Relevance of factor of safety based on number of cycles in the prediction of fatigue crack initiation as per A16 sigma-d approach

摘要

The focus of this paper is to bring out the conservatism present in the fast reactor design standard RCC MRx A16 in predicting fatigue crack initiation for a component that has a crack-like defect. Full-scale pipe bend test results were used for comparing the fatigue life predicted based on the RCC MRx A16 approach. A representative pipe bend of size 570 mm outer diameter and 15 mm thickness has been used for the purpose. The selected pipe bend is made up of austenitic stainless steel (SS 316 LN) and is provided with a surface notch 22.5 mm long and 2.1 nun in depth to simulate the potential flaw, which could be present in the piping system. Cyclic testing has been carried out to determine fatigue life. These tests were conducted in two stages. During the first stage, the finite-sized notch was sharpened into a crack-like defect allowing it to develop into a natural crack (85,150 cycles). In the second stage, the testing was continued to estimate the number of cycles required to advance the crack by > 50 pm and thereby compare prediction capability of the sigma-d approach given in the RCC MRx A16. The number of cycles required for crack incubation and advancement was obtained by periodic measurement of the measured readings obtained from the crack depth gauge. A comparison of the number of cycles was predicted by the RCC MRx A16 method, and the experimental results show that the A16 prediction was very conservative. Useful suggestions were proposed in this paper for improving the fatigue crack initiation prediction capability.