THE DEVELOPMENT OF A NEW METHOD TO COMPARE THE FATIGUE CRACK GROWTH RATES OF AUSTENITIC STAINLESS STEEL OPERATING IN A PWR PRIMARY COOLANT SUBJECTED TO PLANT REALISTIC TEMPERATURE LOADING

Fatigue Crack Growth Rates (FCGRs) of austenitic stainless steels can be significantly enhanced when tested in a high temperature water environment compared to those tested in air. Existing FCGR models are based on simple isothermal waveform loading. Recent work has highlighted that there may be a potential benefit into taking account of plant realistic loading waveforms in fatigue assessments as these may be less damaging than predictions based on simple loading conditions. As a result, new methods to account for these plant realistic loads have been developed to reduce excess conservatism of existing methods for predicting FCGRs. To provide confidence in these methods, a previous UK thermomechanical fatigue testing programme has been conducted on Compact Tension (C(T)) specimens subjected to plant realistic loads, with the crack length and Crack Growth Rates (CGRs) being monitored in-situ using the Direct Current Potential Drop (DCPD) technique. This paper utilizes three different methodologies to evaluate the CGR of samples that underwent corrosion fatigue in different conditions namely; DCPD, post-mortem measurement of crack advance using Scanning Electron Microscopy (SEM), and the measurement of the spacing between striations to infer CGR. It was found that DCPD provided a good global average of FCGRs at the crack front but does not capture local changes associated with the local microstructure. Overall, it was shown that post-mortem examination for stage measurements can be reliably applied to infer CGR on samples that were not instrumented with DCPD.