Abstract
ASME Code Case N-809 provides Fatigue Crack Growth (FCG) expressions for austenitic stainless steels operating in a primary water environment within a Pressurised Water Reactor (PWR). The code case currently contains different expressions for nominally low-carbon (304L, 316L) and conventional (304, 316) grades.
Since the original work that provided the technical basis for N-809 was completed, an increased amount of FCG data has become available through industry testing, particularly for low-carbon stainless steels. A large database is now available that contains significantly more data than the one used in the original development of the code case. The data cover a wider range of testing conditions (temperature, loading rate, and mean stress) and represent a more diverse population of material types, including multiple heats.
In this paper, the N-809 laws are re-analysed in terms of these new data, with a focus on each of the environmental dependencies that are currently included in the law. In particular, alternative R-ratio expressions from the literature are shown to provide an improved description of the effect of R-ratio for nominally low-carbon materials. The statistical distribution of FCG rates and the treatment of partially retarded data are also investigated as part of the derivation of revised descriptions of best-fit and bounding FCG rates.
The analysis highlights a small amount of potential non-conservatism in the current N-809 description of best-fit FCG rates at higher R-ratios. The current description of upper-bounding behaviour is shown to still be valid, however significant over-conservatism exists at lower R-ratios.
