Abstract
The factors which control fatigue crack growth behaviors in Ti-6Al-4V in the ΔK range less than 30 kgf/mm3/2 in sea weter were discussed with respect to those in the air. The results obtained are as follows ;
1) Fatigue crack growth rate, da/dN, is higher in sea water than that in the air at the stress ratios of 0.1 to 0.7. The trend is much remarkable at lower levels of stress ratio and ΔK.
2) The value of da/ dN in sea water is not influenced by the crack tip closure at the stress ratios larger than 0.5, while da/ dN in the air is influenced by the crack tip closure at stress ratios larger than 0.9.
The different crack growth behavior in sea water from that in the air is explained in terms of roughness of fracture surfase. Rougher fracture in sea water is resolved into a smoother one.
3) The da/ dN value plotted against ΔKeff in sea water is higher than that in the air. However, maximum ratio of da/ dN in both the environments is less than two.
4) Crack in sea water does not grow at ΔK levels less than the effective stress intensity threshold, ΔKth, eff, in the air.
5) Fracture surface consists of cleavage-like patterns in sea water and in the air, and it shows of α grain boundary cracking and transgranular cracking of β phase in both the environments at ΔK levels close to ΔKth.
