Mixed-Mode, High-Cycle Fatigue-Crack Growth Thresholds in Ti-6Al-4V: A Comparison of Large and Small Crack Behavior

There are few experimental results to date describing the crack-propagation threshold behavior of short fatigue cracks under multiaxial loading conditions. To address this need, in the present study, the variation in mixed-mode, high-cycle fatigue-crack growth thresholds with crack size and shape are reported for a Ti-6Al-4V turbine blade alloy, heat treated to two widely different microstructural conditions, namely fine-grained bimodal and coarser fully lamellar microstructures. Specifically, fatigue thresholds are examined for through-thickness large cracks (as compared to the microstructural dimensions), throughthickness short cracks (< 200 μm in length), and small, semi-elliptical surface cracks (comparable with microstructural dimensions), under the effect of combined mode I and mode II loading for load ratios (ratio of minimum to maximum load) ranging from 0.1 to 0.8. For a range of mode-mixities, large crack, mode I thresholds, ∆KI,TH, were found to be decrease substantially with increasing phase angle. However, by characterizing in terms of the range in strain energy release rate, ∆GTH, incorporating both mode I and mode II contributions, it was observed that the pure mode I threshold could be regarded as a "worst case" large crack threshold under mixed-mode loading in this alloy. An estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip was also performed. For such neartip (shielding-corrected) thresholds, the influence of mode-mixity was dramatically reduced. Corresponding thresholds for through-thickness short cracks and small surface cracks, where the effect of such shielding is minimized, were also substantially less sensitive to mode-mixity and corresponded in magnitude to the shielding-corrected large crack thresholds.