The variation, with decreasing load ratio, in the threshold peak and cyclic stress intensities that were required for fatigue crack growth in stage I were considered (for mode-II loading) by using a simple model that simulated dislocation motion near to a crack tip. According to this model, the crack grew when dislocations ran into the crack during the loading and unloading phases. The behavior of a crack was initially studied for the case where a dislocation source was relatively far from the crack tip. The crack propagation rates exhibited a Paris regime at high stress intensities, and an abrupt threshold value was observed below which no crack growth occurred. The variation, as a function of the load ratio, of the peak and cyclic stress intensities at the fatigue threshold showed that 2 different processes controlled the observed behavior. At high load ratios, dislocations were easily generated during loading, and the threshold was controlled by the need for sufficient unloading to permit dislocations to run back into the crack. At negative load ratios, the generation of dislocations during the loading stage controlled the threshold condition since, once generated, the large unloading and reversed loading easily forced dislocations back into the crack.

A.J.Wilkinson, S.G.Roberts: Scripta Materialia, 1996, 35[11], 1365-71