Fatigue crack growth under non-proportional mixed mode loading considering mode coupling effects

• Applicant: Professorin Dr.-Ing. Manuela Sander
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 387114789

During their service life, machines and components are exposed to complex operating loads, which are often characterized by non-proportional parts. For the calculation of the remaining lifetime of cracked structures, spatial mixed-mode loading resulting from a superposition of mode I, II and III must therefore be taken into account. For in-plane, proportional loads, existing crack propagation concepts tend to be able to describe fatigue crack propagation. In contrast, there are only a few, not universally valid approaches for the inclusion of all three crack modes in a concept for the description of fatigue crack propagation. In addition, non-proportional loading during fatigue crack growth leads to a temporal and local change in the mode-mixity, so that a change in the dominant fracture behaviour can also occur. The aim of the proposed research project is the development of concepts within the framework of linear-elastic fracture mechanics for a reliable remaining lifetime prediction under non-proportional spatial mixed-mode loading in metallic materials. The investigations focus on the experimental determination of crack paths and crack geometries during the fatigue crack growth, accompanied by an understanding of the effective mechanisms, as an essential basis for the development of corresponding crack propagation concepts. Based on this, basic knowledge on the crack propagation direction as well as on the crack growth rate during fatigue crack growth shall be gained.The intended investigations are carried out using clamped single-edge notched specimens (SEN(TC)-specimens) on a servohydraulic tension/torsion testing machine for achieving a targeted coupling of all three crack modes. In order to identify the effective mechanisms of fatigue crack growth, different load paths are generated. The three-dimensional geometries of the cracks are made accessible for measuring by exposing the fracture surfaces using various experimental techniques. The digital image correlation (DIC) is used for the automated evaluation of crack growth and crack loading on the specimen surfaces. The experimental investigations are accompanied by linear-elastic finite element simulations for the determination of stress intensity factors (SIF) based on the experimentally measured spatial crack geometries. The crack loading is numerically determined by the J-Integral, while the SIF solutions are separated by means of the interaction integral (M-integral).

• DFG Programme: Research Grants