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  • 1.
    Casellas, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Frómeta, David
    Fundació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa, Spai.
    Lara, Toni
    undació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa, Spai.
    Molas, Silvia
    undació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa, Spai.
    Jonsén, Pär
    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.
    A fracture mechanics approach to develop high crash resistant microstructures by press hardening2017In: 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. 101-107Conference paper (Refereed)
    Abstract [en]

    Crashworthiness is a relevant engineering property for car parts. However it is not easy to measure at laboratory scale and complex impact tests have to be carried out to determine it. Crash resistance for high strength steel is commonly evaluated in terms of cracking pattern and energy absorption in crashed specimens. Accordingly, the material resistance to crack propagation, i.e. the fracture toughness, could be used to rank crashworthiness. It has been proved in a previous work by the authors, so the measure of fracture toughness, in the frame of fracture mechanics in small laboratory specimens, would allow determining the best microstructure for crash resistance parts. Press hardening offers the possibility to obtain a wide range of microstructural configurations, with different mechanical properties. So the aim of this work is to evaluate the fracture toughness following the essential work of fracture methodology for ferrite-pearlite, bainite, ferrite-bainite, martensite and martensite-bainite microstructures. Results showed that bainitic microstructures have high fracture toughness, similar to TWIP and CP steels, which allows pointing them as potential candidates for obtaining high crash resistance in parts manufactured by press hardening.

  • 2.
    Casellas, Daniel
    et al.
    Fundació CTM Centre Tecnològic / Universitat Politècnica de Catalunya, Universitat Politècnica de Catalunya, Fundació CTM Centre Tecnològic, Avda.
    Lara, Antoni
    Fundació CTM Centre Tecnològic.
    Molas, Silvia
    Fundació CTM Centre Tecnològic.
    Gironès, Anna
    Fundació CTM Centre Tecnològic.
    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.
    Fracture resistance of tailor tempered microstructures obtained by different press hardening conditions2015In: 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. 221-229Conference paper (Refereed)
    Abstract [en]

    Tailored tempering can produce different amounts of martensite, upper and lower bainite and ferrite depending on the press hardening conditions are usually obtained. The tensile properties of such microstructures are quite well known, but the intrinsic fracture properties, as fracture toughness, have not yet been characterized. This is mainly related the experimental difficulty to measure fracture toughness in thin sheets. Recently, the authors have proposed to measure it through the application of the Essential Work of Fracture methodology. The knowledge of facture toughness would give valuable information to increase their applicability in automotive components and would also allow a further understanding of fracture and crack propagation mechanisms in B steel hardened at different cooling conditions. Thus the aim of this paper is to determine the fracture toughness in microstructures of a 22MnB5 steels obtained by tailored tempering. Results indicate that the EWF methodology can be applied to measure fracture toughness and the energy for crack initiation in ferrite-pearlite and ferrite-bainite microstructures. Fracture initiation energy is the same in ferrite-pearlite and ferrite-bainite microstructures, which will indicate that ferrite has a clear effect on fracture initiation. Moreover, fractography results reveal the effect of inclusion content on fracture energies

  • 3.
    Frómenta, David
    et al.
    Eurecat, Centre Tecnològic de Catalunya.
    Parareda, Sergi
    Eurecat, Centre Tecnològic de Catalunya.
    Lara, Antoni
    Eurecat, Centre Tecnològic de Catalunya.
    Casellas, Daniel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Eurecat, Centre Tecnològic de Catalunya.
    Pujante, Jaume
    Eurecat, Centre Tecnològic de Catalunya.
    Jonsén, Pär
    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. Gestamp R&D.
    Sieurin, Henrik
    SCANIA AB.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Fracture Toughness Evaluation of Thick Press Hardened 22MnB5 Sheets for High Crash Performance Applications in Trucks2019In: CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel, 2019 / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, 2019, p. 113-121Conference paper (Refereed)
  • 4.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A Study on Microstructure-Dependent Deformation and Failure Properties of Boron Alloyed Steel2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Developments in the automotive industry are driven by customer desires and legislative authorities. Legislation has restricted the emissions standards for vehicles, and has mandated the need for higher safety standards. The emission of carbon dioxide is directly related to fuel consumption, and the reduction in fuel consumption can be achieved by reducing the vehicle mass.

    A variety of methods have been used to reduce a vehicle’s mass while maintaining its crashworthiness. A technique using low-alloyed boron steel has been developed, and it enables the design of lighter body-in-white, while maintaining passenger safety. The technique is called press-hardening or hot stamping, and it involves the simultaneous forming and quenching of sheet metal. Press-hardened components have superior material properties compared to components made of mild steel. Another feature of compo-nents formed at elevated temperatures is the possibility of tailoring material properties in desired regions of the component. This is realized by using specially designed tools that allow differential in-die cooling rates and thus direct control of the formed microstructure. Using this technique, it is possible to manufacture a high-strength region next to a high-ductility section divided by a transition zone of mixed microstructure.

    The present work aims to determine the influence of mixed microstructures on the mechanical properties of low-alloyed boron steel. An experimental heat-treatment process is used to form multi-phase microstructures with a variety of phase volume fractions present in the composite. Digital image correlation is used to investigate the deformation of tensile specimens under loading. This full-field technique and a suitable constitutive model enables us to evaluate the flow and fracture properties of heat-treated samples. Microstructural characterization is used to determine the type of phases present and their average volume fraction in the composites.

    The findings from experimental studies are compared to results predicted by a constitutive model. A modeling strategy is employed to determine the effective material properties depending on the properties of single-phase characteristics. Failure of the material is indicated by stress-based fracture criteria. Numerical issues in finite-element modeling concerning the mesh-size sensitivity are addressed using a regularization method.

    The results of the experimental work aids the calibration and validation of the proposed microstructure-based modeling approach, and a knowledge of the processing history enables the prediction of the overall hardening behavior and fracture elongation. A comparison of experimental results, which are not used for calibration, with numerical results shows that there is good agreement.

  • 5.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Prediction of hardening, localization and fracture of multiphase microstructure in boron alloyed steel2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Over recent years the demand of press hardened ultra-high strength steel for safety structures in automobiles has increased and continuation of this trend is expected.Components with tailored material properties can be manufactured using a thermo-mechanical process with heated and cooled areas in a tool. In industry the cooling rate of the blank is controlled this utilizes the formation of different microstructures with varying mechanical properties within a single component.The structural response in a crash situation can be altered by the design of the component with formation of different material grades based on the microstructure in designated areas of the component.Material models used in finite element analysis are required by the automotive industry and its suppliers. These models contribute to the improvement and quality of press hardened components.In this work a set of tensile test specimens with different volume fractions of phases are produced. As material is the boron steel 22MnB5 chosen which is a common material used in press hardening due to its good hardenability. The specimens are austenitized before starting the heat treatment at different temperatures and holding times. In total fourteen different microstructures are produced. Reference material grades for pure phases are ferrite, bainite and martensite. The produced samples consist of ferrite-bainite, ferrite-martensite and bainite-martensite with different volume fractions, additionally a microstructure consisting of three phases, ferrite-bainite and martensite, is available. Using measured mechanical properties of pure phases and the volume fraction of formed phases different homogenization methods are compared in their ability to represent the mechanical response of mixed microstructures. The homogenization methods account for elastic deformation and the hardening of the material. The onset of necking is seen as the last valid point for all homogenization methods. After this point a localization and damage function for the prediction of softening and fracture is applied. Strain localization and fracture are mesh dependent, therefore an analysis length scale is introduced to account for different element sizes. A weakest link criterion is used for fracture. This means failure of the composite is assumed to occur if one phase fails. The material model for homogenization of mixed microstructures including damage is implemented in the commercial available finite element code LS-Dyna and validated by comparison to experimental results.

  • 6.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Frómeta, David
    Fundació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa, Spai.
    Casellas, Daniel
    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.
    Jonsén, Pär
    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.
    Determination of the essential work of fracture at high strain rates2017In: 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. 261-269Conference paper (Refereed)
    Abstract [en]

    During the last decades, the use of ultra-high strength steel (UHSS) has increased as its favorable ratio between strength and mass allows the design of lighter body-in-white while maintaining passenger safety. Modeling impact loads of components made of UHS steel requires reliable descriptions of the material deformation and fracture behavior.

    Traditional stress or strain based fracture criteria are used in finite element modeling. A different approach in modeling fracture in components uses the fracture energy as a model parameter.

    Fracture toughness is difficult to measure in thin sheets; a method termed Essential Work of Fracture (EWF) provides the possibility to determine the fracture toughness in sheet metal. With knowledge of the fracture toughness the understanding of fracture behavior and crack propagation in ultra-high strength steel can be increased. The obtained EWF is related to the fracture energy and can be used in numerical models as a material parameter.

    In the present work results from preliminary testing are shown and a discussion on cross-head speed and strain rate in the critical specimen cross section is given. The use of digital image correlation provides information about the displacement field in the vicinity of the notch and hence about the strain- and strain rate distribution. Furthermore, the difficulties in reliable measurement of force and elongation in high speed tensile testing machines are elucidated. Issues encountered during the development of the high-speed DENT specimen are not limited to the specific geometry presented in this paper.

    The present work aims at the development of a test specimen to obtain the Essential Work of Fracture (EWF) at high test speed. This work contributes to the overall goal to model fracture behavior and crack propagation, dependent on the strain rate. For the investigation, a high-speed tensile testing machine equipped with an in-house developed load cell and an optical elongation measurement system was used with a high-speed camera to obtain data for digital image correlation.

  • 7.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östling, Rickard
    Gestamp Hardtech AB, Luleå.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Characterization of ductile fracture properties of quench-hardenable boron steel: Influence of microstructure and processing conditions2016In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 658, p. 472-483Article in journal (Refereed)
    Abstract [en]

    Developments of the hot stamping technology have enabled the production of components with differential microstructure composition and mechanical properties. These can increase the performance of certain crash-relevant automotive structures by combining high intrusion protection and energy absorption. This paper presents a comprehensive experimental investigation on the flow and ductile fracture properties of boron-alloyed steel with a wide range of different microstructure compositions. Three types of dual phase microstructures at three different volume fractions, and one triple phase grade, were generated by thermal treatment. Flow curves extending beyond necking and the equivalent plastic strain to fracture for each grade was determined by tensile testing using full-field measurements. The influence of phase composition and microstructural parameters were further investigated by means of a multi-scale modeling approach based on mean-field homogenization in combination with local fracture criteria. Inter-phase and intra-phase fracture mechanisms were considered by adopting two separate fracture criteria formulated in terms of the local average stress field. The micromechanical model captures with useful accuracy the strong influence of microstructure and processing conditions on the flow and fracture properties, implying promising prospects of mean-field homogenization for the constitutive modeling of hot stamped components.

  • 8.
    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.

  • 9.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    Gestamp HardTech.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A stress-based fracture criteria validated on mixed microstructures of ferrite and bainite over a range of stress triaxialities2016In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 674, p. 232-241Article in journal (Refereed)
    Abstract [en]

    Hot stamping is a sequential process for formation and heat-treatment of sheet metal components with superior mechanical properties. By applying different cooling rates, the microstructural composition and thus the material properties of steel can be designed. By controlling the cooling rate in different sections of a blank, the material properties can be tailored depending on the desired toughness. Under continuous cooling, various volume fractions of ferrite and bainite are formed depending on the rate of cooling. This paper focuses on the ductile fracture behavior of a thin sheet metal made of low-alloyed boron steel with varying amounts of ferrite and bainite. An experimental setup was applied in order to produce microstructures with different volume fractions of ferrite and bainite. In total, five different test specimen geometries, representing different stress triaxialities, were heat treated and tensile tested. Through full field measurements, flow curves extending beyond necking and the equivalent plastic strain to fracture were determined. Experimental results were further investigated using a mean-field homogenization scheme combined with local fracture criteria. The mean-field homogenization scheme comprises the influence of microstructure composition and stress triaxiality with usable accuracy, connoting auspicious possibilities for constitutive modeling of hot-stamped components

  • 10.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    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.
    A study on homogenization methods for steels with varying content of ferrite, bainite and martensite2016In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 228, p. 88-97Article in journal (Refereed)
    Abstract [en]

    The demand of ultra high strength steel (UHSS) components increased in the last decade due to their high strength to weight ratio. The driving force in this development is the automotive industry and regulations concerning passenger safety and fuel consumption. The use of ultra high strength steel enables design of lighter car bodies with equal or better passenger safety compared to earlier car generations. The automotive industry and their suppliers need predictive tools in the development of components with tailored material properties. Components with tailored material properties are produced by hot stamping, in this process a blank is austenitized before it is formed and quenched in one step. By use of sequential heated or cooled tools, different mechanical properties distributed within the same component are achieved.In order to develop a constitutive model for components consisting of regions with varying phase content, a suitable method to describe the elasto-plastic part of the yield curve is needed. The focus of this work is on the description of the elasto-plastic constitutive model of an ultra high strength steel depending on the phase content in the material. Different volume fractions of ferrite, bainite and martensite are experimentally formed. In this study the capability of different homogenization methods on the prediction of the material response of a multi-phase steel depending on the volume fraction of formed phases is investigated. The modeling results are compared to experimental results.The prediction of the composite response using the micromechanical based double-inclusion model and pure phase measured data as well as experimentally obtained phase volume fractions of present phases showed good agreement throughout all samples tested in this study.

  • 11.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    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.
    Implementation of homogenization scheme for hardening, localization and fracture of a steel with tailored material properties2013In: 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. 75-82Conference paper (Refereed)
  • 12.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    Gestamp HardTech.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modeling of multi-phase microstructures in press hardened components plastic deformation and fracture in different stress states2017In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, article id 012053Article in journal (Refereed)
    Abstract [en]

    Hot stamping or press hardening is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production stage. Within the field of press hardening the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. This paper presents an approach for modeling the mechanical response of mixed microstructures under different stress states. A homogenization method is used to predict the hardening of the material; the strain decomposition provides the possibility of applying a fracture criterion per phase. To validate the modeling approach for different stress states a set of samples with different notch and hole geometries as well as microstructural composition are produced. The combination of a homogenization method and a fracture criterion show good agreement with experimental results. The homogenization method is suitable to predict the hardening of the material with good accuracy. Fracture for different microstructural compositions is well predicted over a range of stress triaxialities relevant for sheet metal applications. 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 for a range of different stress states.

  • 13.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    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.
    Study on fracture in heat affected zones in the vicinity of spot welds in a steel with tailored material properties2015In: 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. 211-219Conference paper (Refereed)
    Abstract [en]

    The demand for ultra-high strength steel (UHSS) increased in the last decade due to their favourable properties in the process of press hardening. Press hardening is a process where an austenitized blank of boron steel is formed and quenched in one production step to achieve superior mechanical properties compared to the blank in as-delivered condition. Driving force for the development of this technique is the automotive industry and regulations concerning passenger safety and fuel consumption. In this work fracture in spot welds and their proximity in low alloyed boron steel have been investigated using finite element simulation and digital speckle photography. This study is motivated by the use of spot welding as major joining method in automotive components. The investigation includes blanks with three different base microstructures, ferritic, bainitic and martensitic. These types of microstructures are commonly found in press hardened components with tailored material properties. All test samples are spot welded in a centred position with a coupon. The welding process introduces heat into the blank and affects the microstructure in the vicinity of the weld leading to varying mechanical properties. During tensile testing of the spot welded samples the mechanical response of the material has been recorded by digital speckle photography (DSP), extensometer and force measurement. The experimental method of DSP measurement allows to resolve displacement and strain fields in the specimen up until fracture. For finite element analysis the commercial available code LS-Dyna together with a user defined material model is used. The material model predicts the mechanical response of the bulk material, i.e. the composite, depending on the phase volume fractions of present phases.

  • 14.
    Golling, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Östlund, Rickard
    Gestamp HardTech.
    Schill, Mikael
    DYNAmore, Linköping.
    Sjöblom, Reimert
    Scania AB, Södertälje.
    Mattiasson, Kjell
    Volvo Car Corporation, Trollhättan.
    Jergeus, Jonas
    Volvo Car Corporation, Trollhättan.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A comparative study of different failure modeling strategies on a laboratory scale test component2017In: 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. 37-46Conference paper (Refereed)
    Abstract [en]

    Ultra-high strength steel (UHSS) has become a common material in the automotive industry during the last decades. The technique of press hardening allows modifying and tailoring the material properties of the blank in accordance with desired performance.

    In the present work, a laboratory scale test component is developed. On basis of tests on the component it is intended to investigate the deformation and fracture behavior of a boron alloyed steel after different heat treatments. The tooling is developed to allow the production of single phase microstructures like martensite and bainite as well as mixed microstructures containing ferrite. Testing of the component is performed in a standard tensile testing machine with additional digital speckle measurements to determine the strain to fracture in the critical cross section. The initial geometry shape introduces bending in the critical cross-section during tensile loading of the specimen.

    The aim of this work is to compare different material models on a component like level, including the prediction of failure. A finite element model of a laboratory scale component is analyzed using LS-Dyna. To compare different failure modeling approaches a set of damage models is calibrated to full hardened, martensitic steel. The deformation and fracture behavior of the component is presented in terms of load-displacement, plastic strain-stress triaxiality as well as in principal strain space.

  • 15.
    Häggblad, Hans-åke
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gustafsson, 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.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling And Simulation The Cracking Of Green Metal Powder Body2016In: Proceedings World PM2016 Congress: International Powder Metallurgy Congress & Exhibition 9-13 October 2016, Hamburg, Germany, European Powder Metallurgy Association , 2016Conference paper (Refereed)
    Abstract [en]

    During the ejection stage of the powder metallurgy (PM) pressing process the elastic behaviour is important. Especially, if the in the powder compact is unloaded in a non-beneficially manner crack formation can occur. Experiments show a non-linear and also stress dependent elastic behaviour of green bodies. Calibrated against experimental data, a fracture model controlling the stress versus crack-width relationship is used. The softening rate of the fracture model is obtained from the corresponding rate of the dissipated energy. The model is implemented with a smeared crack approach in a finite element code and tested in simulation of a diametral compression testing. Results from simulations correlates well with experimental results. The smeared crack method combined with a cohesive fracture model is an interesting tool for predicting fracture in powder compacts.

  • 16.
    Jonsén, Pär
    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.
    Frómeta, David
    Fundació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa, Spai.
    Casellas, Daniel
    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 mechanics based modelling of failure in advanced high strength steels2017In: 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. 15-23Conference paper (Refereed)
    Abstract [en]

    In the last decade, the favorable properties of the press hardening process for advanced high strength steel (AHSS) have increased the demands concerning passenger safety and lightweight design. AHSS show excellent mechanical properties from e.g. tensile test measurements, but it has previously been shown that results from tensile elongation or energy calculation of un-notched and smooth specimen are not appropriate to classify the crash behavior of steel grades. This is because they completely underestimate the post-uniform region from start of necking to failure. Another issue, the mechanical behavior of a notched or cracked component is different than a smooth and un-notched component. If the mechanical behavior in some loading is dominated by crack propagation, it should be rationalized in terms of the materials crack propagation resistance. Therefore, the evolution of the material property that controls crack propagation, i.e. the fracture toughness, is an interesting approach to evaluate loading and deformation of AHSS. Process modelling including fracture toughness depending properties gives valuable information and additional understanding of fracture behavior and crack propagation mechanisms in AHSS components. Fracture toughness in thin sheets can be readily measured through the application of the Essential Work of Fracture (EWF) methodology. The damage evolution law can be specified in terms of fracture energy (per unit area) or in terms of equivalent plastic failure strain as a function of triaxiality and lode angle. In this work, DENT test samples have been experimentally evaluated and finite element simulations of the DENT tests have been performed. By this approach the numerical study includes mechanical response of AHSS specimen including sharp cracks. In the numerical model, the J-integral was evaluated using the virtual crack-tip extension (VCE) method. From the comparison of the numerical and experimental results of load-displacement for different ligament length cases it is obvious that there are in agreement. Also, the numerically obtained value of fracture toughness Jc, is in agreement with the experimentally measured value of essential work of fracture we.

     

    When finite element based fracture mechanics is applied to practical design, the fracture toughness can be used as design criteria. One appealing property of the evaluation of the J-integral is that it can be evaluated from the far field solution, which facilitates computation as many numerical errors arise close to the crack tip. Evolution of stress- and strain field, plastic zone, J-integral value and other mechanical properties is interesting to study with the combination of experimental and numerical investigations.

  • 17.
    Marth, Stefan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Golling, Stefan
    Gestamp R&D, Box 828, 97 125 Luleå, Sweden.
    Östlund, Rickard
    Gestamp R&D, Box 828, 97 125 Luleå, Sweden.
    Barrero Pijoan, Anna
    Eurecat, Centre Tecnològic de Catalunya, Unit of Metallic and Ceramic Materials, Plaça de la Ciència 2, 08243 Manresa, Spain.
    Häggblad, Hans-Åke
    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.
    Failure Modelling and Experimental Evaluation of a Press-Hardened Laboratory Scale Component with Multi-Phase Microstructure2019In: CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel, 2019 / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, 2019, Vol. 7, p. 39-50, article id B1Conference paper (Refereed)
    Abstract [en]

    Hot stamping of boron alloyed steel has become a standard in the automotive industry for safety relevant body in white components. This process allows the design of complex geometries with superior mechanical properties. Special tool design enables to manufacture components with special properties based on varying microstructures in designated areas. This is a challenge for finite element (FE) simulations of deformation and failure for multi-phase microstructure components.

    In the present work, a laboratory scale test component with multi-phase microstructure is studied from blank to fractured component. Using different tool temperatures and adding an air-cooling step before transfer to the press hardening tool, the microstructure of the component is varied. By this, components with four different multi-phase microstructures are produced. These components are tested under tensile deformation until fracture, where force, elongation and the strain field on the components surface are measured.

    The laboratory scale test component is evaluated using FE-modelling. The complete production process is modelled starting with the pre-cut austenitized blank, subsequent transfer, air-cooling, forming operation, and the final post-cooling. The resulting multi-phase micro structures are evaluated using manual optical microscope image analysis and compared with the simulated phase composition. Furthermore, the deformation and fracture of the manufactured component under tensional loading is studied using a mean-field homogenization scheme for the multi-phase composition combined with the OPTUS failure model. This finite element investigation is conducted taking the microstructure composition, shape and thickness deviations from the forming simulation into account.

    The present work shows the feasibility of modelling methods of the complete process chain for press-hardened components with multi-phase microstructures, from blank to fractured component.

  • 18.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Absolute Measurement of Elastic Waves Excited by Hertzian Contacts in Boundary Restricted Systems2017In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 65, no 1, article id 7Article in journal (Refereed)
    Abstract [en]

    In most applied monitoring investigations using acoustic emission, measurements are carried out relatively, even though that limits the use of the extracted information. The authors believe acoustic emission monitoring can be improved by instead using absolute measurements. However, knowledge about absolute measurement in boundary restricted systems is limited. This article evaluates a method for absolute calibration of acoustic emission transducers and evaluates its performance in a boundary restricted system. Absolute measured signals of Hertzian contact excited elastic waves in boundary restricted systems were studied with respect to contact time and excitation energy. Good agreement is shown between measured and calculated signals. For contact times short enough to avoid interaction between elastic waves and initiating forces, the signals contain both resonances and zero frequencies, whereas for longer contact times the signals exclusively contained resonances. For both cases, a Green’s function model and measured signals showed good agreement.

  • 19.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    The influence of contact time and event frequency on acoustic emission signals2017In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 231, no 10, p. 1341-1349Article in journal (Refereed)
    Abstract [en]

    Studies of acoustic emissions (AE) of rotating machine elements is often performed under a constant speed. There are few investigations on speed and contact time dependency of acoustic emission signals, even though some investigations have reported difficulties at elevated rotational speeds. Simplified experiments are, therefore, presented in this article to increase the understanding of the time dependency of acoustic emission signals. Hertzian impacts and tensile tests are used to study contact time, the time duration of an event and offset time, the time between events and accordingly the event frequency. The results of these model experiments indicate an indirect quadratic proportionality of acoustic emission amplitudes and contact time (umax∝1/(tc2)">u max ∝1/(t 2 c ) umax∝1/(tc2) ), as well as a proportional relationship between the root mean square and event frequency (RMS∝fevent∝1/toffset">RMS∝f event ∝1/t offset  RMS∝fevent∝1/toffset ). The relationship between contact time and the root mean square of acoustic emission signals is strongly dependent on the damping of the system

  • 20.
    Schnabel, Stephan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Marklund, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Larsson, Roland
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Golling, Stefan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    The Detection of Plastic Deformation in Rolling Element Bearings by Acoustic Emission2017In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 110, p. 209-215Article in journal (Refereed)
    Abstract [en]

    The detection of plastic deformation caused by particle contamination in rolling element bearings using acoustic emission is reliable at low speeds as shown in several studies. However, there are no studies at greater speeds of the detection of plastic deformation by acoustic emission in rolling element bearings. The acoustic emission signals of rolling element bearings have, however, been shown to be dominated by transient force signals which are elastic waves caused by transient forces acting at the raceway surface. The results of the test showed a dominance of transient force signals at elevated speeds, which masks signals caused by plastic deformation and prohibits the detection of particle contamination, while at low rotational speed plastic deformation is detected by acoustic emission.

  • 21.
    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.

1 - 21 of 21
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