Environment and R - Ratio Effects on Fatigue Crack Propagation Transitions in Ti - 6Al - 4V (ELI)

다국어 초록 (Multilingual Abstract)

The multiple-sloped relationship between da/dN and ΔK in stage Il fatigue crack propagation regime in most engineering alloys has been a subject of considerable research efforts. Even though the crack closure is recognized as a crucial factor affecting FCP rates, closure effect on transition behavior is often ignored due to the uncertainties associated with ΔK_(eff) value and the difficulties in measuring closure especially in aqueous environment. The present study therefore focused on the closure-free transition behavior in mill annealed Ti-6A1-4V in moist air and 1% NaCl at a fixed potential of -500 ㎷_(SCE) as a function of R ratio. The complex da/dN-ΔK relationship in Ti-6Al-4V remains after closure-correction for each R ratio and environment, suggesting that closure is not a controlling factor for a transition, while it may complicate transition behavior. After closure-correction, the transition points and slopes are insensitive to R ratio in moist air, suggesting that R ratio effect on transition behavior in moist air is mostly due to crack closure. The 1% NaCl solution further complicates the da/dN-ΔK_(eff) shape in Ti-6Al-4V at R ratios of 0.1 and 0.4 by superimposing cyclic stress corrosion cracking (SCC)-induced transition at intermediate ΔK regime. Varying R ratio in 1% NaCl is complex and appears to modify the cyclic and/or static SCC contribution.