PhD student at Institut P' (Poitiers, France), in collaboration with Framatome. Working on crack closure of nuclear steels under negative load ratios, using both experimental techniques and FEM modelling.
The design of a nuclear power plant component requires a sound assessment of the risks of crack initiation and propagation. The field experience of French Pressurized Water Reactors (PWR) indicates that fatigue and stress corrosion cracking are the two main damage mechanisms observed. Therefore, it is necessary for the manufacturer and the operator to have a better understanding of those mechanisms to improve the associated design rules. In this context, a part of the research conducted at Framatome focuses on the understanding of fatigue crack propagation for specific loadings in relation with crack closure effects to accurately determine the crack propagation life.
During their life, components must endure primary (stress-controlled) and secondary (strain-controlled) loadings. As regards crack propagation, the commonly used CT-specimen geometry is however limited to primary loadings at positive load ratios, which are not representative of the actual loading conditions. To overcome this limitation, a cylindrical specimen containing an initial defect as starter is here introduced, allowing to conduct tests under primary or secondary loading, as well as negative load or strain ratios. Yet, as this geometry is not standardized, it is necessary to compare the results obtained with it to a standard CT specimen. Standard tests conducted using CT specimens at different load ratios on two different steel grades (304L Stainless Steel and French Reactor Pressure Vessel steel 18MnD5) including crack closure measurements are replicated on these cylindrical specimens where the crack propagation is monitored by the Potential Drop Method. The present paper compares and analyses the results in terms of fatigue crack growth rates and crack closure effects.