Strain Control Correction for Fatigue Testing in LWR Environments

Abstract

Abstract During strain-controlled fatigue testing of solid bar specimens in a LWR environment within an autoclave, it is common practice to avoid the use of a gauge length extensometer to remove the risk of preferential corrosion and early crack nucleation from the extensometer contact points. Instead, displacement- or strain-control is applied at the specimen shoulders, where the cross-sectional area of the specimen is higher and so surface stress levels are lower. A correction factor is applied to the control waveform at the shoulder in order to achieve approximately the target waveform within the specimen gauge length. The correction factor is generally derived from trials conducted in air by cycling samples with extensometers attached to both the shoulders and the gauge length; typically, the average ratio between the strains or the ratio at half-life in these locations is taken to be the correction factor used in testing. These calibration trials may be supplemented by Finite Element Analysis modelling of the specimens, or by other analysis of results from the calibration trials. Within the INCEFA+ collaborative fatigue research project, a total of six organizations are performing fatigue testing in LWR environments within an autoclave. Of these, one organization is performing tests in an autoclave using extensometers attached to both the specimen shoulders and the specimen gauge length. Therefore the INCEFA+ project provides a unique opportunity to compile and compare methods of shoulder control correction used by different organizations when fatigue testing in LWR environments. This paper presents the different methods of correcting shoulder control waveforms used by partners within the INCEFA+ project, compares the correction factors used, and assesses sensitivities of the correction factor to parameters such as specimen diameter. In addition, correction factors for air and PWR environments are compared. Conclusions are drawn and recommendations made for future fatigue testing in LWR environments within autoclaves.