Notch Stress Intensity Factor (NSIF)-Based Fatigue Design to Assess Cast Steel Porosity and Related Artificially Generated Imperfections

Abstract

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.

A Numerically Efficient Method to Assess the Elastic–Plastic Strain Energy Density of Notched and Imperfective Cast Steel Components

“…Six specimens were tested utilizing a stress ratio of R = 0.5 and stress ranges between 100 MPa and 175 MPa at a testing frequency of approximately 75 Hz. The test strategy introduced in preceding studies [10,11,17] was followed for the experimental investigations. This strategy involved testing of each individual specimen at up to two target load levels.…”

“…In order to define also the lower reversal points, the elastic-plastic stress ranges ∆ σij,k have to be calculated. As the investigated cast steel alloy G12MnMo7-4+QT exhibits Masing behaviour [109], the cyclic strains can be described as a function of the stress ranges according to Equation (17).…”

“…Hence, the respective contributions to the total SED can be evaluated simply by calculating the corresponding areas in the stress-strain diagram. In the uniaxial load case, the ascending and descending branches of the hysteresis loop are described by Equation (17). When dealing with a local multiaxial stress state, simply inserting the stress range component ∆ σij,k into Equation ( 17) will therefore not result in the calculated total strain range ∆ε (t) ij,k .…”

“…Due to high-volume contraction, these spatial defect structures often reach dimensions of several millimetres [9][10][11][12]. Hence, the fatigue strength of the respective components is significantly affected by the presence of such macroscopic imperfections [13][14][15][16][17].…”