High cycle thermal fatigue of two austenitic stainless steels

Abstract

Thermal fatigue is one of the main damage mechanisms that engineers must take into account in the periodic assessment of current nuclear power plants or the design of future ones. Fluctuations of temperature in the fluid is indeed inevitable, as is a partial transfer of these fluctuations to the solid walls, and the resulting oscillations of thermal gradients must stay low enough to prevent the formation of typical crack networks. In this study, high cycle thermal fatigue tests are performed under helium environment on two types of austenitic stainless steels, with an original apparatus called FLASH for thermal Fatigue by pulsed LASer beam and Helium jets. Crack initiation and crack network developments are detected by an InfraRed (IR) camera that is also used with IR pyrometers to accurately measure the cyclic thermal loading. The influence of temperature variation, pulse duration and an additional constant mechanical stress on crack initiation, crack propagation rate and crack network morphology is evaluated. Eventually, a numerical analysis of thermal fatigue tests, fed by proper experimental measurements on the crack initiation surface, allows deducing that the number of cycles to crack initiation is similar to the one obtained in more classical uniaxial isothermal fatigue results.