Study On Low Cycle Fatigue Of DD3Ni-based Single Crystal Superalloy Under Multiaxial Non-proportional Loadings

A single crystal nickel-base superalloy has been widely used as akey material for advanced aircraft gas turbine blades owing to itsexcellent strength characteristic under elevated temperatures Fatiguefracture is a pervasive problem because of elevated temperature，pressureand rotating speed and iterative fatigue loading.Therefore，researchedlow cycle fatigue on nickel-base single crystal superalloy undermultiaxial loadings，established a suitable model for predicting hastheoretical significance and application value for aircraft gas turbineblades fatigue strength design.Firstly，the paper present orthogonal experimental design（OED）method for the thin-wall cylindrical tensile-torque experiment schemes ofDD3single crystal Ni-based superalloy.It was the first time that thetension-torsion experiments at680℃and850℃on DD3thin-wall tubespecimens with 〈001〉 orientation were successfully completed ininteriorly.Based on the low cycle fatigue test results，Study on theequivalent strain range，strain path angle， tension/torsion load phaseangle，cycle characteristics and temperature factors of nickel-base singlecrystal superalloy of multiaxial low cycle fatigue life influence，Secondly， combining with the microstructure characteristics ofnickel-based single crystal alloy，according to the test data，the stress andstrain of the macroscopically smooth thin sample and γ/γ’ two-phasesingle cell microscopic scale mechanical model were analyzed by the3Delastic-plastic finite element method.Results of microscopic unit cellmodel shows that the maximum equivalent stress range and the maximumequivalent strain range appears in the phase angle，precipitation phase ofmaximum equivalent strain range and the maximum equivalent stressrange appears in γ/γ’ two intersecting surface fillet transition，and themicroscopic single cell model results has the small error compared withthe test results.macro and the micro finite element model，and low cycle fatigue test data at680℃and850℃were applied to fit the low cycle fatigue life modelby multiple linear regression analysis.The results show that thecalculation accuracy of the unit cell model is better than the macro modelsignificantly.Fourthly，dividing diamond strain loading path into proportionalloading and non-proportional loading segment，the equivalent strain rangeparameter to characterize the effects of non-proportional loading wasproposed，and a stress triaxiality factor for single crystal superalloy wasintroduced to reflect the tension/torsion strain path angle on multiaxialfatigue life.A formula of cyclic plastic strain energy, which is composedof the equivalent strain range taking account of non-proportional loadingeffect，equipment stress range and the stress triaxiality factor was putforward as failure parameter.Multiple linear regression analysis showthat a power law of the failure parameter has a good correlation with thefailure cycle， and all the test data fall into the factor of2.0scatter band.Finally，the author concludes the main work and results in the thesisand outlooks the research work in future.