Fatigue life and reliability of aero-engine blade are always of important significance to flight safety.The establishment of damage model is one of the key factors in blade fatigue research.Conventional linear Miner's sum method is not suitable for aero-engine because of its low accuracy.A back propagation neutral network(BPNN) based on the combination of Levenberg-Marquardt(LM) and finite element method(FEM) is used to describe process of nonlinear damage accumulation behavior in material and predict fatigue life of the blade.Fatigue tests of standard specimen made from TC4 are carried out to obtain material fatigue parameters and S-N curve.A nonlinear continuum damage model(CDM),based on the BPNN with one hidden layer and ten neurons,is built to investigate the nonlinear damage accumulation behavior,in which the results from the tests are used as training set.Comparing with linear models and previous nonlinear models,BPNN has the lowest calculation error in full load range.It has significant accuracy when the load is below 500 MPa.Especially,when the load is 350 MPa,the calculation error of the BPNN is only 0.4%.The accurate model of the blade is built by using 3D coordinate measurement technology.The loading cycle in fatigue analysis is defined from takeoff to cruise in 10 min,and the load history is obtained from finite element analysis(FEA).Then the fatigue life of the compressor blade is predicted by using the BPNN model.The final fatigue life of the aero-engine blade is 6.55 104 cycles(10 916 h) based on the BPNN model,which is effective for the virtual design of aero-engine blade.
LIN JieweiZHANG JunhongZHANG GuichangNI GuangjianBI Fengrong
Based on the nonlinear continuum damage model(CDM) developed by Chaboche,a modified model for high cycle fatigue of TC4 alloy was proposed.Unsymmetrical cycle fatigue tests were conducted on rod specimens at room temperature.Then the material parameters needed in the CDM were obtained by the fatigue tests,and the stress distribution of the specimen was calculated by FE method.Compared with the linear damage model(LDM),the damage results and the life prediction of the CDM show a better agreement with the test and they are more precise than the LDM.By applying the CDM developed in this study to the life prediction of aeroengine blades,it is concluded that the root is the most dangerous region of the whole blade and the shortest life is 58 211 cycles.Finally,the Cox proportional hazard model of survival analysis was applied to the analysis of the fatigue reliability.The Cox model takes the covariates into consideration,which include diameter,weight,mean stress and tensile strength.The result shows that the mean stress is the only factor that accelerates the fracture process.
Many simple nonlinear main journal bearing models have been studied theoretically,but the connection to existing engineering system has not been equally investigated.The consideration of the characteristics of engine main journal bearings may provide a prediction of the bearing load and lubrication.Due to the strong non-linear features in bearing lubrication procedure,it is difficult to predict those characteristics.A non-linear dynamic model is described for analyzing the characteristics of engine main journal bearings.Components such as crankshaft,main journals and con rods are found by applying the finite element method.Non-linear spring/dampers are introduced to imitate the constraint and supporting functions provided by the main bearing and oil film.The engine gas pressure is imposed as excitation on the model via the engine piston,con rod,etc.The bearing reaction force is calculated over one engine cycle,and meanwhile,the oil film thickness and pressure distribution are obtained based on Reynolds differential equation.It can be found that the maximum bearing reaction force always occurs when the maximum cylinder pressure arises in the cylinder adjacent to that bearing.The simulated minimum oil film thickness,which is 3 μm,demonstrates the reliability of the main journal bearings.This non-linear dynamic analysis may save computing efforts of engine main bearing design and also is of good precision and close connection to actual engine main journal bearing conditions.
The effects of journal misalignment on a journal bearing caused by an asymmetric rotor structure are presented in this study.A new model considering the asymmetric deflection is applied.Also,the thermo-hydrodynamic of the oil film in the journal bearing and straightforward elasticity theory are considered in the analysis.Based on the structure stiffness equivalent characteristic,a simple stepped shaft can reflect the entire complex structure model.The existing lubrication model,which does not consider this angle component,is not very precise for journal bearings.Film pressure,misalignment angle,velocity field,oil leakage,and temperature field were calculated and compared in the journal bearing analysis.The results indicate that bearing performances are greatly affected by misalignment caused by the asymmetric structure.A simple stepped shaft can effectively represent a misaligned journal bearing in a rotor-bearing system.
To comprehensively consider the effects of strength degeneration and failure correlation, an improved stress-strength interference (SSI) model is proposed to analyze the reliability of aeroengine blades with the fatigue failure mode. Two types of TC4 alloy experiments are conducted for the study on the damage accumulation law. All the parameters in the nonlinear damage model are obtained by the tension-compression fatigue tests, and the accuracy of the nonlinear damage model is verified by the damage tests. The strength degeneration model is put forward on the basis of the Chaboche nonlinear damage theory and the Griffith fracture criterion, and determined by measuring the fatigue toughness during the tests. From the comparison of two kinds of degeneration models based on the Miner's linear law and the nonlinear damage model respectively, the nonlinear model has a significant advantage on prediction accuracy especially in the later period of life. A time-dependent SSI reliability model is established. By computing the stress distribution using the finite element (FE) technique, the reliability of a single blade during the whole service life is obtained. Considering the failure correlation of components, a modified reliability model of aero-engine blades with common cause failure (CCF) is presented. It shows a closer and more reasonable process with the actual working condition. The improved reliability model is illustrated to be applied to aero-engine blades well, and the approach purposed in this paper is suitable for any actual machinery component of aero-engine rotor systems.
