SCALING OF SN CURVES FOR VARYING 'INITIATION' CRACK DEFINITIONS FROM STRIATION COUNTED ENVIRONMENTAL FATIGUE SPECIMENS: A 250 MICRON AUSTENITIC STAINLESS STEEL SN CURVE AND ASSOCIATED FEN FACTORS

摘要

Future code development to adopt a risk-informed design methodology will require improved accuracy of fatigue initiation predictions. The ability to account for through wall strain gradients in plant components, particularly over the first 3 mm of wall thickness is one area where conservatism can be reduced. This is due to extant design fatigue curves being derived from strain controlled membrane loading tests where the 25% load drop definition for end of test equates to approximately a 3 mm crack. Being able to define initiation fatigue curves for much shorter crack depths would enable fatigue crack growth methods to then predict the additional cycles, taking into consideration the strain gradient, required before the defined end of life crack size is reached. The R5 procedure provides a method to adjust an existing Stress-Life (SN) curve representing an initiation crack depth, to a smaller depth. This method was developed for materials at higher temperature and for a CO_2 cooling environment, thus its validity was uncertain for application to a Pressurised Water Reactor (PWR) plant. This paper details the development of best estimate and design basis SN curves and environmental fatigue enhancement factors (F_(en)) for crack initiation to a depth of 250 urn. It is concluded that the general methodology in R5 was found, through this work, to adequately describe fatigue initiation lives for stainless steel in a PWR environment when augmented with a crack size dependent F_(en) equation and with modified coefficients determined through regression. The resulting method is similar to R5 in that an SN curve can be derived for any crack size, however the current data set only provides validation down to a crack size of 250 μm, as striations at shorter depths were not visible with existing methods.