Fatigue Resistance of Fillet Welds of Traction Rod Brackets on a Locomotive Bogie Based on International Union of Railways Standards and Improvement Measures Adopted

To solve the problem of fatigue failure in fillet welds of traction rod brackets on locomotive bogies of a given model, the cause for failure and the improvement method were studied. The results show that when there is maximum clearance at weld roots, maximum incomplete fusion of sidewalls, maximum incomplete fusion at weld roots, and maximum pores allowable in the ISO 5817 standard, the stress amplitude separately increases by 70~97%, 53~55%, 40~46%, and 19~34%. Despite this, when various types of defects of the maximum size are present in the weld alone, the static and fatigue strengths of fillet welds with a throat depth of 6 mm on the traction rod bracket can still meet the requirements in the UIC615-4 standard. In practical fillet welds, defects including clearance at weld roots, incomplete fusion, and pores are very likely to occur at the same time, which may induce fatigue failure in fillet welds of traction rod brackets within the original design life. According to the size of the frame and the traction rod brackets, a strengthening scheme for increasing the throat depth of fillet welds of traction rod brackets to 8 mm was designed. Calculation results of the strengthening scheme show that for new structures subjected to overall post-weld stress-relief thermal treatments, the maximum stress amplitude decreases by 5~29% when increasing the throat depth of fillet welds from 6 to 8 mm. For structures in service with the throat depth of fillet welds increased from 6 to 8 mm through repair welding, peak residual stress at the weld root after repair welding can reach 383 MPa. Because overall stress-relief thermal treatments cannot be performed on repair-welded structures, the fatigue strength of repair-welded fillet welds cannot meet the requirements of UIC615-4; therefore, local stress-relief treatments have to be performed in the welded zone. The results are of guiding significance for the treatment of locomotives in service and performance improvement of new locomotives and suggest that the current standard is relatively conservative.