Effects of surface roughness and notch on fatigue properties for Ti–5Al–2.5Sn ELI alloy at cryogenic temperatures

Abstract

To evaluate material risk caused by human-error, the effects of surface roughness and notch on the fatigue properties of Ti–5Al–2.5Sn ELI alloy have been investigated at cryogenic temperatures. Specimens with surface roughness changed by emery 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 #100-roughness specimens was a little lower than those of the #600-roughness specimens. Fatigue crack initiation sites of each surface roughness specimen at 4 K were found in the specimen interior (internal type fracture). On the other hand, the fatigue strength of the notched specimens was substantially lower than those of the surface roughness specimens. Although fatigue crack initiation sites of the Kt = 3 notched specimen were at the notch root (surface type fracture), those of the Kt = 1.5 notched specimen were in the specimen interior. The location of the fatigue crack initiation sites changed from the internal type fracture for Kt = 1.5 notched specimens to the surface type fracture for Kt = 3 notched specimens. Therefore, the Kt values of the internal fatigue crack initiation sites correspond between 1.5 and 3. The root area analysis, which is the size of the crack propagation plane as a shape parameter, the fatigue strength depends on the size (internal fatigue crack initiation site size). 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.