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Similar fractures occurred in two gas turbine engine fan blades made of Ti-8Al-1Mo-1V alloy. Their analyses and a supplementary test were performed with the aid of scanning electron microscope (SEM) and transmission electron microscope (TEM) fractography to identify the common mechanism(s).
The initial fracture surface of each fan blade exhibited an area of subcritical crack growth. The area in one fan blade contained three blue discolored zones along the crack initiation side, whereas that in the other fan blade was discolored to white-gray. Intergranular separation and transgranular cleavage were evident at the three discolored zones in one fan blade and near the crack initiation site in the other. Cleavage-like facets were predominant in the remaining area of subcritical crack growth. On some of those facets, striations were seen.
A hot salt stress corrosion cracking test of Ti-8Al-lMo-lV alloy plates resulted in discoloration, intergranular separation, and transgranular cleavage, similar to those observed in one fan blade.
The mechanism of initial crack growth is stress corrosion cracking at an elevated temperature in one fan blade and at ambient temperature in the other. The mechanism of subsequent subcritical crack growth is fatigue at ambient temperature in both fan blades.
fractography, fractures (materials), crack initiation, subcritical crack growth, intergranular separation, transgranular cleavage, stress corrosion, fatigue (materials), striations, stress intensity factor, microvoid coalescence
Metallurgist, Materials Engineering Laboratory, Naval Air Rework Facility, Norfolk, Va.
Metallurgist, Naval Air Development Center, Warminster, Pa.
Manufacturing research and development technologist, Pratt and Whitney Aircraft, East Hartford, Conn.