Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti-6Al-4VI. 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. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles beta = tan(-1)(Delta K-II/Delta K-I) from 0 degrees to 82 degrees (Delta K-II/Delta K-I similar to 0-7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R = 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 stress-intensity range at the threshold, Delta K-I,K-TH, is to decrease with increasing mode mixity, Delta K-II/Delta K-I, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate; Delta G, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases significantly with the applied ratio of Delta K-II/Delta K-I The pure mode I threshold, in terms of Delta G(TH), 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 similar to 200 mu m 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 significantly reduced. Comparison of the large- and short-crack data suggests that the increase in the large-crack fatigue threshold, Delta G(TH), With an increasing mode mixity (Delta K-II/Delta K-I) is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quantification of such shielding is described in Part II of this paper. (C) 2000 Elsevier Science Ltd. All rights reserved.