Page 350 - Failure Analysis Case Studies II

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Fig. 3. Appearance of internal and external surfaces of the cracked tube specimen D,.

but rather by repeated oxidation. Hence, the crack initiation mechanism assumed in the previous
section is confirmed.
The primary crack in the specimen D, which was opened to fracture is illustrated in Fig. 7. The
top line of the specimen represents a fracture surface of the primary crack where a deep oxide layer
is present. Beneath the primary crack, the formation of a secondary crack with a shallow oxide
layer is observed. It is evident that the characteristics of the crack are similar to the crack shown