High cycle fatigue of austenitic stainless steels under random loading

Abstract

The effect of high cycle random loading at 330°C and at 550°C on the fatigue resistance of austenitic stainless steels Type 316LN has been investigated. The aim was to improve the predictions of the consequences of thermal fluctuations occurring in some reactor components. The experimental means used for this purpose was a minicomputer, which generated random stationary Gaussian load sequences in a Markov matrix, and several servo-hydraulic fatigue testing machines. This method has the advantage of taking into account the mean load for each cycle. The specimens tested were taken from two plates which were at two different heats, one 45 mm thick and slightly cold worked, the other 26.5 mm thick. Random fatigue tests were mainly performed in the endurance region of the fatigue curve, using the stair-case method to determine the scatter. The experimental data were analyzed in terms of the test temperature and the irregularity factor, with the aim of establishing design curves for reactor components. The results obtained have shown that the random loading is more damaging than the constant amplitude loading when compared on the basis of the mean square root conventional fatigue limit. They have also shown that the linear damage accumulation, according to the Miner's rule, yields conservative results at least when used in conjunction with the probability density function of peaks for a case where the irregularity factor is near 100%. Based on these observations a method is proposed for predicting the fatigue life from the design curves, which includes random loading effects with the irregularity factor as the indexing parameter.