Three dimensional fatigue propagation modeling of a nozzle corner crack

The problem of nozzle corner fatigue crack propagation has been addressed since more than 25 years (Brookhaven, 1975, Kobayashi, 1979). The high stress concentration in a nozzle corner under pressure and thermal shocks, make necessary the check of fatigue crack initiation and propagation risks in such a zone. Most of the integrity studies carried out over 40 years of service do include fatigue crack propagation assessments. However a realistic analysis of the crack extension is a complex three-dimensional problem. The fatigue growth is driven by the Stress Intensity Factor (SIF) values along the crack front. These SIF depend not only on the crack size but also on its shape and the location along the crack front. Therefore the crack shape is likely to change continuously. The current practice for stress intensity factors calculation consists in using influence functions established for a given type of crack front shape (semi-elliptical) and which take into account only in-depth stress gradients. This leads for instance to an overestimation of the crack extension and is not able to take into account the protective role of a thermal sleeve. The aim of the present study is to get through 3 dimensional finite element computations of the SIF, a more realistic estimate of the fatigue crack propagation of nozzle corner cracks. Namely, a reliable description of the crack shape evolution was expected for analyzing the risk of break through and examining the feasibility of non destructive examinations. (authors)