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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