PSI - Issue 75

Mehdi Ghanadi et al. / Procedia Structural Integrity 75 (2025) 457–466 Mehdi Ghanadi et al./ Structural Integrity Procedia (2025)

464

8

Table 2.Variation of scatter data without and with adjustment with Km,eff.

Specimen

( ) Stdv LogC

, ) m eff Stdv LogC withK (

, m eff T withK 

T 

( f charachteristic nom P  −

2.3%)

(

2.3%)

P

withK



=





=

, m eff

charachteristic nom −

f

T2B T2C

0.27 0.42 0.19 0.17

0.91 1.10 0.20 0.19

1.38 1.65 1.49 1.41

2.93 3.66 1.27 1.25

135

173 147 125 124

91

T16B T16C

119 124

A summary of fatigue stress ranges at 2 million cycles with failure probabilities of 50% and 2.3%, referred to as mean and characteristic stress ranges, respectively, in relation to nominal stresses, are also shown in Fig. 7. Thinner plates exhibit higher fatigue strength but also greater angular misalignment. Compensating for this misalignment further enhances fatigue strength, with a more pronounced effect in 2 mm plates. The standard deviation of the logarithmic value of the Basquin coefficient, (log ) Stdv C , is assessed according to the IIW recommendations for individual batches. The larger scatter (0.91-1.1) observed for 2 T = mm thickness indicates that angular misalignment significantly contributes to the increased variability, particularly in thinner plates. 7. Discussion Thin-walled welded joints are particularly susceptible to angular distortion due to transverse shrinkage during welding. This emphasizes the importance of studying angular misalignment, or the technological size effect, as a global shape imperfection that introduces additional stress, though it is not a weld quality criterion. As shown in Fig. 6, angular distortion is more severe in thinner plates due to their lower stiffness and limited structural restraint. Among the tested specimens, batch T2C exhibits the highest angular misalignment, which exceeds that of T2B. This is attributed to the higher welding current and, consequently, greater heat input used for T2C (Tab. 1). Thicker plates, 16 mm, require more heat to ensure full penetration and proper fusion, unlike thinner 2 mm plates (Fig. 3). Correcting for misalignment shows that, for 2 mm thick joints, fatigue strength diminishes with decreasing weld quality (Tab. 2). This trend is likely due to a reduction in weld toe radius and angle when moving from weld quality B to C (Fig. 5), increasing stress concentrations and, thus, fatigue susceptibility. The impact of these geometric changes is more significant in thinner joints, indicating their heightened sensitivity to weld quality variations. Weld geometry also contributes to variability in fatigue performance. For 2 mm plates, lower weld quality leads to greater data scatter, suggesting increased inconsistency. Conversely, in 16 mm plates, scatter decreases as weld quality drops, implying that other factors may dominate the variability in the stress range. An additional observation from the stress range at 2 million cycles (2.3% failure probability) is that fatigue strength improves as plate thickness decreases — reinforcing the thinness effect. The experimental results, uncorrected for misalignment, closely match the FAT80 S-N design curve recommended by IIW (Hobbacher & Baumgartner, 2024). When corrected for misalignment, the results remain conservatively below FAT80. 8. Conclusion This research investigates the effect of weld quality and thickness on the fatigue strength of transverse non-load carrying attachments. Based on laser scanning of weld geometry and fatigue tests showed failure in the weld toe region for all specimens following conclusions can be drawn: • Weld quality played an important role in fatigue performance, with higher-quality welds lowering stress concentration and extending fatigue life. • A thinness effect was observed, as 2 mm plates showed higher fatigue strength due to reduced stress concentration. • Design guidelines mainly address the effects of thickness on fatigue strength and provides no guidance for thin plates. The results from the present work suggest that the current fatigue design rules could be conservative for thinner structures.