ROLLING CONTACT FATIGUE ON TRAMWAY'S RAIL

Head checking cracks as a special type of rail defects become more frequent recently on the high-speed railways. Partly similar defects were observed at the starting and stopping locations of the vehicles' driving axle at urban tram stops. In these places fatigue defects are appearing parallel to the track on the rail's running surface. These defects were first observed by the author of the abstract in Budapest, along tram Line 49. The most significant defects were discovered on sections of that tram line, where only old carriages run without slip protection equipment. On those sections, where other types of the carriages also run, the defects were less frequent. Measurements of eventual defects were performed using Eddy current sensor, digital microscope, wheel mounted inertial sensor and high-speed camera. Measurements with Eddy current sensors were carried out on the running surface of rail. They did not show cracks but average depth of defects could be determined. Surface deformations were detected by a digital microscope and their dimensions were also measured. Images made by digital microscope (from "micro-slip") showed similarity to the "comet-shaped" rail surface corrugation caused by high-intensity acceleration ("macro-slip"). Based on this fact, the author assumed that probably the slip plays an important role in the formation of defects. Slip values recorded by a high-speed camera during start and stop of tram carriages were found equivalent to the length of the defect under consideration. Further investigations were made to determine the exact value of the slip during the start and stop of the tram carriage under operating conditions. For that purpose, wheel-and vehicle-mounted accelerometers and the speed acquisition device of the tram carriage were used. Assessing the results of the experiments, it is concluded, that the wheel slip is responsible for the defects discovered, but further investigations are needed.