At an incomplete mixing area of high and low temperature fluids, fluid temperature fluctuation often occurs. It induces cyclic thermal stresses in the wall, which may result in fatigue crack initiation. Kasahara et. al. proposed the thermal fatigue evaluation method based on power spectrum density (PSD) in PVP05. This method generalizes the evaluation procedure by preparing PSD charts of fluid and frequency transfer functions of stress for various kinds of plant components. From design point of view, however, this method is too complicated due to the inverse Fourier transform and wave decomposition procedures named Rain Flow Cycle Counting (RFC). In this paper, simplified damage evaluation method for thermal fatigue is proposed by directly evaluating fatigue damage from PSD of stress. Since analytical treatment for evaluation of fatigue amplitude distribution based on PSD is difficult due to complicated procedure of RFC, direct evaluation method for RFC amplitude distribution from PSD is newly proposed. This method gives fatigue damage evaluation with safety margin. This paper shows the dependency of safety margin on geometry of PSD. Finally, application to design for thermal fatigue will be shown. Since PSD of stress in the wall near temperature fluctuation can be easily evaluated using Kasahara’s method, the proposed method will make thermal fatigue damage evaluation far easier.

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