Failure Analysis of High-Strength Bolts in a Railway Steel Truss Bridge

More than 40 high-strength bolts gradually fractured over a service life of about 8 years in a railway steel truss bridge. The fracture takes place at the connection between the nut and the screw thread, which is the stress concentration area of the bolted joint. Since the fracture surface is perpendicular to the axial direction of the bolt, it is determined that the fracture is caused by normal stress. In addition to the pretension, the vibrations generated by trains passing over the bridge also contribute to the cyclic load. Macroscopic and microscopic examination indicates a multi-source crack initiation. Beach marks, stairs, and striations suggest that the bolt failure is attributed to fatigue crack propagation rather than one-time overload damage. Metallographic analysis shows typical intergranular fracture features with a dendritic distribution in the crack initiation region, consistent with stress corrosion. According to the element analysis result of the corrosion products, sulfur as a key corrosion element likely enters the gap in the steel joints of the bridge, leading to local corrosion of the bolts and initiating the fatigue crack. Therefore, the failure mechanism of the bolts is stress corrosion fatigue, resulting from both cyclic load and exposure to corrosive media. In view of this, it is recommended that replacement of the missing bolt and rust removal of the corrosion regions are effective ways to solve the problem.