Fatigue crack initiation in WASPALOY at 20 °C

In two WASPALOY specimens, the orientations of grains that initiated fatigue cracks and adjacent grains were measured using electron backscattered diffraction patterns (EBSP). Crystallographic relationships were found for crack initiating regions that resulted in slip transmission across areas larger than the initiating grain, and the initiating grain was usually larger than average. A similar evaluation of control areas on each specimen found that there was much less likelihood of slip transmission across grain boundaries. Schmid factors (SFs) were also evaluated. It is concluded that the reason that fatigue cracks formed at these locations was due to the lower stress required for slip initiation in these clusters of grains oriented for slip transmission across grain boundaries. Many of the cracks initiated within grain boundaries. A detailed crystallographic analysis of the adjacent grains suggests criteria for intergranular (IG) crack initiation.