Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti-6Al-4VII. Quantification of crack-tip shielding

The role of crack-tip shielding in influencing mixed-mode (mode I + II) fatigue-crack growth thresholds for large, through-thickness cracks in a Ti-6Al-4V turbine blade alloy is examined under high-cycle fatigue loading conditions, i.e., at a loading frequency of 1000 Hz in ambient temperature air for load ratios (K-min/K-max) of R = 0.1-0.8. Techniques are developed to quantify crack-tip shielding with respect to both the mode I and mode II applied loading, enabling an estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip (Delta K-I.TH.eff and Delta K-II.TH.eff or Delta G(TH.eff)) In Part I, it was shown that when the crack-driving force is characterized in terms of the range in strain-energy release rate, Delta G, which incorporates contributions from both the applied tensile and shear loading, the mixed-mode (I + II) fatigue-crack growth resistance increases monotonically with the ratio Delta K-II/Delta K-I. When the fatigue-crack growth thresholds are expressed in terms of the near-tip (shielding-corrected) crack-driving force, this increase in crack-growth resistance with increasing mode mixity is markedly reduced. Moreover, for all mode mixities investigated, the near-tip mixed-mode fatigue threshold is lower than the applied (global) value, with the effect being particularly pronounced under shear-dominant loading conditions. These observations illustrate the prominent role of crack-tip shielding for the mixed-mode loading of fatigue cracks with crack-wake dimensions large compared with microstructural size scales; specifically, they indicate that the elevation of the Delta G(TH) fatigue-crack growth threshold with increasing applied mode mixity is largely due to a shear-induced enhancement of crack-tip shielding. (C) 2000 Elsevier Science Ltd. All rights reserved.