## 1 Introduction  
A locomotive bogie generally comprises the primary suspension, secondary suspension, wheelset axle box, traction device, brake, and frame [1; 2; 3]. To ensure a light-weight ensemble, bogie frames are generally welded structures. The fatigue failure of each component relates to the safety of personnel and locomotives and therefore has long been of concern among researchers [4; 5; 6; 7; 8]. In an entire bogie, the frame is the skeleton that plays important roles in supporting the car body, ensuring safe operation of locomotives, easing interactions between locomotives and tracks, braking, and steering. In view of this, problems pertaining to locomotive bogie frames remain key topics of research. Based on the DVS 1612 and JIS 4270 standards, [PERSON] et al. [9] assessed the fatigue strength of the bogie frame of a certain model. The results show that the bogie frame meets the requirements for fatigue strength in both standards. Under the JIS 4270 standard, welds of the motor cabinet and gearbox bracket are more remarkably loaded, while welds of joints between sill and side sills, and at the transverse stop base, are more significantly loaded under the DVS 1612 standard. [PERSON] et al. [10] assessed and compared the fatigue strength of welded frames of a certain model separately based on the UIC and JIS standards. The results indicate that the weld toe at the side sills of the frame meets the requirements for fatigue strength when it is evaluated using the UIC standard, while the weld root at side sills of the frame does not meet the requirements for fatigue strength when it is assessed using the JIS standard. [PERSON] et al. [11], [PERSON] et al. [12], and [PERSON] et al. [13] separately evaluated the static and fatigue strengths of different bogie frames based on UIC615-4 and EN13749 standards. They found that the static strengths at key locations of the frames and the fatigue strengths at important locations of welds all meet codified requirements. According to JIS E 4207 and UIC 515/615-4, [PERSON] et al. [14] evaluated the fatigue strength of a bogie make of GFRP composites based on the S-N curves and Goodman diagrams of GEP224 glass/epoxy resin composites. Their results indicate that the fatigue strength of the bogie made of GFRP composites satisfies the design requirements; compared with traditional metallic bogies, the bogie fabricated using GFRP composites shows better fatigue performance considering its weight. According to the UIC615-4 and EN13749 standards, [PERSON] et al. [15] performed static strength, fatigue strength, and modal analysis of a bogie frame fabricated using carbon fiber composites on a subway locomotive through simulation. The calculated results show that the strength of the frame meets working requirements. [PERSON] et al. [16; 17] proposed a novel fatigue-life-prediction theory based on microplasticity and conducted finite element simulation on this basis. Computation indicates that the theory can predict the fatigue life of several selected metallic alloys under multiaxial loads and is accurate. The aforementioned research into bogie frames mainly assesses fatigue strength by combining the standard loads and Goodman diagrams. Considering that the UIC standard can meet stress conditions of various bogie frames, the standard has also been widely used. Meanwhile, many scholars have also studied the failure of welded frames. Taking the Ti6 Al4V/nickel-chromium-iron alloy 625 dissimilar joints obtained by intermediate inserts of vanadium and AISI 304 as a case study, [PERSON] et al. [18] explored fatigue behaviors of joints using a scanning electron microscope and a confocal microscope. Fracture analysis revealed that the failure mode is mainly related to the presence of multiple pores on crack paths. [PERSON] et al. [19] experimentally studied the fatigue performance of 2205 two-phase stainless-steel cross-type welded joints, and the results show that fatigue failure of the 2205 steel cross-type welded joints is influenced by material inhomogeneity and welding defects.