Session: CS-17-02 EAF Testing Outcomes

Paper Number: 107205

107205 - Shoulder Control for Fatigue Endurance Tests Carried Out Under Variable Amplitude Loading Conditions 

Strain-controlled fatigue endurance testing in Pressurised Water Reactor (PWR) environments is a well-established testing method that is undertaken in many laboratories around the world. Jacobs have performed such testing over a number of years using both hollow specimens where the water sample flows through the specimen, and bar specimens where the water sample is provided by housing the specimen in an autoclave. Hollow specimen rigs are no longer used in the UK  due to uncertainties over the impact of pressure loading on fatigue life. The method of strain control for bar specimens is a challenge in an autoclave, where space is limited, and high pressure water is directly impinging on the specimen surface. Shoulder controlled testing is currently the most reliable way of producing data at Jacobs. In order for shoulder controlled tests to be set up with the correct gauge length strain amplitude, a correlation is made between gauge length extension and the extension between specimen shoulders, i.e. a calibration curve. This has been derived by Jacobs by running a series strain-controlled dual extensometer fatigue endurance tests in air over a range of strain amplitudes.

Variable Amplitude (VA) fatigue endurance testing in both air and PWR conditions has recently been undertaken by Jacobs. It is considered to be an area where the results have a large potential to reduce conservatisms in fatigue assessments. VA testing typically involves running repeating blocks of cycles, where each block includes a small number of cycles at a large strain amplitude (i.e. overload or underload cycles) and a larger number of cycles at a small strain amplitude (i.e. baseline cycles). In an attempt to achieve the nominal test strains the calibration curve has been used to inform shoulder displacement for both large and small cycles.

There were initially concerns about using the calibration curve for VA loading, given that it was generated from single strain amplitude tests. It wasn’t clear whether the shoulder to gauge correction factor for one strain amplitude might change once a specimen had been cycling at another strain amplitude, and that complex softening and hardening responses in the specimen may affect the validity of the calibration curve. There was a need to understand how much variability there would be in gauge length strain, both throughout a test and within individual blocks, and whether some form of cycle-by-cycle correction might be necessary. Furthermore there was a need to understand the potential level of error in the maximum, minimum and mean strains throughout the test. A series of dual extensometer VA tests were carried out in air. Detailed analysis of both the dual extensometer VA air test data and VA PWR test data suggests that it is possible for VA tests to viably be carried out under shoulder control using the calibration curve to inform shoulder displacement for both large and small cycles. The supporting evidence, which consists of hysteresis loops and stress vs cycles plots, is presented and discussed in this paper.

Presenting Author: Peter Gill Jacobs

Presenting Author Biography: Peter is a Structural Integrity Engineer with 13 years experience in the Nuclear Industry. He lead multi—disciplinary research programmes and has a background in assessment according to various design and fitness for service codes.

Authors:

James Meldrum Jacobs
Peter Gill Jacobs
Jonathan Mann Jacobs
Alec McLennan Jacobs
Norman Platts Jacobs
Michael Toal Jacobs