Alarm limits for wheel–rail impact loads – part 1: rail bending moments generated by wheel flats
Wheel tread defects, such as wheel flats, generate wheel–rail impact loads that in severe cases may promote fracture from pre-existing cracks in the rail. The present Swedish wheel removal criterion relates the alarm limit to the size (length) of a wheel flat. This is not an optimal situation since it may be both difficult and dangerous to locate and measure the length of a wheel flat. Further, a given size of a wheel flat will result in different impact loads if present on different types of vehicles due to differences in train speed, axle load, etc, and on different tracks with differences in track properties. In this study, the influence of wheel–rail impact load magnitude on resulting rail bending moment is studied using a numerical model for simulation of dynamic vehicle–track interaction. Three different types of traffic are investigated: heavy haul at 60 km/h (axle load 30 tonnes), freight at 100 km/h (25 tonnes) and passenger at 200 km/h (21 tonnes). Positive bending moments generating tensional stresses in the rail foot and negative bending moments leading to tensional stresses in the rail head are calculated. The model is calibrated with respect to field measurements. An idealised time history of the impact load due to a wheel flat is suggested, and worst-case scenarios are sought for in parametric studies. It is shown that magnitudes of rail bending moments are significantly influenced by sleeper support stiffness and in particular if adjacent sleepers are not well supported (hung sleepers).