(a) (b)
Fig. 14. FIB cuts perpendicular to the surface and to the stress axis in
a specimen subjected to OPD-TMF cycling; (a) a crack starting in the
thick oxide layer; (b) well developed oxidized crack propagating in the
material volume.
Fatigue cracks initiated from the thick oxide layer enter into the
material independent whether they are at the grain boundary or within
the grain. Fig. 15 displays the image of such two secondary cracks
developed at the end of the fatigue life from the uniform cracked oxide
layer of the specimen subjected to OPD-TMF cycling with total strain
amplitude 6×10−3. Transgranular crack propagation is
evident from the EBSD image (Fig. 15b).