ROLLING CONTACT FATIGUE DAMAGE
Rolling contact fatigue damage is a group of rail damages which can occur on the surface or near surface of rails due to overstressing of the rail material.
The following types of RCF are the more common types:
Gauge Corner Cracking (GCC)
Running Surface Cracking (Head checking)
Squats (and Studs)
Shelling
Spalling
Wheelburns
All RCF type defects appear as a result of repeated overstressing of the rail surface or subsurface by wheel-rail contact cycles. Once cracks have initiated, the level of propagation of these cracks into the rails depend on the following factors:
Wheels and rail profiles
Track geometry
Friction conditions
Vehicle chracteristics
Rail metallurgy
Environmental conditons
…..and many other factors
Note: The presence of grease and water at the wheel rail contact area often plays a critical role in the rate of surface crack propagaton. Where surface cracks only propagate some 3mm deep in dry environments, under wet conditions those surface cracks may propagate to a much greater depth i.e some 7-15mm deep. This is largely due to the hydraulic crack propagation mechanism.
Gauge Corner Cracking
Running Surface Cracking
Squat
Shelling
Spalling
Wheelburn
Gauge Corner Cracking
Damage caused by excessive contact stresses between wheel and rail in the gauge corner/shoulder area (mainly on high rails in curves).
Typically cracks are closely spaced only a few mm a part along the rail. They start at the rail surface and develop into the rail at an angle of about 20 degrees.
Presence of liquids such as water or lubricant can potentially increase crack growth into the rail (hydraulic entrapment)
Cracks can either develop or turn along the rail thereby joining up with another crack causing for a spall (piece of metal falling out) or it can turn down into the rail potentially causing for a rail break.
Running Surface Cracking
Damage caused by excessive contact stresses between wheel and rail in the running surface area and can occur on both rails in curves and tangents. Running surface cracking is often referred to as headchecking
Squats
Thermal damage likely to be initiated by some form of wheel slip or “micro” slip and can be identified as kind of dark spots often accompanied by a V shaped surface crack.
Squats mostly occur within the rail running band on tangents or mild curves (mostly high rails) and can lead to rail breaks if left untreated.
Unlike wheel burns which occur instantly after “macro” wheel slip incidents, squats develop gradually over a period of months/years. Squats often appear in isolated areas where there is a “disturbance” in the wheel rail contact area (areas such as welds, imprints, wheelslip damage and corrugation) and in most cases there is no corresponding defect on the opposite rail indicating that they occur in areas where there are different friction levels between the 2 running rails.
Squats do not appear to occur in tunnels indicating cracks propogate as a result of hydraulic entrapment
Studs
Damage with similar appearance to squats. Typically associated with areas of high traction and they usually occur on one rail.
Multiple studs are common and the defect often tends to grow back to the rail surface causing for a large spall.
Unlike Squats which can develop of a period of months/years (literature mentions >40MGT) Studs can develop in rails within 10MGT of laying
Shelling
Fatigue damage which generally initiates at a depth of 2-8mm below the gauge corner of the high rails in curves. Shelling typically results in loss of large chunks of metal as a result of surface or subsurface cracks linking up with other cracks below the surface such that the material above is no longer connected to the parent rail.
Spalling
Damage caused by excessive contact stresses between wheel and rail and is a typically a further development from GCC and headchecking damage in which small/shallow sections of the rail breakout,mostly due to hydraulic entrapment
Wheelburns
Damage caused by continuous slipping of wheels creating a thermal defect.
Always found on both rails i.e they always come in pairs on both rails
