## 3 Results and Discussion
### Analysis Results of the Fatigue Strength under Operational Loads  
All root nodes of fillet welds of traction rod brackets were taken as stress measuring points, and the stress of all models under each condition of operational loads was calculated. The mean stress and stress amplitude were calculated (Table 4). Goodman diagrams of fatigue strength curves were drawn for the calculation results of all models, and the results are illustrated in Figures 11 and 12. The strengths of fillet weld roots in all models were found to meet the design requirements for fatigue strength. The calculated results show that when various defects allowable in the UIC standard are present in the welds, the strengths at the fillet weld roots of traction rod brackets all meet the design requirements for fatigue strength; because locations with the maximum stress amplitude are locations most prone to failure of welds, influences of various defects on the fatigue strength were compared from the perspective of stress amplitudes. It can be seen from Figure 13 that if the maximum clearance at weld roots, maximum incomplete fusion of side walls, maximum incomplete fusion at weld roots, and maximum pores allowable in the ISO 5817 standard are present, the maximum stress amplitude of the fillet welds of type A traction rod brackets separately increases by 70%, 53%, 40%, and 19%; meanwhile, that of the type B traction rod bracket increases by 97%, 55%, 46%, and 34%. This indicates that among the four types of defects allowable in the ISO 5817 standard, clearance exerts the greatest influences on the fatigue strength of fillet welds of the two traction rod brackets, while pores show the lowest influences thereon. \begin{table}
\begin{tabular}{c c c c} \hline  & **Model** & **Type A Traction Rod** & **Type B Traction Rod** \\  & & **Brackets** & **Brackets** \\ \hline
1 & Fillet weld A6 of the existing welded structure & 42.68 & 30.49 \\
2 & Clearance (\(h=1.7\) mm) & 72.63 & 60.02 \\
3 & Incomplete fusion of sidewalls (\(l=10\) mm, \(h=2\) mm) & 65.28 & 47.27 \\
4 & Incomplete penetration of weld roots (\(l=10\) mm, \(h=2\) mm) & 59.82 & 44.41 \\
5 & Pores (\(d=2\) mm, \(s=20\) mm) & 50.95 & 40.91 \\ \hline \end{tabular}
\end{table} 
Table 4: Maximum stress amplitudes at root nodes of fillet welds of traction rod brackets of the frame (unit: MPa). Figure 10: Maximum equivalent stress of attachment welds on the type A and type B traction rod brackets. Analysis indicates that fatigue cracks occurred in the locomotive bogie before the driving mileage probably reached the design value because of the following reasons: (1) locomotives observed to have fatigue cracks operate on railway lines under poor conditions; and (2) different types of defects are present in practical fillet welds at the same time. In practice, dozens of locomotives of this model were produced in the same batch and multiple locomotives in which fatigue cracks occurred in traction rod brackets before the  
Figure 11: Goodman diagrams of fatigue strength curves at the fillet weld roots in five models of the type A traction rod brackets. Figure 12: Goodman diagrams of fatigue strength curves at the fillet weld roots in five models of the type B traction rod brackets. accumulated mileage driven reached the design value all operated on the same railway line in the mountainous area. In comparison, fatigue cracks were not found on other locomotives operating on railway lines beyond mountainous areas before the mileage reached its design value. Dissection of fatigue cracks shows that defects, including the large assembly clearance at weld roots, pores, and incomplete fusion, occur simultaneously in the crack initiation zone. To guarantee the safety of locomotives that have operated, and will operate, on a railway line, defects in fillet welds need to be controlled on the one hand; on the other, the throat depth of fillet welds can be increased to enlarge the bearing area of weld metal, thus prolonging fatigue life.