PSI - Issue 75

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

651

2

Fatigue performance is a critical consideration for structural materials subjected to cyclic loading. While the elevated strength of HSS suggests potential improvements in fatigue resistance, its reduced ductility raises concerns about its overall fatigue behaviour. Factors such as microstructural characteristics, manufacturing processes, and environmental conditions, including corrosion (Gorash et al., 2022), can significantly influence the fatigue life of HSS. For instance, corrosion can lead to the formation of pits that act as stress concentrators, thereby reducing fatigue strength (Wang et al., 2022). Fatigue behaviour was compared for S690 (HSS) and S355, S690 with martensite microstructure offered greater resistance for the crack initiation when compared to S355 steel (de Jesus et al., 2012). Dantas et al., (2022) research demonstrates that fatigue life in the VHCF region follows a nearly horizontal asymptote, indicating a stable fatigue limit beyond a certain number of cycles for S690. In steels, mean-stress sensitivity increases with material strength; high-strength steels show a more pronounced reduction in fatigue strength with tensile mean stress compared to mild steels (Schönbauer et al., 2022; Schuller et al., 2015). Conventional steels when tested at very high cycle domain at 20kHz experienced heating issues (Gorash et al., 2022). Milne et al., (2022) observed that ferritic steels with Body-Centred Cubic (BCC) crystal lattice when tested at very high cycle domain experiences i) strain rate sensitivity ii) heat generation during testing. The lack of extensive VHCF data for high-strength steels like S690 makes it difficult to predict fatigue life beyond 10⁹ cycles with confidence. Whether a distinct transition from HCF to VHCF exists remains uncertain. Additionally, the strong frequency sensitivity observed in ultrasonic fatigue testing poses challenges in translating laboratory findings into practical engineering applications. This study conducts a comprehensive fatigue life assessment of various steel grades, ranging from conventional structural steels to high-strength variants. Fatigue crack initiation tests are performed using the Shimadzu ultrasonic fatigue testing system USF-2000A, while fatigue crack propagation tests are conducted with the servo-hydraulic testing machine Instron 8801. Additionally, the impact of corrosion on fatigue life is evaluated to provide a holistic understanding of the performance of HSS in real-world applications. By comparing the total fatigue life across different steel grades, this research aims to offer valuable insights into the practical advantages and limitations of HSS in fatigue-critical applications, contributing to informed material selection and structural design decisions in The first material tested, referred to as Grade 1, is a high-strength steel extracted from a 16 mm quenched tempered plate, characterised by a martensitic microstructure as shown in Figure 1(a). Its chemical composition conforms to the requirements of EN 10025-6, ensuring compliance with the standard specifications. The second material, Grade 2, is also a high-strength steel but possesses lower yield strength compared to Grade 1. It was extracted from an 80 mm diameter rod, 1 m in length, and exhibits a ferritic, over-tempered microstructure displayed in Figure 1 (b). The chemical composition of all the tested grades of steel is listed in Table 1. For comparative analysis, these grades are evaluated alongside Q355B and S355JR structural steels, which were previously tested and published by the co-authors (Milne et al., 2022). Both these grades are Conventional strength steel (CSS) extracted from hot-rolled plates exhibiting ferritic-pearlitic microstructure with the difference being the larger-sized grains in Q355B as indicated in Figure 1 (c, d). Due to the confidentiality constraints of this research, the exact grades of the high-strength steels cannot be disclosed, and they will be referred to as Grade 1 and Grade 2 throughout this paper. engineering. 2. Materials

Table 1. Chemical composition of the tested steels, according to Spark OES. Material C Si Mn S P Cr Ni Cu Al N Grade 1 0.13 0.28 1.17 0.003 0.007 0.24 0.07 0.01 - 0.005 Grade 2 0.32 0.27 0.48 0.006 <0.003 1.01 0.11 0.25 - - Q355B 0.193 0.20 0.88 0.005 0.021 0.020 0.010 0.010 0.030 0.004 S355JR 0.15 0.03 1.24 0.005 0.007 0.04 0.03 0.04 0.048 0.006