Experimental analysis and phase field modelling of the interaction between plastic zone and fatigue crack propagation in ductile materials under complex loading conditions

• Applicants: Dr. Eric Breitbarth; Professor Dr.-Ing. Markus Kästner
• Subject Area: Mechanics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454147675

With the present proposal we want to continue the project on the experimental and numerical analysis of fatigue crack growth in aluminium sheets. In the first phase of the project, we focused on the development of the plastic zone. Due to the process-related texture of the rolled AA2024-T3 aluminium sheets, a significant directional dependence of the crack propagation was found. On the experimental side, we therefore focused on the characterisation of the anisotropic crack resistance and the investigation of the crack deflection and crack propagation rate. The 2D phase field model for crack propagation was extended to include anisotropic material behaviour and can reproduce both processes. An autonomous DIC system was developed to analyse crack tip loading, crack path and plastic zone. During the investigations, it became apparent that not only the direction of crack propagation varies, but also the three-dimensional shape of the fracture surface: There is a twisting around the horizontal crack propagation axis and the crack changes from a plane fracture surface to a "slant" or "V" mode. Our investigations indicate that the phenomenon is dependent on the loading condition, the stress state at the crack tip and the specimen thickness and is reflected in the expression of the plastic zone. In addition, the fracture surface characteristics influence the crack propagation rate and thus the service life of the component. For this reason, more detailed investigation of the phenomenon is required. Within the framework of the applied project, the fracture surface characteristics during fatigue crack propagation will therefore to be investigated experimentally and numerically. On the experimental side, parameters will be determined in order to adjust the fracture mode and to investigate the causes using the microscope DIC system. On the numerical side, the existing phase field model for the simulation of the crack propagation is extended by the 3D case and a new fatigue and plasticity model in order to be able to represent the plastic shear failure of the slant and V-mode. By combining experiment and simulation, the cause of the fracture surface manifestations based on continuum mechanics and microstructure will be investigated. Finally, simplified methods for the prediction of the fatigue crack propagation will be developed considering the fracture surface distortion. The data and methods will be published.

• DFG Programme: Research Grants