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98137 EFFECTS OF TESTING CHARACTERISTICS ON OBSERVED SCC BEHAVIOR IN BWRs

Product Number: 51300-98137-SG
ISBN: 98137 1998 CP
Author: Peter L. Andresen
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There is a growing awareness that differences in testing procedure in high temperature water can produce very large scatter in the stress corrosion crack (SCC) growth rate data. For example, data from single or multiple laboratories often show scatter of 100 - 1000X or more, which is too high to establish reliable quantitative dependencies unless very large data sets are generated. However, obtaining large data sets is of limited value if the scatter in the data result from measurement errors or flaws in experimental technique, particularly if they consistently bias the data in one direction so that the “mean’ is mean of measurement errors, not of the physical SCC process. While environmental cracking is more complex than, e.g., fatigue or fracture toughness measurements - both as a physical process and an experimental measurement - it should not inherently possess a scatter involving a factor of 100 - 1000X, but perhaps a factor of 2 - 5X. A clear distinction must be made between the inherent in a physical process (e.g., propagation of sound or cracks in a metal) and scatter associated with inadequate experimental controls and flaws in the measurements. Generating reproducible, high quality SCC growth rate data requires that a wide variety of experimental controls and optimized test procedures be employed. Common issues relate to pre- cracking; stress intensity - specimen size validity; significance of “no observed growth”; water chemistry controls; stability of load, temperature, water chemistry, etc.; inadequate resolution or accuracy in crack length measurements; etc. There are additional issues related to testing philosophy (e.g., changing test conditions to demonstrate dependence on a given variable and to return to prior test conditions to demonstrate data reproducibility); use of periodic unloading to maintain a straight crack front and help prevent artificial crack arrest; etc. Issues such as these need to be vigorously pursued to eliminate as much of the scatter as possible from SCC data. Keywords: Stress corrosion cracking, high temperature water, data quality, experimental techniques, data reproducibility, data interpretation.
There is a growing awareness that differences in testing procedure in high temperature water can produce very large scatter in the stress corrosion crack (SCC) growth rate data. For example, data from single or multiple laboratories often show scatter of 100 - 1000X or more, which is too high to establish reliable quantitative dependencies unless very large data sets are generated. However, obtaining large data sets is of limited value if the scatter in the data result from measurement errors or flaws in experimental technique, particularly if they consistently bias the data in one direction so that the “mean’ is mean of measurement errors, not of the physical SCC process. While environmental cracking is more complex than, e.g., fatigue or fracture toughness measurements - both as a physical process and an experimental measurement - it should not inherently possess a scatter involving a factor of 100 - 1000X, but perhaps a factor of 2 - 5X. A clear distinction must be made between the inherent in a physical process (e.g., propagation of sound or cracks in a metal) and scatter associated with inadequate experimental controls and flaws in the measurements. Generating reproducible, high quality SCC growth rate data requires that a wide variety of experimental controls and optimized test procedures be employed. Common issues relate to pre- cracking; stress intensity - specimen size validity; significance of “no observed growth”; water chemistry controls; stability of load, temperature, water chemistry, etc.; inadequate resolution or accuracy in crack length measurements; etc. There are additional issues related to testing philosophy (e.g., changing test conditions to demonstrate dependence on a given variable and to return to prior test conditions to demonstrate data reproducibility); use of periodic unloading to maintain a straight crack front and help prevent artificial crack arrest; etc. Issues such as these need to be vigorously pursued to eliminate as much of the scatter as possible from SCC data. Keywords: Stress corrosion cracking, high temperature water, data quality, experimental techniques, data reproducibility, data interpretation.
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