Characterization of damage at the friction surface of wheel mounted brake disks for railway applications

摘要

The surface regions in friction pairs experience significant thermo-mechanical fatigue (TMF) loading. Due to out-of-phase TMF loading, high compressive stresses occur at the spot of the temperature peak. In combination with reverse loading during cooling the brake disks undergo cyclic plastic deformation near the surface and residual stresses will develop. Under harsh loading conditions such as emergency stops hot spots form at the friction surface. The temperature at these hotspots is so high that a phase transition takes place. All these effects influence the formation and growth of heat checks at the friction surface which appear most commonly as a system of parallel cracks propagating in thickness direction. In the current study two wheel-mounted steel disks tested at a test rig with two different loading scenarios are available for a detailed damage analysis. The residual stresses are analyzed systematically at various positions at the surface and near the surface up to a depth of about 1.5 mm by X-ray diffraction. In a subsequent step the reasons for the detected residual stress distributions are investigated by means of metallography and SEM techniques, thus revealing the type of microstructural changes as a function of the depth from the contact surface. Furthermore, the micro-crack network is characterized. The understanding of the damage mechanisms appearing during braking and their spatial distribution is essential for defining a modeling strategy. The damage analyses performed provide information on local heating and the depth of cyclic plastic deformation. Furthermore the microstructural changes are investigated in view of their importance for a global simulation scheme. The knowledge obtained in this study will be essential for subsequent finite element modeling of the response of railway brake disks in service.