RESULTS FROM ENVIRONMENTALLY-ASSISTED SHORT CRACK FATIGUE TESTING ON AUSTENITIC STAINLESS STEELS

The effect of environment on fatigue life is currently assessed using methods (such as NUREG/ CR-6909) that may be excessively conservative when applied to plant components and loading transients. To reduce this conservatism, the ASME WGEFEM has proposed the development of an improved assessment methodology for environmental fatigue based on a Total Life Prediction approach that would be adequately, but not excessively, conservative. Such an approach necessitates the development of analytical methods for the various stages of crack nucleation, short crack growth and long crack growth. Hence, there is a requirement to undertake testing within the short crack growth regime that would bridge the gap between fatigue nucleation and long crack growth (Paris Law) enabling better prediction of total life measured by fatigue endurance. A test methodology has been developed by Wood to enable short crack growth testing with in-situ monitoring using DCPD. Testing has been undertaken in both high temperature air (300 degrees C) and simulated end-of-cycle primary water chemistry at 300 degrees C on cold-worked stainless steel specimens, which were subject to a range of load ratios and rise times. FEA modelling has been undertaken to determine the effective stress intensity factors applied under the loading conditions based on the specific material properties. This paper presents the results from a testing program conducted with EPRI, to measure fatigue crack growth data for short cracks from 0.15 mm to 1.0 mm. Crack growth rates have been compared to those predicted in ASME, Code Case N-809 and results from material specific in-house testing to assist the understanding of the behaviour of mechanically short cracks.