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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These variations in stress amplitude emphasise the influence of microstructural characteristics on fatigue performance. The fatigue lives of the samples varied significantly, with some specimens reaching run-out conditions at 1.00E+10 cycles, while others failed at lower cycles depending on the applied stress amplitude.

Fig.6. Crack length vs Cycles

4.2. Fatigue Crack Growth Results The (FCG) performance of the tested steels, presented as da/dN(mm/cycle) v/s ΔK(MPa√m ), reveals distinct differences associated with microstructure, processing route, and loading ratio. Grade 1 demonstrates relatively good resistance to crack propagation, particularly at lower values of ΔK, consistent with the higher strength. Grade 2, which exhibits slightly higher crack growth rates compared to Grade 1, especially in the mid-to- high ΔK regime, reflecting the detrimental influence of over-tempering and casting-related microstructural heterogeneity on crack growth resistance. The ferritic Q355 steel shows the least resistance at higher ΔK values, with noticeably elevated da/dN which is consistent with the relatively low strength and low crack-tip shielding capability of ferritic microstructures. The influence of stress ratio R is also evident across the dataset, with R=0.1 producing consistently higher growth rates than R=0 attributable to reduced crack closure and increased effective stress intensity range. Overall, these results demonstrate that Grade 1, offer superior resistance to fatigue crack propagation compared to other 2 tested grades, especially under high crack driving forces. To improve statistical reliability, four specimens will be tested per condition. This will help capture data, provide stronger validation of the observed trends, and allow more confident comparison of crack growth resistance across the selected grades of steel.