PSI - Issue 76

Jürgen Bär et al. / Procedia Structural Integrity 76 (2026) 27–34

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In case of Steel V, the models are also not conservative for all values except those obtained on specimens with notch 1. In the representation over the notch depth (Fig. 7a), notches 2 and 4 are also clearly separated indicating that this representation is not suitable for this material. In the representation over the Murakami parameter (Fig. 7b), all measured values are lying very close to the line of the El Haddad approach. In this representation, notches 3 and 4 are lying close together. Obviously, the KT diagram in the Murakami representation describes the fatigue limit in an adequate manner. 5. Discussion The results have shown that the method introduced in this work allows a simple and fast determination of data points in the KT diagram. Sharp notches with different sizes can be produced easy and fast with a laser engraving system. Unfortunately, the production of several notches with the same depth is not possible even when the same laser parameters are used. In combination with the laser notches, the determination of crack arrest and failure loadings with block wise increasing loads is a suitable method to determine test data points in the KT diagram. The use of the potential probe allows for simple and reliable determination of crack arrest. Due to this, the length of the loading blocks and therefore the test time can be reduced significantly. The tests on a low alloyed steel with two different heat treatments yielded the following results: for the normalized steel (Steel N), none of the three models and neither the notch depth a nor the Murakami parameter √ fits to the measured data. Except for notch 1, all three models are strongly non-conservative. In case of the hardened steel (Steel V), the Murakami parameter √ in combination with the El Haddad approach fits nearly the measured values. But, for the larger notches, this approach is also not conservative. The results have shown that, for both steels, all three models give non-conservative predictions. This clearly indicates that an experimental validation of the KT diagram is needed. Acknowledgements The authors like to thank Yusuf Kiyak (Division 9.4 from BAM) for the FE Analysis and Romeo Saliwan Neumann (Division 5.1 from BAM) for the EBSD measurements. The research project "Rissarrest und Schwingfestigkeit: Zuverlässige und praxisnahe Methode zur Ermittlung von Kitagawa-Takahashi-Diagrammen" is funded by the Federal Ministry of Economic Affairs and Energy as part of the "Industrial Collective Research" programme on the basis of a resolution of the German Bundestag. This project IGF 01IF22523N / P 1655 from the Research Association for steel Application (FOSTA), Düsseldorf, is carried out at Bundesanstalt für Materialforschung and – prüfung and the Universität der Bundeswehr Munich. Chapetti, M.D.: Hardness as a tool for the estimation of the microstructural threshold. Procedia Structural Integrity 7 (2017) 229–234. DOI: 10.1016/j.prostr.2017.11.082 DIN e.V., 2022. DIN 50100:2022-12 Load controlled fatigue testing - Execution and evaluation of cyclic tests at constant load amplitudes on metallic specimens and components. DOI: 10.31030/3337109 DIN e.V. 2023. DIN EN 10025-6:2023-06 Hot rolled products of structural steels - Part 6: Technical delivery conditions for flat products of high yield strength structural steels in the quenched and tempered condition. DOI: 10.31030/3427127 El Haddad M.H., Topper T.H., Smith K.N.: Prediction of non propagating cracks. Engineering Fracture Mechanics 11 (1979) 573 – 584. DOI:10.1016/0013-7944(79)90081-X Kitagawa H., Takahashi S.: Applicability of Fracture Mechanics to very small Cracks or the Cracks in the early stage. Proceedings of the Second International Conference on Mechanical Behavior of Materials; Boston, MA, (1976) 627 – 31. Maierhofer, J.; Gänser, H.-P.; Pippan, R.: Modified Kitagawa–Takahashi diagram accounting for finite notch depths. International Journal of Fatigue 70 (2015) 503–509, DOI: 10.1016/j.ijfatigue.2014.07.007 McEvily, A.J.; Minakawa, K.: On crack closure and the notch size effect in fatigue. Engineering Fracture Mechanics 28 (1987) 519–527, DOI: 10.1016/0013-7944(87)90049-X Tanaka, K.;Akiniwa,Y.: Resistance-curve Method for Predicting Propagation Threshold of Short Fatigue Cracks at Notches. Engineering Fracture Mechanics 30 (1988) 863–876, DOI: 10.1016/0013-7944(88)90146-4 References