Prediction on the fatigue behavior of Ti-6Al-4V components treated by split sleeve cold expansion with different final reaming depth

The fatigue life of fastening holes in metal components is a critical issue in structural design. Many methods have been proposed to enhance the fatigue performance of the components, among which the split sleeve cold expansion method is an effective and widely used one. Previous research has shown that the final reaming process will increase the residual stress level in the components, but this does not always improve the fatigue life. Therefore, this paper presents a comprehensive study of the residual stress distribution in the components treated by the split sleeve cold expansion technique and also estimates the fatigue life of the components using numerical methods with a new mean stress correction model specifically modified for Ti-6Al-4V alloy. The calculation results agreed with the previous experiments well and the changing pattern of fatigue life to the reaming depth is acquired, which shows that a small ratio of reaming benefits the fatigue life by providing extra compressive stress, but the benefit of excessive reaming on residual stress does not bring about further life extension. This phenomenon can be well explained using the proposed four-area model of the cumulative damage in the components. Apart from increasing the residual stress level near the hole edge, the reaming process will cause another area with maximum tensile stress to move inward and bear a higher level of load, which is harmful to the fatigue life. The movement of the fatigue crack initiation area is also in good agreement with the experimental results, which verifies the reliability of the proposed model.