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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as a reference to validate its accuracy. Visual crack monitoring setup is added to track the crack growth symmetrically on both the faces as depicted in Figure 4.

Fig. 4. : Visual crack monitoring technique

4. Results and discussion 4.1. Fatigue Crack Initiation Results

Fatigue performance is a critical consideration in the evaluation of structural steels, particularly under high frequency cyclic loading conditions. This study investigates the comparative analysis of the total fatigue life assessment of different steel grades (from low to high) under ultrasonic fatigue testing (UFT) at 20 kHz. The results provide insights into the fatigue lives of different grades based on stress amplitude versus cycles, highlighting variations due to material characteristics and test conditions. The obtained fatigue testing results for the four steel grades are summarised in Figure 5, presenting data points and trendlines. Ultrasonic fatigue testing (UFT) of Grade 1 and Grade 2 high-strength steels was compared against conventional steels S355JR and Q355B. A major issue observed in S355JR and Q355B was excessive heat generation during testing. Peak temperatures reached up to 160 °C just before failure, likely due to crack tip plasticity. This highlights a limitation of UFT for ferritic steels, where heat generation at amplitudes ≥400 MPa becomes unmanageable. In contrast, Grade 1 and Grade 2 showed minimal heating, likely due to their martensitic microstructure, making them more suitable for ultrasonic testing under high-cycle loading. One notable issue was the high scatter in fatigue life for Grade 1 steel. The underlying reason for this scatter is not yet fully understood; however, a similar scatter has also been reported in the literature for steels of comparable strength, such as S690 (Dantas et al., 2022). To clarify this behaviour, additional tests on further specimens, which are prepared in the corrosive environment, are planned to build more statistics, together with detailed fractographic analyses. Based upon the available results, the fatigue limit is expected to be in the range 650 660 MPa. It should be noted that corrosion has a very detrimental effect on the fatigue strength of Grade 1. It can be seen in Figure 5, that the fatigue strength of Grade 1 drops by over 200 MPa in terms of stress amplitude, therefore, eliminating the advantages of HSS when used in a corrosive environment. Available fatigue testing results for the pre-corroded condition of Grade 1 are too scarce to draw conclusions, but even a few data points available for Grade 1 indicate a brittle nature of this HSS with a very short phase of the crack propagation.