Fatigue Behaviour and Notch Sensitivity of Welded Ultra-High-Strength Steel Plates

Ultra-high-strength steels (UHSS), such as S700, S960, and S1100, are gaining popularity in the structural engineering and automotive industries owing to their exceptional strength-to-weight ratios. However, their fatigue performance in welded conditions remains a significant problem, since welding induces residual stresses, microstructural changes, and geometric discontinuities that influence crack initiation and growth. This thesis investigates the fatigue behaviour of welded ultra-high-strength steels (UHSS) plates with a comprehensive experimental and analytical framework, focusing on fatigue crack propagation, high-cycle fatigue life under notched conditions, and the assessment of notch sensitivity. This research is categorised into three main experimental phases. The first phase evaluates fatigue crack growth rates (FCGR) using compact tension (CT) specimens extracted from different zones near the welded joints, including base metal (BM), weld metal (WM), and heat-affected zone (HAZ), using both gas metal arc welding (GMAW) and laser welding (LW) methods. Tensile tests equipped with a Digital Image Correlation (DIC) system using a 3D optical measurement device (ARAMIS), Vickers micro-hardness testing with a Duramin-40 in accordance with Australian Standard AS1817.1–2003, and microstructural characterisation using a Nikon Eclipse ME600 optical microscope were conducted to examine the mechanical and microstructural properties of each weld region. Fatigue crack growth rate (FCGR) tests were performed in accordance with ASTM E647 to determine Paris’ Law constants (C and m) for these grades. The results consistently showed that the heat-affected zone (HAZ) exhibited the lowest fatigue resistance due to microstructural softening and coarsening within the HAZ region. Compared to gas metal arc welding (GMAW), laser welding produced narrower HAZ widths and improved fatigue performance in higher-strength steel grades. The second phase of the study involved high-cycle fatigue testing on notched specimens, including both centrally and edge-notched configurations, designed to resemble real-world fatigue problems commonly associated with geometric stress concentrations in structural components. Tests were conducted under a set of different load ranges to generate S–N curves and analyse fatigue life trends. Lower fatigue life was found in notched specimens with edge-notched configurations due to higher local stress intensities at the notch tip compared to their centre-notched counterparts. Among the three grades, S700 BM displayed superior fatigue life, while laser welding significantly enhanced the fatigue resistance in S960 and S1100. These findings emphasise the influence of weld quality and configuration on the fatigue performance of UHSS plates. The final phase assessed the notch sensitivity index (NSI), which is an indicator used to quantify fatigue strength reductions caused by the presence of notches. Endurance limits for both notched and unnotched specimens were experimentally determined. In contrast, the stress concentration factor (K_t )for V-notched specimens was calculated using finite element analysis in ANSYS 2022 R2. The fatigue strength reduction factor 〖(K〗_f) was derived from experimental data, and the notch sensitivity index (NSI) was subsequently calculated using the classical theoretical relationship, q=(K_f-1)/(K_t-1). Based on the test results, higher-strength steel grades demonstrated greater notch sensitivity, particularly in the heat-affected zone (HAZ) region. This finding validates the well-established principle of the inverse relationship between material strength and notch tolerance. In laser-welded joints, lower NSI values were observed, which can be attributed to improved microstructural control and narrower heat-affected zones (HAZs). To complement the experimental work, a fatigue life prediction model based on linear elastic fracture mechanics (LEFM) was employed. The model incorporated experimentally derived Paris’ Law parameters and stress intensity factor solutions to estimate crack propagation life from initial to critical crack sizes. The predicted fatigue lives showed a strong agreement with experimental results that justify the applicability of LEFM for welded UHSS under high-cycle fatigue conditions. Overall, this research provides a thorough assessment of fatigue crack propagation behaviour, high-cycle fatigue life prediction, and notch sensitivity in welded ultra-high-strength steel (UHSS) plates. It also highlights the detrimental effects of welding-induced heterogeneities, particularly within the heat-affected zone (HAZ) and demonstrates the advantages of laser welding in enhancing fatigue resistance. The findings offer valuable data and predictive models for fatigue design, which are directly related to welded UHSS structures, such as bridges, towers, and automotive applications. Recommendations for future work include fatigue investigations of welded UHSS to variable amplitude loading, diverse stress ratios, and environmental effects such as temperature extremes. Advanced modelling approaches, including XFEM, and the 4R method, are recommended to improve predictive accuracy beyond LEFM. Large-scale structural validation and incorporation into advanced design codes will further support the practical application of UHSS in modern engineering structures.