Thermal Fatigue This damage results from cyclic and excessive thermal fluctuations accompanied by mechanical constraint. Excessive temperature gradients can add to internal strain to initiate or enhance the cracking process. Once initiated, freshly exposed metal within the cracks can undergo oxidation, creating a wedge effect at the crack tip, since the oxide occupies a greater volume than the base metal. Thermal fatigue can occur in waterwalls or other areas subjected to DNB or rapidly fluctuating flows. Low-amplitude vibrations of entire superheaters can lead to thermal fatigue.