Research On Thermomechanical Fatigue Behavior And Life Prediction Approaches Of Nickel-based Superalloy

The high-temperature components of aero-engines always serviced in the environment of thermal and mechanical cyclic load,which would induce thermal mechanical fatigue and threatens the safety of aero-engines.Therefore,the research on the thermomechanical fatigue has important theoretical significance and practical value.In this paper,the Chaboche unified viscoplasticity constitutive model under isothermal and variable temperature conditions was established to predict cyclic stress-strain behavior of IN718 superalloy.Meanwhile,the thermomechanical fatigue life prediction method of IN718 superalloy was studied by the improved three-parameter energy function.The main research results of this paper are as follows:(1)The isothermal and variable temperature Chaboche unified viscoplasticity constitutive model was built.A step by step method was implemented to determine the initial values of the constants for the viscoplasticity model,then the initial values were optimized by genetic algorithm.The constants were fitted to a temperature correlation function and applied to the variable temperature Chaboche unified viscoplasticity constitutive model.Then,the established constitutive model was used for finite element simulation under isothermal and variable temperature.(2)The effects of temperature,strain amplitude,loading rate and asymmetric loading on the isothermal cyclic stress-strain relationship of IN718 superalloy were investigated.Under symmetrically cyclic loading,the hysteresis loop of the superalloy is symmetric about the coordinate origin,and it exhibits a characteristic of cyclic softening first and then stablizing.As the temperature increases,the ability to resist deformation of the superalloy decreases,and the higher the strain amplitude,the more energy consumed by the deformation.The more the loading rate increases,the more the cyclic stress range increases.Under asymmetric cyclic loading,the superalloy hysteresis loop is no longer symmetric about the coordinate origin,and the half-cycle of higher strain amplitude exhibits a characteristic of higher cyclic softening.(3)The research on cyclic stress-strain relationship of IN718 superalloy under variable temperature loads was conducted.The results show that the stress of the superalloy under variable temperature loads is higher than that of isothermal loads;The hysteresis loop of the superalloy is no longer symmetric about the origin,the average stress is always biased towards the low temperature half cycle,and the inelastic strain mainly happens at the high temperature half cycle;The superalloy exhibits cyclic softening,but cycle stability stage is not obvious;as the loading rate increases,the cyclic stress response decreases.(4)A method for predicting the thermomechanical fatigue life of IN718 superalloy was developed by introducing a total strain energy to improve the three-parameter energy function.The research shows that the elastic strain also plays an important role in the thermomechanical fatigue damage.Therefore,the three-parameter energy function is improved by comprehensively considering the total strain energy of elastic strain and inelastic strain.the influence of phase relationship of strain-temperature and temperature range were considered in the improved model,which comprehensively reflects the asymmetry of hysteresis loop and can effectively predict the thermomechanical fatigue life of IN718 superalloy.