Fatigue Life Prediction Research Of Turbine Rotor Grooves Based On Mixed Hardening Constitutive Model

The rotor disc of steam turbine is mainly subjected to the alternating action of the centrifugal load of the blade during the service process.This paper focuses on the shortcomings of the empirical model of average stress relaxation used in the current fatigue life prediction of structures,based on the local stress-strain method,the transient mechanical response and the average stress dynamic relaxation behavior of the disk material under the action of fluctuating alternating load are studied by using the mixed hardening constitutive model(N5L1).Based on this,a method for predicting the fatigue life of rotor wheel disc considering the dynamic relaxation of the average stress at the notch root of the wheel groove is studied.Therefore,it has important theoretical and practical significance.In this paper,the cyclic deformation behavior and average stress relaxation law of the disk material under different strain amplitude are studied based on the existing low-cycle fatigue test data of the smooth sample of the disk material.The results show that there is a significant average stress relaxation in the wheel material during the cycle,and the initial average stress decreases and the average stress relaxation rate increases with the increase of the strain amplitude.Through the analysis of the hysteresis loop of the wheel material under different strain amplitude,it is found that the wheel material shows the characteristic of cyclic softening with the cycle under the same strain amplitude.Two methods(translation method and bausinger strain fitting method)were used to further prove that the roulette material conforms to Mashing characteristics.Based on Chaboche N5L1 mixed hardening constitutive model,combined with the genetic algorithm(GA),through parameter optimization,this paper studies the wheel material back stress and isotropic hardening parameters on roulette material cyclic stress-strain curve and the effect of hysteresis loop,under different strain amplitude and the initial average stress relaxation,hysteresis loop and steady state simulation,the results show that: after parameter optimization,five back stress model is better than one,four,six levels of back stress model,overlay linear item 5 back stress model(N5L1)is better than not overlay linear scale back stress model.This provides a basis for further research on the fatigue life prediction method of rotor disk by using the mixed hardening constitutive model(N5L1).In this paper,the mixed hardening constitutive model(N5L1)and the average stress relaxation empirical model(Landgraf,MaxWell and Kodama formulas)were used to simulate the average stress relaxation behavior of the wheel material under pulsating loading and the five-order strain amplitude(0.4%,0.5%,0.6%,0.8%,1%),and the results were coMPared with the experimental results.The results show that the mixed hardening constitutive model adopted in this paper can simulate the average stress relaxation behavior more accurately than the empirical model of average stress relaxation.Based on the basic principle of local stress-strain method and combined with the mixed hardening constitutive model(N5L1),the uniform method was used to determine the local stress and strain of the notch and fatigue damage are calculated by use of a SWT model and the linear cumulative damage rule,this paper further studies to establish a given wheel groove root average stress dynamic relaxation of the rotor wheel fatigue life prediction method.Through the test of a certain type of wheel groove components and the model based on the average stress relaxation experience(Landgraf,Kodama,MaxWell model)method coMParison showed that based on the mixed hardening constitutive model(N5L1)the fatigue life prediction and experimental life overall control error between 7-25%(within 1.5 times of the dispersion band),and based on the average stress relaxation experience model prediction error with the test results about life is between 50-130%.Therefore,the fatigue life prediction method based on the mixed hardening constitutive model(N5L1)established in this paper has higher prediction accuracy,which is worthy of attention and recommendation in the anti-fatigue design of turbine rotor disc.