PSI - Issue 19

Kim Bergner et al. / Procedia Structural Integrity 19 (2019) 140–149 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

148

9

4. Results of the fatigue analyses The calculated component Woehler curves for the bending specimens with a rim zone thickness of t RZ = 1 mm, achieved using the three different fatigue approaches, are compared in Fig. 10. The fatigue test results are shifted with (5) to a rim zone thickness of t RZ = 1 mm to make them comparable with those considered in the three fatigue assessments.

Fig. 10. Woehler curves for the different fatigue analysis

The fatigue assessment according to the FKM guideline leads to a non-conservative result, by overestimating the fatigue strength at N = 10 6 by a factor of 1.6 compared to the fatigue tests. In the second fatigue approach, two hot spots are evaluated, one in the rim zone layer and one in the bulk material. The fatigue assessment for the first hot spot (rim zone) leads to results below the fatigue tests. For the second hot spot (bulk material), the results shows that the failure will occur at hot spot 1. The third fatigue approach, using the strain-life approach with consideration of the rim zone factor, fits well with the fatigue tests even if it is slightly too low and the slope is too flat. 5. Discussion The discrepancies between the Woehler curve according to the FKM guideline and the fatigue tests can be partly explained by the low mean stress sensitivity specified in the FKM guideline. The tests were carried out with a stress ratio of R = 0 and thus exhibit a high mean tensile stress. According to the FKM guideline, the mean stress sensitivity is M = 0.21, whereas, in [19], medium mean stress sensitivities of about M = 0.5 were determined for GJS materials. Using a mean stress sensitivity of M = 0.5 for the fatigue assessment regarding the FKM guideline leads an overestimation in the fatigue strength at N = 10 6 by a factor of 1.3. The fatigue analysis shows a crack initiation in the rim zone long before the bending specimen fails. However, the crack growth in the rim zone and in the bulk material

has not yet been determined. 6. Conclusion and Outlook

The results show that the FKM guideline leads to non-conservative results for a fatigue assessment of a cast iron component with a casting skin. The developed fatigue approach, using the rim zone factor, leads to a good fit to the fatigue tests and tends to be conservative. These investigations were focused on the rim zone of EN-GJS-400-15. This should be extended to further materials and also the surface roughness should be investigated. So far, it could only be shown that there is an influence of the roughness on the fatigue life of the cast iron component, but without determination of the effects of different surface roughnesses.