Orientation Imaging Microscopy of fatigue crack formation in Waspaloy: Crystallographic conditions for crack nucleation

Effects of grain orientation on fatigue crack nucleation in Waspaloy were investigated. Fatigue testing was carried out at 85–99% of the yield stress of the material, at R = 0.1 using electropolished specimens extracted from the web of a forged aircraft engine compressor disk. Multiple nucleations of cracks of the order of a few grain diameters in size, and the propagation of the dominant crack were documented for a total of fourteen cracks. Orientation Imaging Microscopy (OIM) was used to investigate individual orientations of grains in 10 of the cracks’ nucleation and adjacent areas. Cracks appeared to nucleate in groups of similarly-oriented grains, due to the closeness of Schmid factors between the grains in these groups. This neighborhood effect was seen in nine of the 10 cracks investigated.