PSI - Issue 68

496 Lucia Morales-Rivas et al. / Procedia Structural Integrity 68 (2025) 493–499 Lucia Morales-Rivas, Eberhard Kerscher / Structural Integrity Procedia 00 (2025) 000–000 intensity factor range at the propagation/non-propagation threshold for long cracks ( ),_345* ), experimentally determined in the MECBAIN project –ref. Sourmail et al. (2017)- from tensile testing and crack growth experiments, are listed in Table 1. Notch fatigue was studied by the present authors in the MECBAIN project -Sourmail, et al.(2017)-, leading to published results that will be shown and discussed in this manuscript. Only those nanobainitic conditions for which a complete fatigue testing procedure was conducted in MECBAIN will be considered now, in order to enable the analysis that will be described next. Table 1. Mechanical properties ( UTS and YS ) and results from crack growth experiments ( ΔK !"_$%&' ) and from notched fatigue ( Δσ ',!" and σ )*+,,-.!,/0 ) on the advanced bainitic microstructures considered in the present work -from MECBAIN, ref. Sourmail et al. (2017)-. YS refers to the 0.2% offset yield strength. The parameter a stands for the notch depth according to Fig. 3. , /MPa , , /MPa UTS /MPa YS /MPa ΔK <"_$%&' /MPa √ (notch depth) /mm < =2 < =4 < =2 < =4 0.6C-1.5Si-890°C-220°C (114h) 2102 1643 4.6 2 700 480 1556 2133 0.6C-1.5Si-890°C-250°C (16h) 1990 1448 4.3 2 730 460 1622 2044 1C-2.5Si-950°C-250°C (16h) 2106 1738 4.8 2.25 620 420 1378 1866 Experimental -from MECBAIN, ref. Sourmail et al. (2017)- 4

For notch fatigue testing, in total, two different specimen geometries were used for each microstructure, each geometry corresponding to a particular theoretical stress concentration factor, ) ( ) =2 or ) =4). The geometries of the fatigue specimens were cylindrical and had a V-shaped notch, as represented in Fig. 3, with the geometric parameters listed in Table 2. Fatigue tests with a stress ratio R=0.1 will be considered in this work. MECBAIN -Sourmail et al. (2017)- results for Δσ *,), , corresponding to both specimen geometries, are listed in Table 1.

Fig. 3. Drawing of the notched fatigue specimens .

Table 2. Dimensions of the notched fatigue specimens according to Fig. 3, from MECBAIN, ref. Sourmail et al. (2017)).

< D /mm /mm /mm 10 6 1

Variant

0.6C-1.5Si-890°C-220°C (114h) 0.6C-1.5Si-890°C-250°C (16h) 1C-2.5Si-950°C-250°C (16h) 0.6C-1.5Si-890°C-220°C (114h) 0.6C-1.5Si-890°C-250°C (16h) 1C-2.5Si-950°C-250°C (16h)

2

9.5

5 6 5

0.85

10

0.2

4

9.5

0.18

3. Application of Neuber-based approaches on advanced bainitic steels 3.1. Application of Neuber´s rule

Mueller et al. (2016) proposed Kitagawa-Takahashi-based diagrams for the study of microstructural features critical to the fatigue response of the nanostructured bainitic microstructures considered in the project MECBAIN - Sourmail et al (2017)-. As opposed to the common practice, the authors did not use gross (nominal) values for the construction of such diagrams, but local stresses originated from the effect of the notch. The authors calculated, first, the maximum fictitious local stress at the fatigue limit, 678,19:),;< , for the described specimens with a V-shaped notch, that is, the product of the gross stress, ∆ *,), , and the theoretical stress concentration factor, ) ( ) =2 or ) = 4 ). Then a so-called “critical crack length”, designated in the present work as ( ′′ , was calculated as the intersection