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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5. Conclusions This study investigated the fatigue performance of two grades of high-strength steels (HSS) through ultrasonic fatigue testing (UFT) and fatigue crack growth (FCG) experiments, with results compared against conventional structural steel (Q355/S355). The key findings can be summarized as follows: • HSS exhibit a better fatigue resistance compared to conventional steels such as S355/Q355 under UFT in atmospheric conditions with the Grade 1 being superior and Grade 2 slightly better; • Grade 1 steel showed a noticeable scatter in the UFT results with the underlying cause of this variability remaining under investigation; • Grade 2 steel showed much less scatter in the UFT results, but also significantly lower fatigue strength compared to the Grade 1; • Fatigue failure mechanisms were found differed for the investigated HSS grades: ✓ Grade 1: Failures predominantly initiated at the surface; ✓ Grade 2: Surface Failure was found for the Main group of dataset, and the Long-Life behavior was associated predominantly with internal crack initiation; • Pre-corroded (4-week exposure) UFT samples revealed that corrosion significantly reduces the fatigue advantage of HSS, particularly for HSS, diminishing their benefits in corrosion-prone environments. • Fatigue crack growth tests were conducted in accordance with ASTM E647 with the Paris Law coefficients reported in Table 3. • Interpretation of Paris Law coefficients from the Table 3 indicates: ✓ The lower m values for Grades 1 and 2 suggest reduced sensitivity to ΔK increases compared to Q355, which is beneficial for long-life fatigue resistance. But, the higher C values (Grade 1 at R = 0, Grade 2 at R = 0.1) imply a tendency for faster crack initiation and growth near threshold conditions, compared to Q355. ✓ HSS grades may slow crack growth under higher ΔK conditions, their early -stage crack growth resistance is more variable and can be less favourable than conventional steels depending on stress ratio. • Although HSS offers advantages in fatigue resistance under clean atmospheric conditions, its performance in corrosion-prone environments and near-threshold crack growth regimes requires careful consideration.

Table 3: Paris Law estimates for the selected grades of structural steel Material Stress Ratio (R) m C Grade 1 0 2.0415 1.50E-10 Grade 1 0.1 2.7216 1.35E-12 Grade 2 0.1 2.454 3.24E-11 Q355B 0 3.0486 4.06E-12

6. Future Work To build upon the current understanding of fatigue performance, future work will focus on exploring the synergistic effects of corrosion and fatigue, particularly in high-cycle and very-high-cycle regimes. One of the primary objectives is to investigate the influence of corrosion on fatigue crack initiation by pre-corroding Ultrasonic Fatigue Testing (UFT) samples in a 3.5% NaCl solution. This will help simulate working environments, especially for components exposed to environment or humid conditions. Following this, studies will be conducted to evaluate the effect of frequency on fatigue life, specifically for Grade 1 and Grade 2 UFT samples. Any anomalies observed on the fracture surfaces during these tests will be