| Due to the excellent mechanical property at medium-high temperature,Ti-6Al-4V titanium alloy becomes the main manufacturing material of turbofanengine low pressure (LP) rotor blade including the fan blade (FB) installed in the veryfront of the LP rotor. For high-bypass ratio turbofan engines, the fan supplies most ofthe thrust and fatigue fracture under the complex alternating loads becomes one of themain failure causes of the FBs. In this paper, based on the continuous damage theoryand crack growth theory and combined with numerical simulation and fatigue tests, ansystematic study was made on the damage evolution, crack growth, life prediction,and reliability anlysis of FB made of Ti-6Al-4V. The work can be summarized asfollows:(1) A series of fatigue tests were conducted with Ti-6Al-4V standard specimen,including tensile test, push-pull fatigue test, plane strain fracture toughness test andcrack growth rate test, in order to obtain static mechanical properties, stress fatigueperformance and crack growth properties. And the test data was used for thefoundation and verification of the damage model and crack growth rate function. Thefatigue limits and damage variables of Ti-6Al-4V under different stress ratios weredetermined, and the effects on crack growth rate of Ti-6Al-4V of stress ratio, crackclosure effect, and fracture toughness was analyzed.(2) A corrected nonlinear continuum damage model (CDM) of fan blade wasproposed based on the Chaboche’s uniaxial model. This correction model takes theeffect of torque brought by twisting into consideration.The S-N curve of the specimenwas simulated to verify the damage model, and the calculation results were agreedwith the experimental data. The approach that use the neural network (NN) to fit thenonlinear damage accumulation function was attempted and the back propagation (BP)NN based on L-M arithmetic is established. By determining the hidden neuronnumbers through numerous computations, the prediction results by BPNN shows agood agreement with the test data.(3) A multi-parameter crack growth rate function considering stress ratio, crackclosure, fracture toughness, and the peak value of stress intensity factors was given,and the values of parameters in the equation were also provided. The crack growth rate and crack growth life of specimen were simulated, and the results ofmulti-parameter equation have a great agreement with the experimental data.(4) The power spectral distribution of the blades in1000flight hours wasanalyzed, and four typical operating conditions used in the life evaluation of thisarticle were formulated. The3D model of the blade was dawned and the FE modelwas meshed. The stress distributions of the blade under centrifugal loads,aerodynamic loads, and vibration loads in different working conditions werecalculated. The designed fatigue life and remaining fatigue life of the blade areevaluated by using the life estimation model proposed above.(5) A nonlinear strength degradation equation for Ti-6Al-4V was established andintroduced to the Stress-Strength interference model of the fan blade reliability.Moreover, common cause failure (CCF) model was used to popularize the reliabilitycalculation from single blade into a group of blade. Statistical method of survivalanalysis is also applied to the reliability analysis of fan blades, and Cox proportionalhazards model was used to analyze the contribution of covariates to the blade failure.The survival rate and failure risk of the fan blades in the life cycle were calculated. |
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