Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti±6Al±4V I. A comparison of large- and short-crack behavior

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti±6Al±4V turbine blade alloy with a bimodal microstructure. Speci®cally, the e€ect of combined mode I and mode II loading, over a range of phase angles b ˆ tanÿ1…DKII=DKI† from 0° to 82° …DKII=DKI 0±7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R ˆ 0:1 to 0.8 at a cyclic loading frequency of 1000 Hz in ambient temperature air. Although the general trend for the mode I stressintensity range at the threshold, DKI;TH, is to decrease with increasing mode mixity, DKII=DKI, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate, DG, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases signi®cantly with the applied ratio of DKII=DKI. The pure mode I threshold, in terms of DGTH, is observed to be a lower bound (worst case) with respect to mixed-mode (I‡ II) behavior. These results are compared with mixed-mode fatigue thresholds for short cracks, where the precrack wake has been machined to within 200 lm of the precrack tip. For such short cracks, wherein the magnitude of crack-tip shielding which can develop is greatly reduced, the measured mixed-mode fatigue-crack growth thresholds are observed to be markedly lower. Moreover, the dependence of the mixed-mode fatigue-crack growth resistance on the applied phase angle is signi®cantly reduced. Comparison of the largeand short-crack data suggests that the increase in the large-crack fatigue threshold, DGTH, with an increasing mode mixity …DKII=DKI† is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quanti®cation of such shielding is described in Part II of this paper. Ó 2000 Elsevier Science Ltd. All rights reserved.