Fatigue Life Prediction And Reliability Analysis Of Aero-engine Main Shaft Bearings

The aero-engine main bearing is a key component of aero-engine.Once it fails,it will cause catastrophic consequences.The low life and reliability of the main bearing is one of the bottlenecks restricting the development of aero-engines in China.Compared with foreign aviation powerhouses,China's research on aero-engine main bearings started relatively late.Although China has made certain progress in the design and manufacture of main bearings,but China's main bearings have low lifespan and poor reliability,and cannot cope with complex environments such as high temperature,high speed,high DN value,variable load,and variable working conditions.Therefore,to carry out research on the fatigue life prediction method and reliability analysis method of the main bearing,as the basis for bearing design and application,it has important scientific significance and engineering value for finding the problems existing in the actual application of the bearing and supporting the improvement of the bearing.At present,some theoretical achievements have been made in the life prediction and reliability of main bearings in China,but there are still many scientific problems to be solved urgently.For constant loading conditions,the life prediction model commonly used in engineering is the statistical-based life model,and in the case of variable loading conditions,the life prediction is based on the linear damage accumulation theory.However,these two methods have many defects and deficiencies,and cannot cope with the life prediction and reliability analysis of the main bearing under the actual complex operating conditions.To this end,this dissertation will carry out effective research on fatigue life prediction and reliability analysis methods of aero-engine main bearings.The main research contents include:(1)During the high-speed operation of the main bearing,a large amount of heat generated by the friction between the moving pairs will cause the temperature of the bearing components to rise sharply,affecting the working clearance of the bearing,resulting in changes in the load distribution of the bearing,which in turn affects the fatigue life of the bearing.In this dissertation,the quasi-static method is used to calculate the load distribution of the bearing,and the contact stress between the rolling elements and the inner and outer rings is calculated according to the Hertz contact theory.The finite element simulation software ANSYS Workbench is used for thermal-mechanical coupling analysis to obtain the thermal stress distribution of the bearing,and then the contact stress is corrected by linear superposition,which provides a theoretical basis for subsequent fatigue life prediction and reliability analysis.(2)During the operation of the main bearing,the contact stress at the contact point fluctuates periodically,which will generate a non-zero mean stress,which seriously affects the fatigue life of the main bearing.In view of this,based on the mean stress correction criterion proposed by Smith,Watson and Topper(SWT),this dissertation introduces a material sensitivity coefficient to characterize the sensitivity of different materials to the effect of mean stress,and proposes a fatigue life prediction model considering mean stress and fatigue limit.In addition,the model is applied to the fatigue life prediction of the bearing.(3)The main bearing of aero-engine works under variable load conditions,and the fatigue failure under variable load is the result of the gradual accumulation of damage caused by different amplitude loads.The existing methods for predicting the fatigue life of rolling bearings under variable loading conditions are all based on the linear damage accumulation theory,and do not consider the effect of the loading sequence and the load interaction.The evolution of fatigue damage is not only dependent on the current load,but also closely related to the previous load level.Based on this,an improved nonlinear fatigue damage accumulation model is proposed in this dissertation to predict the fatigue life of rolling bearings under variable loading conditions.(4)In view of the extreme pursuit of long life and high reliability,and the high price of the main bearing of aero-engine,the traditional reliability analysis method is no longer applicable.In this dissertation,based on Physics of Failure theory,considering the influence of uncertain factors such as geometry,load and material properties,a probabilistic fatigue life prediction model based on generalized polynomial chaos expansion is established,and the statistical distribution of bearing fatigue life under various loads is obtained.On this basis,the method of fatigue reliability analysis of main bearing is proposed.