PSI - Issue 35

Vera Friederici et al. / Procedia Structural Integrity 35 (2022) 106–114 V. Friederici et al. / Structural Integrity Procedia 00 (2019) 000–000

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For every load amplitude the number of cycles for each increment da is added up until K IC is reached. By correlating this resulting crack propagation curve with the previously experimentally measured curve (only fatigue life region of the SN-curve), a good estimate for a crack initiation curve can be predicted, see Figure 4. Basquin and scattering parameters for the displayed curves are shown in Table 4.

Table 4. Basquin and scattering parameters corresponding to Figure 4.

Fatigue life S 0 / MPa N 0

k

m L 2.3

Fatigue test results 520

178,431 11.5

Crack propagation 520

36,619

2.8

Crack initiation

520

106,844 10.2

0.7

The slope of the crack propagation curve is much steeper than that of the experimentally determined curve. Thus, crack propagation at higher stress amplitude has a smaller contribution to the total lifetime of a specimen than at lower stress amplitude. The calculated crack initiation duration, except for very high load levels, takes a larger share in the

total life of the specimens. 4. Conclusion and outlook

It was possible to predict a crack initiation curve for rotation-bending specimens by correlating results from crack propagation and rotation-bending tests. So far, these calculations have been completed only for the material in the core condition. They must be continued for the material in the transition and raceway condition. The transfer of this approach to large rolling bearings, taking into account the volume ratio, is still in progress. The FE sub-model of a ring segment with the local stress distribution as a result of a specific loading condition (worst case scenario after FE-modelling of different rotation angles and wind speeds was chosen) has already been obtained from a project partner. A segment of the region of maximum stresses was selected. After estimating the highly loaded volume fraction the next steps will contain a simulation to determine locations of highest probability for crack initiation (1), thereafter a FE calculation for the time needed until crack initiation takes place at the previously indicated spots (2), and finally also a crack propagation simulation (3) on those sites. The next question to be addressed is whether cracking will occur at the locations of the highest stress under normal conditions, or whether additional forces (overloads) or initial defects (inclusions, corrosion) significantly determine structural failure. Not surprisingly, the serious influence of corrosion on service life was demonstrated experimentally on rotating bending specimens. Acknowledgements The project is funded by the German Ministry of Economic Affairs and Energy (BMWi), based on a decision by the German Bundestag (Grant number: 0324303E). References Adasooriya, N.D., Pavlou, D., Hemmingsen, T., 2020. Fatigue strength degradation of corroded structural details: A formula for S ‐ N curve. Fatigue and Fracture Engineering Materials and Structures, 43, 721–733. DOI: 10.1111/ffe.13156 Álvarez, M., Muñiz-Calvente, M., Urresti, I., 2021. Fatigue life estimation of pre-corroded 42CrMo4 subjected to accelerated pitting corrosion method. International Journal of Fracture. DOI: https://doi.org/10.1007/s10704-021-00533-y Barsom, J. M., Rolfe, S.T., 1987. Fracture and Fatigue Control in Structures: Applications of Fracture Mechanics, Second Edition, Prentice-Hall Inc., Englewood Cliffs, New Jersey, p.21. ISBN 0-8031-2082-6 Chahardehi, A., Mehmanparast, A., 2016. Fatigue crack growth under remote and local compression – a state-of-the-art review. Frattura ed Integrità Strutturale, 35, 41-49. DOI: 10.3221/IGF-ESIS.35.05