Study On Stress-weakening Damage And Low Cycle Fatigue Of Ni-based Single Crystal Superalloy

A single crystal nickel-base superalloy has been widely used as a keymaterial for advanced aircraft gas turbine blades owing to its excellentstrength characteristic under elevated temperatures. Fatigue fracture is apervasive problem because of elevated temperature, pressure and rotatingspeed and iterative fatigue loading, as the effect of centrifugal force, theblade will be under stress weaken damage at the same time. Therefore, thestudy of cycle feature and stress relaxation of low cycle fatigue onnickel-base single crystal superalloy under asymmetric cyclic loadings andestablish a suitable model for life predicting have theoretical guidingsignificance and application value for aircraft gas turbine blades fatiguestrength design.Firstly, based on tension-compression and tension-torsion low cyclefatigue test of DD3nickel-based single crystal superalloy, Throughprocessing test data, Drawing the stress-cycle curves and axial/shear stressstrain curves with different crystal orientation, temperature, loading phaseangle, The material’s properties of fatigue cyclic hardening, softeningunder different loading phase angle, the strain loading paths, temperatureand stress relaxation behavior at the high temperature were researchedobviously, the behavior will produce stress-weakening damage whichaffect the fatigue life.Secondly, The numerical simulation of smooth cylindrical specimenunder uniaxial tensile/compressive and thin-walled cylinder specimenunder multiaxial were carried out by using parametric programminglanguage APDL,compare and analyze the stress strain curves with the testresults. Based on the microstructure characteristics of nickel-based singlecrystal alloy, γ/γ’ two-phase multi-cell microscopic scale mechanicalmodel were established by finite element method, the numericalsimulation of multiaxial non-proportional cyclic strain loading was carriedout. This model was avoid the boundary distortion effect of single cellmodel and verified the rationality, use this model to simulation on the stress relaxation behavior and analysis the damage mechanism of materialthe first to be destroyed is matrix phase.Through study on stress relaxation behavior of single crystalnickel-base superalloy deeply, a parameter F which characteristicstress-weakening damage of single crystal nickel-base superalloy was putforward, it has exponential function relationship with fatigue life. Basedon energy dissipation theory, a low cycle fatigue life prediction model forsingle crystal superalloy was proposed by combining with Mises stressrange, the stress triaxiality factor and crystallographic orientation functionbuild cyclic plasticity strain energy as damage parameters, Multiple linearregression analysis of low cycle fatigue test data shows that the model hasa good correlation with the failure cycle, the analysis results of macromodel and micro cell、multi-cell model were used to have a forecast at thesame time.Finally, the author concludes the main work and results in the thesisand outlooks the research work in future.