Fatigue crack growth of multiple load path structure under combined fatigue loading: Part II experiment study

The fir-tree mortise of a turbine is a typical multiple load path structure which can reallocate the loading during operation. Performing experiments on a full scale turbine blade attached to a part of actual turbine disc at elevated temperature can present the reallocation phenomenon and accurately predict the crack growth of the fir-tree mortise under combined cycle fatigue (CCF). The purpose of this experiment is to determine the allowable crack size of this multiple load path structure under the circumstance of a firm overhaul period. The experimental load spectrum was determined by using a linear cumulative damage rule based on the flight load spectrum. And three turbine components were tested by using a Ferris Wheel system to access the effect of HCF loading amplitude on life. Then the experiments on crack growth were carried out to investigate crack growth lives of the actual mortise teeth with a pre-crack under combined cycle fatigue at elevated temperature. The effects of the contact state of the turbine mortise on crack propagation rates were investigated by comparing the turbine components with nominal dimensions and limiting dimensions. Fatigue fracture analysis by SEM certified that the failure was owing to combined cycle fatigue. Combined with numerical simulations, the crack growth behavior of the turbine components under combined cycle fatigue loading was estimated. Based on the above work, this paper provides a new way to establish a life-time criterion for withdrawal from service of turbine components.