Abstract: Objective This study aims to investigate on fatigue crack initiation behavior of root incomplete penetration defects in welded joints of railway vehicle bogie frames under various loading conditions, and to establish a rational criterion for determining the allowable defect size. Methods Butt and T-joint samples made of SMA490BW steel with simulated incomplete penetration defects were prepared. High-frequency fatigue tests were conducted to obtain S-N curves. Furthermore, finite element analysis was used to study stress intensity factors along the defect front. Results Results showed that fatigue cracks in butt joints consistently initiated at the weld root defect. In contrast, T-joints exhibited two initiation modes at the weld root defect and the weld toe, influenced by stress ratio and stress level. In addition, fracture mechanics parameter describing the condition under which defects did not develop into macroscopic cracks, crack growth threshold ΔK_th, was determined. Conclusion A quantitative relationship between weld root defect depth and critical stress range is established for the two types of welded joints. This provides a clear assessment basis for determining allowable defect sizes in welded structure design.