Shells are RCF defects that initiate several millimetres under the gauge corner of the high rail. A close relationship has been found between the defect rate from shells and a shell index (the so-called “Clayton number”):
The Sugino Index, SI, represents the length of oxide stringers.
A fatigue crack propagates initially parallel to the rail just under the gauge corner, then turns down into the rail to form a transverse defect (TD). The TD appears similar to that which occurs from a squat or from GCC.
The cleanliness of rail steel is now controlled deliberately to limit shells. Routine reprofiling is an important treatment for both squats and shells.