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- Forsberg, Jimmy, et al.
(författare)
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Evaluation of response surface methodologies used in crashworthiness optimization
- 2006
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 32:5, s. 759-777
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Tidskriftsartikel (refereegranskat)abstract
- Optimization of car structures is of great interest to the automotive industry. This work is concerned with structural optimization of a car body with the intent to increase the crashworthiness properties of the vehicle or decrease weight with the crashworthiness properties unaffected. In this work two different methodologies of constructing an intermediate approximation to the optimization problem are investigated, i.e. classical response surface methodology and Kriging. The major difference between the two methodologies is how the residuals between the true function value and the polynomial surface approximation value at a design point are treated.Several different optimization problems have been investigated, both analytical problems as well as finite element impact problems.The major conclusion is that even if the same kind of updating scheme is used both for Kriging and linear classic response surface methodology, Kriging improves the sequential behaviour of the optimization algorithm in the beginning of the optimization process. Problems may occur if a constraint is violated after several iterations and then classic response surface methodology seems to more easily be able to find a design point which satisfies the constraint.
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| 4. |
- Forsberg, Jimmy, et al.
(författare)
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On polynomial response surfaces and Kriging for use in structural optimization of crashworthiness
- 2005
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Ingår i: Structural and multidisciplinary optimization (Print). - : Springer Science and Business Media LLC. - 1615-147X .- 1615-1488. ; 29:3, s. 232-243
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Tidskriftsartikel (refereegranskat)abstract
- The accuracy of different approximating response surfaces is investigated. In the classical response surface methodology (CRSM) the true response function is usually replaced with a low-order polynomial. In Kriging the true response function is replaced with a low-order polynomial and an error correcting function. In this paper the error part of the approximating response surface is obtained from “simple point Kriging” theory. The combined polynomial and error correcting function will be addressed as a Kriging surface approximation.To be able to use Kriging the spatial correlation or covariance must be known. In this paper the error is assumed to have a normal distribution and the covariance to depend only on one parameter. The maximum-likelihood method is used to find the latter parameter. A weighted least-square procedure is used to determine the trend before simple point Kriging is used for the error function. In CRSM the surface approximation is determined through an ordinary least-square fit. In both cases the D-optimality criterion has been used to distribute the design points.From this investigation we have found that a low-ordered polynomial assumption should be made with the Kriging approach. We have also concluded that Kriging better than CRSM resolves abrupt changes in the response, e.g. due to buckling, contact or plastic deformation.
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| 5. |
- Forsberg, Jimmy, et al.
(författare)
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The optimisation process of an energy absorbing frontal underrun protection device
- 2008
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Ingår i: International Journal of Vehicle Design. - 0143-3369 .- 1741-5314. ; 46:3, s. 271-293
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the methodology used during the development of an energy absorbing Frontal Underrun Protection device (eaFUP). The aim of this study is to show how different optimisation methods can be used at different stages during the design process. It also shows one approach to derive an optimal design taking several different design alternatives into account, each of which consists of several different materials. The outcome of the optimisation process is three different designs of the eaFUP.
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- Jansson, Mikael, 1972-
(författare)
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Hydro-mechanical forming of aluminium tubes : on constitutive modelling and process design
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tube hydroforming is a forming method which has several advantages. By using pressure in combination with material feeding it is possible to manufacture products with high structural integration and tight dimensional tolerances. The forming method is especially suited for aluminium alloys which have a relatively low ductility. Finite Element simulations are used extensively in the sheet metal stamping industry, where the methodology has contributed to a better understanding of the process and the new prediction capability has significantly reduced costly die tryouts. Similarly, the tube hydroforming industry can benefit from Finite Element simulations, and this simulation methodology is the topic of this dissertation.Deep drawing and tube hydroforming have a basic difference, namely that the latter process essentially is a force controlled process. This fact, in combination with the anisotropic behaviour of aluminium tubes, enforces a need for accurate constitutive descriptions. Furthermore, the material testing needs to account for the specifics of tube hydroforming. The importance of proper material modelling is in this work shown for hydrobulging and hydroforming in a die with extensive feeding.The process parameters in hydroforming are the inner pressure and the material feeding, where a correct combination of these parameters is crucial for the success of the process. It is here shown, that Finite Element simulations together with an optimisation routine are powerful tools for estimating the process parameters in an automated procedure.Finally, the reliability and quality of the simulation results depend on how failure is evaluated, which in the case of hydroforming mainly oncerns wrinkling and strain localisation. Since tube hydroforming often is preceded by bending operations this fact also demands the criteria to be strain path independent. In this work, it is shown that the prediction of strain localisation depends on the ability to predict diffuse necking, which in turn is strongly related to the chosen constitutive model.
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| 7. |
- Jansson, Mikael, et al.
(författare)
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On constitutive modeling of aluminum alloys for tube hydroforming applications
- 2005
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Ingår i: International Journal of Plasticity. - : Elsevier BV. - 0749-6419. ; 21:5, s. 1041-1058
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Tidskriftsartikel (refereegranskat)abstract
- The increased interest in lightweight materials for automotive structures has also lead to a search for efficient forming methods that suit these materials. One attractive concept is to use hydroforming of aluminum tubes. The advantages of this forming method includes better tolerances, decreased number of parts and an increased range of forming options. By using FE simulations, the process can be optimized to reduce the risk for failure, i.e. bursting or wrinkling. However, extruded aluminum is highly anisotropic and it is crucial that the material model used for simulations is able to accurately describe this behavior. Also, tube hydroforming occurs predominantly in a biaxial stress state which should be considered in the material testing, where uniaxial tests are used extensively in the industry today. The present study accentuates the need for improved constitutive models. It is shown that a material model, which accurately describes the anisotropic behavior of aluminum tubes, can be obtained from simple and robust experiments.
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| 8. |
- Jansson, Mikael, et al.
(författare)
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On process parameter estimation for the tube hydroforming process
- 2007
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 190:1-3, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- Tube hydroforming is a forming process where an inner pressure combined with axial feeding deforms the tube to the shape of a die cavity. One of the main concerns when designing such a process is to avoid burst pressure, i.e. the process state where the hardening of the material is unable to resist the increase in inner pressure and wall thickness reduction. The success of a hydroforming process strongly depends on the choice of process parameters, i.e. the combination of material feeding and inner pressure. Especially in hydroforming processes, where the free forming phase is substantial, the process is proved to be very sensitive to the inner pressure. By transforming the problem into a deformation controlled rather than a force controlled process, the results from the process parameter estimation become more reliable but on the other hand less intuitive. In this context, three distinct parameter estimation procedures are suggested. Firstly, a self feeding based procedure is proposed with the intention of being a fast method to be used as a first estimate of suitable process parameters. Secondly, an iterative optimization problem set up is presented. Thirdly, and finally, an adaptive simulation procedure based on process response approximations is proposed, which only requires a limited number of simulation runs.
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| 9. |
- Jansson, Mikael, et al.
(författare)
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On strain localisation in tube hydroforming of aluminium extrusions
- 2008
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 195:1-3, s. 3-14
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Tidskriftsartikel (refereegranskat)abstract
- One important issue when simulating tube hydroforming is to predict bursting, i.e. when the increase in cavity pressure cannot be compensated by hardening of the tube material. Traditionally, this is made by a forming limit diagram (FLD), where the limit strains determine whether a material point is experiencing necking or failure. However, the experimental FLD depends on the strain path, and the methods which are used to determine the FLD are adapted to conventional deep drawing which, depending on the process characteristics, could make it unsuitable for tube hydroforming applications. In this work, analytical and numerical forming limit predictions are studied from a hydroforming point of view. These predictions are then applied to free bulge cases, and a case with extensive feeding in a die where the results from the latter case is compared to experiments. Further, the influence from extrusion welds and a circumferential thickness distribution on the forming limit is also evaluated.
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| 10. |
- Jansson, Mikael, et al.
(författare)
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The use of biaxial test data in the validation of constitutive descriptions for tube hydroforming applications
- 2007
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 184:1-3, s. 69-76
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Tidskriftsartikel (refereegranskat)abstract
- When considering finite element simulations of aluminium tube hydroforming, the user is facing several challenges. Firstly, extruded aluminium is anisotropic in yield stress and plastic flow. Secondly, the hydroforming process introduces new issues concerning friction and process control. This imposes a demand for accurate constitutive models as well as for hydroforming process related testing methods. The present study focuses on how biaxial tests can be used to calibrate and validate a constitutive model. It is also shown that by using inverse modelling, additional information can be obtained through these types of tests, such as, e.g. the frictional behaviour for different lubrication conditions.
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