Implemetation of residual stress to the microstructurally based fracture mechanics at the example of a martensitic steel and application on the fracture-mechanical analysis of the fatigue strength of springs

• Applicants: Professor Dr. Robert Brandt; Professor Dr.-Ing. Hans Jürgen Christ; Professor Dr.-Ing. Claus-Peter Fritzen
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 414274929

This project is targeted on a characterization of the mechanisms of fatigue in martensitic spring steels by the propagation of short cracks. Furthermore, a materials science based simulation of the propagation of short cracks should be elaborated by means of a model which is based on the experimentally observed mechanisms. Thus, a better understanding of the metal physical processes in martensitic steels under cyclic loading in the HCF regime is established considering also residual stress in order to conduct more specific investigations of phenomena induced by residual stress, e.g. crack closure of short cracks.Modern experimental methods of materials testing as well as materials characterization will be applied for this purpose. The use of a miniaturized materials fatigue testing device allows for in-situ-analysis of the fatigue damage of the material under uniaxial, cyclic loads by means of confocal laser microscopy. An identical testing device will be redesigned for biaxial, cyclic loads by application of a torsional moment to a specimen. The effect of residual stress induced crack closure can be measured and quantified by an interferometric determination of the displacement. The condition of residual stress, i.e. macro and micro residual stress, within volumes near to the surface will be determined by means of x-ray analysis. Furthermore, for gathering a comprehensive fatigue data base additional fatigue testing will be conducted at standard samples under tensile-compression load or torsional load, respectively, by using conventional fatigue testing methods.Based on the experimentally identified fatigue mechanisms of the considered material a reasonable approach for simulation of short crack propagation will be implemented into a mechanism-oriented model. A model of short crack propagation which can be solved by using a boundary element method serves as a basis. It will be modified for the simulation of short crack propagation in martensitic steels. The martensitic structure and the residual stress condition shall be realistically described and the effect of the spatial residual stress profile on the short crack propagation into the depth of the specimen shall be simulated.Due to the close collaboration of experts in mechanics and materials sciences, respectively, a deeper understanding of the fundamental mechanisms of fatigue is emerging by the results and insight is gained by the combined efforts in experiments and simulation. After finalizing the project, it will be feasible to improve the application-oriented fatigue assessment of martensitic steels and to enhance the materials utilization within structures based on the consideration of its fatigue strength.

• DFG Programme: Research Grants