Equilibrium of crack growth and wear rates during unlubricated rolling-sliding contact of pearlitic rail steel

It is generally accepted that large rolling contact fatigue cracks in rails do not develop during unlubricated rolling-sliding contact, and damage under these conditions is restricted to wear of the rail steel. However, close examination of a worn rail steel surface reveals the presence of a multitude of wear flakes, the roots of which closely resemble shallow rolling contact fatigue cracks. Experiments have been conducted under unlubricated rolling-sliding conditions to examine the early development of flakes, or cracks, using a laboratory-based, twin-disc test machine to simulate the contact pressure and slip characteristic of the contact between a rail and a locomotive driving wheel. Small defects were found after as few as 125 unlubricated contact cycles. It was found that an equilibrium between crack growth rate and surface wear rate was established after approximately 10 000 cycles, leading to a shallow steady state crack depth. Initial crack growth by ratchetting (accumulation of unidirectional plastic strain until the critical failure strain of the material is reached), followed by shear stress-driven crack growth described by fracture mechanics, was found to be a sequence of mechanisms in qualitative agreement with the observed crack growth and steady state crack depth.