Cracks interfacial region

With time (under increased temperature and humidity) the crack tip continues to a weaker region which for this surface treatment appears to be near the oxide/alloy interface. Figure 11 summarizes the analysis of the bond failure for this particular surface treatment. The important aspect here is that under identical conditions, different surface preparations show different modes of failure. Weak boundary layers are not developed using some treatment/bonding combinations. Processes have been developed in which the locus of failure remains in the adhesive ("a cohesive failure") and it is necessary to use a mechanical test in which even more stress is placed on the interfacial region (19). 

Fig. 7.7 (a) Transmission electron micrograph of the as-received, untested microstructure of AI2O3/33 vol.% SiC whisker composite, (b-d) Transmission electron micrographs of the fatigue crack tip region in the composite in 1400°C air (R = 0.1S and vc = 0.1 Hz) showing the nudeation of interfacial cavities. Part (e) shows the development of a diffuse cavitation zone ahead of the fatigue crack tip. From Ref. 25. 

Fig. 15 (a) Schematic representation of the stress profiles in an adherent portion of film at a distance x from a free surface of a through-thickness crack. Interfacial shear stress x. and peel stress p correspond to the action of the substrate on region (1 The normal stress parallel to the x axis, is supposed to remain constant through the film thickness h. 

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