From the testing laboratory: damage analysis of a railway mounting bracket
1. April 2019
A lucky escape: the fracture was discovered before a derailment occurred (illustrative image).
1) The damage case
After 16 years in service, the mounting bracket of a torque reaction arm on a regional train fractured – significantly earlier than its expected service life of 30 years. In the worst case, the torque reaction arm could have become detached while the train was moving and fallen onto the track, potentially causing a derailment. Fortunately, this did not happen.
2) Material and operating conditions
The torque reaction arm transfers torque from the motor and brake to the bogie frame. The mounting bracket was a welded fabrication, attached to the frame on both sides by double fillet welds. The material used was S355 structural steel.
3) Damage analysis
The damage analysis focused on optical microscopy of the welded joint and scanning electron microscopy of the fracture surface. These examinations showed that the brackets had failed by fatigue fracture originating in the welded joint (Fig. 2).
The joint was a double fillet weld without full penetration. Fatigue cracks formed in the unfused areas of the plate joint as a result of the increased notch effect. Metallographic examination of the opposite side, which had not yet fractured, also revealed initial fatigue cracks in the welded joint (Fig. 3).
4) Conclusions from the fatigue fracture analysis
The selected welded design, using a joint without full penetration, is only conditionally suitable for components subjected to dynamic loading. If the internal cavities in the joint have an unfavourable geometry, as in this case, internal notches can arise. Fatigue cracks can initiate at these notches and ultimately cause the component to fail.
The international standard DIN EN 15085-3, which specifies design requirements for welded components on rail vehicles, therefore recommends full-penetration welded joints wherever possible for dynamically loaded structures. This avoids internal notches.
W.S. Spezial seminar: Weld Testing and Evaluation
The seminar “Weld Testing and Evaluation” (SemSNB) provides practical guidance on evaluating welds, identifying typical damage mechanisms and improving the quality of welded fabrications.
Participants learn about:
- the essential principles of welding processes
- the materials engineering fundamentals of welding processes
- typical weld imperfections and their effects on components
- materials engineering strategies for avoiding imperfections
- testing approaches for weld evaluation
- an overview of relevant standards for quality evaluation
Further information about the seminar is available at https://www.werkstoff-service.de/en/trainingcenter/kurse-und-seminare-buchen/weld-testing-and-evaluation-snb-seminar/
