Fatigue crack growth behaviour for a mill-annealed Ti-6Al-6V-2Sn alloy tested in air was found to be very sensitive to frequency and microstructure near threshold and again at higher stress intensities. Fracture near threshold was characterized by a combined failure mode involving striation formation, cleavage, and grain boundary separation. A modified Dugdale–Barenblatt model, which quantifies microstructural effects, predicted reasonably good threshold values as a function of grain size. At high stress intensities and a low cyclic frequency, crack growth rates exhibited enhanced values over those observed at a higher frequency. Fracture associated with the enhanced rates was characterized by both cleavage and striation formation. The appearance of the cleaved grains was identical to that observed in sustained load testing and associated with hydrogen embrittlement.
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The support of the Air Force Office of Scientific Research through contract 78-3133 is gratefully acknowledged.