DEVELOPMENT OF FATIGUE CRACKING IN AIRCRAFT ENGINE COMPRESSOR DISKS OF TITANIUM-ALLOY TI-6AL-3MO-2CR

Fractographic features related to fatigue growth in a titanium alloy T i-6Al-3Mo-2Cr were examined in compressor disks that had failed in ser vice. In-flight block cyclic loading that included hold-time loading a nd variable amplitudes of vibrational loading produced Fracture charac teristics which mainly reflected the two-phase (alpha + beta) lamellar structure of the titanium alloy and contained localized areas of fati gue striations. Blocks of cyclic loads which included cycles of bold-t ime loading during the testing of disks resulted mainly in the formati on of fatigue striations with no evidence of the lamellar microstructu re of the titanium alloy. An analytical equation V-i = [(1 -nu(2))/(16 . pi . E . sigma(0.2))][K-e](i)(2) is suggested to describe the relat ionship between the crack propagation rate, V, and the equivalent stre ss intensity factor, K-e, as applied to the semi-and then to the quart er-elliptical-shaped cracks. Fractographic Features are discussed and the disks' fatigue crack growth behaviour over 50 flights is demonstra ted.