Fatigue crack growth of austenitic stainless steels can be enhanced significantly in high temperature light water reactor coolant environments and an ASME Code Case, N-809, has recently been developed to provide fatigue crack growth rate curves for these alloys in pressurized water reactor environments. However, under some conditions, the enhanced rates can decrease to rates close to those in air at long rise times, a process referred to as retardation which is not taken account of in the Code Case. An improved understanding of the mechanisms of both enhancement and retardation would be beneficial to determining whether advantage could be taken of these retarded rates in plant assessment. A number of studies have been undertaken to evaluate fatigue crack growth behavior in both air and water environments in order to provide mechanistic insight. Progress on this work will be described. The data from air and inert environments support the proposed mechanism of environmentally enhanced fatigue by environmental enhancement of planar slip, although it is not yet possible to differentiate between the impact of oxidation and corrosion hydrogen on the level of enhancement in aqueous environments. Testing in high temperature water environments suggests that both corrosive blunting and/or oxide-induced closure mechanisms may contribute to crack growth rate retardation under specific circumstances.
Small specimen fatigue testing is challenging in simulated LWR coolant environments at elevated temperatures and pressures. Two approaches to isothermal uniaxial testing in such environments have been developed: use of an autoclave to contain the environment around the specimen, which is conventionally of a solid design (e.g. circular cross-section, parallel sided gauge length); and use of a thin-walled hollow or tubular specimen, where the coolant environment passes through the bore of the specimen. It is often assumed that fatigue lives measured using these two specimen designs are equivalent. However, recent isothermal strain-controlled fatigue endurance tests on a single heat of Type 304L stainless steel at Amec Foster Wheeler — on behalf of Rolls-Royce — have indicated a significant difference in life from testing of these different specimen designs in high temperature PWR coolant, with hollow specimens consistently giving shorter lives. This paper presents those test results, and identifies a range of possible reasons for the differences in fatigue life through consideration of relevant literature and laboratory examination of failed specimens. These new test results have potentially significant implications for test programmes in which solid specimen test results in air are compared to hollow specimen results in LWR environments, and for fatigue databases that include results from testing of both specimen types. The use of a conversion factor, to be applied to fatigue lives from hollow specimens tests to allow comparison to solid specimen test results, is discussed. Further work to investigate the relevance of findings to further heats of material and to a wider range of loading conditions is identified.
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