Failure behavior, stress-flaw size

The exposure of a strueture to a sudden change of temperature may cause its ruin either because locally, most often on the surface, the tension stress reaches the critical rapture value, or because a flaw of the structure propagates and finally reaches a critical size, causing a catastrophic failure. In the former case, a breaking criterion can easily be obtained from the equations which govern the thermoelastic behavior (see section 8.3.1). In the latter case, the fracture occurs only after several cycles, or at the end of a certain period of time, depending on the difference of temperature and the initial microstracture this is the phenomenon of thermal fatigue, which is more difficult to model. 

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