One of the most important applications of Iron-based Shape Memory Alloys (Fe-SMA) in the construction sector is utilizing their recovery stress for pre-stressing structural elements such as concrete and metallic beams and plates, which are subsequently subjected to cyclic loads. This makes it crucial to develop a reliable fatigue design process for the attached Fe-SMA members. It is known that the fatigue strength of metals decreases with an increase in the stress ratio (R). Since Fe-SMA members in civil engineering are used in a prestressed form, the mean stress level plays a significant role in determining the fatigue strength of the alloy. This is particularly important because the alloy is generally used as a prestressing element with superimposed cyclic service loads, resulting in large stress ratios, potentially up to R = 0.7. Recent studies have shown the potential of tailoring Fe-SMA properties to achieve higher recovery stresses, reaching up to 600 MPa. On one hand, this results in higher potential pre-stress levels, but on the other hand, it also increases the stress ratio. While recent research has focused on enhancing the recovery stress of Fe-SMA pre-stressing members, the urgent need to assess the changing structural fatigue behavior has been somewhat neglected. Therefore, this study aims to provide initial insights into this research gap by systematically analyzing the influence of different heat-treated conditions of Fe-SMA on their structural fatigue behavior. This analysis includes studying the impact of varying stress ratios due to tailored pre-stress levels (recovery stress) and different service load levels. This work will help establish a more comprehensive understanding of the fatigue behavior of Fe-SMA members, facilitating the development of safer and more reliable applications in the construction sector.

This work investigates the impact of the real weld geometry and geometric weld imperfections on the fatigue strength of butt welded joints. Numerical calculations of the fatigue test clamping process, based on the real weld geometry, are employed to quantify stress raising effects due to welding imperfections. In addition, stress concentration and fatigue notch factors are computed based on the theory of critical distance according to Peterson that takes into account the material micro-support effect. The real weld geometry is incorporated by using the reverse engineering process. For that, the real weld geometry has first been scanned and transformed into an equidistantly ordered point cloud. Using a self-developed algorithm, the point cloud has been inserted into a CAD model, which exhibited the same geometric welding imperfections, i.e., axial misalignment and angular distortion as the real specimen. Additionally, this study examines the correlation between clamping-induced stresses and these geometric welding imperfections. The results indicate that clamping-induced stress increases with greater misalignments. Furthermore, the fatigue strength of butt welded specimens made from S355MLO mild steel is comparatively investigated using the established fatigue assessment methods nominal stress approach and the conventional notch stress approach, based on the numerically computed fatigue notch factors and fatigue test results. For a more comprehensive and realistic fatigue assessment, this work introduces a modified notch stress approach for assessing the fatigue strength of welded joints, including the effects of real weld geometry and geometric welding imperfections. Although, this approach necessitates further investigations, preliminary results are promising.