353

a Run out.

The fatigue behavior of titanium alloys can be divided into S-N fatigue and fatigue crack growth rate (FCGR, or da/dn vs. 8K). Within S-N fatigue a further subdivision can be made between low-cycle fatigue (LCF) and high-cycle fatigue (HCF). For LCF, failure occurs in 104 cycles or less, while HCF failure occurs at greater than 104 cycles.

Different uses favor different techniques for determining LCF, specifically strain-controlled and load-controlled tests, Table 8.5 and Fig. 8.8 [26]. Both notch concentration (Kt) and overall surface condition can strongly influence LCF. The beneficial effect of relatively gentle surface conditioning is shown in Fig. 8.9 [26]; more severe working of the surface can result in the formation of cracks and a degraded LCF behavior. The effect of Kt and crack propagation of LCF life on preloaded Ti-6Al-4V at 205°C is shown in Fig. 8.10 [26].

Surface condition can also strongly influence HCF (Fig. 8.11) [26]. The fatigue endurance limit is relatively flat to at least 315°C (600°F) (Fig. 8.12) [26]; with benefits apparent for titanium alloys over steels.

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