Liquid–solid interaction at opening in rolling contact fatigue cracks

The interaction between liquid and solid in rolling contact fatigue (RCF) cracks at the opening phase was modelled by coupling the 2D FE model of surface braking shallow angle crack and the theoretical squeeze fluid film (SFF) model of liquid contained between the crack faces. In the FE model, the moving contact load was simulated by the theoretical Hertz contact pressure distribution approaching the crack mouth from the position situated far from the crack. The liquid flow inside the crack was described by the one-dimensional SFF Reynolds equation derived for compressible fluid with pressure dependant viscosity. This form of Reynolds equation was analytically integrated after approximating the profile of opening velocity along the crack length with an analytical function. As a result of numerical FE analysis the kinematics of motion of the crack faces was determined, which was the input data to the analysis of the SFF model. The purpose of analysis was to verify commonly taken assumption about the ability of liquid sucking in (for instance oil or water if present in the wheel/rail contact area) into the crack interior during one cycle of opening. The analysis was carried out for load, crack and contact geometry typical for rail/wheel contact. The results obtained from the analysis indicate on the existence of “limiting speed” of moving contact load (train speed) for which the phenomenon of continuous liquid sucking in into the crack is possible. For the RCF rail head crack of length a = 13.5 mm inclined at 15° to the horizontal, this speed is about 0.9 m/s. For higher speeds, the SFF model does not give the reasonable solution because pressure in the section of fluid film near the crack tip drops deeply below zero. These conditions indicate that cavitation can occur in the fluid film inside the crack or fluid flow can be broken or confined to fraction of crack length. Additionally, the lack or very insignificant influence of the existence of fluid film inside the crack on the kinematics of crack opening in the analysed phase of loading cycle was observed.