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  • 101.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindkvist, Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Tool thermal conditions for tailored material properties2011In: HTM: Journal of Heat Treatment and Materials: Zeitschrift für Werkstoffe, Wärmebehandlung, Fertigung, ISSN 0341-101X, Vol. 66, no 6, p. 329-334Article in journal (Refereed)
    Abstract [en]

    With the press hardening technology it is possible to produce components with tailor- made material properties. The present work is based on a comprehensive thermo-mechanical material model for simulation of press hardening and similar processes. The model accounts for and predicts time dependent and timeindependent phase transformations, mechanical and thermal properties as well as transformation plasticity during the complete cooling and deformation process from 900 °C to room temperature. The model predicts the microstructure evolution during the complete process as well as the final fractions. The thermal contact properties and the thermal properties in the tools have influence on the final material state. Solutions for tool heating and cooling for manufacturing of components with soft zones are studied. Based on the results from the press hardening simulations, thermal properties of special tool materials are taken into account in order to optimise the manufacturing thermal cycle.

  • 102.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindkvist, Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Tool thermal cycle design for manufacturing of components with tailored material properties2011In: Hot sheet metal forming of high-performance steel, CHS2: 3rd international conference, June 13-17 2011, Kassel, Tyskland. Proceedings / [ed] Mats Oldenburg; Kurt Steinhoff; Braham Prakash, Auerbach: Verlag Wissenschaftliche Scripten , 2011, p. 203-2010Conference paper (Refereed)
    Abstract [en]

    With the press hardening technology, it is possible to produce automotive structural components with a tailor-made material property distribution. The properties can range from a soft ferrite dominated micro-structure to ultra-high strength martensitic structure. Tailor-made material prop-erty distribution is especially useful in components subjected to crash deformation. With this tech-nique it is possible to account for structural integrity and in the same time maximize the energy absorption in the structure. The present work is based on a comprehensive thermo-mechanical material model for simulation of press hardening and similar processes. The model accounts for and predicts time dependent and time-independent phase transformations, mechanical and thermal properties as well as transformation plasticity during the complete cooling and deformation proc-ess from 900 oC to room temperature. The model predicts the microstructure evolution during the complete process and the final fractions of martensite, bainite, perlite and ferrite. The thermal properties in the contact between the blank and the tools as well as the thermal properties in the tools have influence on the processes for tailor-made material properties. In this work, solutions for tool heating and cooling in the design of tools for manufacturing of components with soft zones are studied. Based on the results from the press hardening simulations, thermal properties of special tool materials are taken into account in order to optimise the manufacturing thermal cycle.

  • 103.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Nilsson, Larsgunnar
    Linköping Institute of Technology.
    Position code algorithm for contact searching1994In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 37, no 3, p. 359-386Article in journal (Refereed)
    Abstract [en]

    A new global search method for general contact systems is developed and implemented in the DYNA3D program, along with a recent contact interface algorithm. The concept of `position codes' for efficient global contact searching is presented. With the position code algorithm, the problem of sorting and searching in three dimensions is transformed to a process of sorting and searching within a one-dimensional array. The cost of contact searching is of the order of N log2 N, where N is the number of nodes in the system. The proposed algorithms are uncomplicated and the implementation into any finite element code is straightforward. Numerical experiments are presented in order to examine the behaviour of the algorithms in different aspects.

  • 104.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Sandberg, L.
    Plannja HardTech AB.
    Bergman, Greger
    Plannja HardTech AB.
    Jonsson, Mikael
    Experiments with one-sided spray-cooling of steel plates for evaluation of thermo-mechanical material models1996In: Quenching and the control of distortion: proceedings of the 2nd International Conference on Quenching and the Control of Distortion, 4 - 7 November 1996, Cleveland Marriott Society Center, Cleveland, Ohio / [ed] George E. Totten, Materials Park, Ohio: ASM International, 1996, p. 267-273Conference paper (Refereed)
    Abstract [en]

    The quenching process of thin plates is studied. In many industrial applications of quenching, there is a need for simple tests for evaluation of analysis methods and material models for simulations. The presented study is part of a program for the simulation of the combined pressing and quenching process of thin-walled structures. This type of manufacturing process is used in, e.g., the production of side-impact protection beams for cars. By the use of an experiment with one-sided spray-cooling of a thin strip, large deformations can be studied and followed in time. The prediction of the deformation of the strip can be regarded as a one-dimensional problem where the curvature of the strip is sought. The thermo-mechanical behavior of the material can be evaluated throughout the time-history of the tests. In order to match the deformation of the plate, the material model must predict the temperature distribution through the thickness of the plate in each point in time, the phase transformations taking place in each layer of the thin plate and the stress and strain components in each layer. Experiments have been performed with a material (Inconel 600) without phase transformations in the actual temperature and time scale and with a material with only martensitic transformations. Analyses with a thermo-elastic-plastic material model with transformation plasticity included have been performed

  • 105.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Steinhoff, KurtUniversity of Kassel.Prakash, BrahamLuleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hot sheet metal forming of high-performance steel, CHS2: 2nd international conference, June 15-17 2009, Luleå, Sweden. Proceedings2009Collection (editor) (Other academic)
  • 106.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Steinhoff, KurtUniversity of Kassel.Prakash, BrahamLuleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hot sheet metal forming of high-performance steel, CHS2: 3rd international conference, June 13-17 2011, Kassel, Germany. Proceedings2011Collection (editor) (Other academic)
  • 107.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Steinhoff, KurtUniversity of Kassel.Prakash, BrahamLuleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Proceedings: 4th International Conference Hot Sheet Metal Forming of High Performance Steel June 9 - 12, 2013, Luleå, Sweden2013Collection (editor) (Other academic)
  • 108.
    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)
  • 109.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Wikman, Bengt
    Häggblad, Hans-Åke
    Study of metal powder component pressing with use of numerical simulations1998In: Proceedings of the 1998 powder metallurgy world congress and exhibition: Granada, Spain, October 18 - 22, 1998, Shrewsbury: European powder metallurgy association , 1998, Vol. 2, p. 2.51-2.56Conference paper (Refereed)
    Abstract [en]

    The global behaviour of the cold pressing process of iron powder is dependent on the process kinematics, material response in the powder as well as the friction effects in the contact between the powder and the tool walls. The analysis of powder pressing requires accurate material models of the various powder mixes that are used. The modelling is dependent on tri-axial compression tests for the material model and friction tests for the nonlinear friction model. The modelling parameters have been determined in international cooperation within the COST 512 and MMPF (Modelling of Metal Powder Forming) research programs. Component pressing with varying kinematic conditions have been performed by the Sintertech company. In this work, a set of process and model parameters have been established for numerical simulations of powder pressing process.

  • 110.
    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.

  • 111.
    Oldenburg, Mats
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Gestamp Hardtech AB.
    Bergman, Greger
    Gestamp Hardtech AB.
    Salomonsson, Per
    Microstructure evolution and mechanical response in the hot stamping process2007In: 6th European LS-Dyna Users' Conference: Gothenburg, Sweden, ERAB , 2007, p. 135-144Conference paper (Refereed)
  • 112.
    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.

  • 113.
    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)
  • 114.
    Pérez Caro, Lluís
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Forming group OSAS, Industrial Development Centre in Olofström AB, Vällaregatan 30, Olofström.
    Marth, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Schill, Mikael
    DYNAmore Nordic AB, Linköping.
    Odenberger, Eva-Lis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Forming group OSAS, Industrial Development Centre in Olofström AB, Vällaregatan 30, Olofström.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Calibration of a damage and fracture model for alloy 7182016In: Advancements in Theoretical and Applied Plasticity: Proceedings of PLASTICITY ’16: The Twenty Second International Symposium on Plasticity and its Current Applications / [ed] Akhtar S. Khan, Fulton, Maryland 20759-0591, USA: NEAT PRESS , 2016, p. 223-225Conference paper (Refereed)
    Abstract [en]

    Nickel-based superalloys are primarily used in the hot sections of aircraft engines because they can maintain their mechanical properties and chemical stabilities at high temperatures under severe corrosive environments i.e. for a long time. In order to simulate forming procedures in Alloy 718 sheets, the GISSMO damage and failure model is coupled with both isotropic von Mises and anisotropic Barlat YLD2000 material models in the finite element code LS-DYNA. In this study, the calibration of the GISSMO model for forming simulations at room temperature is discussed. The calibration requires failure strains for different stress states as a function of triaxiality, which are obtained by testing six different specimen geometries up to fracture. Numerical predictions will be compared with experimental observations from forming tests.

  • 115.
    Pérez Caro, Lluís
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. RISE IVF AB.
    Schill, Mikael
    DYNAmore Nordic AB.
    Haller, Kristian
    AcousticAgree AB.
    Odenberger, Eva-Lis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. RISE IVF AB.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Damage and fracture during sheet-metal forming of alloy 7182019In: International Journal of Material Forming, ISSN 1960-6206, E-ISSN 1960-6214Article in journal (Refereed)
    Abstract [en]

    Forming nickel-based superalloy aero-engine components is a challenging process, largely because of the risk of high degree of springback and issues with formability. In the forming tests conducted on alloy 718 at room temperature, open fractures are observed in the drawbead regions, which are not predicted while evaluating the formability using the traditional forming-limit diagram(FLD). This highlights the importance of an accurate prediction of failure during forming as, in some cases, may severely influence the springback and thereby the accuracy of the predicted shape distortions, leading the final shape of the formed component out of tolerance. In this study, the generalised incremental stress-state dependent damage model (GISSMO) is coupled with the isotropic von Mises and the anisotropic Barlat Yld2000-2D yield criteria to predict the material failure in the forming simulations conducted on alloy 718 using LS-DYNA. Their effect on the predicted effective plastic strains and shape deviations is discussed. The failure and instability strains needed to calibrate the GISSMO are directly obtained from digital image correlation (DIC) measurements in four different specimen geometries i.e. tensile, plane strain, shear, and biaxial. The damage distribution over the drawbeads is measured using a non-linear acoustic technique for validation purposes. The numerical simulations accurately predict failure at the same regions as those observed in the experimental forming tests. The expected distribution of the damage over the drawbeads is in accordance with the experimental measurements. The results highlight the potential of considering DIC to calibrate the GISSMO in combination with an anisotropic material model for forming simulations in alloy 718.

  • 116. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Investigation of heat transfer in the press hardening process2009In: Hot sheet metal forming of high-performance steel, CHS2: 2nd international conference, June 15-17 2009, Luleå, Sweden. Proceedings / [ed] Mats Oldenburg; Kurt Steinhoff; Braham Parkash, Auerbach: Verlag Wissenschaftliche Scripten , 2009, p. 239-246Conference paper (Refereed)
    Abstract [en]

    The use of thin ultra high strength steel automotive passive safety components manufactured with the press hardening process has increased drastically the last two decades. The application and complexity in design has increased and thermo mechanical forming simulation is an important tool to optimize the final component properties early in the product development process. The transfer of heat is a key process that affects formability and the evolution of the mechanical properties in the product. Therefore it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulation code. In this study, an experimental setup and inverse simulation approach is used to predict the interfacial heat transfer coefficient (IHTC) in the press hardening process. Different process conditions such as contact pressure, air gaps in the contact and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation approach, the improved-advance-retreat and golden section method is used to solve the ill-posed inverse heat conduction problem (IHCP). In the finite element analysis, a thermo-mechanical coupled simulation model is used 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 heat transfer coefficient is dependent of the contact pressure and air gap and varies in time. The variation in time is assumed to be the temperature dependence. When heat transfer is under mechanical contact, the variation of the heat transfer coefficient is higher for higher contact pressures compared to lower. If there is heat transfer over a thin gap of air, the heat transfer coefficient also varies but tend to be close to constant for bigger air gaps. The present work will be used as a base in a future model of the heat transfer coefficient influence at different contact conditions in press hardening process.

  • 117. 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.

  • 118. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Åkerström, Paul
    Gestamp Hardtech AB, Luleå.
    Bergman, Greger
    Gestamp Hardtech AB, Luleå.
    Experimental and numerical evaluation of the heat transfer coefficient in press hardening2009In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 80, no 11, p. 841-845Article in journal (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 where 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.

  • 119.
    Sandberg, Marcus
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Industrilized and sustainable construction.
    Kokkolaras, Michael
    Larsson, Tobias
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Boart, Patrik
    Volvo Aero Corporation, Sverige.
    Projekt: Strukturell konceptuell konstruktion och analys av helajetmotorer - en METodik för OPtimering, Integration och Automatisering (METOPIA)2009Other (Other (popular science, discussion, etc.))
    Abstract [sv]

    Detta projekt är en fortsättning på förstudieprojektet NFFP4202 - Helmotormodell för systemanalys av mekaniska egenskaper där en plattform, samt en pilot som demonstrerade plattformens förmåga i ett industriellt sammanhang, utvecklades. Detta projekt fokuserar på vidareutveckling av den framtagna plattformen med optimeringsteknologi och simuleringsdriven produktutvecklingsmetodik för att via produktdefinitionen (jetmotorkomponenterna) balansera flera olika funktionsbehov (t ex strukturdynamiska, aerodynamiska, termodynamiska) och skapa möjligheter till effektivare analyser av helmotorkoncept. Resultatet är en metodik innefattande systemangreppssätt, modelluppbyggnad, modellarkitektur samt analys, som tillämpas på ett realistiskt scenario genom en pilot.

  • 120.
    Sandberg, Marcus
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Industrilized and sustainable construction.
    Kokkolaras, Michael
    Larsson, Tobias
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Projekt: NFFP4 - Helmotormodellering2009Other (Other (popular science, discussion, etc.))
    Abstract [sv]

    Nationella Flygtekniska Forskningsprogrammet NFFP Projekt: V4202 Helmotormodellering

  • 121.
    Sjöberg, Ted
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kajberg, Jörgen
    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.
    Calibration and Validation of Three Fracture Criteria for Alloy 718 Subjected to High Strain Rates and Elevated Temperatures2018In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 71, p. 34-50Article in journal (Refereed)
    Abstract [en]

    The aerospace industry has an important role to play in lowering greenhouse gas emissions and thereby reducing its ecological footprint. Aerospace manufacturers do therefore push components and materials closer to their limits in order to design for minimal weight. An example of component is the metallic turbine case of the aircraft engine for protection of the surrounding structure in situations such as blade-off events. This implies design, where knowledge regarding mechanical material response from onset of yield to fracture, at both high strain rates and elevated temperatures, are needed. Alloy 718 is a nickel-based superalloy commonly used in the containment structures of the hot parts of the aircraft engines. Three established criteria, Modified-Mohr-Coulomb, Maximum Shear Stress and Magnitude of Stress Vector, have been applied to characterise the fracture behaviour of Alloy 718 supplied to conditions present at a blade-off event. The calibrations of the criteria were based on high-speed tensile tests of thin sheet specimens with different geometries for varying different stress states. The fracture strains were determined using high-speed photography combined with digital image correlation. The temperature and strain rate were varied from 20 to 650 °C and 1 to 1000 s−1 respectively. The calibrated criteria were validated through finite element analyses and reverse impact testing at room temperature up to 650 °C. Discs were fired against a instrumented rod with different shapes of their tips for obtaining varying stress states. It was found that the Modified-Mohr-Coulomb locus showed the best agreement with measured fracture strains in the calibration experiments. This criterion did also predict the fractures of discs in the reverse impact tests fairly well. The possibility to accurately predict fracture facilitates the use of modern numerical software for containment design as a complement to time-consuming and expensive full-scale tests.

  • 122.
    Sjöberg, Ted
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kajberg, Jörgen
    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.
    Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities2017In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 178, p. 231-242Article in journal (Refereed)
    Abstract [en]

    A methodology for fracture characterisation at strain rates up to 1000 s−1, temperatures up to 650 °C, and various stress triaxialities is presented. High-speed photography combined with digital image correlation is used to evaluate the strain at fracture. The methodology was successfully demonstrated on aged nickel based Alloy 718, commonly used in the containment structure of aircraft engines. Tensile specimens with four different geometries were loaded to get a wide range of positive stress triaxialities. All specimens originated from one single heat and batch to ensure consistent mechanical properties. The results showed evident stress state dependency on the failure strain, where lower failure strains were observed at higher stress triaxialities for all combinations of temperatures and strain rates. A coupled relationship between the temperature and the stress triaxiality controlling the fracture strain was found. However, any clear dependency on strain rate was hard to detect.

  • 123.
    Sjöberg, Ted
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Sundin, Karl-Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kajberg, Jörgen
    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.
    Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures2016In: Thin-walled structures, ISSN 0263-8231, E-ISSN 1879-3223, Vol. 107, p. 671-677Article in journal (Refereed)
    Abstract [en]

    An experimental technique has been developed which allows loading of heated sheet material under impact conditions with simultaneous measurement of the impact force history. The combined characteristics of impact loading at elevated temperature makes the experiment ideal for validation of models used to simulate the containment structure surrounding aircraft engines. In this paper experimental results for Alloy 718 are presented, a nickel based super alloy commonly used in hot parts of the containment structure. The experimental results are then compared to simulations in order to validate previously calibrated material parameters. The basic principle of the validation experiment is based on reverse ballistics, in which a thin circular specimen with free boundaries impacts the end of an instrumented rod. Using induction heating the specimen is heated to temperatures up to 650 °C and a gun driven by compressed air accelerates the specimens to desired velocity. In the reported work velocities are kept low enough to avoid cracking and thus the study is limited to plastic conditions, even though the technique is applicable also for fracture studies. The free boundaries of the experiment makes numerical modelling and simulation straightforward, making it valuable as a validation tool. All numerical simulations are performed using the commercial finite element code LS-Dyna and plastic behaviour of the material was modelled with the Johnson-Cook material model

  • 124.
    Sjöberg, Ted
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Sundin, Karl-Gustaf
    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.
    Calibration and validation of plastic high strain rate models for alloy 7182013In: Computational Plasticity XII / [ed] E. Oñate; D.R.J. Owen; D. Peric; B. Suárez, Barcelona, 2013, p. 469-479Conference paper (Refereed)
    Abstract [en]

    Alloy 718 (Inconel™ 718), composed mainly of nickel, iron and chromium has properties that are of interest in many high temperature applications. One such application is the containment structure in aero engines which prevents fragments from penetrating the structure in case of blade failure. Impact of blade fragments on the containment structure includes both high strain rates and high temperatures and simulation models must therefore have their base in experimental conditions including transient loading and heating. An experimental method has been developed that utilizes induction heating in a high rate tensile test machine. Strain rates up to the order of 1000 s-1 and temperatures up to 650 °C have been included in the test program. Material parameters for the Johnson-Cook and the Zerilli-Armstrong models are evaluated from experimental data using optimisation. These parameters are then used to simulate a specially designed impact experiment and a direct comparison of a calculated and measured quantity can be made in order to validate model and parameters. The result from the validation experiment showed better agreement for the Johnson-Cook model than the Zerilli-Armstrong model

  • 125.
    Somani, Mahesh C.
    et al.
    Uleåborg universitet.
    Karjalainen, Pentti
    Uleåborg universitet.
    Eriksson, Magnus
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dimensional changes and microstructural evolution in a B-bearing steel in the simulated forming and quenching process2001In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 41, no 4, p. 361-367Article in journal (Refereed)
    Abstract [en]

    To improve the modelling of the behaviour of steel profiles in the forming and quenching process, the influences of high-temperature plastic deformation and applied stress on the martensitic transformation were investigated in a B-bearing steel by dilatometric measurements and compression tests. The plastic deformation of austenite was found to enhance ferrite formation so significantly that the dilatation due to the low-temperature transformation decreases even at a cooling rate of 280°C/s. The presence of ferrite in the microstructure results in markedly lower hardness and flow stress than the completely martensitic microstructure. Possibilities to avoid ferrite formation have been discussed. Stress applied during the martensitic transformation increases diametric dilatation by as much as 200 % under axial compression, which seems to result from the preferred orientation of the martensite formed. However, subsequent to a hightemperature plastic deformation, the influence of applied stress remains much smaller.

  • 126. Somani, M.C.
    et al.
    Karjalainen, L.P.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Eriksson, Magnus
    Effects of plastic deformation and stresses on dilatation during the martensitic transformation in a B-bearing steel2001In: Journal of Materials Science & Technology, ISSN 1005-0302, Vol. 17, no 2, p. 203-206Article in journal (Refereed)
    Abstract [en]

    To provide data for improved modelling of the behaviour of steel components in a simultaneous forming and quenching process, the effects of plastic deformation and stresses on dilatation during the martensitic transformation in a B-bearing steel were investigated. It was found that plastic deformation of austenite at high temperatures enhances ferrite formation significantly, and consequently, the dilatation decreases markedly even at a cooling rate of 280°C/s. The created ferritic-martensitic microstructure possesses clearly lower hardness and strength than the martensitic structure. Elastic stresses cause the preferred orientation in martensite to be formed so that diametric dilatation can increase by nearly 200% under axial compression

  • 127.
    Steihoff, Kurt
    et al.
    University of Kassel.
    Oldenburg, MatsLuleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.Prakash, BrahamLuleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hot sheet metal forming of high-performance steel: Proceedings of the 1st International Conference, Kassel, Germany, October 22-24, 20082008Collection (editor) (Other academic)
  • 128.
    Sundin, Karl-Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Granström, Jan
    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.
    Experimental investigation of plastic and fracture behaviour of 22MnB5 at hot stamping conditions2013In: 4th International Conference Hot Sheet Metal Forming of High-performance Steel CHS: June 9-12, Luleå, Sweden : Proceedings / [ed] Mats Oldenburg; Braham Prakash; Kurt Steinhoff, Auerbach: Verlag Wissenschaftliche Scripten , 2013, p. 557-564Conference paper (Refereed)
  • 129.
    Westman, Eva-Lis
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Pederson, Robert
    Luleå tekniska universitet.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical and microstructural evaluation of elevated temperature compression tests on Ti-6AI-4V2004In: Ti-2003 : science and technology: proceedings of the 10. World Conference on Titanium, held at the CCH-Congress Center Hamburg, Germany, 13 - 18 July 2003 / [ed] Gerd Lütjering, Weinheim: John Wiley & Sons, 2004Conference paper (Refereed)
  • 130. Wikman, Bengt
    et al.
    Bergman, Greger
    Gestamp Hardtech AB.
    Oldenburg, Mats
    Häggblad, Hans-åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Estimation of constitutive parameters for powder pressing by inverse modelling2006In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 31, no 5, p. 400-409Article in journal (Refereed)
    Abstract [en]

    Powder metallurgy processes are used in many material technologies for manufacturing of a wide range of industrial parts. Products such as components for cars, cemented carbides and high-speed steels for mechanical cutting, magnets and soft magnetic materials, bearings and refractory metals are made from powder. These parts are manufactured by powder die pressing followed by sintering of the resulting green body in a furnace. Traditionally, experience-based methods have been used to design and adapt the processing variables for optimal performance. Cost savings can be made if the tool design can be based on reliable predictive numerical simulations of the powder compaction process. Computer modelling could aid process and design engineers in selecting and optimizing the best pressing route for many industrial components. The aim of the present work has been to develop an efficient way to determine the necessary constitutive model parameters of the numerical models by means of inverse modelling. An experiment for establishing input data to the inverse problem has been designed and validated. The objective function is formed based on the discrepancy in force-displacement data between the numerical model prediction and the experiment. Minimization of the objective function with respect to the material parameters is performed using an in-house optimization software shell which is built on a modified Nelder-Mead simplex method also known as the subplex method. The completed analyses show that the proposed approach can readily be used to determine material parameters.

  • 131. Wikman, Bengt
    et al.
    Häggblad, Hans-åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Inverse modelling of powder compaction: the influence of material and friction behaviour interaction2000In: Proceedings of 2000 Powder Metallurgy World Congress : November 12 - 16, 2000, Kyoto International Conference Hall, Japan: organized by Japan Society of Powder and Powder Metallurgy, Japan Powder Metallurgy Association / [ed] Koji Kosuge; Hiroshi Nagai, Tokyo: Japan Society of Powder and Powder Metallurgy , 2000Conference paper (Refereed)
  • 132. Wikman, Bengt
    et al.
    Häggblad, Hans-åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Oldenburg, Mats
    Modelling of powder-wall friction for simulation of iron powder pressing1997In: Proceedings of the International workshop on modelling of metal powder forming processes, 1997Conference paper (Refereed)
  • 133. Wikman, Bengt
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-åke
    Modelling methods in pressing simulations of metal powder components1998In: Simulation of materials processing : theory, methods and applications: international conference on numerical methods in industrial forming processes, NUMIFORM '98 / [ed] J. Huétink; F.P.T. Baaijens, Rotterdam: Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1998, p. 397-402Conference paper (Refereed)
    Abstract [en]

    The process of pressing metal powder to net shape with subsequent sintering is very important for the automotive industry and similar industries with production of metal components in large quantities. The development of products and tools for the pressing of the products is a very time consuming and costly process. It is of great importance for the industry to be able to predict the result of the pressing operation with respect to the powder flow and final density distribution. With this knowledge the time and cost for product development, tool manufacturing and tool testing can be significantly reduced. In this work a set of methods used for analyses of the pressing process are presented. The methods used are based on an explicit finite element formulation with arbitrary Lagrangian Eulerian formulation. The material model used is a cap-plasticity model. The contact constraint are imposed with a method based on explicit integration of the contact interface equations in order to determine the contact forces. The contact algorithm includes a non-linear friction model with parameters based on experimental observations. An analysis of a pressed component is presented and compared with experimental results.

  • 134. Wikman, Bengt
    et al.
    Solimannezhad, N.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-åke
    Wall friction coefficient estimation through modelling of powder die pressing experiment2000In: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 43, no 2, p. 132-138Article in journal (Refereed)
    Abstract [en]

    Forming of PM components through powder pressing is a process, which is influenced by the friction between the powder and the tool walls. For good performance of the pressing process it is of great interest to understand and estimate the effects of powder-wall friction. However, quantification of the friction coefficient between the powder and the tool is a delicate task. Local contact conditions, such as contact stress, must be measured or otherwise estimated in order to evaluate the coefficient of friction. Here, the friction coefficient is estimated by combining an experiment with modelling of the experiment. Two methods for assessment of friction are presented, an optimization approach using finite element analyses and an analytical approach. Experimental data are taken from the single action cold pressing of a cylinder shaped component.

  • 135. 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.

  • 136. 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)
  • 137. 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.

  • 138.
    Åkerström, Paul
    et al.
    Gestamp Hardtech AB, Luleå.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical simulation of a thermo-mechanical sheet metal forming experiment2008In: Numisheet 2008: Proceedings of the 7th international conference and workshop on numerical simulation of 3D sheet metal forming processes, September 1-5, 2008, Interlaken, Switzerland / [ed] Pavel Hora, Zurich: Institute of Virtual manufacturing, ETH Zurich , 2008, p. 569-574Conference paper (Refereed)
  • 139. 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)
  • 140. 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.

  • 141.
    Östlund, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Berglund, Daniel
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Failure analysis of a hat profile with tailored properties subjected to axial compression2013In: 4th International Conference Hot Sheet Metal Forming of High-performance Steel CHS: June 9-12, Luleå, Sweden : Proceedings / [ed] Mats Oldenburg; Braham Prakash; Kurt Steinhoff, Auerbach: Verlag Wissenschaftliche Scripten , 2013, p. 23-30Conference paper (Refereed)
  • 142.
    Östlund, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Golling, Stefan
    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.
    Microstructure based modeling of ductile fracture initiation in press-hardened sheet metal structures2016In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 302, p. 90-108Article in journal (Refereed)
    Abstract [en]

    The manufacturing of sheet metal components with spatially varying microstucture composition and mechanical properties using press-hardening technology is now an established practice in the automotive industry. To estimate the performance envelopes of such components, a multi-scale approach to ductile fracture prediction based on mean-field homogenization is proposed. Two non-interacting fracture criteria are formulated in terms of the local average stress field, referring to inter-phase and intra-phase fracture mechanisms. The overall ductility is governed by the weakest constituent or interface present in the multiphase material. Moreover, instabilities related to the strain localization problem at the macroscale are treated by embedding discontinuities in the element formulation. These are triggered by a localization criterion derived via bifurcation analysis of the homogenized material. Issues concerning numerical implementation include a forward Euler scheme for integrating the mean-field equations, suitable for explicit finite element analysis of heterogeneous materials. Tensile specimens with ten distinctly different microstructure compositions are evaluated, for which useful predictions of the overall force-displacement response and fracture elongations are demonstrated.

  • 143. Östlund, Rickard
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Microstructure-Based Modelling of Ductile Failure2015In: Hot Sheet Metal Forming of High-Performance Steel 5th International Conference: May 31-June 3, Toronto, Canada : Proceedings / [ed] Kurt Steinhoff; Mats Oldenburg; Braham Prakash, Auerbach: Verlag Wissenschaftliche Scripten , 2015, p. 149-156Conference paper (Refereed)
    Abstract [en]

    This study is concerned with plasticity and ductile fracture modelling aspects of press hardened steels, where multiple phases of different yield strength, work hardening and ductility are present. Macroscopic constitutive properties are determined using a semi-analytical approach termed Mean Field Homogenization (MFH), based on the properties of individual phases and micro-topology. This is combined with a phenomenological ductile fracture initiation criterion formulated in stress space at the microscopic scale. Localization enhanced kinematics are introduced at the macro-scale when instability is signaled by a local bifurcation analysis of the homogenized material. Discontinuities in the strain field are introduced into a quadrilateral shell based on the weak discontinuity approach. Fracture properties and flow curves up to large strains are determined from single phase tensile experiments using digital image correlation. The predictions of the integrated MFH and fracture model is compared with experimental results on steel sheets with different thermal histories, in terms of fracture prediction and and overall mechanical properties

  • 144.
    Östlund, Rickard
    et al.
    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.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Berglund, D.
    Gestamp R&D.
    Evaluation of localization and failure of boron alloyed steels with different microstructure compositions2014In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, no 3, p. 592-598Article in journal (Refereed)
    Abstract [en]

    Within the press hardening technology, where hot sheet blanks are simultaneously formed fixed and quenched, new methods with differential thermal treatment come to light. With controlled tool temperature variation, components with tailored properties can be produced. Automotive components combining high energy absorption and intrusion protection in a crash situation are feasible. In the present work the mechanical properties of three different material qualities, beginning with the same base sheet metal subjected to different thermal histories, are investigated. A strategy for modelling post-necking response and crack initiation using shell elements larger then the typical bandwidth of the localized neck is used. The model relies on a sequence of full field measurements throughout a tensile test; i.e. Digital Speckle Photography(DSP). The full field experimental method allows for evaluation of mechanical and failure properties at different analysis lengths, providing parameters for a model which accounts for shell element size. Additionally the model contains a strain based failure criteria as a function of stress triaxiality. Good correlations between a simulated tensile test and experimental results were found. A detailed metallographic study of the three grades was performed and is presented.

  • 145.
    Östlund, Rickard
    et al.
    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.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Berglund, Daniel
    Gestamp Hardtech AB, Luleå, Sweden.
    Failure model evaluation for varying microstructure based on material hardness2011In: Proceedings: 3rd International Conference Hot Sheet Metal Forming of High Performance Steel : June 13 - 17, 2011, Kassel, Germany / [ed] Mats Oldenburg, Auerbach: Verlag Wissenschaftliche Scripten , 2011Conference paper (Refereed)
  • 146.
    Östlund, Rickard
    et al.
    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.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Berglund, Daniel
    Gestamp R&D.
    Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion2015In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 56, p. 1-10Article in journal (Refereed)
    Abstract [en]

    Strain localization and fracture initiation of elasto-plastic thin steel sheets is analyzed. A shell element enhanced by embedded discontinuities is developed to improve coarse-mesh accuracy in terms of fracture initiation prediction and to regularize the post-instability response. Discontinuities in the strain field are introduced when instability is signaled by a local bifurcation analysis. The enhancements are implemented for the Belytschko-Lin-Tsay shell element. This is combined with a stress based fracture criterion which relates the magnitude of the stress vector and the first invariant of the stress tensor. A robust experimental procedure based on full-field measurements enable direct calibration in stress space, and provides a flow curve up to large strains. Numerical examples involving tensile samples with different localization behavior are presented to demonstrate significant reduction of spurious mesh dependence. Moreover, the engineering feasibility of the direct use of a stress based fracture criterion in combination with the enhanced element is evaluated by comparison of measured and calculated fracture elongations.

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