| Some key hot-section components represented by aero-engine turbine discs are subjected not only to constant loads during dwell periods,but also to cyclic loads during starts-up and shut-down periods.Additionally,these components are always affected by severe creep-fatigue interactions,posing a new challenge to life design and prediction methodology for such components.However,research on creep-fatigue life prediction for hot-section components starts late in China,especifically in the aspects of material system,theoretical system and technical system,as well as the unclear understanding of structural strength and reliability in the pre-study.Focusing on this issue,the research constents are presented in this paper from three levels:material behavior,life model and extensive structural application.The creep-fatigue macroscopic mechanical behavior of nickel-based superalloy GH4169 at 650 0C is firstly investigated by a large number of experiments.Then,creep-faitigue damage mechanisms under different loading types are revealed by advanced microscopic characterization techniques.Secondly,a new life prediction model based on strain energy density dissipation criterion is developed,and the following life theory is perfected by considering oxidation effect under complicated loading conditions.Thirdly,a new numerical procedure is proposed to predict creep-fatigue life under multi-axial stress states based on finite element analysis.The proposed life theory is further applied by means of a certain type of aero-engine turbine disc.The main conclusions have been drawn as follows:(1)A series of high-temperature monotonic tensile tests,creep tests,pure fatigue tests and creep-fatigue tests are carried out for nickel-based superalloy GH4169 at 650 ?,so as to bulid a relatively complete database.Experimental results show that this alloy has high-temperature tensile properties of high strength and low ductility,an excellent resistance to creep deformation,and is superior to Inconel 718 in terms of creep-fatigue resistance.Moreover,a BIC criterion that takes into account both model complexity and prediction accuracy in the demonstration of the creep-fatigue life model for GH4169 alloy is introduced.Results show that energy-based frequency modified-damage function model and strain energy partitioning method have the strongest comprehensive prediction capablities.(2)Microstructure characterization tests,macroscopic creep-fatigue tests and post failure analysis tests are conducted for radial forging formed GH4169 superalloy at 650 °C at different sampling locations.Significant inhomogeneous microstructures can be observed for radial forged dics from core to edge.The length frequency of ?3 special grain boundaries is the main microscopic factor affecting the creep-fatigue performance.In detail,the length frequencies ?3 special grain boundaries are the highest in the innermost locations among different sampling locations,leading to highest creep-fatigue lives under the same loading conditions.From the fracture morphology,the microscopic characteristics and statistical analysis of secondary cracks,the tensile hold times can result in typical creep-fatigue damage,while the compressive hold times fatigue-oxidation damage and the tensile-compressive-both hold times creep-fatigue-oxidation interactions.(3)With the combination of a linear damage summation and an energy dissipation criterion,a high accuary modifided strain energy density exhaustion method based on Takahashi's model is developed by considering the healing effect of compressive mean stress.A series of creep-fatigue life data points are collected from different materials at different temperatures.Prediction results show that all the creep-fatigue data fall into a range within a scatter band of± 2.5,in particular 99%of the data located in a scatter band of ± 2.Furthermore,with the help of a cycle-by-cycle concept,a time-dependent creep-fatigue interaction diagram is developed.Thereafter,life prediction abilities are further improved covering the loading conditions of small strain ranges and long hold times.(4)Creep and oxidation damage mechanisms are revealed and the generalized strain energy density exhaustion method under complicated loading conditions is perfected in relation to predicting creep-fatigue-oxidation lives.A series life data points under different loading types are collected from dif'ferent materials at different temperatures.Prediction results show that all the data fall into a range within a scatter band of ± 1.5.In addition.the oxidation dimension is included on the basis of the traditional creep-fatigue interaction diagram.Based on this consideration,a creep-fatigue-oxidation damage interaction diagram is proposed,and a three-dimensional enveloping surf-ace is presented simultaneously.(5)A new numerical procedure containing a modified unified viscoplastic constitutive model.a multi-axial fatigue damage model and a multi-axial creep damage model is developed.The feasibility of this numerical procedure is validated throughout a series of single-edge-notch creep-fatigue tests.Life prediction results show that all the data fall into a range within a scatter band of ± 1.5.According to stress-strain behavior of typical nodes around the notch root region and the features of the proposed multi-axial creep-fatigue damage models,the simulation results show that the crack initiation sites of fatigue-dominated specimens are located at notch root surface,while those of creep-dominated specimens are located at interion region around notch root.This phenonmenon is in consistency with the corresponding metallographic observations via the electron backscatter diffraction technique.(6)With the combination of non-unfied creep-fatigue constitutive equations and the proposed multi-axial creep-fatigue damage models,fatigue and creep damages in the steady state cycles are assessed in a certain type of aero-engine turbine discs.It is determined that the creep-fatigue crack initiation sites are usually located at structural geometric discontinuities. |
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