Investigation of a premature failure axle from a road-rail truck under rotary bending fatigue

• Premature failure of a road-rail truck axle resulted from rotary bending fatigue. • Fatigue fracture occurred in the braking system attachment zone. • Critical factors were fretting damage, stress induced by tightening and rotary bending. • Preventive actions were proposed about assembly and inspection of the braking system. • Routine inspection should include a fretting affected region examination. The present study provides a detailed analysis of an axle from a road-rail truck. The main objective of this work was to characterize a premature axle fracture failure in the context of ongoing derailment investigations. A comprehensive material and macro/micro analysis of the axle fracture was conducted which revealed that the premature failure resulted from rotary bending fatigue. The fatigue fracture occurred specifically in the braking system attachment zone. Microscopy and SEM analysis demonstrated that initiation occurred on the surface of the axle. A combination of critical factors resulted in imposing stress below the fatigue strength and leading to a premature failure of the axle. The determined critical factors were fretting damage, stress induced by tightening, and rotary bending. Through the combination of all identified influential variables, multiple crack initiation sites were detected across the braking system press-fitted regions. Following the forensic examination, a plan was proposed outlining the necessary actions, checks, and tests necessary to correct the existing failure through maintenance and/or establishing the time intervals between axle inspections. Corrective actions were recommended regarding the assembly and regular inspection of the braking system, focusing on the contact pressure region with the axle. The routine inspection should include a thorough examination of the fretting affected region for potential cracks and uneven damage utilizing visual amplifying means and ultrasonic testing.