PSI - Issue 75

Fritz Wegener et al. / Procedia Structural Integrity 75 (2025) 363–374

373

Wegener et al. / Structural Integrity Procedia 00 (2025) 000–000 11 Table 5: Calculation results for di ff erent residual compressive stresses σ res and comparison to experimental data of test series M2-M56-b-sg. σ res ∆ σ P J a ini N ini a end , mean N prop N tot N exp , mean ∆ N rel [ N / mm 2 ] [ N / mm 2 ] [ N / mm 2 ] [ mm ] [-] [ mm ] [-] [-] [-] [%]

18,675 9.5 40,235 9.3

250 200 150 250 200 150

6.8 4.3 2.4 2.7 1.9 1.4

29,335 48,010 123,373 -61 52,522 92,757 234,536 -60

0

0.175

121,883 10.2 113,938 235,821 580,416 -59

96,461 9.5 191,711 9.3

29,355 125,796 123,373 52,522 244,233 234,536

+ 2 + 4

-600

0.175

388,783 10.2 113,938 502,721 580,416 -13

5. Conclusion and Outlook

The investigations presented in this article support the suitability of the notch-strain approach for assessing the e ff ects of nominal diameter, hot-dip galvanizations and residual stresses on the fatigue strength of bolting assemblies. The comparison of the results calculated using the notch-strain approach and fracture mechanics with the experimen tal fatigue tests carried out on HV bolting assemblies show a good approximation of the S-N curve. A theoretical consideration of the size e ff ect in the nominal diameter range M30 to M72 with constant material parameters shows good agreement with the new size reduction exponent of n = 0 . 1 introduced for the calculation of k s in the cur rent draft of EC3, even independently of the presence of hot-dip galvanizing. Tests are currently being carried out to experimentally confirm the observed correlations. For hot-dip galvanized assemblies, the fatigue strength is overestimated, unless the e ff ect of the hot-dip galva nizing is explicitly included in the calculation. However, the strain factor S f proposed by Eichsta¨dt is suitable for consideration of the higher notch sharpness and reduces the overestimation. Regarding the positive e ff ect of threads rolled after heat treatment, the thin surface layer model shows the potential to include this e ff ect into the calculation of the crack initiation life using the notch-strain approach. However, actual values for the residual compressive stresses are required to perform a realistic calculation. The determination of these values for the bolting assemblies shown in this paper is planned in the further course of the research project. In order to establish the notch-strain approach in practical applications in the future, the methods for a simplified determination of the cyclic material properties, the local stresses and strains and approaches for the e ff ective consider ation of further relevant influencing parameters must be tested for their performance. In this context, a comparison of the concept with a growing set of test results, e. g. on the e ff ect of mean stress, alternative coatings and the influence of lubrication, is planned. The e ff ect of bolt and nut geometry is also the subject of current investigations. In addition, methods have been developed to gain reference data for a direct validation of the notch-strain approach without the need for a fracture mechanics calculation, see Wegener et al. (2025). For the authors, however, the notch-strain approach is already an important tool for expanding experimental param eter ranges by calculation and thus identifying relevant parameter combinations. This is of particular importance when taking into account the increasing nominal diameters and the associated challenges in experimental fatigue testing in order to utilize the limited machine capacities as e ff ectively as possible.

Acknowledgements

The IGF-project (No. 01IF22748N) of the Research Association for steel Application (FOSTA), Du¨sseldorf has been funded by the DLR and AiF within the program for sponsorship by Industrial Collective Research (IGF) of the German Federal Ministry of Economic A ff airs and Climate Action based of a decision by the German Bundestag. In addition, the authors would like to thank all project partners involved.

References

Ba¨umel, A., Seeger, T., 1990. Materials data for cyclic loading (Suppl 1). Elsevier Science, Amsterdam.