Failure analysis of axle box lower cover plate bolts considering bending damage

This study investigates the premature fatigue failure of axle box lower cover bolts in railway vehicles. Field measurements were conducted to obtain the axial tensile and bidirectional bending loads of the bolts during operation. The results showed that excessive bending was the dominant contributor to the premature failure. Bench tests further showed that the clearance between the axle box and guide column had increased from the designed 0.6 mm to approximately 1.1 mm, and the measured response corresponded to a bending moment of about 19 N·m. An angle-dependent cumulative damage method was used to synthesize the axial and bidirectional bending stresses around the bolt circumference. The maximum cumulative damage of the Grade 8.8 bolt occurred near 60°, with a value of approximately 6.0, consistent with the observed crack-initiation direction. Based on the bench and line tests, clearance restoration and a replacement configuration using Grade 10.9 bolts with an increased tightening torque were evaluated. Using the observed service life of the Grade 8.8 bolt as a reference, the fatigue life of the replacement configuration was predicted to increase from approximately 4 months to 44 months.