Research And Application Of Creep Damage Assessment Method Under Variable Load History

In order to solve the problem of equivalent acceleration of creep damage in the compilation of load spectrum of aeroengine acceleration mission test,the creep problems of metal materials at high temperature are comprehensively investigated.The creep constitutive model,micro mechanism of deformation and fracture and creep damage accumulation model are analyzed.On this basis,the high temperature tensile and creep tests of GH4169 alloy,a common high temperature material for engine,were conducted at elevated temperature to study the tensile and creep properties and their relationship.Then,the microstructure of GH4169 alloy was studied by Metallographic Microscope,Scanning Electron Microscope and Electron Backscatter Diffraction.The crystal structure characteristics and dislocation density of GH4169 alloy before and after creep were revealed,and the microstructure morphology and corresponding damage state from the original state to different creep stages were obtained.A constitutive model which can describe the three stages of creep is established based on the study of macroscopic test law and micro deformation mechanism.Finally,the application of the constitutive model in the commercial finite element software is realized by the joint compilation software.Then,based on the analysis of the existing creep damage accumulation models,a new nonlinear creep damage assessment method is proposed.The influence of loading history on creep damage of materials can be considered.The effectiveness of the new model is evaluated by variable load creep tests of TC11 alloy and GH4169 alloy.Then,based on the principle of consistent damage parameters,the creep damage equivalent criterion of materials is preliminarily established,which can be applied to the accelerated creep damage equivalence.Finally,in order to consider the effect of local multiaxial stress state and stress concentration on the high-temperature properties of the material,three notched bar specimens were used to simulate the local stress characteristics of the actual structure.High temperature tensile and constant load creep tests were carried out,and the influence of notch on the tensile and creep properties was analyzed.The finite element analysis of notched specimen is carried out by using the creep constitutive model developed previously,and the relationship between stress and strain in notched specimen with time is obtained.Combined with the results of finite element analysis,creep performance evaluation model and creep damage accumulation model,the damage assessment method of notched specimen under variable load creep condition is developed.Finally,the damage assessment method is verified by the creep test of notched specimen under variable load.Based on the above research,the main conclusions of this paper are as follows:（1）The tensile curves of GH4169 alloy at 650℃show obvious nonlinear hardening plasticity characteristics.When the material enters into plasticity,it reaches its tensile strength quickly.The creep curves show typical three-stage characteristics,but the proportion of the first stage of creep to the whole life is less than 10%,and the accumulated creep strain in the first and second stages is less than 2%,so the main creep strain accumulation occurs in the third stage.In addition,the modified Wilshire equation based on the normalization of tensile parameters proposed in this paper can well characterize the creep properties of various high-temperature materials.The temperature induced creep property changes are considered in the parametersQ_c^*,k’₁,u’,k’₂ andn’of the equation,and the extrapolation prediction error of creep life of GH4169alloy is within 30%.（2）In the GH4169 material studied in this paper and the test conditions,the microstructure of most regions of the samples after creep has no obvious change.With the increase of creep life,the density of GND decreases,and large plastic deformation and microcracks are concentrated in the material near the fracture surface.At 650℃,the cracks initiate near the surface in intergranular mode and continue to propagate along the grain boundary.However,when the stress increases gradually,the number of holes and cracks in the material will decrease.In addition,oxidation plays an important role in the creep process,and when the deformation accumulation reaches a certain degree,cracks tend to initiate near the oxide layer.When the crack penetrates the oxide layer,oxygen diffuses along the crack and forms oxide particles on the grain boundary,which affects the creep crack growth performance of the material.（3）According to the results of creep test and the conclusion of micro analysis,the new creep constitutive model can describe the three stages of creep curve,and can well reflect the relationship between creep properties and stress,and also can characterize the relationship between creep life and stress.In addition,compared with theθmapping method,BAT model and CMD model,the new model has the best agreement with the experimental data in the characterization of creep properties and deformation curves of TC11 alloy and GH4169 alloy.Finally,on the basis of the above model,the finite element subroutines with different creep constitutive models are compiled and linked with commercial finite element subroutines to form an executable finite element program.（4）Based on the existing creep damage assessment methods under variable load history and the damage assessment model developed by material creep performance characteristics,the influence of load history on material creep damage is considered.Different creep damage assessment models are evaluated by using variable load creep test data of TC11 and GH4169.The results show that the new creep damage accumulation model can better predict the residual life of TC11 alloy and GH4169 alloy,but the time fraction method or strain fraction method is difficult to accurately predict the creep damage under variable load history.Based on the damage accumulation model,the creep damage equivalent criterion of the material is preliminarily established.（5）Under the research materials and experimental conditions,based on the creep damage cumulative model developed in this paper and finite element analysis of creep constitutive model,a creep damage assessment method for notched specimens is proposed,which can consider the load history and the effect of notch.The effectiveness of this method is verified by the creep test of notched specimens under variable load.The results show that the proposed method can well characterize the creep damage of notched specimens and predict the residual life within 35%.