Some components in French nuclear power plants are submitted to thermo-mechanical loadings during theirlifetime. Thermal fatigue cracking (striping) is observed in the mixing zones of the reactor in some areas of theresidual heat removal system (RHR). Edge cracks located on the internal surface of the pipe are observedessentially near weld tips but also far from the weld. Crack initiation has been related in a qualitative way to thethermal fluctuation and to the detrimental effects of residual weld stress and surface finishing. The aim is tosimulate the damage accumulation for two industrial structures : a mock-up of a RHR and the RHR itself. Forthat we used two endurance limit criteria for multiaxial loading : MATAKE and DANG VAN both based on theconcept of critical plane. We extend these models to damage accumulation and constant or variable amplitudeloading. Furthermore, in these models we have introduced a new parameter to take into account an initialhardening. These criteria have been implemented in Code_Aster?, the structural analysis finite element code ofEDF [http://www.code-aster.org]. Finite Element simulations are carried out on a RHR structure and anexperimental T-piece, denoted FATHER, in which cold water and hot water are mixed. In our computations thethermal loading used results from a thermo-hydraulic calculation realized at EDF R&D [1,2]. For the momentthere is no guaranty about the validity of thermal loading, but we consider that it is sufficiently realistic to beemployed.From the comparison of the numerical results we conclude that the damage is at least three times moreimportant with models extended to variable amplitude than with those extended to constant amplitude.Furthermore when we compare the largest fatigue damage obtained with a variable amplitude criterion in whichthe pre-hardening coefficient is equal to 1.25, with a constant amplitude criterion the ratio is about ten.
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