Jp. Campbell et Ro. Ritchie, Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti-6Al-4VI. A comparison of large- and short-crack behavior, ENG FRACT M, 67(3), 2000, pp. 209-227
Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for thr
ough-thickness cracks (large compared to microstructural dimensions) in a T
i-6Al-4V turbine blade alloy with a bimodal microstructure. Specifically, t
he effect of combined mode I and mode II loading, over a range of phase ang
les beta = tan(-1)(Delta K-II/Delta K-I) from 0 degrees to 82 degrees (Delt
a K-II/Delta K-I similar to 0-7), is examined for load ratios (ratio of min
imum to maximum loads) ranging from R = 0.1 to 0.8 at a cyclic loading freq
uency 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 t
he strain-energy release rate; Delta G, incorporating contributions from bo
th the applied tensile and shear loading, the threshold fatigue-crack growt
h resistance increases significantly with the applied ratio of Delta K-II/D
elta 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 c
racks, where the precrack wake has been machined to within similar to 200 m
u m of the precrack tip. For such short cracks, wherein the magnitude of cr
ack-tip shielding which can develop is greatly reduced, the measured mixed-
mode fatigue-crack growth thresholds are observed to be markedly lower. Mor
eover, 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, wh
ich reduces the local crack-driving force actually experienced at the crack
tip. Quantification of such shielding is described in Part II of this pape
r. (C) 2000 Elsevier Science Ltd. All rights reserved.