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  • 701.
    Wedberg, Dan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling of high strain rate plasticity and metal cutting2013Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Metal cutting is one of the most frequently used forming processes in the manufacturing industry. Extensive effort is made to improve its process and simulation has become an integrated part, not only in the product development process but also in the customer relations. However, simulation of metal cutting is complex both from numerical as well as physical point of view. Furthermore, modelling the material behaviour has shown to be crucial. Errors in the material model cannot be reduced by the numerical procedures. The magnitudes of strain and strain rate involved in metal cutting may reach values of 1-10 and 103-106 s-1. The dissipative plastic work together with the chip tool friction also leads to locally high temperatures. These extreme ranges of conditions imply that a diversity of physical phenomena is involved and it is a challenge to develop a material model with adequate accuracy over the whole loading range. Furthermore, this intense and severe deformation represents thermo-mechanical behaviour far from what is generated from conventional material compression and tension testing. A highly desirable feature is also a material model that can be extrapolated outside the calibration range. This is not trivial since materials exhibit different strain hardening and softening characteristics at differentstrains, strain rates and temperatures. Models based on modelling some aspects of the underlying physical process, e.g. the generation of dislocations, are expected to have a larger range of validity than engineering models. Though, engineering models are the far most common models used in metal cutting simulations.The scope of this work includes development of validated models for metal cutting simulations of AISI 316L stainless steel. Particular emphasis is placed on the material modeling and high strain rate plasticity phenomena. The focus has been on a physically based material model. The approach has been to review the literature about flow stress models and phenomena and particularly at high strain rates. A previous variant of a dislocation density model has thereafter been extended into high strain rate regimes by applying different mechanisms. Some of the models have been implemented in commercial finite element software for orthogonal cutting simulations. Experimental measurements and evaluations that include SHPB-measurements, cutting force measurements, quick-stop measurements and some microstructural examinations has been conducted for calibration and validation. The compression tests, within a temperature and strain rate range of 20-950 °C and 0.01-9000 s-1 respectively, showed that the flow stress increased much more rapidly within the dynamic loading range and hence depends on the strain rate. The dynamic strain aging (DSA) that has been observed at lower strain rates is non-existent at higher strain rates. The temperature and strain-rate evolution is such that the DSA is not necessary to include when modelling this process. Furthermore the magnetic balance measurements indicate that the martensite transformation-strengthening effect is insignificant within the dynamic loading range.In the present work the concept of motion of dislocations, their resistance to motion and substructure evolution are used as underlying motivation for description of the flow stress. A coupled set of evolution equations for dislocation density and mono vacancy concentration is used rendering a formulation of a rate-dependent yield limit in context of rate-independent plasticity. Dislocation drag due to phonon and electron drag, a strain rate dependent model of the subcell formation, a strain rate and temperature dependent recovery function and a structural dependent thermally activated stress component have among others been considered. Best predictability was obtained with a strain rate dependent subcell formulation. Dislocation drag did not improve the predictability within the measured testing range. Although showed to has a greater influence outside the range of calibration when extrapolated. It has been shown that extrapolation is uncertain. Results from experiments and modelling of material behaviour and metal cutting together with the literature indicate that the predictability of the material behaviour within and outside the measured testing range can be further tuned by implementing models of the phenomenon mechanical twining and recrystallization.

  • 702.
    Wedberg, Dan
    et al.
    AB Sandvik Coromant, Metal Cutting Research.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling flow stress of AISI 316L at high strain rates2015Ingår i: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 91, nr 1, s. 194-207Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modelling of the material behaviour is crucial for machining simulations. Strain and strain rates can reach values of 1–10 and 103–106 s−1 during the severe deformations associated with machining. An existing dislocation density model for AISI 316L based on a coupled set of evolution equations for dislocation density, mono vacancy concentration is enhanced in order to accommodate plastic deformation at high strain rates. Two mechanisms are evaluated with respect to their contribution in this respect. One is rate dependent cell formation and the other is dislocation drag due to phonons and electrons. Furthermore a temperature and strain rate dependent recovery and a proportionality interaction factor and short range component that both depends on the dislocation density are also considered. High strain rate compression tests are performed using Split-Hopkinson technique at various initial temperatures. Experimental results are then used to calibrate the models via an optimization procedure. Evaluation of various flow stress models shows that the flow stress behaviour of 316L stainless steel is best modelled by the model with a rate dependent cell formation. Its numerical solution is implemented in a format suitable for large-scale finite element simulations

  • 703.
    Wedberg, Dan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Predictive capability of constitutive model outside the range of calibration2011Ingår i: Computational Plasticity XI - Fundamentals and Applications: proceedings of the XI International Conference on Computational Plasticity - fundamentals and applications held in Barcelona, Spain, 07 - 09 September 2011, Barcelona: CINME , 2011, s. 307-317Konferensbidrag (Refereegranskat)
    Abstract [en]

    Machining one of the most common manufacturing processes within the industry but it is also a process with extreme conditions in the vicinity of the cutting insert. Due to diversity of physical phenomena involved machining has proven to be complex and difficult to simulate. The chip formation process is in the vicinity of the cutting insert associated with highly localized severe deformations accompanied by high local temperatures rise. Furthermore, the strain rate can in the primary zone be very high (>50000 s -1), far beyond what can be reached with conventional mechanical material tests. Therefore, the possibility to extrapolate the material model outside the calibration range with respect to strain rate is a wanted feature. It is recognized that the mechanical behavior at high strain rate differs considerably from that observed at low strain rates and that the flow stress increase rapidly with the strain rates above ∼1000 s -1. The predictive abilities outside as well as inside the calibration range of the empirical Johnson-Cook plasticity model and a dislocation density based model are compared and discussed with reference to AISI 316L stainless steel. The results clearly show the difficulty of obtaining a comprehensive material model that predicts the material behavior across the loading conditions that can occur in machining with good accuracy and that the accuracy of extrapolation is uncertain

  • 704.
    Wedberg, Dan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Svoboda, Ales
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling high strain rate phenomena in metal cutting simulation2012Ingår i: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 20, nr 8, s. 85006-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Chip formation in metal cutting is associated with large strains and high strain rates, concentrated locally to deformation zones in front of the tool and beneath the cutting edge. Furthermore, dissipative plastic work and friction work generate high local temperatures. These phenomena together with numerical complications make modelling of metal cutting difficult. Material models, which are crucial in metal cutting simulations, are usually calibrated based on data from material testing. Nevertheless, the magnitude of strains and strain rates involved in metal cutting are several orders higher than those generated from conventional material testing. A highly desirable feature is therefore a material model that can be extrapolated outside the calibration range. In this study, two variants of a flow stress model based on dislocation density and vacancy concentration are used to simulate orthogonal metal cutting of AISI 316L stainless steel. It is found that the addition of phonon drag improves the results somewhat but the addition of this phenomenon still does not make it possible to extrapolate the constitutive model reliably outside its calibration range.

  • 705.
    Westman, Eva-Lis
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Pederson, Robert
    Luleå tekniska universitet.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical and microstructural evaluation of elevated temperature compression tests on Ti-6AI-4V2004Ingår i: 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, 2004Konferensbidrag (Refereegranskat)
  • 706. Wikander, Lars
    et al.
    Karlsson, Lennart
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oddy, Alan S.
    Luleå tekniska universitet.
    Plane thermo-mechanical finite element modelling of welding with special reference to the material behaviour1994Ingår i: International conference proceedings on modeling and control of joining processes: December 8 - 10, 1993, Orlando, Florida / [ed] Thomas Zacharia, Miami, Fla: American Welding Society , 1994, s. 517-524Konferensbidrag (Refereegranskat)
    Abstract [en]

    The influence of phase changes on the residual strains in a welded hollow steel beam were investigated. A D-section beam is fabricated by U-bending 6 mm thickness SS142712 plate and welding in 10 mm thickness SS142132 plate under restraint at 0.993 MJ/m heat input. Finite element analyses were used to determine residual stresses due to cold bending and for thermomechanical welding analysis. Previously published modelling was modified to include the effect of transformation induced plasticity; cross weld residual strain distribution predicted by the two models and measured experimentally were compared; discrepancies are discussed

  • 707. Wikander, Lars
    et al.
    Karlsson, Lennart
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oddy, A.S.
    Plane thermo-mechanical finite element modelling of welding with special reference to the material behaviour1993Ingår i: First International Symposium on Process Industry Piping: December 14 - 17, 1993, Orlando, Florida USA / [ed] J.E. Aller, Houston, Tex: NACE International, 1993Konferensbidrag (Refereegranskat)
  • 708. Wikman, Bengt
    et al.
    Bergman, Greger
    Gestamp Hardtech AB.
    Oldenburg, Mats
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Estimation of constitutive parameters for powder pressing by inverse modelling2006Ingår i: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 31, nr 5, s. 400-409Artikel i tidskrift (Refereegranskat)
    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.

  • 709. Wikman, Bengt
    et al.
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Design of the container for hot isostatic pressing of a diesel engine piston top2003Ingår i: P / M Science & Technology Briefs, ISSN 1527-2478, Vol. 5, nr 1, s. 10-14Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Numerical simulations of the hot isostatic pressing (HIP) process for manufacturing of a piston top have been used to achieve near net shape in this work. The shape of the container is optimised by repeated simulations of the manufacturing process. The application is a piston top for a large diesel engine. The aim is to contribute to the development of an integrated computer based tool for the design and manufacture of hot isostatically pressed products. The computer aided concurrent engineering (CACE) system which is integrated using a relational database includes the solid modeller of a computer aided design system, codes for non-linear finite element analysis (FEA), and data from a coordinate measuring system (CMM). Predictions of near net shape geometry obtained numerically are in tolerable agreement with experimental measurements. The model is based on an associated flow rule and linear isotropic hardening is assumed, the material properties correspond to AISI 304 steel.

  • 710. Wikman, Bengt
    et al.
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Inverse modelling of powder compaction: the influence of material and friction behaviour interaction2000Ingår i: 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 , 2000Konferensbidrag (Refereegranskat)
  • 711. Wikman, Bengt
    et al.
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Modelling of powder-wall friction for simulation of iron powder pressing1997Ingår i: Proceedings of the International workshop on modelling of metal powder forming processes, 1997Konferensbidrag (Refereegranskat)
  • 712. Wikman, Bengt
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-åke
    Modelling methods in pressing simulations of metal powder components1998Ingår i: 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, s. 397-402Konferensbidrag (Refereegranskat)
    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.

  • 713. Wikman, Bengt
    et al.
    Reyre, M.
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Three dimentional finite element analysis of powder compaction2002Ingår i: Process modeling in powder metallurgy & particulate materials: proceedings of the 2002 International Conference on Process Modeling in Powder Metallurgy & Particulate Materials, [held October 28 - 29, 2002, Newport Beach, California] / [ed] Alan Lawley, Princeton, NJ: Metal powder industries federation , 2002, s. 115-122Konferensbidrag (Refereegranskat)
  • 714. Wikman, Bengt
    et al.
    Solimannezhad, N.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-åke
    Wall friction coefficient estimation through modelling of powder die pressing experiment2000Ingår i: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 43, nr 2, s. 132-138Artikel i tidskrift (Refereegranskat)
    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.

  • 715. Wikman, Bengt
    et al.
    Svoboda, Ales
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    A combined material model for numerical simulation of hot isostatic pressing2000Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 189, nr 3, s. 901-913Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In modelling of hot isostatic pressing (HIP) of powder materials the constitutive model should be able to describe different deformation mechanisms during the consolidation process. In the early stage, the consolidation is dominated by granular behaviour. As temperature and pressure increase in the powder the deformation can be described by a viscoplastic model. Experimental observations show substantial time-independent deformation in the early stage. At this stage of the densification process, pores in the powder are still interconnected. This cannot be described properly by a viscoplastic model. The inconsistency between the deformation mechanisms can be treated by a combined elasto-plastic and elasto-viscoplastic model. Here a granular plasticity model is combined with a viscoplastic model. In previous works the viscoplastic model, power-law breakdown, has been used to describe the entire deformation process. The combined model is implemented into an in-house finite deformation code for the solution of coupled thermomechanical problems. The simulation of a hot isostatic pressing test with dilatometer is performed in order to compare calculated results with the experimental measurement. The results from previously performed analysis carried out with a viscoplastic model only are also compared. Analysis with the combined material model shows good agreement with the experiment for the whole densification process.

  • 716.
    Wärmefjord, Kristina
    et al.
    Department of Product and Production Development, Chalmers University of Technology.
    Söderberg, Rikard
    Department of Product and Production Development, Chalmers University of Technology.
    Ericsson, Mikael
    University West, Department of Engineering Science, 461 32 Trollhättan.
    Appelgren, Anders
    University West, Department of Engineering Science, 461 32 Trollhättan.
    Lundbäck, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Lööf, Johan
    GKN Aerospace Engine Systems, Trollhättan.
    Lindkvist, Lars
    Department of Product and Production Development, Chalmers University of Technology.
    Svensson, Hans-Olof
    GKN Aerospace Engine Systems, Trollhättan.
    Welding of Non-nominal Geometries: Physical Tests2016Ingår i: Procedia CIRP, ISSN 2212-8271, E-ISSN 2212-8271, Vol. 43, s. 136-141Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The geometrical quality of a welded assembly is to some extent depending part positions before welding. Here, a design of experiment is set up in order to investigate this relation using physical tests in a controlled environment. Based on the experimental results it can be concluded that the influence of part position before welding is significant for geometrical deviation after welding. Furthermore, a working procedure for a completely virtual geometry assurance process for welded assemblies is outlined. In this process, part variations, assembly fixture variations and welding induced variations are important inputs when predicting the capability of the final assembly.

  • 717.
    Zakrisson, Björn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical and experimental studies of blast loading2010Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In the past decade, there has been an increasing demand from governments for high level protections for military vehicles against explosives. However, designing and validation of protection is a time consuming and expensive process, where previous experience plays an important role. Development time and weight are the driving factors, where the weight influences vehicle performance. Numerical simulations are used as a tool in the design process, in order to reduce development time and optimise the protection. The explosive load acting on a structure is sometimes described with analytical functions, with limitations to shape and type of the explosive, confinement conditions etc. An alternative way to describe the blast load is to use numerical simulations based on continuum mechanics. The blast load is determined by modelling the actual type and shape of the explosive in air or sand, where the explosive force transfers to the structure of interest. However, accuracy of the solution must be considered, were methods and models should be validated against reliable experimental data. Within this work, tests with explosive placed in air, sand or a steel pot has been performed. For all tests, the dynamic and residual deformation of steel plates was measured, while the impulse transfer was measured for some tests. This thesis focuses on continuum based numerical simulations for describing the blast load, with validation against data from the experiments. The main conclusion of this work is that numerical simulations of air blast loading in the near-field can be described with sufficient accuracy.

  • 718.
    Zakrisson, Björn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical simulations of blast loaded steel plates for improved vehicle protection2013Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In the past decade, there has been an increasing demand from governments for high level protections for military vehicles against explosives. However, the design and validation of protection is a time consuming and expensive process, where previous experience plays an important role. Development time and weight are the driving factors, where the weight influences vehicle performance. Numerical simulations are used as a tool in the design process, in order to reduce development time and successively improve the protection. The explosive load acting on a structure is sometimes described with analytical functions, with limitations to shape and type of the explosive, confinement conditions etc. An alternative way to describe the blast load is to use numerical simulations based on continuum mechanics. The blast load is determined by modelling the actual type and shape of the explosive in air or soil, where the explosive force transfers to the structure of interest. However, accuracy of the solution must be considered, where methods and models should be validated against experimental data. Within this work, tests with explosive placed in air, soil or a steel pot have been performed, where the blast load acts on steel target plates resulting in large deformations up to fracture. For the non-fractured target plates, the maximum dynamic and residual deformations of steel plates were measured, while the impulse transfer was measured in some tests. This thesis focuses on continuum based numerical simulations for describing the blast load, with validation against data from the experiments. The numerical and experimental results regarding structural deformation of blast loaded steel plates correlates relatively well against each other. Further, simulations regarding fracture of blast loaded steel plates show conservative results compared to experimental observations. However, more work needs to be undertaken regarding numerical methods to predict fracture on blast loaded structures. The main conclusion of this work is that numerical simulations of blast loading on steel plates, leading to large deformations up to fracture, can be described with sufficient accuracy for design purposes.

  • 719.
    Zakrisson, Björn
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling and simulation of explosions in sand2011Konferensbidrag (Övrigt vetenskapligt)
  • 720.
    Zakrisson, Björn
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling and simulation of explosions in soil interacting with deformable structures2012Ingår i: Central European Journal of Engineering, ISSN 1896-1541, E-ISSN 2081-9927, Vol. 2, nr 4, s. 532-550Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A detonating explosive interacting with a deformable structure is a highly transient and non-linear event. In field blast trials of military vehicles, a standard procedure is often followed in order to reduce the uncertainties and increase the quality of the test. If the explosive is buried in the ground, the state of the soil must meet specific demands. In the present work, laboratory experiments have been performed to characterize the behaviour of a soil material. Soil may be considered a three-phase medium, consisting of solid grains, water and air. Variations between the amounts of these phases affect the mechanical properties of the soil. The experimental outcome has formed input data to represent the soil behaviour included in a three-phase elastic-plastic cap model. This unified constitutive model for soil has been used for numerical simulations representing field blast trials, where the explosive load is interacting with a deformable structure. The blast trials included explosive buried at different depths in wet or dry sand. A dependence of the soil initial conditions can be shown, both in the past field trials along with the numerical simulations. Even though some deviations exist, the simulations showed in general acceptable agreement with the experimental results.

  • 721.
    Zakrisson, Björn
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Wikman, Bengt
    Numerical simulations of blast loads and structural deformation from near-field explosions in air2011Ingår i: International Journal of Impact Engineering, ISSN 0734-743X, E-ISSN 1879-3509, Vol. 38, nr 7, s. 597-612Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Numerical simulations of air blast loading in the near-field acting on deformable steel plates have been performed and compared to experiments. Two types of air blast setups have been used, cylindrical explosive placed either in free air or in a steel pot. A numerical finite element convergence study of the discretisation sensitivity for the gas dynamics has been performed, with use of mapping results from 2D to 3D in an Eulerian reference frame. The result from the convergence study served as a foundation for development of the simulation models. Considering both air blast setups, the numerical results under predicted the measured plate deformations with 9.4–11.1%. Regarding the impulse transfer, the corresponding under prediction was only 1.0–1.6%. An influence of the friction can be shown, both in experiments and the simulations, although other uncertainties are involved as well. A simplified blast model based on empirical blast loading data representing spherical and hemispherical explosive shapes has been tested as an alternative to the Eulerian model. The result for the simplified blast model deviates largely compared to the experiments and the Eulerian model. The CPU time for the simplified blast model is however considerably shorter, and may still be useful in time consuming concept studies. All together, reasonable numerical results using reasonable model sizes can be achieved from near-field explosions in air.

  • 722.
    Zakrisson, Björn
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Wikman, Bengt
    BAE Systems Hägglunds.
    Johansson, Bo
    FOI, Totalförsvarets Forskningsinstitut.
    Half scale experiments with rig for measuring structural deformation and impulse transfer from land mines2008Ingår i: 24th International Symposium on Ballistics: proceedings, New Orleans, Louisiana, September 22-26, 2008 / [ed] Stephan Bless; James Walker, Lancaster, Penn: DEStech Publications, Inc. , 2008, s. 497-504Konferensbidrag (Refereegranskat)
  • 723.
    Zamani, Mohammadreza
    et al.
    Department of Materials and Manufacturing, School of Engineering, Jönköping University.
    Dini, Hoda
    Department of Materials and Manufacturing, School of Engineering, Jönköping University.
    Svoboda, Ales
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Seifeddine, Salem
    Department of Materials and Manufacturing, School of Engineering, Jönköping University.
    Andersson, Nils-Eric
    Department of Materials and Manufacturing, School of Engineering, Jönköping University.
    Jarfors, Anders E.W.
    Department of Materials and Manufacturing, School of Engineering, Jönköping University.
    A dislocation density based constitutive model for as-cast Al-Si alloys: Effect of temperature and microstructure2017Ingår i: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 121, s. 164-170Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow stress of an as-cast Al-Si based alloy was modeled using a dislocation density based model. The developed dislocation density-based constitutive model describes the flow curve of the alloy with various microstructures at quite wide temperature range. Experimental data in the form of stress-strain curves for different strain rates ranging from 10−4 to 10−1 s−1 and temperatures ranging from ambient temperature up to 400 °C were used for model calibration. In order to model precisely the hardening and recovery process at elevated temperature, the interaction between vacancies and dissolved Si was included. The calibrated temperature dependent parameters for different microstructure were correlated to the metallurgical event of the material and validated. For the first time, a dislocation density based model was successfully developed for Al-Si cast alloys. The findings of this work expanded the knowledge on short strain tensile deformation behaviour of these type of alloys at different temperature, which is a critical element for conducting a reliable microstructural FE-simulation.

  • 724.
    Zhang, Zhao
    et al.
    State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Faculty of Vehicle Engineering and Mechanics, Dalian University of Technology.
    Wan, Zhenyu
    State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Faculty of Vehicle Engineering and Mechanics, Dalian University of Technology.
    Lindgren, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Tan, Z.J.
    State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Faculty of Vehicle Engineering and Mechanics, Dalian University of Technology.
    Zhou, Xia
    State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Faculty of Vehicle Engineering and Mechanics, Dalian University of Technology.
    The Simulation of Precipitation Evolutions and Mechanical Properties in Friction Stir Welding with Post-Weld Heat Treatments2017Ingår i: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 26, nr 12, s. 5731-5740Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A finite element model of friction stir welding capable of re-meshing is used to simulate the temperature variations. Re-meshing of the finite element model is used to maintain a fine mesh resolving the gradients of the solution. The Kampmann–Wagner numerical model for precipitation is then used to study the relation between friction stir welds with post-weld heat treatment (PWHT) and the changes in mechanical properties. Results indicate that the PWHT holding time and PWHT holding temperature need to be optimally designed to obtain FSW with better mechanical properties. Higher precipitate number with lower precipitate sizes gives higher strength in the stirring zone after PWHT. The coarsening of precipitates in HAZ are the main reason to hinder the improvement of mechanical property when PWHT is used.

  • 725. Åhrström, Bert-Olof
    et al.
    Lindqvist, S.
    Höglund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Sundin, Karl-Gustaf
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modified split Hopkinson pressure bar method for determination of the dilatation-pressure relationship of lubricants used in elastohydrodynamic lubrication2002Ingår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 216, nr 2, s. 63-74Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In theoretical calculations of film thickness, pressure distribution and friction in an elastohydrodynamically lubricated (EHL) conjunction it is necessary to model the physical/mechanical behaviour of the lubricant. It is important to know, for example, the dilatation-pressure or the density-pressure relationship. In this paper a modified split Hopkinson pressure bar system for determination of the compressibility of oil is presented. It makes it possible to test oils under conditions similar to those found in real EHL contacts: loading duration in the range of 100-300 μs and pressures of almost 2 GPa. An empirical model has been suggested for mathematical description of the dilatation-pressure relation of the specific oils. A naphthenic mineral oil and a synthetic oil, 5P4E, have been tested under adiabatic conditions and at pressures up to 1.5 and 1.9 GPa respectively. The adiabatic results have been recalculated to isothermal conditions for comparison

  • 726.
    Åkerström, Paul
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    A Novel Tooling Technology for Hot Forming Processes2017Ingår i: 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, s. 243-250Konferensbidrag (Refereegranskat)
  • 727.
    Åkerström, Paul
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling and simulation of hot stamping2006Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

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

  • 728. Åkerström, Paul
    et al.
    Bergman, Greger
    SSAB HardTech.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical implementation of a constitutive model for simulation of hot stamping2007Ingår i: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 15, nr 2, s. 105-119Artikel i tidskrift (Refereegranskat)
    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.

  • 729. Åkerström, Paul
    et al.
    Bergman, Greger
    Gestamp R&D.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Salomonsson, Per
    Utveckling av mikrostruktur och mekanisk respons vid presshärdning2007Ingår i: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, s. 98-Konferensbidrag (Övrigt vetenskapligt)
  • 730. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Austenite decomposition during press hardening of a boron steel: computer simulation and test2006Ingår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 174, nr 1-3, s. 399-406Artikel i tidskrift (Refereegranskat)
    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.

  • 731.
    Åkerström, Paul
    et al.
    Gestamp Hardtech AB, Luleå.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical simulation of a thermo-mechanical sheet metal forming experiment2008Ingår i: 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, s. 569-574Konferensbidrag (Refereegranskat)
  • 732. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Studies of the thermo-mechanical material response of a boron steel by inverse modelling2004Ingår i: 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, 2004Konferensbidrag (Refereegranskat)
  • 733. Åkerström, Paul
    et al.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Material parameter estimation for boron steel from simultaneous cooling and compression experiments2005Ingår i: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 13, nr 8, s. 1291-1308Artikel i tidskrift (Refereegranskat)
    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.

  • 734.
    Östlund, Rickard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure based modelling of ductile fracture in quench-hardenable boron steel2015Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Reduction of fuel consumption and emissions by vehicle weight minimization constitute a major driving force for the development of new materials and manufacturing processes in the automotive industry. Simultaneously formed and quenched boron steel components have higher strength to weight ratio than conventional mild steel components. Additionally, hot formed components can be tailored to have regions with lower strength and higher ductility, improving their crash performance. This is often realized via dierential in-die cooling rates, thus yielding a variable microstructure compositiongiving rise to distributed mechanical properties. Predicting the performance envelopes of these types of components poses some challenges in terms of constitutive modelling, due to the dierential material composition and mechanical properties. Moreover, fractureinitiation is often a limiting design factor. This thesis aims to contribute to the constitutive and ductile fracture modelling of quench-hardenable boron steels, with reference to microstructure composition and hence process history. Modelling techniques which in an approximate manner can estimate the eective material properties based on the properties of the constituents in combination with ductile fracture models are presented.Computational issues concerning numerical nite element modelling of material instabilities are also addressed, essentially via two dierent methods. Introducing a discretization dependent parameter in the constitutive description, or by kinematic enhancements with respect to the localization problem. Both aim to reduce mesh sensitivity and provide improved predictions of post-instability response with industrially relevant mesh sizes.Additionally, an experimental investigation on the ow and fracture properties of boron steel, with a comprehensive range of dierent microstructure compositions, is presented. A full-eld measurement technique enabled the direct evaluation of mechanical properties and fracture relevant data from tensile tests. These results have supported the establishment of models and enabled their calibration, and they provide further insight to the inuence of microstructure and processing conditions on the ductile fracture properties. Comparisons between simulations and experiments indicate that useful predictions of the overall hardening behaviour and fracture elongations can be obtained by the suggested microstructure based modelling approach.

  • 735.
    Östlund, Rickard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modelling and characterisation of fracture properties of advanced high strength steels2011Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Growing demands for passenger safety, vehicle performance and fueleconomy is a continuous driving force for the increase in use of advancedhigh strength steels (AHSS) in the automotive industry. Thesesteels area characterised by improved formability and crash worthinesscompared to conventional steel grades. An important prerequisite of theapplication of new material grades is the characterization of its mechanicalproperties. Post-localization and fracture predictive technologiesgreatly facilitate the design of components which make optimal use ofthese steel grades. In this thesis, press hardened boron alloyed steelsubjected to differential thermo-mechanical processing is characterized.Fracture properties in relation to the different microstructures obtainedis studied. Furthermore a dual phase (DP) cold forming steel is chosenfor evaluation of ductility limit in shear loading. throughout thiswork a strategy for modelling post-localization response and predictingductility limit using shell elements larger then the typical width of thelocalized neck is used. The studied material is assumed to be in a stateof plane stress. Mesh dependency is alleviated by the introduction of aelement size dependent parameter into the constitutive description. Thisparameter acts as a hardening parameter, controlling the evolution ofthe yield surface depending on loading, strain history and shell elementsize. Model calibration relies on a full field measurement technique, DigitalSpeckle Photography (DSP), to record the plane deformation field oftensile specimens. Quantitative measurements of the severely localizeddeformation preceding crack initiation are feasible. With the proposedstrategy, mesh sensitivity in terms of post localization load responseand fracture elongation predictions is reduced significantly compared toresults obtained without the element size dependent parameter. It wasfound that high strain hardening favours strain localization of shear band type, and accelerates the formation of a localized neck. The hardeningcharacteristics is determinant to which deformation mode dissipates theminimum energy. For the DP steel, the Tresca yield surface more accuratelydescribes the yielding point compared to the von Mises planestress elipse. Furthermore, the exponential ductility function dependenton the stress triaxiality parameter agrees well with experimental fracturedata in the ductile loading regime for both DP and boron steel.In shear loading, the maximum shear (MS) stress criterion successfullydescribes the ductility limit. Due to the significantly different ductilityof the various microstructures obtainable by the thermo-mechanicalprocessing of boron alloyed steel, a modelling strategy is needed. It wasfound that in ductile loading, local equivalent fracture strain can be relatedto the hardness of that material point. An exponential decrease inductility with increased hardness describes experimental data collectedfor five different microstructures.

  • 736.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Failure analysis of a hat profile with tailored properties subjected to axial compression2013Ingår i: 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, s. 23-30Konferensbidrag (Refereegranskat)
  • 737.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Golling, Stefan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure based modeling of ductile fracture initiation in press-hardened sheet metal structures2016Ingår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 302, s. 90-108Artikel i tidskrift (Refereegranskat)
    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.

  • 738. Östlund, Rickard
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Microstructure-Based Modelling of Ductile Failure2015Ingår i: 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, s. 149-156Konferensbidrag (Refereegranskat)
    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

  • 739.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, D.
    Gestamp R&D.
    Evaluation of localization and failure of boron alloyed steels with different microstructure compositions2014Ingår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, nr 3, s. 592-598Artikel i tidskrift (Refereegranskat)
    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.

  • 740.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Gestamp Hardtech AB, Luleå, Sweden.
    Failure model evaluation for varying microstructure based on material hardness2011Ingår i: 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 , 2011Konferensbidrag (Refereegranskat)
  • 741.
    Östlund, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Berglund, Daniel
    Gestamp R&D.
    Numerical failure analysis of steel sheets using a localization enhanced element and a stress based fracture criterion2015Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 56, s. 1-10Artikel i tidskrift (Refereegranskat)
    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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