PSI - Issue 75

Mattias Clarin et al. / Procedia Structural Integrity 75 (2025) 467–473 Clarin et al./ Structural Integrity Procedia 00 (2025) 000 – 000

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At lower stress ranges, several specimens reached the predefined run-out limit without failure, as indicated by 151 hollow triangles. These run-out results suggest a pronounced extension of fatigue life due to the application of 152 overload, particularly noticeable at the higher overload percentages (75% and 100%). It should also be noted that the 153 regression lines and the associated data points used for deriving mean curves exclude results beyond the defined upper 154 cycle limit (dashed vertical line at five million cycles). This exclusion ensures consistency and reliability of the fatigue 155 life predictions from the obtained data. Overall, these findings strongly support the beneficial role of initial overload 156 in enhancing fatigue resistance and extending the structural integrity of welded joints under cyclic loading conditions. 159 160 161 162 163 164 165 166 167 To further illustrate this, Fig. 6 presents a metallographic cross-section of a tested specimen that has been sectioned, 168 mounted, polished, and etched. The image shows the weld regions from both the failed and non-failed sides of the 169 specimen. Notably, a clear difference in weld geometry can be observed between the two sides. The unfailed side 170 exhibits a greater leg length along the main plate, which results in a shallower weld toe angle. In contrast, the failed 171 side displays more equal leg lengths and a steeper weld toe angle. This sharper transition is likely to have introduced 172 a higher local stress concentration, thereby contributing to the earlier initiation of fatigue cracking on that side. Series Slope m Mean strength @ 2M cycles Stdv log C 0 % 3.45 124 0.045 25 % 3.57 125 0.071 50 % 7.12 200 0.077 75 % 7.21 255 0.128 100 % 11.6 336 0.030 157 158 Table 1. Tabulated data for the mean fatigue strength curves corresponding to the different overloaded specimen groups.

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175 Figure 6. Etched cross-section of a welded specimen where both the side with failure and the side without failure are visible. 176 Concluding remarks 177 The experimental investigation presented in this study demonstrates a significant beneficial effect of nominal 178 preloading on the fatigue performance of welded, non-load-carrying circular flat studs made of S960 ultra-high- 179 strength steel. Key findings from the study are summarised as follows: 180 • A clear correlation was observed between the magnitude of applied preload and the improvement in 181 fatigue life, with substantial increases noted at higher preloading levels (75% and 100% of the yield 182 strength). 183 • Fatigue test results consistently showed increased cycles to failure in preloaded specimens, validating the 184 hypothesis that controlled initial overload can effectively enhance fatigue resistance. 185 Overall, these findings underscore the potential of preloading as a practical strategy for improving fatigue life in 186 welded structures utilising high-strength steels such as S960.

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