Crack propagation and microstructural transformation on the friction surface of a high-speed railway brake disc

While brake disc wear represents a significant problem in high-speed rail systems, the progressive development of fatigue cracks during successive braking cycles also plays a great role in braking integrity. The modified microstructure consisting of a white etching layer (WEL) containing nanosized ferrite was observed on the friction surface of worn brake discs. In order to analyze how sequential thermal and mechanical stress affected crack propagation and microstructure evolution in brake discs, successive braking cycles were simulated on a full-scale braking bench test rig. Crack initiation and propagation mechanisms were proposed based on the experimental results, i.e., (i) occurrence of heat checking caused by heating and cooling transients during braking; (ii) heat checking increasing the roughness of the friction surface which in turn caused a local stress concentration and (iii) localized friction stress and thermal stress driving the heat checking to propagate and coalesce with the radial main crack. Analysis of the thermal-mechanical conditions that exist at the friction surface during braking indicates that WEL formation can be attributed to severe plastic deformation caused by the repeated friction between the disc and pads. Mechanical testing also indicated that WEL formation is not detrimental to brake disc integrity.