Caused by the enormous maintenance and repair costs of large industrial applications, the interest in extended fatigue life of components is increasing. Therefore, the demand for additional knowledge about the fatigue behavior in the VHCF regime grows steadily. In the case of high strength steels, fatigue tests in the VHCF regime reveal that multiple failure mechanisms occur. However, common statistical analysis of constant amplitude tests generate summarized multiple-flaw S-N curves which neglect the differentiation according to the type of the failure cause, e.g. oxides or sulfides 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 also to be taken into account that some mechanisms occur less frequently only 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 AISI 52100 (DIN 100Cr6) were tested uniaxially at two stress ratios up to 2x109 load cycles using two ultrasonic testing devices. The tests exhibited crack initiation at different types of inclusions within the volume and at the surface. A mathematical solution is given to calculate single-flaw S-N curves including the regime of the fatigue limit from these results.