Investigations on crack propagation under cyclical isothermal and thermo-mechanical loadings for a type 304-L stainless steel used for pressurized water reactor

The integrity of structures exhibiting flaws in Pressurized Water Reactor (PWR) has to be assessed to meet safety criteria. This paper deals with crack-propagation under cyclic thermo-mechanical loadings, as encountered in class I austenitic pipes of PWR's. To have a conservative and reliable assessment of the crack propagation due to the in-service loading, various codes and standards use simplified method. For example, RSE-M introduces a plastic correction depending on the proportion of the mechanical loading. An improvement of the current method requires additional investigations. Moreover, components loaded with transient or thermal fluctuations are not really in strength-controlled conditions. To this end, a new device called PROFATH was designed. The specimen is a pre-cracked thick-walled tube undergoing a set of thermal cycles and loaded with a static mechanical force. During the first part of the cycle, a high frequency induction coil heats the external wall. Then, the heating system stops and the specimen is cooled down by running water inside the tube. Finite element calculations show that only a region half-way along the tube should be heated to ensure adequate structural effect. In the heated zone, the machining of a sharp circumferential groove ensures the propagation of a unique crack. An electromechanical jack controls the level of the mechanical static load. To obtain a very precise thermal mapping, a specific specimen with dedicated instrumentation is used: 20 thermocouples are appropriately located on the outer surface and along the wall thickness. During the test, the crackpropagation is estimated through crack-opening measurements (compliance method). Now, three first tests have been carried out. These tests allow having an evaluation of the pertinence of the correction proposed by the RSE-M for a significant plasticity. Following tests are planned in order to have a confirmation or to propose an extension of the validity domain.