Research On The Thermomechanical Fatigue Behavior And Life Prediction Approaches Of Ni-base Superalloy

For the aeroegines' turbine components,the thermomechancial fatigue(TMF)failures of which are caused by the action of alternating gas temperature and alternating mechanical load,have been a serious threat to the security of aeroegines.In this thesis,isothermal fatigue(IF)tests at 650?,including low-cycle fatigue(LCF)tests and creep/fatigue interaction(C/F)tests,and in-phase(IP)and out-of-phase(OP)TMF tests in the 350~650 ? temperature range are conducted on Ni-base superalloy GH4169,getting widely used in turbine components.The response behaviors in IF and TMF tests are comparatively analyzed and the TMF life prediction approaches are studied.The main work of this thesis is as followed:Investigation on the IF behavior is done.Under the condition of LCF,the cyclic response hysteresis loop is symmetrical about origin.Stress and strain are in the monotonic relation,which means stress and strain increase or decrease at the same time.Also,Manson-Coffin equation can properly describe the LCF life distribution.Under the condition of C/F,strain holding notably changes the fatigue behavior,leading to stress relaxation and life reduction subjected to the same strain level,compared with LCF.In both LCF and C/F tests,GH4169 exhibits cyclic softening followed by being stable.Investigation on the TMF behaviors is done.Results suggest that temperature variation has a strong influence on the fatigue behavior.More harmful damage is produced,resulting in life reduction.The cyclic response behavior is changed,the hysteresis loop being asymmetrical.Unexpectedly,stress and strain still maintain the monotonic relation and no sign of stress decreasing during the loading period,like other Ni-base superalloys exposed to TMF,is observed.Cyclic stress response behavior is changed and the high-temperature half-cycle and the low-temperature half-cycle are different in hardening or softening.In the high-temperature half-cycle,GH4169 exhibits cyclic softening followed by being stable,which is similar to the IF-tests.However,the low-temperature half-cycle remains stable from the beginning to the end.For OP specimen,there is no rapid decrease in stress at the final stage of life.Some improvement is brought to the Walker strain-life model and a TMF life prediction approach is developed.Results suggest that the improved Walker model can properly predict the TMF life of GH4169 under both IP and OP condition by calibrating on isothermal C/F data only,which achieves the goal to predict TMF life by using the IF data from one temperature condition and one load type,and show a better capacity than other phenomenological models and damage-partitioning model.What's more,when used to model other Ni-base superalloys' C/F life in different load types and TMF life in different phases,the improved Walker model also gives good answers and indicates a great ability to character damage.