PSI - Issue 75

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

371

Wegener et al. / Structural Integrity Procedia 00 (2025) 000–000

9

Due to the assumptions regarding the material parameters and the approximation of the local loading conditions, the results can only give an indication for plausible size exponents. However, the parameter study shows that the calculations according to the notch-strain approach tend to better match the new size exponent n = 0 . 1 in the new Eurocode draft FprEN 1993-1-9 (2024-02) and proposed in Glienke et al. (2023) with regard to the crack initiation life, compared to the exponent n = 0 . 25. The results also indicate that a higher exponent could be required starting from the nominal diameter M72 onwards. This can possibly be explained by the constant pitch P = 6 mm for bolting assemblies with nominal diameters ≥ M64, which leads to a greater increase in notch sharpness. This fact is already taken into account, for example, in the IEC 61400-6 / AMD1 (2025) for nominal diameters > M72. According to Ungermann et al. (2015), Simonsen (2015) and Oechsner et al. (2015), the fatigue strength of hot-dip galvanized bolting assemblies is reduced by shrinking cracks initially present in the zinc coating, like shown in Figure 7 (a), (c), since they introduce microstructural stress peaks. To assess the capability of the notch-strain approach to cover the e ff ect of hot-dip galvanizations, calculations are performed for the series M2-M12-hdg-sv and M2-M56 hdg-sv. For this purpose, Eichsta¨dt (2019) proposes the use of a strain factor of S f = 1 . 25 ... 1 . 30, which is considered in the calculation of the strain range according to Equation 7. As Eichsta¨dt states, equivalent results can be achieved by reducing the P-life curve with an additional modification factor. However, since di ff erent modification factors would be required for di ff erent damage parameters, Eichsta¨dt favours the modification of the local loading conditions. ε ∗ = S f · ε (7) 4.3. Consideration of hot-dip galvanization

Fig. 7: (a) Zinc coating with shrinking cracks at thread root of bolt M2-M12-hdg-sv; (b) Comparison of fatigue life calculations with di ff erent strain factors S f for series M2-M12-hdg-sv; (c) Zinc coating with shrinking cracks at thread root of bolt M2-M56-hdg-sv; (d) Comparison of fatigue life calculations with di ff erent strain factors S f for series M2-M56-hdg-sv.

Table 4: Results of calculations regarding the incorporation of coating e ff ects.

M2-M12-hdg-sv

M2-M56-hdg-sv

S f

m var , exp

m var , calc

m var , exp

m var , calc

∆ σ c , 50% , exp [ N / mm 2 ]

∆ σ c , 50% , calc [ N / mm 2 ]

∆ σ c , 50% , exp [ N / mm 2 ]

∆ σ c , 50% , calc [ N / mm 2 ]

[ − ]

[ − ] 3.34 2.94 2.89

[ − ]

[ − ] 3.73 3.15 3.08

79.3 70.8 69.9

74.7 64.3 63.2

1.00 1.25 1.30

73.3

2.48

62.1

2.83

Figure 7 (b), (d) and Table 3 show the calculation results when neglecting the e ff ect of hot-dip galvanizing ( S f = 1 . 00) in comparison with the experimental data for the M2-M12-hdg-sv and M2-M56-hdg-sv series. It can be seen