Elbow pipes are commonly used in the piping systems of power plants and chemical plants. The stress states at the elbow part are complex and quite different from those of the straight pipes. It is well known that the fatigue lives of metals under simple push-pull conditions can be successfully predicted by Manson’s universal slope method. However, the low-cycle fatigue lives of elbow pipes under combined cyclic bending and inner pressure cannot be predicted by this method, though the reasons for this have not been clarified. In this work, the low-cycle fatigue tests and finite element analysis of elbows under cyclic bending and inner pressures were carried out. The results showed that the multi-axial stress factor, which is a ratio of hoop stress and axial stress, at elbows is quite high. Considering the multi-axial stress factor, a revised Manson’s universal slope method is proposed in this paper. Using the proposed method, we were able to predict conservatively the low-cycle fatigue lives of elbows under combined cyclic bending and inner pressure.

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