Fatigue Life Prediction And Reliability Analysis Of Aero-engine Turbine Blades

Aero-engine is a complex system coupling with interdisciplinarity and multi-components,and operates under complex conditions,such as high temperature,high pressure and high rotational speed,which must meet the requirements of large thrust,long life and high reliability.Turbine blades are the important rotor parts of the aero-engine,which mainly transmit power to the engine.Due to the continuous starting,acceleration,deceleration,braking and parking of aero-engines in practice,the blades are subjected to complex loadings.There are many stress concentrations of blades owing to the irregular shapes,and the dangerous area is normally the blade and disk attachment region.Because of the concealment and sudden failure of turbine blades,it occurs without any warning,which may lead to serious disasters.Fatigue life of turbine blades shows a great scatter owing to the uncertainties,and it is difficult to model the fatigue life and the reliability.Therefore,in order to ensure the high reliability of aero-engines,it is of great significance to predict the fatigue life and analyze the fatigue reliability of turbine blades.Until now,various methods on fatigue life prediction and reliability analysis of turbine blades have been established,but there are still many scientific problems needed to be solved.In order to consider the mean stress effect,failure mode identification,loading history and uncertainty quantification,researches on turbine blades have been conducted in this dissertation.The main work can be drawn as follows:(1)Development of an equivalent strain model considering the effect and sensitivity of mean stressIn service,the blade is subjected to cyclic loadings,its load amplitude and mean stress change with different working conditions.Various models were proposed based on monotonic tensile tests,which provided simple modifications of mean stress using the static properties.However,the Walker model can characterize the effect and sensitivity of mean stress.According to the finite element analysis results of turbine blades,an equivalent strain model based on the Walker model is proposed for fatigue life prediction.(2)Development of an energy-critical plane model considering the shear and tensile behaviorsAccording to the failure mechanism of turbine blades,there are two modes of crack failure,shear-type and tensile-type failure modes.Most fatigue models are proposed based on shear-type failure.If shear-type prediction model is used to estimate the fatigue life under tensile-type failure,which may lead to great errors.In fact,the tensile behaviors will attribute to shear-type failures,and shear behaviors will attribute to tension-type failures.An energy-critical plane fatigue model considering the interaction of tensile and shear behaviors is established,which is also applied to estimate the fatigue life of high pressure turbine blades.(3)Development of a non-linear cumulative damage model considering the loading historyAccording to influence of loading history on fatigue damage,under low-high loading,it will enhance the material properties,and the residual stress is beneficial when the load changes,and the interaction will delay the damage process and extend its fatigue life.Otherwise,under high-low loading,the residual stress is harmful,and it will accelerate the damage process and shorten its fatigue life.The process of damage accumulation can be regarded as the process of material memory degradation,and considering the loading interaction and loading sequence,a nonlinear cumulative damage model is established,which is also applied to estimate the fatigue life and fatigue damage of low pressure turbine blades.(4)Development of a fatigue reliability analysis method of turbine blades under uncertaintiesThere are lots of uncertainties affecting the fatigue performance of turbine blades in operation.The deterministic life prediction method often ignores the uncertainties,and the probabilistic method can provide satisfactory prediction results.Firstly,in order to describe the influence of various uncertainties on fatigue life of turbine blades,Bayesian theory,Latin hypercube sampling and finite element analysis methods are imcoporated to quantify the uncertainties from model parameters,material parameters and loads;secondly,one-to-one probability density function(PDF)transformation is employed to change the PDF of life cycles into that of damage accumulation;finally,the reliability analysis is performed to assess the turbine blades.