Low Cycle Fatigue Lifetime Grey Prediction Of Nickel-based Single Crystal Superalloy Based On Micro Cell Model

The nickel-based single crystal superalloy has been widely used tomake blade of aircraft gas turbines in China and US industry. The low cyclefatigue damage of blades occurs under frequently start-stop cyclic loadings.The study of low cycle fatigue damage mechanism and lifetime predictionmodels are always the hot and difficult problems at home and abroad. Inthis paper, theoretical models, numerical simulation analysis andexperimental tests were used to study the low cycle fatigue characters ofDD3nickel-based single crystal superalloy. A micro unit cell finite elementmodel was firstly established, after deep investigation of the method of theelastic-plastic mechanical behavior and low cycle fatigue properties ofnickel-based single crystal superalloy at high temperature. GreyRelationship analyses were then used to find the major factor of affectingthe fatigue lifetime. A GM (1, N) model was built to predict the low cyclefatigue lifetime and to improve the accuracy of fatigue lifetime prediction.The main content of the paper is as follows,1, From a micro-structure view of point, a micro unit cell finiteelement model of single crystal superalloy was established. The model wasthen analyzed under the single axial loadings. The results showed that thematrix and precipitated phase interface are in complex stress condition, andthe yield intensity of the precipitated phase is much higher than the matrixphase. The fatigue damage starts from the matrix phase and the precipitatedphase improves the macro single crystal alloy material strength. Theaverage stress deduced by the law of composite material mixturecalculation formula is consistent with the uniaxial tension test stress value,which verifying accuracy of the single cell finite element model.2, For DD3single crystal nickel-based alloy thin-walled cylindricalspecimens multiaxial low cycle fatigue test respectively at680℃hightemperature, the macro/micro finite element model (Tension-Compression,Tension-torsion) was analyzed under cycle loading thermal elastic-plasticnumerical simulation. Results show that finite element simulation results ofthe micro-unit cell model is much closer to the experimental results.3, Based on the grey relation analysis method, the correlation effects were analyzed with the use of nickel base single crystal alloy low cyclefatigue test data, and the main factors of fatigue lifetime were determined.4, A grey GM (1, N) life prediction model was established based onthe single cell model and the grey theory. DD3nickel base single crystalalloy single axial/multiaxial low cycle fatigue test data were used tovalidate Grey Model. The average residual error is within10%underverification of residual error, and all single axle and multiaxial fatigue testdata are falling in the2.6,2.3times scatter band, this shown that singlecrystal alloy low cycle fatigue GM (1, N) life prediction model predictionaccuracy is higher, can be used for nickel base single crystal alloy lowcycle fatigue life prediction. The method provides a new way to study thelow cycle fatigue lifetime prediction of Nickel-based single crystalsuperalloy.