Multiscale modeling of failure initiation in a ferritic–pearlitic steel

A representative volume element (RVE)-based strategy for modeling the hardening and failure behavior of a ferritic–pearlitic steel at different length scales – mesoscale and microscale – is presented. At first, pearlite properties were considered to be isotropic and homogeneous. Micrographs taken on the undeformed material were transformed to a finite element mesh by using the software OOF2, a public domain FE-analysis software created at the National Institute of Standards and Technology (NIST) for the investigation of microstructures. Boundary conditions of the RVE were defined based on the macroscopic deformation history of a region of interest of an axisymmetric impact extrusion part. Crack initiation in pearlite is modeled within the extended finite element method (XFEM) framework. Pearlite cracking modeled at the mesoscale corresponds well to the observed cracks on SEM-micrographs. In a further approach, the crystallographic orientation of ferrite as well as various distributions of cementite lamellae were considered to take the inhomogeneous structure of the pearlite into account. For this purpose, in the framework of RVE computation, a spectral solver of the code DAMASK (Dusseldorf Advanced Material Simulation Kit, an open source crystal plasticity general purpose solver) was applied to model strain localizations at grain level. X-ray measurements were carried out to determine the orientation of ferrite grains and to determine the parameters of the applied crystal plasticity material model (critical resolved shear stress and slip hardening parameters). Investigations showed that the orientations of cementite lamellae have a significant influence on strain localization. The concept of coupling the FE-method to simulate the macroscopic behavior of the material and the spectral solver to achieve a high resolution of the microstructure in the framework of RVE computations leads to an efficient strategy regarding computational time and modeling of the microstructure.