Rolling contact fatigue: Spalling versus transverse fracture of rails

Rolling contact fatigue (RCF) defects in the running band of the rail may develop, as a function of born tonnage, either superficially and spall off, or penetrate into the subsurface. In practice, the first type is found to occur notably (but not exclusively) on heat-treated pearlitic rails. Both possibilities involve an essentially different operational risk with respect to transverse rail fracture and require therefore different inspection and maintenance regimes. This study presents a validated hypothesis that explains both similarities and differences of spalling defects and classical squat defects that develop also in depth. It is shown that their microstructural/-mechanical initiation mechanism is different and not necessarily governed by the local tangential stress history in the case of spalling. A model is set up and validated for subsurface crack propagation directivity, distinguishing a spalling and a transverse fracture domain for development of running band defects for both straight track and high and low legs of curves. This model allows for understanding and recognition of the nature of running band defects and for adjustment of control actions.