Derivation of shakedown factors (Ks) for Type 316 and Type 321 stainless steel at high temperature

Abstract

The shakedown factor, Ks, is defined in R5 Volume 2/3 as the factor applied to the minimum 0.2% proof stress, Sy, in order to obtain the material ratchet limit, in terms of the maximum stress level at which the materials exhibits cyclically stable behaviour. This experimentally derived factor is used in the R5 assessment procedure to perform simple checks for shakedown and also when a more detailed shakedown analysis is required. In addition, it is also used to determine the start-of-dwell stress as part of a creep-fatigue damage assessment.This paper presents the derivation of this shakedown factor, Ks, from a number of cyclic load-controlled tests, which were carried out on several casts of Types 316H and 321 steel between room temperature and 650°C. These tests involved subjecting uniaxial specimens to blocks of continuous load controlled cycles, the stress range remaining constant during each block, but then being increased until material cyclic instability was observed. Since the type of cycle was expected to be related to reactor trip or fault conditions, and only a few hundred were expected in the lifetime of the plant, the tests were based on blocks of 500 repeated cycles. Tensile tests were also carried out on the same materials in order to provide values of the 0.2% and 1% proof stress which are used to derive the Ks parameter. The Ks factor is sensitive to the strain rate used in the tensile tests.Finally, the technique used to evaluate the shakedown factor is discussed, along with possible modifications to the R5 methodology to improve the representation of stress-strain cycles at elevated temperatures.