1.2 Rolling contact fatigue / gauge corner cracking
“Fatigue” describes damage that occurs as a result of the incremental growth of a crack or series of cracks. ”Rolling contact fatigue” (RCF) describes the process in which cracks grow as a result of the contact stresses between a rolling wheel and the rail. Both normal and tangential stresses are required in order for RCF to develop.
RCF that is continuous or quasi-continuous is often found along the gauge corner or gauge shoulder of the high rail. This type of RCF is known as “head checking” or “gauge corner cracking” (GCC).
GCC is initiated by high tangential forces required for curving. The angle of the crack mouth on the rail indicates the angle of the force that initiated the crack: in the photograph above, the train would be moving towards the viewer. The force on the high rail would be backwards and towards the inside of the curve. Cracks start at the rail surface and develop into the rail at an angle of about 20° to the running surface. Typically there are many closely-spaced cracks, only a few millimetres apart along the rail as shown below.
Development of GCC beyond the first few millimetres into the rail requires the presence of a liquid, in particular water. If the crack is oriented so that water can be trapped within it, this gives extremely high pressures and stress at the tip of the crack (a mechanism known as “hydraulic entrapment”). This mechanism drives the crack down through the layer of compressive residual stress near the running surface. At the edge of this layer, the crack tends to turn. If it turns down into the rail, it develops subsequently under bending and other bulk stresses. Such cracks can break a rail. Since there are usually many cracks at a similar stage of development, there may be multiple breaks. There is a great danger of derailment in such circumstances.
If the crack “turns” along the rail, it joins up with other cracks and a small piece of metal falls out. This is known as spalling.
GCC initiates only occasionally on the low rail because tangential loads are usually low; and contact stresses are also lower because of the more conformal contact conditions. An example is shown of GCC on the low rail that has developed because of incomplete reprofiling around a weld in an area where trains are under high traction.
Quasi-continuous RCF occasionally occurs in straight track. If so, it is often the result of high locomotive traction, poor transverse profile or tight gauge, which forces the wheels to run along the gauge corner giving high contact stresses. RCF may occur on one rail if there is cant in straight track, since bogies then steer towards one rail.
A successful and widely accepted treatment of quasi-continuous RCF is routinely to reprofile rails to establish a transverse profile that unloads critical areas of the rail (in particular the gauge shoulder) and also removes short cracks before they propagate into the rail. It is not necessary to remove cracks if the rail is profiled in such a way that cracks are no longer touched, since hydraulic entrapment can no longer operate to propagate the cracks.
RCF develops less quickly in harder rails provided these are profiled satisfactorily and this profile is maintained. There is also less plastic flow and wear on hard rails, so the profile changes more slowly than on a softer rail, and less maintenance is required in the long term. If the profile is inappropriate, RCF can develop more quickly because of the sustained high contact stresses.