DEVELOPMENT OF A NEW SPECIMEN TO STUDY CRACK PROPAGATION THRESHOLD AND NON-PROPAGATION CONDITIONS

The extension of nominal service-life of nuclear power plants leads to calculate more involved safety analyses of the nuclear structures for the different scenario of failures. In order to prevent catastrophic fracture, it is important to minimise the initiation of cracks or if considering that a crack can initiate, to determine if the extension of these cracks can stop or lead to the failure of the structure integrity. As a result, this work requires better understanding and characterisation of the crack propagation or non-propagation behaviour. Existing practises to determinate fatigue crack growth rate and fatigue thresholds are mainly based on compact tension (CT) specimen testing. For high loading ratio, the maximum of stress intensity factor (SIF) usually remains constant whereas the minimum of the SIF is increased to reduce the SIF range down to the threshold. On the contrary, for low loading ratio, complex loading control is required to make maximum and minimum SIF decrease. Furthermore, the results of these tests are mainly dependent on the accuracy of the piloting of the test bench, the specimen instrumentation and the force loading cell capacity. This paper presents details of adaptation of a specimen initially developed to study crack arrest problems under cleavage fracture. A bulk compact tension (BCT) specimen has been designed based on a standard CT specimen. Specifically, a reinforced heel at the back of the specimen enables a reduction of the loading at the crack front. In the case of fatigue, this reduces the SIF range while the loading condition remains constant during the test. A SIF calibration for these BCT specimens has also been established to estimate the compliance using finite element analysis with the French code, Cast3M.