Observations and Recommendations for Further Research Regarding Environmentally Assisted Fatigue Evaluation Methods 1

The U.S. Nuclear Regulatory Commission (NRC) and Argonne National Laboratory (ANL) have completed research activities on environmentally assisted fatigue (EAF) methods. This work has led to a revision of NUREG/CR-6909, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” in its entirety. This report was issued for public review and comment in April 2014. These revisions addressed the following areas: • A much larger fatigue (e–N) database was used to recalculate the air and water fatigue curves. The additional data expanded the e–N data previously used by the NRC and ANL by as much as 74%. Despite the large increase in data, the NRC’s previous best fit air and water curves did not change appreciably. • The environmental fatigue multiplier (Fen) expressions for carbon, low-alloy, stainless, and nickel-alloy steels and comments from interested stakeholders related to: o constants in previous Fen expressions that results in Fen values of approximately 2.0 even when the strain rate is very high or the temperature is very low, o temperature dependence of the Fen expression for carbon and low-alloy steels, and o dependence of Fen on water chemistry for austenitic stainless steels. • Validation of the Fen expressions using the results of five different experimental data sets obtained from fatigue tests that simulated actual plant conditions. • The appropriateness of a strain threshold and the possible effects of hold periods. • The potential effects of dynamic strain aging (DSA) on cyclic deformation and environmental effects. In the course of performing the foregoing EAF research activities, the NRC and ANL identified the following areas where further research could yield reduced conservatism in EAF evaluation: