Cross-scale modeling of the formation of FGA and WEC and the associated crack propagation processes

• Applicants: Dr.-Ing. Marion Cornelia Kreins; Professor Dr.-Ing. Ulrich Krupp
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523648868

The damage behavior of metallic materials under cyclic loading is of crucial importance for the service life and integrity of components. Cyclic mechanical stress can cause microstructural changes and cracks, which can ultimately lead to structural damage and component failure. Under high- and very high cycle fatigue loading, even very low stresses lead to the formation of so-called fine-granular areas (FGA) in the immediate vicinity of crack initiation. The formation mechanisms of these microstructural changes are the subject of controversial debate, especially as they determine more than 90% of the fatigue life. Against this background, consideration of such effects in the modeling of fatigue damage is essential for an accurate service life prediction. This project aims to enable the microstructure-based modeling of FGA formation, crack initiation and short crack growth under VHCF loading. A particular focus is on the interactions between the crack tip and the locally modified material properties due to FGA formation. Methodologically, this is realized by a mesoscopic coupling of the Crystal Plasticity Finite Element Method (CPFEM) and the Boundary Element Method (BEM). Based on a real microstructure, a representative volume element (RVE) with a non-metallic inclusion is created using 3D EBSD (electron backscatter diffraction). The stresses around the inclusion are analyzed with CPFEM taking into account (i) process-induced residual stresses, (ii) matrix, inclusion and interface properties, (iii) inclusion geometry and (iv) external loading in order to derive crack initiation criteria. Building on this, the subsequent crack growth in the martensitic microstructure is modeled with the BEM. The implementation of FGA formation and the integration of the associated microstructural modification into the crack growth behavior represent a particular challenge. In combination with the experimental results of the research group, the model enables a comprehensive analysis of the interactions between microstructural defects, crack initiation, crack growth and FGA formation. This holistic approach provides valuable insights into the damage behavior of cyclically loaded components and thus contributes to increasing the service life and reliability of metallic components in numerous industrial applications.

• DFG Programme: Research Units
• Subproject of: FOR 5701: Identification of the formation mechanisms of white etching cracks and fine granular dark areas during fatigue loading – parallels and differences (White and Dark)