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

Abstract: 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 composition giving 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, fracture initiation 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 in uence 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.