Notch Effects on High-Cycle Fatigue Properties of Titanium Alloy at Cryogenic Temperatures

摘要

To evaluate material risk caused by human-error, the notch effects on the fatigue properties of forged Ti-5A1-2.5Sn ELI alloy have been investigated at cryogenic temperatures. Smooth and notched specimens with the K_t = 1.5, 2 and 3 were prepared. High-cycle fatigue tests were carried out at 4, 77 and 293 K. The S-N curves of each specimen shifted to higher stress level with a decrease of the test temperature. At 4 K, the fatigue strength of the K_t = 3 specimens were substantially lower than those of the smooth specimens. Although the notch effects on the fatigue properties of the K_t = 1.5 and the K_t = 2 notched specimens are not clear or significant. Fatigue crack initiation sites of the smooth and the K_t = 1.5 notched specimens were in the specimen interior (internal type fracture) and those of the K_t = 2 and the K_t = 3 notched specimens were at the notch root (surface type fracture). The location of the fatigue crack initiation sites showed transition from the internal type fracture for the K_t = 1.5 notched specimens to the surface type fracture for the K_t = 2 notched specimens. Therefore, the K_t values of the internal fatigue crack initiation sites correspond between 1.5 and 2. The sizes of the internal fatigue crack initiation sites were calculated by the (area)~(1/2). The (area)~(1/2) sizes of the K_t = 1.5 notched specimens and the smooth specimens were in a similar range and the fatigue strength of the two are not significant.