Spalling
What is it
Spalling is the breaking away of small patches of metal from the rail surface. Caused by shallow rolling contact fatigue, cracks join up and small chips or flakes of steel come away from the rail head. In UK practice it is usually treated as a surface manifestation of rolling contact fatigue; overseas, closely related terms include flaking, shelling and chipping, although usage varies between North America, Australia and India.
Why it matters
Spalling matters because it removes load-bearing metal from the railhead, roughens the running surface and increases impact loading between wheel and rail. Left unmanaged, small spalls can grow into larger pits and wedge-shaped losses that accelerate noise, vibration, wheel damage and further fatigue, while in severe cases they can contribute to rail breaks or degraded switch and crossing performance.
Where, when and who
Engineers have long recognised spalling as a late-stage outcome of surface fatigue, but understanding improved as railways linked it more clearly to rolling contact fatigue, wear balance and traffic patterns on heavily used curves and braking zones. In the UK, spalling is commonly associated with high-stress sites such as curves, switches and crossings, and approaches to signals or level crossings where traction and braking forces are high.
Comparable evidence is reported overseas on heavy-haul, metro and turnout assets, including research on turnout spalling failure mechanisms and defect classification methods used beyond the UK.
How it works
Spalling begins when repeated wheel loads generate high contact stresses and initiate shallow cracks at or just below the running surface. As these cracks lengthen and intersect, a thin wedge or chip of steel becomes partly detached; repeated wheel passages then break it out, leaving a pit or flaked area on the railhead. Early stages may look like light flaking, but deeper or more extensive cracking can produce larger spalls, especially where wear no longer removes the cracked layer quickly enough. On switches and crossings, impact loads and complex contact conditions can make spalling especially aggressive.
Monitoring and maintenance
Spalling is often found by visual inspection and confirmed through rail defect monitoring, especially at known high-risk sites. Management typically includes rail grinding or milling to remove the cracked surface layer, control contact stresses, and restore profile before defects deepen.
Within One Big Circle’s product suite, AIVR Focus provides automated underbody and line-scanning capture for early detection of plain-line and conductor-rail anomalies, while AIVR’s computer-vision workflows identify spalling defects. AIVR combines synchronised imagery and positioning data to provide defect intelligence that supports repeatable remote inspection and targeted maintenance planning.