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 Ti–6Al–3Mo–2Cr were examined in compressor disks that had failed in service. In-flight block cyclic loading that included hold-time loading and variable amplitudes of vibrational loading produced fracture characteristics which mainly reflected the two-phase (α + β) lamellar structure of the titanium alloy and contained localized areas of fatigue striations.
Blocks of cyclic loads which included cycles of hold-time loading during the testing of disks resulted mainly in the formation of fatigue striations with no evidence of the lamellar microstructure of the titanium alloy. An analytical equation Vi = [(1 − ν2 )/(16 π E σ_{0.2 )][Ke ]2i} is suggested to describe the relationship between the crack propagation rate, V, and the equivalent stress intensity factor, K_e , as applied to the semi- and then to the quarter-elliptical-shaped cracks. Fractographic features are discussed and the disks' fatigue crack growth behaviour over 50 flights is demonstrated.