<p>In welding, improper operation or an excessively low welding current can easily cause incomplete penetration defects. These defects significantly reduce the cross-sectional area of the weld, leading to stress concentrations that can initiate fatigue cracks and, ultimately, cause premature fatigue failure of the welded joint. To investigate the effects of different incomplete penetration defects and weld geometry parameters on the fatigue life of butt welds, this paper first verifies the validity of the structural stress method for fatigue life prediction by comparing the fatigue life of actual specimens with incomplete penetration to that of the finite element model. The effects of different incomplete penetration shapes, depths, and weld widths on fatigue life were then analyzed using the structural stress method. Finally, based on the shock absorber seat model and the bogie model, the influence of the structural location of lack-of-welding defects on the overall fatigue life of the component was quantitatively analyzed. Engineering case studies demonstrate that fatigue life prediction based on the structural stress method can be effectively applied to evaluate incomplete penetration defects in welds in practical engineering applications.</p> Graphical abstract <p></p>