(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).