Abstract:

Several high-strength fastening bolts were found broken on a railway steel bridge. Failure analysis of these bolts was performed to determine why they fractured and to prevent future bolt fractures. The fracture morphology, microarea-chemical analysis of the fracture, microstructure and hardness of the bolts, compositional contents, and mechanical properties of the bolt steel were analyzed by using scanning electron microscopy, energy dispersive X-ray spectrometry, optical microscopy, a Rockwell hardness tester, X-ray fluorescence spectrometry, and an electronic universal testing machine, respectively. The results indicate that the compositional contents, hardness, and mechanical properties are consistent with the 20MnTiB steel standard. In addition, dendritic cracks can be observed in the fracture initiation zones of the fastening bolts and the corrosion products in the cracks contain sulfur.The fastening bolts are subjected to cyclic loads in normal working conditions. Therefore, fractures of the fastening bolts were caused by corrosion fatigue fractures under cyclic loads. The cracks initiated at the root of the screw thread or the connecting point between the stud and nut, which are particularly vulnerable to stress corrosion cracks caused by the presence of sulfur.

Key words: steel bridge, high strength bolt, stress corrosion crack, fatigue, fracture