Effect of chronological order of cyclic loads with different amplitudes on fatigue crack growth

When cracked bodies are subjected to cyclic loading, fatigue crack growth evaluation is often required from the viewpoint of the assurance of fitness-for-service. For cyclic loading with constant amplitude, crack growth can be calculated by integrating a fatigue crack growth rate law provided by the Paris law in regard to stress intensity factor range. However, the cause of cyclic loading assumed in actual plants is not unique. For an example of LWR plants, a number of datasets of cyclic loading with different amplitudes is necessary for specific transient events. And the chronological order of individual transients cannot be determined. In this paper, a general treatment of the multiple transients was proposed in case that the chronological order of the transients was indefinite. It was found that cyclic loading sequence such that the loadings were lined up in the order corresponding to larger amplitude gave the most conservative crack growth prediction among the possible sequences from the set of the specific transient events. Nevertheless the effect of the sequence was quite limited and the differences in fatigue crack growth could be comparable to the order of possible rounding errors.