Comparison of different fatigue assessment methods for HFMI-treated longitudinal stiffeners under variable amplitude loading

Abstract Due to the extensive research on High-Frequency Mechanical Impact (HFMI) treatment in recent years, regulations regarding HFMI treatment are increasingly being incorporated in the revision of design codes and recommendations. Generally, these trends promise a greater application of HFMI treatment in practice and therefore offer the potential of realising resource-efficient and durable lightweight constructions. However, the increasing number of guidelines is also accompanied by some differences regarding the assessment methods and the evaluation of the fatigue life of HFMI treated welds which may lead to divergent estimations of fatigue life. Therefore, this study analyses differences between the assessment methods, provided by current and upcoming design codes and recommendations for HFMI treated welds, specifically in terms of nominal and effective notch stress approach. Overall, the results indicate that current design rules lead to a conservative fatigue assessment, implying that the potential of an increased fatigue strength cannot be fully considered within the fatigue life estimation. This study primarily focuses on options of taking different R-ratios into account, as well as the impact of the intersection point of AW and HFMI S–N curve emphasising the assessment of variable amplitude loading. It is observed that particularly at R = − 1 the fatigue assessment tends to be overly conservative due to the inability to account for this effect. Additionally, the results suggest that neglecting the intersection point between AW and HFMI S–N curve may result in an even more conservative fatigue assessment, especially at higher stress ranges. The comparison of different fatigue assessment methods within this study focuses on current IIW-recommendations as well as the upcoming revised version of Eurocode. Applying different assessment methods reveals no significant differences in fatigue estimation, as conservative results were obtained for both guidelines considered in this study. The comparison is carried out based on fatigue test data on the example of the structural detail “longitudinal stiffener” under variable amplitude loading.