Micromechanics-based modelling of properties and failure of multiphase steels

Within this study mechanical properties and damage mechanisms including crack initiation and propagation of multiphase steels have been investigated. Starting with one chemical composition, five different multiphase microstructures have been realised by different annealing treatments. A wide variety of mechanical properties and of failure mechanisms has been observed using tensile tests of round tensile bars without and with notch.A description of flow curves for these different microstructures has been realised using finite element calculations of representative volume elements (RVE) within a continuum mechanical framework using the isotropic von Mises plasticity law for individual phases. In creating the RVE, carbon partitioning as well as metallographically identified phase fractions has been taken into account.Two failure modes have been detected experimentally in parallel: dimple and cleavage fracture. The mode-affecting parameters have been identified in terms of microstructural features and triaxiality.For ductile damage it can be shown, that two void nucleation mechanism are in competition: those due transformation of austenite to martensite and due to the presence of inclusions. In order to model the ductile dimple failure mode within the ferritic phase, the Gurson–Tveergard–Needleman approach is used. A model parameter study has been realised for both nucleation mechanisms.