Microstructure based modelling of ductile fracture in quench-hardenable boron steel

• Main Author: Östlund, Rickard
• Format: Doctoral Thesis
• Language: English
• Published: Luleå tekniska universitet, Material- och solidmekanik 2015
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-26413; http://nbn-resolving.de/urn:isbn:978-91-7583-262-3; http://nbn-resolving.de/urn:isbn:978-91-7583-263-0

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. === <p>Godkänd; 2015; 20150219 (ricost); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Rickard Östlund Ämne: Hållfasthetslära /Solid Mechanics Avhandling: Microstructure Based Modelling of Ductile Fracture in Quench-Hardenable Boron Steel Opponent: Professor Michael Worswick, Dept of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada Ordförande: Professor Mats Oldenburg, Avd för Material- och solidmekanik, Institutionen för Teknikvetenskap och matematik, Luleå tekniska universitet, Luleå. Tid: Fredag 17 april kl 10.00 Plats: E231, Luleå tekniska universitet</p>