PSI - Issue 75

Monisha Manjunatha et al. / Procedia Structural Integrity 75 (2025) 650–659 Monisha Manjunatha et al. / Structural Integrity Procedia (2025)

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Fig.5. Experimental SN curves for UFT samples tested at 20kHz Compared to Grade 1, Grade 2 steel demonstrates a considerably lower fatigue strength with the fatigue limit being about 200 MPa lower than Grade 1, which is a significant drawback. On the other hand, Grade 2 looks slightly stronger compared to the medium-strength structural steel grades, namely S355JR and Q355B, as can be seen in Figure 5. The advantage in fatigue strength for the Grade 2 comparing the corresponding fatigue limits is about 50 MPa. However, shifting the focus from the long-term fatigue life to the range of higher stresses that are limited by 1 million cycles, the fatigue performance of the Grade 2 looks much more beneficial with about 100 MPa advantage in terms of stress amplitude. The reason for that is the difference in the slope of SN curves, which is much steeper for the Grade 2 compared to S355JR/Q355B. Another distinctive feature of the Grade 2 fatigue dataset is that it consists of two distinctive groups. Both fatigue data groups are fitted with SN curves having a very similar slope. The main group includes the datapoints obtained at relatively high stresses with the duration from 100k up to 20 million cycles. It corresponds to the classical definition of High Cycle Fatigue domain, which is dominated by the surface crack initiation. The Long Life (LL) group is smaller, obtained at lower stress levels, and has a duration in the range from 9 million to 2 billion cycles. It is a lower boundary of the Very High Cycle Fatigue domain, which is dominated by the internal crack initiation mode. This fatigue behaviour is very similar to the classification of S-N curves using the concept of duplex SN curves reported by(Shiozawa et al., 2001) for a range of structural steels and alloys. Grade 2 falls very well under the definition of Type C, which is called a pure duplex SN curve in this study. The position of the probability distribution for the internal fracture mode (LL group) is close to that of the surface failure mode (main group), and both partially overlap. Grade 2 steel also shows much smaller scatter for its both groups, indicating less variability in its microstructure and mechanical properties.