  Surface
fatigue / fatigue fracture |
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A shallow angle crack initiated at the surface is turned down into the rail, a serious failure mode. From Hertz theory it is known that with pure rolling the crack is subsurface initiated and propagates to the surface, resulting in a shell or spall.
In the case of rolling with traction (µ>1/9) the
crack is surface initiated (Chapter 2) and may propagate to the
surface or down into the rail
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