Evaluation of multiple-flaw failure of bearing steel 52100 of different heats in the VHCF regime and mathematical determination of single-flaw behaviour

Caused by different factors, the interest in extended fatigue life of components is increasing. Therefore, the demand for additional knowledge about the fatigue behaviour in the VHCF regime grows. In the case of high strength steels, fatigue tests mostly reveal that multiple failure mechanisms occur. However, the common statistical analysis of constant amplitude tests generates summarized multiple-flaw S-N curves, which neglect the differentiation between the type of failure origin, e.g. oxides or sulphides as non-metallic inclusions. To improve the fatigue life of materials it is essential to determine single-flaw S-N curves for the assessment of the harmfulness of each failure type. It has to be taken into account that some mechanisms occur rarely because they are covered by others. Furthermore, it has to be considered that the probability of the occurrence of different failure types depends not only on the stress amplitude but also on the applied mean stress. In this investigation specimens made of different heats of the bearing steel 52100 were tested uniaxially at two stress ratios up to 2·109 load cycles. The tests exhibited crack initiation at different types of inclusions and at the surface depending on the chemical composition of the heat and the applied mean stress. A mathematical procedure for the determination of single-flaw S-N curves from multiple-flaw test results derived from the adaption of the competing risks theory is presented.