Cracking of spring planks in Wagon Truck K4

Based on the American swing motion truck techniques and some improvements, Wagon Truck K4 is manufactured as a new type truck. The spring plank, a key part on the truck, plays an important role in reliability of the truck. After being in service for about half a year, it was found that there were cracks 10–15 mm in length on the planks for many trucks. All cracks were located at the same positions, i.e., at the two ends of welded seams between the plank body and the fender, and then propagated into the plank body in the longitudinal direction of the plank. Chemical analysis, mechanical tests and metallurgical inspections show that the elemental composition, tensile strength and hardnesses correspond to the technical requirements and there are no abnormal phenomena in microstructures either in the bodies or in the welded seams. Fracture surface observation reveals that there are a few origins and obvious beach marks, which demonstrates the cracking is a typical fatigue failure and implies there are high stresses at the cracking positions. Mechanical simulation tests and FEM indicate that the cracking sites are vulnerable spots on the planks and there is constructive stress concentration interference. Therefore, the failures are mainly due to the high stresses at the seams. In addition, weld quality is another important factor contributing to the failures. Preventative measures, such as changing the structure of the plank or the length of the seam so as to diminish constructive interference, and polishing the weld seam so that the stress concentration becomes as low as possible, are put forward.