Fatigue damage assessment for high-cycle thermal fatigue at mixing tee

This study was aimed at showing a procedure for assessing the fatigue life at a junction. The elastic and elastic-plastic structural analyses were performed for a mixing tee pipe made from stainless steel using the temperature transient obtained by the mock-up tests. Bi-linear strain curves were assumed for the elastic-plastic analysis. Then, the stress and strain change and the difference between obtained by the elastic-plastic analyses were compared to investigate the influence of the plastic deformation. The analysis results showed that the strain range obtained by the elastic-plastic analysis was almost the same as that obtained by the elastic analysis even if significant plastic deformation was considered. Since the fatigue life correlated not with stress range but with strain range, the fatigue life can be predicted using the strain range obtained by the elastic analysis without considering the plastic deformation. Moreover, mean stress observed in the elastic analysis results became negligibly small in the elastic-plastic analysis. Therefore, the mean stress is not necessary to be considered in the fatigue life assessment even if the elastic analysis exhibits significant mean stress. (author)