Investigation On Low Cycle Fatigue Life Prediction Method Of Single Crystal Turbine Blades Based On Critical Plane Approach

Single crystal nickel-based superalloys,with their superior high temperature fatigue properties,are widely used in gas turbine blades and vane materials.However,because of the material's anisotropic characteristics,fatigue life estimation of single crystal nickel-based superalloys is closely related to crystallographic orientation and rather complex.At present,it is still deficient in fatigue life prediction method which is suitable for single crystal fatigue damage mechanism as well as engineering application.Based on the background above,the low cycle fatigue damage problem for high turbine during the gas turbine start-up periods were discussed in this paper.The main research contents are as follows:1.For the high pressure turbine during the gas turbine start-up periods,a numerical simulation method based on conjugate heat transfer was presented,which a boundary parameters determination method was studied and validated by total gas turbine testing.The results showed that this numerical method is reasonable and effective,and could reflect accurately the flow and temperature fields variation of turbine blades during the the gas turbine start-up periods.2.The thermal-structure conjugate numerical simulation method was performed based on conjugate heat transfer results which using heat transfer coefficient and fluid temperature as thermal boundary conditions.The turbine blades transient temperature and stress fields method was studied,moreover,the effect of cooling structure on temperature and stress was analyzed.For different cooling structure guide vane,the maximum thermal stress of hub ejection was smaller than that of trailing edge ejection.3.Based on method combining the critical plane with crystallographic slip theory,an anisotropic low cycle fatigue life model,which considering the maximum resolved shear stress amplitude,resolved shear strain amplitude,normal strain,and normal strain ratio,was proposed.Besides,another anisotropic low cycle fatigue life model considering the maximum resolved shear stress amplitude,resolved shear strain amplitude,the mean stress and stress weakening damage under asymmetrical cyclic loading was also proposed in this paper.Validation results indicated that for two developed models,the life predictions agreed well with experimental results for verified materials,and they were found to be applicable not only for uniaxial loading but also for multi-axial loading.4.Based on linear cumulative damage theory and critical plane method,the low cycle fatigue life models were applied for the gas turbine high pressure turbine life prediction.Wu model was applicable for turbine guide vane and disk life prediction,the model considering the asymmetrical cyclic loading factor was more suitable for rotor blades fatigue damage.