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 und er these conditions is restricted to wear of the rail steel. However, close examination of a worn rail steel surface reveals the presence of a multitu de of wear flakes, the roots of which closely resemble shallow rolling cont act fatigue cracks. Experiments have been conducted under unlubricated rolling-sliding conditio ns to examine the early development of flakes, or cracks, using a laborator y-based, twin-disc test machine to simulate the contact pressure and slip c haracteristic 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 shall ow steady state crack depth. Initial crack growth by ratchetting (accumulat ion of unidirectional plastic strain until the critical failure strain of t he material is reached), followed by shear stress-driven crack growth descr ibed by fracture mechanics, was found to be a sequence of mechanisms in qua litative agreement with the observed crack growth and steady state crack de pth.