Modelling and characterisation of fracture properties of advanced high strength steels

Sammanfattning: Growing demands for passenger safety, vehicle performance and fuel economy is a continuous driving force for the increase in use of advanced high strength steels (AHSS) in the automotive industry. These steels area characterised by improved formability and crash worthiness compared to conventional steel grades. An important prerequisite of the application of new material grades is the characterization of its mechanical properties. Post-localization and fracture predictive technologies greatly facilitate the design of components which make optimal use of these steel grades. In this thesis, press hardened boron alloyed steel subjected to differential thermo-mechanical processing is characterized. Fracture properties in relation to the different microstructures obtained is studied. Furthermore a dual phase (DP) cold forming steel is chosen for evaluation of ductility limit in shear loading. throughout this work a strategy for modelling post-localization response and predicting ductility limit usingshell elements larger then the typical width of the localized neck is used. The studied material is assumed to be in a state of plane stress. Mesh dependency is alleviated by the introduction of a element size dependent parameter into the constitutive description. This parameter acts as a hardening parameter, controlling the evolution of the yield surface depending on loading, strain history and shell element size. Model calibration relies on a full field measurement technique, Digital Speckle Photography (DSP), to record the plane deformation field of tensile specimens. Quantitative measurements of the severely localized deformation preceding crack initiation are feasible. With the proposed strategy, mesh sensitivity in terms of post localization load response and fracture elongation predictions is reduced significantly compared to results obtained without the element size dependent parameter. It was found that high strain hardening favours strain localization of shear band type, and accelerates the formation of a localized neck. The hardening characteristics is determinant to which deformation mode dissipates the minimum energy. For the DP steel, the Tresca yield surface more accurately describes the yielding point compared to the von Mises plane stress elipse. Furthermore, the exponential ductility function dependent on the stress triaxiality parameter agrees well with experimental fracture data in the ductile loading regime for both DP and boron steel. In shear loading, the maximum shear (MS) stress criterion successfully describes the ductility limit. Due to the significantly different ductility of the various microstructures obtainable by the thermo-mechanical processing of boron alloyed steel, a modelling strategy is needed. It was found that in ductile loading, local equivalent fracture strain can be related to the hardness of that material point. An exponential decrease in ductility with increased hardness describes experimental data collected for five different microstructures.