Study of the initiation and the propagation of cracks under 3D thermal cyclic loading

The incident which has occurred on the Civaux power plant has shown the noxiousness of thermal loading and the difficulty to take it into account at design level. The objective of this report is to study the initiation and the propagation of crack under thermal loading. In this aim the CEA has developed a new experiment named FAT3D. The various experiments carried out showed the harmfulness of a thermal loading, which makes it possible to rapidly initiate a network of cracks and to propagate one (or some) cracks through the totally thickness of the component under certain conditions. These experimental results associated with a mechanical analysis put at fault the usual criteria of damage based on the variations of the equivalent strain. In addition, the study of the propagation stage shows the importance of the plasticity which, in the case of a thermal loading, slows down the propagation of the crack. (author)