Study of antecedent stages of thermal fatigue damage: experiments and simulations at the scale of dislocations

The thermal fatigue is a generic problem that occurs in stainless steel pipes used in cooling systems of industrial installations. The thermal fatigue may generate a grid of cracks or a series of isolated cracks, these cracks are trans-granular and their propagation begins from the surface. The Cythia experiment consists in heating periodically through high-frequency induction the outer part of a pipe whose inner part is steadily cooled by an adequate flow of water. After a series of about 100 thermal cycles, samples of the external skin of the pipe have been examined through transmission electronic microscopy. These radiographs show that dislocation structures are localized on 30 nm broad bands that are separated by at least 100 nm. A numerical simulation of the dynamics of these dislocations is presented. (A.C.)