Effects of Surface Finish and Loading Conditions on the Low Cycle Fatigue Behavior of Austenitic Stainless Steel in PWR Environment: Comparison of LCF Test Results With NUREG/CR-6909 Life Estimations

Abstract

During mid 2006, ANL issued a NUREG/CR-6909 [2] report that is now applicable in The US for evaluations of PWR environmental effects in the fatigue analysis of new reactor components. In order to assess the conservativeness of the application of this NUREG report, low cycle fatigue (LCF) tests were performed by AREVA NP on austenitic stainless steel specimens in a PWR environment. The selected material exhibits in an air environment a fatigue behavior consistent with the ANL reference “air” mean curve. Tests were performed for two various loading conditions: for fully reverse triangular signal (for comparison purpose with tests performed by other laboratories with same loading conditions) and complex signal, simulating strain variation for actual typical PWR thermal transients. Two surface finish conditions were tested: polished and ground. The paper presents on one side the comparison of environmental penalty factors (Fen = Nair,RT/Nwater) as observed experimentally with the ANL formulation (considering the strain integral method for complex loading), and, on the other hand, the actual fatigue life of the specimen with the fatigue life predicted through the NUREG/CR-6909 application. Low Cycle Fatigue test results obtained on austenitic stainless steel specimens in PWR environment with triangle waveforms at constant low strain rates gives Fen penalty factors close to those estimated using the ANL formulation (NUREG report 6909). On the contrary, it was observed that constant amplitude LCF test results obtained under complex signal reproducing an actual sequence of a cold and hot thermal shock exhibits significantly lower environmental effects when compared to the Fen penalty factor estimated on the basis of the ANL formulations. It appears that the application of the NUREG/CR-6909 [2] in conjunction with the Fen model proposed by ANL for austenitic stainless steel provides excessive margins whereas the current ASME approach seems sufficient to cover significant environmental effect for components.