An overview of the principles of modeling charpy impact energy data using statistical analyses
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INTRODUCTION
MAGNOX reactor steel pressure vessels now operated by Magnox Electric plc were designed in the early 1960s against a requirement that failure in service should be incredible.[1] These pressure vessels are 20 m in diameter and fabricated from nominally 75- to 100-mm-thick C-Mn steel plates and forgings, joined together by either manual metal arc or, machine-made, submerged arc welds.[2] Since construction, all the vessels have operated satisfactorily, and their structural integrity is subject to periodic review. The mechanical properties of these steels change as a result of exposure to neutron irradiation at different temperatures; to monitor the changes, in-reactor surveillance schemes were established. For these, specimens of the pressure vessel steels were installed in canisters, at defined locations within an individual reactor, to monitor principally the tensile and Charpy impact energy properties in different reactors. Surveillance specimens have been withdrawn from these reactors after different periods of service to provide measurements of these mechanical properties for a range of neutron irradiation doses and temperatures. The resulting data obtained from the separate reactor schemes have been analyzed to derive relationships which provide trends for changes of mechanical properties as a function of neutron irradiation dose and temperature.[2] The trend curves for the yield and ultimate tensile R. MOSKOVIC, Research Engineer, and P.E.J. FLEWITT, Manager, are with the Structural Integrity Branch, Technology and Central Engineering Division, Magnox Electric plc, Berkeley, Gloucestershire, United Kingdom GL13 9PB. Manuscript submitted October 17, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
strengths describe the effect of neutron irradiation dose and temperature by comparing, for example, the yield strength of the material irradiated to different neutron dose levels with the unirradiated control condition.[3] Analyses of the measured tensile properties are relatively simple for two main reasons. First, the trends can be assessed by using measurements at a single test temperature, and second, each test provides a pair of yield stress and ultimate tensile strength values that can be used as an observation for the dependent variable. Such data can be analyzed by simple linear regression methods. In contrast, changes of the ductile-to-brittle transition temperature (DBTT) derived from Charpy impact energy vs temperature curves assess the variability of the temperature at the 40J absorbed energy level.[4] Hence, estimates of both the Charpy absorbed impact energy as a function of test temperature and the variability in the transition temperature (T40J) with neutron irradiation dose and service temperature are required. The trend curves that are obtained from these data are then used to adjust the fracture toughness values for the steel. For this estimation procedure, it is important to have the most precise statistical estimates of the mean and the associated uncertainties for the T40J DBT
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