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Modelling and characterisation of fracture properties of advanced high strength steels
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2011.
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Research subject
Solid Mechanics
URN: urn:nbn:se:ltu:diva-26695Local ID: f9684548-7bbf-4860-bc1f-6e7924e51affISBN: 978–91–7439–320–0OAI: diva2:999865
Godkänd; 2011; 20110927 (ricost); LICENTIATSEMINARIUM Ämnesområde: Hållfasthetslära/Solid Mechanics Examinator: Professor Mats Oldenburg, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Diskutant: Dr, Forskningsingenjör Greger Bergman, Gestamp Hardtech AB, Luleå Tid: Torsdag den 3 november 2011 kl 10.00 Plats: E246, Luleå tekniska universitetAvailable from: 2016-09-30 Created: 2016-09-30Bibliographically approved

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