Microstructure based evaluation of real structures in cold formable steels
Multi-phase steel has unique mechanical properties because of its interplay between high strength and good uniform elongation properties. This is due to the fact that the microstructure is made up of constituents with strong distinctions in strength and ductility. As a result of these inhomogeneities, strong gradients in the local stress distributions can be observed. Moreover, in terms of fracture behaviour, several failure mechanisms occur in parallel because of microstructure compositions. This research project aims to characterise the ductile failure processes in multiphase steels for automotive application and to develop microstructural failure criteria.
To calculate the macroscopic material properties we make use of a micromechanical approach, by setting up a representative volume element (RVE) in which all phases are represented by their correct volume concentration and morphology. Consequently, the properties of the RVE can be homogenized to yield the macroscopic mechanical properties that result from the given microstructure. Voronoi tessellation was applied in order to achieve a realistic representation of the microstructure. A Python script runs in the Abaqus environment to create a random microstructure. The script is parameterized in order to take into account different features of an experimentally observed microstructure, i.e. grain-size distribution, number and volume fraction of phases, orientation-distribution or random crystal orientation and different constitutive behavior.
Within the framework of this project, failure modeling of DP-TRIP aided steel must be performed. This includes investigation of competition between failure modes because several failure mechanisms can be observed such as the ductile failure of ferrite, the brittle failure of martensite and the interface debonding between phases. For this study XFEM technique was used to study the damage evolution in martensitic regions without prescribing the crack path. Moreover, damage curve for the ductile ferritic phase was derived and employed on ferrite. Concerning modeling of interface debonding, it has been excluded for the current model because of convergence problem. Nevertheless, modeling of interface debonding will be carried out in due time. Most probably the application of cohesive elements to the interface between the ferrite and the martensite is the favourable modelling approach for this purpose. As a result of implementing the approach, microcracks initially occurred in the martensite. The resulting strain localisation in the surrounding ductile ferrite later resulted in the initiation and propagation of a crack in the ferritic phase. Afterwards, ductile crack propagation governed the failure of the microstructure.