Thermomechanical Fatigue Initiation in Nuclear Grades of Austenitic Stainless Steel Using Plant Realistic Loading

Abstract

The effect of a Pressurised Water Reactor (PWR) environment on fatigue life is currently assessed using methods such as NUREG/CR-6909 for initiation and ASME Code Case N809 for crack growth, which may be inherently conservative for certain components, especially when considering plant relevant loading. The thermal shock testing with thick-walled specimens as discussed in this paper allows for more plant relevant loading regimes to be utilised in assessments, incorporating through-wall stress gradients, thick walled test specimens and out-of-phase temperature/strain characteristics. This should lead to improvements in reducing the levels of excess conservatism in current assessment methodologies. The capability of the test facility was first presented in PVP2016-63161 [4]. Since then, significant modifications have been made in order to maximise the achievable strain amplitudes in the thick-walled specimen geometry, alongside minimising typical test durations. This was achieved by maximising the temperature differential between the hot and cold cycles and tuning the cycle length in order to ensure that the cycle is long enough to achieve a target strain amplitude, whilst ensuring that it is not so long as to unreasonably increase test durations. This paper details the results of the thermal shock testing performed to date, the development of accompanying Finite Element Analysis (FEA), preliminary initiation data and the development of the various Non Destructive Testing (NDT) techniques used to detect fatigue crack initiation on the thick-walled specimens. Owing to the long testing times needed to achieve the required cycling, various NDT techniques were developed and employed to confirm the presence of fatigue cracking in the thick-walled test specimens before considering more in-depth characterisation using destructive techniques. Eddy Current Array (ECA) testing has been specifically developed for this testing and uses a 360-degree custom bore probe to conduct non-contact ECA measurements on the inner surface of the test specimens. Calibration blocks containing various sized Electrical Discharge Machining (EDM) notches were used to provide a calibration (amplitude and phase) of eddy current responses for prospective flaw depth sizing from indications. The ECA testing performed has provided indications that fatigue cracking is present within the thick-walled specimens tested and subsequent Visual Testing (VT) was performed to assess the highlighted indications from the ECA testing. The VT methods employed included a video borescope for imaging the inner walls of the specimen. In order to increase the detection capabilities (by improving the contrast) the VT was used in conjunction with fluorescent Dye-Penetrant (fDP) testing, whereby a method was developed for using fDP within the inside bore of the specimen alongside a custom ultraviolet (UV) source to better highlight cracking. This paper discusses the success of the NDT developments and testing performed to date and details the latest complementary crack growth assessment work.