Combined fatigue life test and extrapolation of turbine disk mortise at elevated temperature

To investigate the crack failure of a certain turbine disc mortise, a test system with a new low- and high-cycle noninterfering loading scheme was established to study the low- and high-cycle combined fatigue (L-HCCF) of fir-tree mortise at elevated temperature. Then, for 50 and 100 Hz vibrations with constant stress amplitude superimposed on the same low cycle loading, respectively, the contrast L-HCCF life tests of light- and serious-corrosive disc mortises were carried out. Through real-time crack detection of the first teeth, the crack propagation rule of fir-tree mortise, a typical multiple load path structure, was obtained. Finally, a new method, two-frequency-ratio method, was presented to relate the load and life of laboratory test to the equivalent those of service in order to help the definition of the failure criteria for serious-corrosive discs in actual practice. The extrapolative results show that the life policies in use for light-corrosive turbine disc can not be applied to serious-corrosive disc directly because its reliable life is greatly less than an overhaul schedule, and it is urgent to establish criteria for withdrawal from service of the serious-corrosive discs to assure the structural integrity of this aeroengine.