PSI - Issue 23

Ulrich Krupp et al. / Procedia Structural Integrity 23 (2019) 517–522 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Conclusions

The fatigue strength of tempered steels at very high numbers of load cycles (VHCF regime) is mainly determined by the occurrence of microstructure inhomogeneities, like the structure, shape and size of nonmetallic inclusions, or the arrangement of prior austenite grains, martensite pakets, blocks and lathes, as well as carbides. By combining high-frequency testing with in-situ thermography monitoring, the detrimental effect of segregation bands was revealed. By means of ultrasonic fatigue testing, 10 9 cycles can be reached in reasonable testing times. However, the high-strain-rate effect needs to be taken into account that pretends a somewhat higher fatigue limit as it can be expected during low-frequency loading. Depending on the heat treatment / strength condition of the tempered steels, VHCF cracks are either initiated at the surface, where they are non-propagating (moderate strength => fatigue limit), or internally. In the latter case, early crack propagation is driven by a FGA mechanism (fine-granular area) that eliminates the fatigue limit.

Acknowledgements

The authors gratefully acknowledge the financial support of the German Federal Ministry of Education and Research (BMBF project Microlife) and the Volkswagen Foundation (project OptiHeat). Additionally, the authors would like to express their thanks to Robert Bosch GmbH and Georgsmarienhütte GmbH for the technical and materials support.

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