PROPAGATION OF SHORT FATIGUE CRACKS IN NICKEL SINGLE CRYSTALS

Short fatigue cracks have four special characteristics that differ them from "long" fatigue cracks: there is no threshold of stress intensity factor below which the short cracks do not grow, their propagation rate is higher than that of long cracks, their behavior may not be reproducible but at some stage their propagation rate converges to that of the long cracks. One definition of a short crack is that it is shorter than the grain size. This raises the possibility that some of their special characteristics relies on their growth within one grain. Therefore we suggest that single crystals with long fatigue cracks may serve as model materials for short fatigue cracks in polycrystalline materials. In the present work we studied the dependence of macroscopic fatigue crack growth (FCG) on their crystallographic orientation in single crystals of nickel and observed the slip activity on the free surface by means of SEM and AFM. We found that the FCG rate is higher in single crystals relative to polycrystals, which implies all the four special characteristics of short cracks. We also observed peculiar slip behavior associated with the fatigue FCG: selection of different slip plans on each face of the crack, dependence of the slip activity on the distance from the crack and the length of the crack and the activation of a new slip system in isolated regions. The engineering implications of the proposed model to the study of short fatigue cracks will be considered.