Analysis of Sleeper Screw Failures

A small but critical component in the railway is the sleeper screws fastening the baseplates with the rails to the sleepers. High loads from the passing trains, are transferred into the screws. Flexing and displacement of the rails can cause cyclic tensile and shear loads on the screws which together with corrosion may lead to cracks and failures. Due to the complex conditions, the failure mechanisms of screws are neither well characterised nor understood in spite of their importance for safety and reliability of the railway network. In the present study, more than 20 cracked and failed screws were collected at Switches and Crossings on concrete sleepers close to the Central Station of Copenhagen and characterized by four complementary techniques: 3D X-ray tomography, optical microscopy (OP), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). 3D X-ray tomography was used for three-dimensional mapping of changes in screw shapes and of cracks. OP and SEM were applied to study the characteristics of the fractured surfaces. SEM and EBSD were combined to investigate the sub-surface deformation and corrosion. All this information was put together for a comprehensive analysis of failure. The failure cause and mechanisms for the present screws are discussed, and the type of investigation may be used as an example for screw failure characterization and analysis in general.