Assessment Method to Account for the Rise Time of Complex Waveforms in Stainless Steel Environmental Fatigue Crack Growth Calculations

Abstract

The reference Fatigue Crack Growth (FCG) rate behaviour for austenitic stainless steel in a deaerated Pressurised Water Reactor (PWR) environment is provided in ASME Section XI Code Case N-809. This FCG law is dependent on temperature, R ratio and environment, which is defined through a load rise time parameter. The basis for this law is contained within a large dataset of testing carried out by various industry sources. Code Case N-809 defines rise time as the time for which the stress is increasing during a stress cycle, based on specimen testing in which sawtooth loading (typically 85% rise/15% fall) was primarily used. However it was found from testing of more complex waveforms carried out by Amec Foster Wheeler, which is reported in a separate paper at this conference, that the FCG rate is not always well characterised by this definition of rise time and was often found to be overly conservative. A number of different waveforms were considered, including simplified two-stage linear waveforms where the loading rate is different in the top and bottom half of the cycle, and those more representative of plant transients. This paper presents a method to account for the environmental enhancement of complex waveforms with variable loading rate by weighting the loading rate according to its position within the loading cycle. In this way an effective rise time can be obtained for use in Code Case N-809. This method was found to give good agreement with the experimental data generated using complex waveforms.