Assessment of Ratcheting: A Proposed Rule and Engineering Guideline to Prevent Ratcheting

For nuclear power plants, the combination of internal pressure and thermal fluctuations, under certain conditions, can cause ratcheting. Because of that, a ratcheting check is required for the components of a nuclear power plant that are subjected to those types of loadings. In these cases, ratcheting is mainly investigated using simplified rules. According to nuclear design codes (RCC-M, RCC-MRx, ASME, R5, etc), an elastic Finite Element Analysis (FEA) must be performed, and then the stresses have to be compared with criteria which ensure that (elastic or plastic) shakedown occurs. Because plasticity is not considered on the elastic FEA, those methods are often overly conservative. In such cases, alternative methods must be carried out. An alternative method is to perform a direct cycle-by-cycle elastic-plastic FEA in order to evaluate that the component is within shakedown and the accumulated plastic strain is lower than the strain criteria. However, that method is more computationally expensive and complex than the commonly used ones. The purpose of the paper is to propose a rule and an engineering guideline for the ratcheting assessment of austenitic stainless steels. The latter must be less conservative than simplified commonly used rules and without requiring a full cyclic elastic-plastic computation.