Validation of Constant Load C-Ring Apex Stresses for Stress Corrosion Cracking Testing in Supercritical Water

In selecting the materials for the Canadian supercritical water-cooled reactor (SCWR), the effects and extent of stress corrosion cracking (SCC) on candidate alloys of construction, under various operational conditions, must be considered. Several methods of applying stress to a corroding material are available for investigating SCC and each have their benefits and drawbacks; for simplicity of the experimental setup at University of New Brunswick (UNB), a constant load C-ring assembly has been used with Inconel 718 Belleville washers acting as a spring to deliver a near-constant load to the sample. To predict the stress at the apex of the C-ring, a mechanistic model has been developed to determine the force applied by the spring due to the thermal expansion of each component constrained within a fixed length when the temperature of the assembly is increased from ambient conditions to SCWR operational temperatures. In an attempt to validate the mechanistic model, trials to measure the force applied by the washers as the assembly thermally expanded were performed using an Instron machine and an environmental chamber. Accounting for the thermal expansion of the pull rods, the force was measured as temperature was increased while maintaining a constant displacement between the platens holding the C-ring. Results showed the initial model to be insufficient as it could not predict the force measured through this simple experiment. The revised model presented here considers the thermal expansion of the C-ring and all the components of the testing apparatus including the tree, backing washers, and Belleville washers. Further validation using the commercial finite element (FE) package abaqus is presented, as are preliminary results from the use of the apparatus to study the SCC of a zirconium-modified 310 s SS exposed to supercritical water.