5.5 Spalls associated with studs
If studs (Section 4.4) are allowed to develop, they tend to spall out as shown. The stud has initiated towards the field side of the rail (towards the bottom of the photograph), grown into the rail and then propagated transversely. These cracks appear to develop by a mechanism of conventional fatigue (as evidenced by the “beach marks” on the fracture surface) under the surface layer of compressive residual stress, then rise up to the rail surface beyond this layer. Quite a large portion of the rail surface can then spall out, as shown here.