Contact Stress Analysis And Life Prediction Of Blade-disk Fir-tree Attachment

With the development of modern aero engines towards high Mach number,high thrust-to-weight ratio and high reliability,the service environment of the engine becomes more critical,which brings challenges for ensuring engine structural integrity.The fir-tree tenon structure as one of the main connecting forms of engine turbine disk and blade is subjected to high temperature,high pressure and complex thermo-mechanical alternating loads during service.Therefore,predicting the fatigue life of fir-tree tenon is of great significance to prevent structural failure and ensure stable of disk and blade to work safely,so as to ensure the structural integrity and reliability of the aero engine.In this thesis,researches on an aero engine turbine blade-disk fir-tree tenon structure have been conducted as follows:(1)The contact finite element(FE)analysis of turbine blade-disk fir-tree tenon structure is performed.In order to improve the computational efficiency,the turbine disk-slice structure is divided into several equal parts.Combined with the load spectrum of one typical flight mission,FE analysis under the contact condition for blade-disk structure is carried out by using ANSYS software considering the actual loading forces and constraints.The calculated results are in accordance with the position estimated by using contact theory.The calculated equivalent stress value is similar to the tested one.Correct and reasonable FE analysis provides the basis for subsequent fatigue life prediction.(2)Explore multiaxial fatigue life prediction method based on the critical plane approach.According to the critical plane approaches used in multiaxial fatigue life prediction,the calculation process to determine critical plane is summarized.Simultaneously,evaluate and analyze the validity and accuracy of the simulation to ensure accurate multiaxial fatigue life prediction.In addition,the crack failure mechanism of multiaxial fatigue fracture is elaborated and the multiaxial fatigue life prediction models based on critical plane approach is summarized,which provides a reference for the comparative study of multiaxial fatigue models.(3)A new multiaxial fatigue life prediction model is proposed by considering the interaction between normal and shear behavior on the critical plane.Different crack failure mode judgment methods are also proposed for the two failure modes.Based on the fatigue test data of the titanium alloy TC4,the error analysis and evaluation of the three failure mode judgment methods are carried out to determine the most appropriate judgment method,thus establishing the entire model system,which is applied for fatigue life prediction of compressor blade-disk fir-tree structure.(4)Considering additional material constants of fatigue models often require additional fatigue testing data to be determined,a new multiaxial fatigue life prediction model without additional material constants is proposed for ductile materials.Research on the additional material constants of fatigue models is conducted to quantify the influence of additional material constants on the fatigue life prediction ability of the corresponding fatigue model for various materials with different properties.Combined with the modified generalized strain amplitude damage parameters,considering different effect to reflect the contribution of shear strain and normal strain on critical plane for fatigue failure,a new multiaxial fatigue model that considering mean stress effect and non-proportional loadings effect,which is applied for fatigue life prediction of high pressure turbine blade-disk fir-tree structure.