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  • 1.
    Draxler, Joar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Edberg, Jonas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Andersson, Joel
    University West.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A Numerical Model for Simulating the Effect of Strain Rate on Eutectic Band ThicknessIn: Article in journal (Refereed)
  • 2.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    Gestamp HardTech.
    Bergman, Greger
    Gestamp HardTech.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling of Plastic Deformation and Fracture in Hot Stamped Steel with Multi-Phase Microstructure2017In: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 207, p. 687-692Article in journal (Refereed)
    Abstract [en]

    Hot stamping is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production step. Within the field of hot stamping the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. The mechanical properties in a blank are modified by the formation of different microstructures. Martensite is a microstructure with high strength but low ductility, ferrite has lower strength but higher ductility. Using special tooling tough martensite and soft ferrite can be placed in adjacent sections in a blank. Between those sections a transition zone consisting of a mixed microstructure exists with mechanical properties between martensite and ferrite. Transition zones possess intermediate cooling rates, hence formation of bainite and composites of bainite and another phase can from.

    This paper presents an approach of modelling the complete process from austenitized blank to fracture. The method presented relies on the prediction of phases formed during cooling using an austenite decomposition model. In the course of ferrite formation the carbon content in the remaining austenite increases, the carbon content in austenite influences formation of additional daughter phases. The estimated phase composition is used in a homogenization scheme to predict the hardening of the material during plastic deformation. Fracture in the different microstructural phases is predicted using the strain decomposition provided by the homogenization and a fracture criteria. The homogenization scheme and the fracture criteria use measured data from single phase microstructures, i.e. ferrite, bainite and martensite.

    A heat treatment process for tensile test specimens is used to produce samples with different volume fractions of the microstructures ferrite, bainite and martensite. The pre-cut specimens are austenitized, ferrite is formed in a second furnace with lower temperature, bainite and martensite are formed by the use of a temperature controlled plane tool.

    Prediction of the phase content in mixed microstructures showed good agreement with microstructural characterization and therefore results can be used as input value for the homogenization. Comparing experimental and numerical results for a variety of different mixed microstructures good agreement in the prediction of hardening and fracture is found.

    It is concluded that the use of a homogenization method combined with a fracture model can be used to predict the mechanical response of mixed microstructures. The method described in the present work can be applied in the development of hot stamped components.

  • 3.
    Lindgren, Lars-Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Edberg, Jonas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Zhang, Zhao
    Dalian University of Technology, Dalian, China.
    Modeling of thermal stresses in low alloy steels2019In: Journal of thermal stresses, ISSN 0149-5739, E-ISSN 1521-074X, Vol. 42, no 6, p. 725-743Article in journal (Refereed)
    Abstract [en]

    Computing the evolution of thermal stresses accurately requires appropriate constitutive relations. This includes both the thermal and mechanical aspects, as temperature is the driver to thermal stresses. The paradigm of Integrated Computational Materials Engineering (ICME) aims at being able to quantitatively relate process-structure-property of a material. The article describes physics based models, denoted bridging elements, which are one step towards the vision of ICME. They couple material structure with heat capacity, heat conductivity, thermal and transformation strains and elastic properties for hypo-eutectoid steels. The models can account for the chemical composition of the steel and its processing, i.e. thermomechanical history, giving the evolution of the microstructure and the corresponding properties.

  • 4.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Sundin, Karl-Gustaf
    Wikman, Bengt
    Kajberg, Jörgen
    Åkerström, Paul
    Material characterisation using advanced experiments and inverse methods2006In: Computational mechanics: Abstracts : abstracts of the papers presented at the regular sessions of the sixth world congress on computational mechanics in conjunction with the second Asian-Pacific congress on computational mechanics, September 5-10, 2004, Beijing, China / [ed] Zhenhan Yao; Mingwu Yuan; Wanxie Zhong, Bejing: Tsinghua University Press, 2006Conference paper (Other academic)
  • 5.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Bergman, G.
    Salomonsson, Per
    Modelling of microstructure and material response in the press hardening process2008In: Conference Best in Class Stamping, June 16 - 18, 2008, Olofström, Sweden: [proceedings] / IDDRG, International Deep Drawing Research Group / [ed] Nader Asnafi, Olofström: Industriellt utvecklingscentrum i Olofström AB , 2008, p. 463-474Conference paper (Refereed)
    Abstract [en]

    The use of ultra-high strength components in automotive structures is rapidly increasing due to strong driving forces to reduce weight in order to minimise fuel consumption. This should also be accomplished with maintained or increased passenger safety. Components manufactured with the press hardening process meet most of the requirements and the market for such products is currently growing very fast. The quenching results in a material with a very high yield and tensile strength falling into the category of martensitic ultra high strength steels. Simulation of the complete press hardening process requires coupled thermo-mechanical transient analysis with the possibility to account for mechanical end thermal contact conditions between the tool and the blank. In addition, one sided or two sided contact areas may occur. Thus, large temperature variations through the thickness of the blank may be present during the process. In an earlier work, Bergman and Oldenburg formulated a thermal shell element with quadratic temperature interpolation through the thickness of the shell while there is a linear interpolation in the plane of the element. This formulation is implemented in the LS-Dyna code and is used in coupled thermomechanical analysis of hot forming processes. The modelling of the press hardening process has been developed in several steps. The model development involves e.g. determination of the flow stress, austenite decomposition modelling, constitutive modelling, experimental studies and evaluation of simulation results. The presented model accounts for the most significant phenomena occurring in the thermo-mechanical press hardening process. The mechanical response and the micro-structure evolution as well as the final material state can be predicted with good accuracy.

  • 6.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Bergman, Greger
    Gestamp Hardtech AB, Luleå.
    Salomonsson, Per
    Simulation and evaluation of phase transformations and mechanical response in the hot stamping process2007In: Materials Processing and Design: Modeling, Simulation and Applications; NUMIFORM 2007: Proceedings of the 9th International Conference on Numerical Methods in Industrial Forming Processes / [ed] Jose M.A.Cesar de Sa; Abel D. Santos, Melville, NY: American Institute of Physics (AIP), 2007, Vol. 908, p. 1181-1186Conference paper (Refereed)
    Abstract [en]

    When producing thin ultra high strength steel components with the hot stamping process it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components. Often the desirable microstructure consists of a mix of martensite and bainite. Therefore, it is of great importance to accurately predict the final microstructure of the component early in the product development process. In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The decomposition model is based on Kirkaldy's rate equations and later modifications by Li et al. The modified model accounts for the effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the simulated volume fractions of micro-constituents and hardness profiles show good agreement with the corresponding experimental observations. The phase proportions affect both the thermal and the mechanical properties during the process of continuous cooling and deformation of the material. A thermo-elastic-plastic constitutive model including effects from changes in the microstructure as well as transformation plasticity is implemented in the LS-DYNA code. The material model is used in combination with a thermal shell formulation with quadratic temperature interpolation in the thickness direction to simulate the complete process of simultaneous forming and quenching of sheet metal components. The implemented model is used in coupled thermo-mechanical analysis of the hot stamping process and evaluated by comparing the results from hot stamping experiments. The results from simulations such as local thickness variations, hardness distribution and spring-back in the component show good agreement with experimental results.

  • 7.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Bergman, Greger
    Salomonsson, Per
    Simulation of the micro structure evolution in a press hardened component2008In: Hot Sheet Metal Forming of High-Performance Steel: Proceedings, 1st International Conference, Kassel, Germany, October 22-24, 2008 / [ed] Kurt Steihoff; Mats Oldenburg; Braham Prakash, Bad Harzburg: GRIPS media , 2008, p. 3-13Conference paper (Refereed)
  • 8. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Bergman, Greger
    Experimental and numerical evaluation of the heat transfer coefficient in press hardening2008In: Hot Sheet Metal Forming of High-Performance Steel: Proceedings, 1st International Conference, Kassel, Germany, October 22-24, 2008 / [ed] Kurt Steihoff; Mats Oldenburg; Braham Prakash, Bad Harzburg: GRIPS media , 2008, p. 267-274Conference paper (Refereed)
    Abstract [en]

    When producing thin ultra high strength steel components with the press hardening process it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components were it is of great importance to accurately predict the final component properties early in the product development process. The transfer of heat is a key process that affects the evolution of the mechanical properties in the product and it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulations. In this study an experimental setup is developed combined with an elementary inverse simulation approach to predict the interfacial heat transfer coefficient (IHTC) when the hot blank and cold tool are in mechanical contact. Different process conditions such as contact pressure and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation, a thermo-mechanical coupled simulation model is used with a thermo-elastic-plastic constitutive model including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the pressure dependence for the two materials is different and the heat transfer coefficient is varying during quenching. This information together with further testing will be used as a base in a future model of the heat transfer coefficient influence at different conditions in press hardening process.

  • 9.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A Novel Tooling Technology for Hot Forming Processes2017In: 6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2: June 4-7 2017, Atlanta, Georgia, USA : proceedings / [ed] Mats Oldenburg, Braham Prakash, Kurt Steinhoff, Warrendale, PA: Association for Iron & Steel Technology, AIST , 2017, p. 243-250Conference paper (Refereed)
  • 10. Åkerström, Paul
    Material characterisation for simulation of press hardening2004Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has drastically increased the demand for ultra high strength steel components. There exist several production methods for such components, where the press hardening technique (hot stamping) is one of the most successful in producing complex components from boron steel. In order to accurately perform numerical Finite Element (FE) simulations of the actual thermo-mechanical forming, it is crucial to use correct material data and models. This work is focusing on two main aspects of the material characterisation as follows. The first is the flow stress of the austenite at elevated temperatures and different strain rates, relevant for the process, which is crucial for correctly predicting the strains in the component and the forming force. During a press hardening cycle, the actual forming is performed at high temperatures and the steel is in the austenitic state. The second, the austenite decomposition into daughter products such as ferrite, pearlite, bainite or martensite is a function of the thermal and mechanical history. To find the mechanical response (flow stress) for the austenite, a method based on multiple overlapping continuous cooling and compression tests (MOCCCT) in combination with inverse modelling has been developed. A validation test (in combination with the compression tests) shows good agreement with the simulated forming force, indicating that the estimated flow stress as a function of temperature, strain and strain rate is accurate in the actual application. The austenite decomposition model is developed and integrated as a material subroutine into the FE-code LS-DYNA. The model is based on the combined nucleation and growth rate equations proposed by Kirkaldy. A separate test to simulate different cooling histories along a boron alloyed steel sheet has been conducted.

  • 11.
    Åkerström, Paul
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling and simulation of hot stamping2006Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has drastically increased the demand for ultra high strength steel components. There are several production technologies for such components. The hot stamping technology (press hardening) is one of the most successful in producing complex components with superior mechanical properties. The hot stamping process can be described by the following steps; punching of blanks, heating to 900C in a furnace to austenitization followed by simultaneous forming and quenching in forming tools. In order to obtain accurate numerical Finite Element (FE) simulations of the actual thermo-mechanical forming, correct material data and models are crucial and mandatory. This work is focusing on three main aspects described below for the numerical simulation of the thermo-mechanical forming of thin boron steel sheets into ultra high strength components. The objective is to predict the shape accuracy, thickness distribution and hardness distribution of the final component with high accuracy. The first aspect is the flow stress of the austenite at elevated temperatures and different strain rates, which is crucial for correctly predicting the strains in the component and the forming force. During a hot stamping cycle, the actual forming is performed at high temperatures and the steel is mainly in the austenitic state. The second aspect is the austenite decomposition into daughter products such as ferrite, pearlite, bainite or martensite that is a function of the thermal and mechanical history. The third aspect is the mechanical material model used, which determine the stress state and consequently the component distortion. To find the mechanical response (flow stress) for the austenite, a method based on multiple overlapping continuous cooling and compression experiments (MOCCCT) in combination with inverse modelling has been developed. A validation test (in combination with the compression tests) shows good agreement with the simulated forming force, indicating that the estimated flow stress as a function of temperature, strain and strain rate is accurate in the actual application. The austenite decomposition model is developed and integrated as a material subroutine into the FE-code LS-DYNA. The model is based on the combined nucleation and growth rate equations proposed by Kirkaldy. A separate test to simulate different cooling histories along a boron alloyed steel sheet has been conducted. Different mixtures of daughter products are formed along the sheet and the corresponding simulation show acceptable good agreement with the experimentally determined temperature histories, hardness profile and volume fractions of the different microconstituents formed in the process. For the mechanical response, a mechanical constitutive model based on the original model proposed by Leblond has been implemented into LS-DYNA. The implemented model account for transformation induced plasticity (local plastic flow in austenite) according to the Greenwood-Johnson mechanism as well as classical plasticity during global yield. Finally, a FE-simulation using the implemented models of the thermo-mechanical forming of a component is compared to the corresponding experiment, including forming force, thickness distribution, hardness distribution and shape accuracy/springback.

  • 12. Åkerström, Paul
    et al.
    Bergman, Greger
    SSAB HardTech.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical implementation of a constitutive model for simulation of hot stamping2007In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 15, no 2, p. 105-119Article in journal (Refereed)
    Abstract [en]

    In order to increase the accuracy of numerical simulations of the hot stamping process, an accurate and robust constitutive model is crucial. During the process, a hot blank is inserted into a tool where it is continuously formed and cooled. For the steel grades often used for this purpose, the initially austenitized blank will decompose into different product phases depending on the cooling and mechanical history. As a consequence, the phase proportions change will affect both the thermal and mechanical properties of the continuously formed and cooled blank. A thermo-elastic-plastic constitutive model based on the von Mises yield criterion with associated plastic flow is implemented into the LS-Dyna finite element code. Models accounting for the austenite decomposition and transformation induced plasticity are included in the constitutive model. The implemented model results are compared with experimental dilatation results with and without externally applied forces. Further, the calculated isothermal mechanical response during the formation of a new phase is compared with the corresponding experimental response for two different temperatures.

  • 13. Åkerström, Paul
    et al.
    Bergman, Greger
    Gestamp R&D.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Salomonsson, Per
    Utveckling av mikrostruktur och mekanisk respons vid presshärdning2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 98-Conference paper (Other academic)
  • 14. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Austenite decomposition during press hardening of a boron steel: computer simulation and test2006In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 174, no 1-3, p. 399-406Article in journal (Refereed)
    Abstract [en]

    In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The model is based on Kirkaldy's rate equations. The basic rate equations has been modified to account for the austenite stabilization effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the obtained simulated volume fractions microconstituents and hardness profiles shows promising agreement to the corresponding experimental observations.

  • 15. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Studies of the thermo-mechanical material response of a boron steel by inverse modelling2004In: Proceedings: 2nd International Conference on Thermal Process Modelling and Computer Simulation : Nancy, France, March 31 - April 2, 2003 / [ed] S. Denis, Les Ulis: EDP Sciences, 2004Conference paper (Refereed)
  • 16. Åkerström, Paul
    et al.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Material parameter estimation for boron steel from simultaneous cooling and compression experiments2005In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 13, no 8, p. 1291-1308Article in journal (Refereed)
    Abstract [en]

    In order to increase the accuracy of numerical simulations of the hot stamping process, reliable material data is crucial. Traditionally, the material is characterized by several isothermal compression or tension tests performed at elevated temperatures and different strain rates. The drawback of the traditional methods is the appearance of unwanted phases for some test temperatures and durations. Such an approach is also both time consuming and expensive. In the present work an alternative approach is proposed, which reduces unwanted phase changes and the number of experiments. The isothermal mechanical response is established through inverse modelling of simultaneous cooling and compression experiments. The estimated material parameters are validated by comparison with data from a separate forming experiment. The computed global response is shown to be in good agreement with the experiments.

1 - 16 of 16
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