Experimental and numerical study of crack initiation and propagation under a 3D thermal fatigue loading in a welded structure

The incident which has occurred in the Civaux power plant has shown the nocivity of thermal loading, the specificity of this type of loading and the difficulty to take it into account at design level. The objective of this study is to examine the initiation and the propagation of crack under thermal loading for the structure with or without weld. In this aim the French Atomic Energy Commission (CEA) developed a new experiment named FAT3D. The various experiments carried out on pipe without weld showed the harmfulness of a thermal loading, which can initiate a network of cracks and propagate one (or some) cracks through the total thickness of the component. The experiments on welded structure showed a faster initiation of cracks and the importance of the orientation of the weld. Indeed it showed the initiation of two cracks both located the feet of the weld joint as observed in the Civaux power plant. These experimental results associated with a mechanical analysis question the usual criteria of damage based on the variations of the equivalent strain.