Numerical crack growth study on porosity afflicted cast steel specimens

Abstract

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.

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

“…Therefore, the casting simulation tool Magma 5 © is utilized for the design process of the representative specimen geometry. In the following, a brief summary about the design process is given, for details see [23]. At first, the specimen's dimensions are set to a diameter of D = 30 mm and a total length of L = 300 mm.…”

“…A subsequent comparison of the numerical results exhibits a similar crack propagation rate between an FT#A defect and an ECD/4 penny-shaped crack. But for the FT#B defects, a penny-shaped crack of size ECD/2 results in a similar lifetime [23].…”

“…The literature offers a wide variety of studied simple geometrical shapes and the corresponding fracture mechanical geometry factor solutions [67,68]. Referring to an engineering-feasible substitutional shape, a correlation between the arbitrarily-shaped defects and a corresponding penny-shaped crack has been studied in [23] by means of numerical crack growth simulations. In the following, the assessment approach is briefly reviewed.…”

“…In the following, the assessment approach is briefly reviewed. Details about the crack propagation rates, corresponding simulations and the results were given in [23].…”

“…As presented by Hardin and Beckermann [21], shrinkage porosity can feature sizes up to several millimetres, which exceed the application limit of Murakami's √ area and therefore leads to an unsatisfying design result. On the other hand, as published in recent works [22,23], the generalized Kitagawa-Takahashi diagram, which can be considered as an extended form of the linear-elastic fracture mechanical design [24], enables an auspicious assessment of shrinkage porosity afflicted large-scale cast steel specimens. Early works by Williams, Gross and Mendelson [25,26] contributed to the development of the notch stress intensity factor (NSIF) concept in terms of an analytical solution of the angle-dependent Williams eigenvalue λ at sharp V-notches, as defined in Equation 1.…”