The mixing tees of PWR nuclear power plant are submitted to complex cyclic thermal loadings that could lead to significant fatigue damage as observed on the RHR mixing area of Civaux 1 PWR in 1998. The characteristics of associated mechanical loading have been investigated by both experimental and numerical studies. A constant loading due to mean temperature and pressure is combined with equi-biaxial variable amplitude loading in the field of high cycle fatigue. This paper compares several fatigue damage criteria applied to both low and high cycle fatigue tests on 304 stainless steel specimens under several loading conditions according to mixing zone thermal fatigue loading amplitude. In order to focus the comparison on intrinsic qualities of the models, each criterion is evaluated directly with stress and strain measured on the specimen. The relevance of each criterion for high cycle thermal fatigue is discussed taking into considerations the precision of the results, and also the “usage cost” (including identification and complexity) and the adaptability to cumulative damage rules.

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