Manuel Schuscha

Shrinkage porosities and non-metallic inclusions are common manufacturing process based defects that are present within cast materials. Conventional fatigue design recommendations, such as the FKM guideline (“Forschungskuratorium Maschinenbau”), therefore propose general safety factors for the fatigue assessment of cast structures. In fact, these factors mostly lead to oversized components and do not facilitate a lightweight design process. In this work, the effect of shrinkage porosities on the fatigue strength of defect-afflicted large-scale specimens manufactured from the cast steel G21Mn5 is studied by means of a notch stress intensity factor-based (NSIF-based) generalized Kitagawa diagram. Additionally, the mean stress sensitivity of the material is taken into account and establishes a load stress ratio enhanced diagram. Thereby, the fatigue assessment approach is performed by utilizing the defects sizes taken either from the fracture surface of the tested specimens or from non-destructive X-ray investigations. Additionally, a numerical algorithm invoking cellular automata, which enables the generation of artificial defects, is presented. Conclusively, a comparison to the results of the experimental investigations reveals a sound agreement to the generated spatial pore geometries. To sum up, the generalized Kitagawa diagram, as well as a concept utilizing artificially generated defects, is capable of assessing the local fatigue limit of cast steel G21Mn5 components and features the mapping of imperfection grades to their corresponding fatigue strength limit.

This paper deals with the fatigue assessment of cast steel defects in terms of macroscopic shrinkage porosity. Within preliminary studies, a generalized Kitagawa diagram GKD was established by numerical analyses of V-notched specimens with varying opening angles. It was experimentally verified by the application of the notch stress intensity factor (NSIF) concept on fatigue tests under rotating bending and axial loading. This paper continuous the work by an application of the GKD to real cast steel pores. At first, casting simulations are performed to design representative cast specimen geometries. The study focusses on macroscopic shrinkage pores with different spatial shapes. At second, fatigue tests under axial loading are conducted. Subsequent fracture surface analysis by light optical and scanning electron microscopy provides fracture mechanical based geometry parameters. Finally, the results of the experiments related to the failure relevant defect sizes are assessed by the GKD. In order to define an equivalent defect size of the complexly shaped defects, numerical crack growth analyses are performed demonstrating crack coalescence path tendencies. Summing up, the application of the NSIF approach based on a GKD shows a sound accordance to the experimental results and thus provides an engineering-feasible fatigue assessment method of cast steel components with macroscopic imperfections.

An accurate assessment of cast steel components including cast imperfections minimizes production rejects and moreover facilitates lightweight design. In order to improve the fatigue assessment of cast steel structures exhibiting macroscopic imperfections, a generalized Kitagawa diagram based on the linear elastic fracture mechanical theory is presented. Therefore, single-edge-notch-bending crack propagation tests as well as extensive fatigue tests utilizing un-notched base material specimens are conducted. Both the stress intensity factor threshold range and the plain material fatigue limit act as basis to set-up the generalized Kitagawa diagram. In addition, fatigue tests under rotating bending as well as axial loading using round V-notched specimens with varying opening angles are additionally conducted for validation. It was observed that the notch-stress-intensity-factor (NSIF) calculation based on El-Haddad’s material length leads to slightly non-conservative designs for this cast material. But if the intrinsic material length is calculated by Neuber’s stress averaging approach or Peterson’s microstructural length instead, the presented NSIF-based fatigue assessment reveals a sound comparability to the experimentally determined fatigue reference strength. Summing up, the presented generalized Kitagawa diagram provides a feasible engineering-applicable fatigue assessment tool incorporating varying imperfection opening angles and load conditions for G21Mn5 cast steel.