Investigations on Crack Propagation and Ratchetting Under Cyclical Thermo-Mechanical Loadings for a Type 304-L Stainless Steel. Development of Structural Integrity Testing Facilities

Abstract The nuclear industry is particularly interested in structural integrity issues, aware of the catastrophic consequences of a major accident. Every effort is made to reduce the risks of such a situation as much as possible. Thus, right from the design stage and throughout the life of the plant, a level of safety is required and verified by the authorities, on the basis of files submitted by the operator. These continuous interactions generate supporting R&D work. We can cite, in particular, the problem of thermal fatigue in the mixing zones, which reappeared in situations not yet envisaged (at the origin of the shutdown of the Civaux power plant in 1998), or the improvement of knowledge of phenomena already identified and codified (case of the Pressurised Water Reactor vessel). Some of these works have a consequent scope because they are the basis for the development, validation and codification of rules and criteria for the integrity analysis of metallic structures. These structures, often of large dimensions or particular geometry (elbow, thickness variations), are subjected during their operating cycle to complex loads combining variable, multiaxial mechanical loads with non-zero mean values associated with temperature fluctuations. Before applying the concepts and methods observed on analytical specimens to these structures, it is necessary to consolidate them by implementing an original approach combining tests on quasi-structures, characterisation tests on specimens and fine interpretations by finite element calculation. It is then necessary to set up experimental means to examine and re-evaluate the codified approaches ensuring the integrity of industrial components subjected to complex loading with consideration of the environment. Among the CEA teams concerned by this theme, the Structures Integrity and Standardisation Laboratory of the CEA, has the vocation to study the integrity problems of metallic nuclear structures by relying mainly on exceptional experimental means: the RESEDA experimental platform makes it possible to implement scale or analytical tests (representative of the phenomena sought) in order to reproduce industrial situations, associated with varied and powerful instrumentation and analysis equipment. This article will present different experimental devices developed at the CEA concerning the study of crack initiation, and propagation under complex loads (for example with thermal + mechanical loadings). Another structural integrity can be also highlighted, concerning the development of ratchetting.