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Disc brakes mounted on intercity trains can be exposed to severe thermomechanical loads during emergency braking, with conditions at the disc-pad interface exceeding 1.5MPa, 30m/s and 1000°C for several tens of seconds. This does not affect their integrity and performance under standard service use but may become an issue in the case of repeated loads during intensive testing. Recent experiments showed that repeated exposure to high energy braking may durably affect friction behaviour of disc brakes equipped with steel discs and sintered copper-iron pads. Such conditions can be met during UIC standard tests and could modify the outcome of the test programs performed afterward. In this study, friction and wear mechanisms are compared before and after repeatedly exposing a disc brake to high energy braking. The results should bring new highlights on how severe thermomechanical loads govern braking performance, and help analysing UIC test results regarding the brake’s thermomechanical history. On a reduced scale pin on disc tribometer, a steel disc and a sintered copper-iron pad were exposed to repeated high energy braking sequences adapted from UIC 541-3 C2 test program. To understand how such conditions may durably affect performance during subsequent train service use, lower energy braking sequences, adapted from a field service route, were performed before and after high energy braking to characterize the evolution of friction and wear related to the brake’s thermomechanical history. To make sure this softer braking sequence does not alter performance, and to allow comparative analysis of rubbed surfaces, another pad was submitted to only service braking. During the test, the friction coefficient was calculated from recorded normal and tangential forces. Temperatures were measured by thermocouples embedded in the pad and disc, and thermal localizations were observed at the disc surface using a high speed infrared camera. An indicator of wear was given by a pad displacement measurement. After the test, post-mortem analysis of rubbed surfaces was achieved by a combination of Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX), in order to better understand differences between friction and wear mechanisms after low and high energy braking. First, friction behaviour was characterized after low energy braking. Results show that instantaneous friction is very dependent on the interface’s temperature, even under service use conditions. Different starting temperatures induce different thermal distributions in the contact and different friction levels over time. This concerns durations of one brake application but does not affect subsequent ones. Second, friction evolution was studied after high energy braking as compared to low energy one. Each high energy brake significantly modified friction mechanisms, and the disc’s surface exhibited thermal localization marks and uneven, deeply scratched surface. Afterwards, the contact needed several brake applications to recover stable friction levels and homogeneous disc surface, but friction levels were irreversibly lowered after each high energy braking sequence. These changes can be attributed to thermomechanical history, yet the induced evolution in friction and wear mechanisms is still to be determined.
University of Lille: Prof. Yannick Desplanques, Dr. Anne-Lise Cristol, Dr. Philippe Dufrénoy; Alstom Flertex: Mr. Sylvain Delattre, Dr. Michèle Henrion