Study of the fracture of the last stage blade in an aircraft gas turbine

The fracture on the blade in an aircraft gas turbine has been studied to see the cause of crack initiation. The turbine blades were made of Ni-base superalloy of MAR 200Hf grade and fabricated by DS investment casting. From the visual examination of the fractured surface, the turbine blade had initially cracked by a fatigue mechanism over a period of time and then fractured by the overload at the last moment. On further examination, the crack initiated at the trailing edge of the blade, and propagated by the fatigue under the cyclic loading experienced by the blade during service. The pores and segregated area of Hf and Ti at the trailing edge of the blade, caused by DS investment casting, is found by the microstructure and EDX analysis of the blade. The crack initiated at this area and was due to the stress concentration at the pores and segregation of Hf and Ti.