Thermal fatigue appears to be more damaging than uniaxial isothermal fatigue: A complete analysis of the results obtained on the CEA thermal fatigue device splash

Thermal fatigue sometimes occurs in different types of nuclear reactor components. To estimate the crack initiation damage, uniaxial fatigue curves are often used. They were deduced from strain control tests using normalised cylindrical specimens. However, for in-service components, thermal loadings induce biaxial mechanical loadings. In that frame, the questioning of an eventual influence of the triaxiality factor on fatigue damage can be asked. To investigate differences between uniaxial fatigue damage and in service biaxial thermal fatigue damage, a campaign has been conducted on a specific thermal fatigue device (SPLASH). The specimen is continuously heated by Joule effect and cyclically cooled by a water spray. To deduce the thermo-mechanical state of the specimen, two uncoupled computations are performed: a thermal analysis and a mechanical analysis with the previous computed temperature field as a given loading parameter. All the computations have been performed using the object-oriented finite element code cast3M. All the analysed tests show clearly that initiation under thermal fatigue occurs before initiation under uniaxial isothermal fatigue: thermal fatigue is the most damaging case. Such difference is not due to difference on microscopic mechanisms for that temperature range. It results from a pure mechanical origin: a biaxial state corresponds to an increase of the hydrostatic stress.