Failure of turbine rotor blisk of an aircraft engine

Abstract

Cracks were observed at the trailing edge of the blades of a turbine rotor blisk of a gas turbine engine during inspection following an endurance test. The integrally bladed disk was made of Mar-M-247 alloy, cast and hipped. Visual inspection, micro and macro fractography, and metallography of the failed blades indicated that the failure was caused by stress rupture at the trailing edge followed by fatigue crack propagation. A systematic analysis was carried out to ascertain the cause for stress rupture. Microstructural studies revealed that the columnar grains at the blade root airfoil region were oriented unfavorably leading to poor stress rupture property in the transverse direction. Coupled with this, the high operating temperatures were responsible for the failure of the blade by stress rupture. It was recommended that the casting parameters be controlled so that the columnar grains are oriented parallel to the principal stress axis of the blade. Coarse columnar grains with high aspect ratios in the longitudinal direction will improve the stress rupture property. It was also suggested to monitor the operating temperature to see that it does not exceed the specified temperature limit of the alloy.