Failure characterisation of Ti6Al4V gas turbine compressor blades

In this study, the failure process of Titanium compressor blades of an industrial gas turbine was investigated. Several premature failures occurred in the high-pressure section of the compressor due to the fracture of the blade roots. Macro- and micro-fractographic investigations were carried out on the fracture surfaces. Optical and scanning electron microscopy of the blade airfoil and root were also performed. Mechanical properties of the blade alloy were evaluated and compared with the standard specifications. Next, a 2D finite element model of the blade root was constructed and used to provide accurate estimates of stress field in the dovetail blade root and to determine the crack growth initiation in the dovetail. Based on the normal service operation of the compressor, the centrifugal and shear forces applied to the blade-disc configuration were considered in the model. The experimental results showed no metallurgical and mechanical defects for the blade materials. Microstructure of the blade root and airfoil, and hardness and tensile properties were all comparable with those reported in the standard specification. Fractography experiments clearly showed multiple crack initiation sites and fatigue beach marks. Debris particles were observed on the fracture surfaces and in the mouth of initiated cracks. The blade surface in contact with the disc in the dovetail region showed a higher surface roughness than the other surfaces. The numerical model clearly showed stress concentration at the corner on the contact facet of the blade dovetail between the blade dovetail and the wheel dovetail. Based on the results obtained, the fretting fatigue mechanism was proposed for the premature failures.