Session: CS-17-02 EAF Testing Outcomes

Paper Number: 105910

105910 - Assessment of Gauge Length Effects on Cylindrical Austenitic Stainless Steel Fatigue Specimens in Air and High-Temperature Water Enviornment 

This paper addresses the specimen gage length on the fatigue behavior of 316L austenitic stainless steels when tested in air and light water reactor environment. In fatigue testing, the overall spread of the data tends to be high due to high natural random error due and also unknown systematic errors due to ambiguous or poorly constrained standards. Identifying and mitigating sources of systematic error can reduce the high spread in fatigue data and significantly increase the reliability of fatigue life approximations under a specific set of test conditions. In order to reduce sources of systematic error, we have explored the effect of the gage length of 316L stainless steel specimens in two different environments. For this purpose, a minimum of three low-cycle fatigue tests were performed for each set of conditions with 0.3% and 0.6% strain amplitude using gage lengths of 20, 27 and 34 mm. Fatigue tests were performed first using solid specimens with a diameter of 8 mm at room temperature in ambient air. Then, environmental effects were introduced by testing hollow specimens with an outer diameter of 10 mm and an inner diameter of 5 mm in simulated boiling water reactor (BWR) conditions, which consist of elevated temperature up to 300^oC and a constant internal pressure of around 150 bar with a medium of high-purity, neutral water with 150 ppb dissolved hydrogen. Tests were also performed at 0.6% strain amplitude with a tenth of the strain rate of the standard tests in order to increase the influence of the environmental effects. The effect of the high-temperature BWR environment results in a strong reduction of the fatigue life in comparison with tests carried out in air at room temperature. The effect is usually quantified in literature using the Fen factor defined as the ratio of the fatigue life in air at room temperature to the fatigue life in a light water reactor (LWR) environment. Fen factors were calculated for each set of conditions and specimen gage lengths using room temperature air tests as reference and compared with existing literature. According to the results, gauge length effects had a strong dependence on the strain amplitude. At 0.3% amplitude, gage length had a negligible effect on fatigue life in room temperature air environment. At 0.6% strain amplitude, fatigue life in air unexpectedly increased with a longer specimen gage length. In BWR environment with 0.3% strain amplitude the gage length effect was insignificant just like in air, but with 0.6% strain amplitude it was found that specimens with longer gauge length showed significantly lower fatigue life, by up to 40%. The observed Fen factors were higher than values found in literature. Increasing gage length by 1.4d (where d is the diameter of the fatigue specimen) caused the Fen factor to rise by 45-55% at 0.6% strain amplitude in BWR environment. This study thus shows that the specimen geometry had an influence on fatigue life at higher strain amplitude in elevated temperature and pressure, and therefore the requirement to revise specimen design guidelines presented in ISO and ASTM standards in order to avoid inconsistent results is established.

Presenting Author: B. Aydin Baykal Paul Scherrer Institut (PSI)

Presenting Author Biography: Aydin Baykal obtained his Bachelor's degree in Mechanical Engineering with a minor in Fluids and Energy from Istanbul Technical University in 2011, his Master's degree from the University of Michigan-Ann Arbor in 2012 and his PhD in Materials Science and Engineering with a minor in Paper Science and Engineering from the Georgia Institute of Technology in 2018. He is currently working as a post-doctoral scientist at the Structural Integrity Group of the Nuclear Energy and Safety Division at Paul Scherrer Institut in Villigen, Aargau, Switzerland. He specializes on corrosion and fatigue in wet environments of interest in particular to the paper, petroleum and nuclear industries. His research interests include low-cycle fatigue, corrosion fatigue, design and testing guidelines of miniaturized fatigue specimens, multiphase flows, localized corrosion, fluid-surface interaction, crystal structure and its effects on long-term structural integrity.

Authors:

B. Aydin Baykal Paul Scherrer Institut (PSI)
Philippe Spätig Paul Scherrer Institut (PSI)
Hans-Peter Seifert Paul Scherrer Institut (PSI)