Effects of Surface Roughness and Notch on Cryogenic Fatigue Properties of Titanium Alloy

To evaluate material risk caused by human-error, fatigue properties of Ti-5Al-2.5Sn ELI alloys and the effects of surface roughness and notches on the properties have been investigated at cryogenic temperatures. Specimens with surface roughness changed by plural SiC papers (Grade #600, #100) and notched specimens were prepared (Kt=1.5, 3). The S-N curves shifted to higher stress level with a decrease of the test temperature. Regarding the effect of surface roughness, the fatigue strength of the Grade #100-surface-roughness specimens was a little lower than those of Grade #600-surface-roughness specimens. Fatigue crack initiation sites of each surface roughness specimen at 4 K were found in the specimen interior. Using root area analysis, which is the size of the crack propagation plane as a shape parameter, the fatigue strength depends on the size. In notched specimens, the fatigue strength of the Kt=3 specimen were lower than those of the Kt=1.5 notched specimens. Although fatigue crack initiation sites of the Kt=3 notched specimen were at the notch root, those of the Kt=1.5 notched specimen were in the specimen interior.Fatigue properties of surface roughness and notched specimens at cryogenic temperatures were expected to be more improved when the grain size of the materials was minimized, i.e., fatigue crack initiation sites were minimized.