapidzic, Zlatan, 1978-
(författare)
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Static and Fatigue Failure of Bolted Joints in Hybrid Composite-Aluminium Aircraft Structures
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of fibre composites in the design of load carrying aircraft structures has been increasing over the last few decades. At the same time, aluminium alloys are still present in many structural parts, which has led to an increase of the number of hybrid composite-aluminium structures. Often, these materials are joined at their interface by bolted connections. Due to their different response to thermal, mechanical and environmental impact, the composite and the aluminium alloy parts are subject to different design and certification practices and are therefore considered separately.The current methodologies used in the aircraft industry lack well-developed methods to account for the effects of the mismatch of material properties at the interface.One such effect is the thermally induced load which arises at elevated temperature due to the different thermal expansion properties of the constituent materials. With a growing number of hybrid structures, these matters need to be addressed. The rapid growth of computational power and development of simulation tools in recent years have made it possible to evaluate the material and structural response of hybrid structures without having to entirely rely on complex and expensive testing procedures.However, as the failure process of composite materials is not entirely understood, further research efforts are needed in order to develop reliable material models for the existing simulation tools.The work presented in this dissertation involves modelling and testing of bolted joints in hybrid composite-aluminium structures.The main focus is directed towards understanding the failure behaviour of the composite material under static and fatigue loading, and how to include this behaviour in large scale models of a typical bolted airframe structure in an efficient way. In addition to that, the influence of thermally induced loads on the strength and fatigue life is evaluated in order to establish a design strategy that can be used in the industrial context.The dissertation is divided into two parts. In the first one, the background and the theory are presented while the second one consists of five scientific papers.
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| 4. |
- Rentmeester, Rikard, et al.
(författare)
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On mixed isotropic-distortional hardening
- 2015
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Ingår i: International Journal of Mechanical Sciences. - : Elsevier. - 0020-7403 .- 1879-2162. ; 92, s. 259-268
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Tidskriftsartikel (refereegranskat)abstract
- Mixed isotropic-distortional hardening allows for individual stress-plastic strain relations in different straining directions. Such hardening can be obtained by allowing the parameters in the effective stress function depend on anisotropy functions of the equivalent plastic strain. A methodology to calibrate these anisotropy functions is proposed in this work, and is demonstrated on an austenitic strainless steel. A high exponent eight parameter effective stress function for plane stress states is utilised. The anisotropy functions are calibrated by the use of experimental data from uniaxial tensile test data in three material directions and a balanced biaxial test. The plastic anisotropy is evaluated at a finite number of plastic strains, and it is assumed to vary piecewise linearly with respect to the equivalent plastic strain. At each level of plastic strain, the anisotropy is correctly represented, even if rather large increments in plastic strain are used in the calibration. It was found that there are at least two sets of anisotropy functions which satisfy the conditions in the calibration procedure. The resulting uniaxial stress-strain relations from the two sets of anisotropy functions in four additional straining directions, not included in the calibration set, were compared to the corresponding experimental data. From this validation, one of the anisotropy function sets could be discarded, whereas the other one gave a good prediction of the stress-strain relations in all the four additional directions. (C) 2015 Elsevier Ltd. All rights reserved.
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| 5. |
- Ryberg, Ann-Britt, et al.
(författare)
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A metamodel-based multidisciplinary design optimization process for automotive structures
- 2015
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Ingår i: Engineering with Computers. - : Springer. - 0177-0667 .- 1435-5663. ; 31:4, s. 711-728
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Tidskriftsartikel (refereegranskat)abstract
- Automotive companies continuously strive to design better products faster and more cheaply using simulation models to evaluate every possible aspect of the product. Multidisciplinary design optimization (MDO) can be used to find the best possible design taking into account several disciplines simultaneously, but it is not yet fully integrated within automotive product development. The challenge is to find methods that fit company organizations and that can be effectively integrated into the product development process. Based on the characteristics of typical automotive structural MDO problems, a metamodel-based MDO process intended for large-scale applications with computationally expensive simulation models is presented and demonstrated in an example. The process is flexible and can easily fit into existing organizations and product development processes where different groups work in parallel. The method is proven to be efficient for the discussed example and improved designs can also be obtained for more complex industrial cases with comparable characteristics.
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| 6. |
- Ryberg, Ann-Britt, 1974-
(författare)
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Metamodel-Based Multidisciplinary Design Optimization of Automotive Structures
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multidisciplinary design optimization (MDO) can be used in computer aided engineering (CAE) to efficiently improve and balance performance of automotive structures. However, large-scale MDO is not yet generally integrated within automotive product development due to several challenges, of which excessive computing times is the most important one. In this thesis, a metamodel-based MDO process that fits normal company organizations and CAE-based development processes is presented. The introduction of global metamodels offers means to increase computational efficiency and distribute work without implementing complicated multi-level MDO methods.The presented MDO process is proven to be efficient for thickness optimization studies with the objective to minimize mass. It can also be used for spot weld optimization if the models are prepared correctly. A comparison of different methods reveals that topology optimization, which requires less model preparation and computational effort, is an alternative if load cases involving simulations of linear systems are judged to be of major importance.A technical challenge when performing metamodel-based design optimization is lack of accuracy for metamodels representing complex responses including discontinuities, which are common in for example crashworthiness applications. The decision boundary from a support vector machine (SVM) can be used to identify the border between different types of deformation behaviour. In this thesis, this information is used to improve the accuracy of feedforward neural network metamodels. Three different approaches are tested; to split the design space and fit separate metamodels for the different regions, to add estimated guiding samples to the fitting set along the boundary before a global metamodel is fitted, and to use a special SVM-based sequential sampling method. Substantial improvements in accuracy are observed, and it is found that implementing SVM-based sequential sampling and estimated guiding samples can result in successful optimization studies for cases where more conventional methods fail.
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| 7. |
- Ryberg, Ann-Britt, et al.
(författare)
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Spot weld reduction methods for automotive structures
- 2016
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Ingår i: Structural and multidisciplinary optimization (Print). - : SPRINGER. - 1615-147X .- 1615-1488. ; 53:4, s. 923-934
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Tidskriftsartikel (refereegranskat)abstract
- Spot welds are commonly used to join steel sheets in automotive structures. The number and layout of these spot welds are vital for the performance of the structure. However, reducing the number of spot welds will cut both production time and cost. This article presents three different methods of reducing the number of spot welds in automotive structures: ranking-based selection, topology optimization and size optimization of a parameterized model. The methods are compared in a simple example and it is found that the latter two methods have the best potential of reducing the number of spot welds. Topology optimization requires less preparation and computational effort as compared to size optimization of a parameterized model. However, the method is primarily suitable for studies where load cases involving linear systems are judged to be most important. Otherwise, size optimization of a parameterized model is probably a better choice. The topology optimization approach is successfully demonstrated in a full-scale industrial application example and confirms that the method is useful within contemporary product development.
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| 8. |
- Shetty, Sandeep, et al.
(författare)
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An evaluation of simple techniques to model the variation in strain hardening behavior of steel
- 2017
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Ingår i: Structural and multidisciplinary optimization (Print). - Bonn : Springer. - 1615-147X .- 1615-1488. ; 55:3, s. 945-957
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Tidskriftsartikel (refereegranskat)abstract
- It is important to consider variations in material parameters in the design of automotive structures in order to obtain a robust and reliable design. However, expensive tests are required to gain complete knowledge of the material behavior and its associated variation. Consequently, due to time and cost constraints, simplified material scatter modeling techniques based on scatter data of typical material properties provided by the material suppliers are used at early design stages in simulation-based robustness studies. The aim of this paper is to study the accuracy of the simplified scatter modeling methods in representing the real material variation. The simplified scatter modeling methods are evaluated by comparing the material scatter obtained by them to the scatter obtained by complete tensile tests, which are obtained after detailed timeconsuming experimental investigations. Furthermore, an accuracy assessment is carried out based on selected responses from an axially-crushed, square tube made from DP600 steel.
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| 9. |
- Shetty, Sandeep, et al.
(författare)
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Multiobjective reliability-based and robust design optimisation for crashworthiness of a vehicle side impact
- 2015
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Ingår i: International Journal of Vehicle Design. - : InderScience Publishers. - 0143-3369 .- 1741-5314. ; 67:4, s. 347-367
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Tidskriftsartikel (refereegranskat)abstract
- Optimisation of vehicle design is necessary to meet increased safety requirements, new emission regulations, and to deal with competition in the global market, etc. However, optimised design using classical optimisation techniques with deterministic models might not meet the desired performance level or might fail in extreme events in real life owing to uncertainties in the design parameters and loading conditions. Consequently, it is essential to account for uncertainties in a systematic manner to generate a robust and reliable design. In this paper, an approach to perform multiobjective, reliability-based, and robust design optimisation is presented using a vehicle side impact crashworthiness application. Metamodels have been used in the optimisation process to decrease computational effort. Variations in material properties, thicknesses, loading conditions, and B-pillar heat-affected zone material strength have been considered for the stochastic optimisation. A comparative study of deterministic, reliability-based, and robust optimisation approaches is performed.
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| 10. |
- Shetty, Sandeep
(författare)
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Optimization of Vehicle Structures under Uncertainties
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Advancements in simulation tools and computer power have made it possible to incorporate simulation-based structural optimization in the automotive product development process. However, deterministic optimization without considering uncertainties such as variations in material properties, geometry or loading conditions might result in unreliable optimum designs. In this thesis, the capability of some established approaches to perform design optimization under uncertainties is assessed, and new improved methods are developed. In particular, vehicle structural problems which involve computationally expensive Finite Element (FE) simulations, are addressed.The first paper focuses on the evaluation of robustness, given some variation in input parameters, the capabilities of three well-known metamodels are evaluated. In the second paper, a comparative study of deterministic, reliability-based and robust design optimization approaches is performed. It is found that the overall accuracy of the single-stage (global) metamodels, which are used in the above study, is acceptable for deterministic optimization. However, the accuracy of performance variation prediction (local sensitivity) must be improved. In the third paper, a decoupled reliability-based design optimization (RBDO) approach is presented. In this approach, metamodels are employed for the deterministic optimization only while the uncertainty analysis is performed using FE simulations in order to ensure its accuracy.In the fifth paper, two new sequential sampling strategies are introduced that aim to improve the accuracy of the metamodels efficiently in critical regions. The capabilities of the methods presented are illustrated using analytical examples and a vehicle structural application.It is important to accurately represent physical variations in material properties since these might exert a major influence on the results. In previous work these variations have been treated in a simplified manner and the consequences of these simplifications have been poorly understood. In the fourth paper, the accuracy of several simple methods in representing the real material variation has been studied. It is shown that a scaling of the nominal stress-strain curve based on the Rm scatter is the best choice of the evaluated choices, when limited material data is available.In this thesis work, new pragmatic methods for non-deterministic optimization of large scale vehicle structural problems have been developed. The RBDO methods developed are shown to be flexible, more efficient and reasonably accurate, which enables their implementation in the current automotive product development process.
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