The effect of an LWR 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 Working Group for Environmental Fatigue Evaluation Methods (WG-EFEM) 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.
Previous negative R long crack growth testing using corner-crack specimens measured the effects of crack closure under compressive loading, and has been used to address some of the conservatism in existing assessment methods. This methodology has been developed further to enable negative R short crack growth testing with in-situ monitoring using DCPD. Testing has been undertaken in both high temperature air (300°C) and a simulated PWR primary water chemistry at 300°C on both cold-worked and non-cold-worked stainless steel specimens at a load ratio of R = −1. One heat of stainless steel has been tested, with another heat of different grain size to be tested imminently, in order to investigate the effect of grain size on short crack growth rates. FEA modelling has been undertaken to both correlate Direct Current Potential Drop (DCPD) response with crack growth measurements, and to determine the effective stress intensity factor ranges applied under the loading conditions based on the specific material properties.
This paper describes the methodology and findings from this negative R short crack growth test programme. Crack growth rates have been compared to ASME Code Sec. XI and Code Case N-809 reference curves and results from material specific in-house testing to assist the understanding of the behaviour of mechanically short cracks.