Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti±6Al±4V II. Quanti®cation of crack-tip shielding

The role of crack-tip shielding in in ̄uencing 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 (Kmin=Kmax) of R ˆ 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 (DKI;TH;eff and DKII;TH;eff or DGTH;eff ). In Part I, it was shown that when the crack-driving force is characterized in terms of the range in strainenergy release rate, DG, which incorporates contributions from both the applied tensile and shear loading, the mixedmode …I‡ II† fatigue-crack growth resistance increases monotonically with the ratio DKII=DKI. When the fatigue-crack growth thresholds are expressed in terms of the near-tip (shielding-corrected) crack-driving force, this increase in crackgrowth 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 e€ect 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; speci®cally, they indicate that the elevation of the DGTH fatigue-crack growth threshold with increasing applied mode mixity is largely due to a shear-induced enhancement of crack-tip shielding. Ó 2000 Elsevier Science Ltd. All rights reserved.