Notch Effects on Cryogenic Fatigue Properties of Titanium Alloy and Internal Type Fracture

To evaluate material risk caused by human-error, the notch effects on the fatigue properties of forged Ti-5Al-2.5Sn ELI alloys have been investigated at cryogenic temperatures. Smooth and notched specimens with the Kt=1.5, 2 and 3 were prepared. High-cycle fatigue tests were carried out at 4 K (liquid helium), 77 K (liquid nitrogen) and 293 K (room temperature). 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 Kt=3 specimens were substantially lower than those of the smooth specimens. Although the notch effects on the fatigue properties of the Kt=1.5 and the Kt=2 notched specimens are not clear or significant. Fatigue crack initiation sites of the smooth and the Kt=1.5 notched specimens were in the specimen interior (internal type fracture) and those of the Kt=2 and the Kt=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 Kt=1.5 notched specimens to the surface type fracture for the Kt=2 notched specimens. Therefore, the Kt 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 \sqrtarea. The \sqrtarea size of the Kt=1.5 notched specimens and smooth specimens were in a similar range and the fatigue strength of the two are not significant.