EVALUATION OF DNB INDUCED FATIGUE IN HEAT TRANSFER TUBES.

In the DNB (Departure from Nucleate Boiling) region of steam generator evaporator, the heat transfer tubes are subjected to oscillating thermal condition with high frequency and the resulting high cycle fatigue damage in these tubes is usually evaluated on the basis of the alternating stress intensity and the mean stress intensity during the thermal cycle. Due to the three-dimensional nature of thermal loading, a three-dimensional stress analysis is required to rigorously determine the stress intensity. This paper presents a systematic numerical comparison of the simplified and three-dimensional analyses and recommends an appropriate simplified model for practical applications.