| In recent years,with the rapid development of fuel oil market,the technical requirements of residual oil purification process are becoming higher and higher.The furnace tube of the heating furnace is in the bad working environment for a long time and faces the critical life period.The fatigue failure of the furnace tube will cause great harm to the enterprise and personnel.Therefore,scholars at home and abroad have done a lot of research on the fatigue life evaluation of materials,but there are few related researches and conclusions on the high temperature and low cycle fatigue of materials under multi-dimensional stress state,so the research in this paper has a certain practical significance in engineering.In this paper,TP347H is selected as the low cycle fatigue test material.Combined with the surface crack observation technology,ANSYS finite element simulation technology,MATLAB data processing technology and other methods,the fatigue failure mechanism of TP347H is studied,and its high temperature and low cycle fatigue characteristics and life are evaluated.In this paper,TP347H material is first tested for low cycle fatigue at room temperature.The cylindrical specimen with middle ring notch is processed to form a complex three-dimensional stress environment.After polishing,the specimen with notch radius r=4 is tested for low cycle fatigue with strain control under 0.2%strain amplitude.The real-time observation of crack initiation and propagation at the notch during the test is carried out by using the interruption test method The mechanism of fatigue failure in the test is studied.It is found that when n=7000 and N=8000,there are two cracks.When n=10000,the two cracks are combined with the short cracks around them and further expand.When n=10480,the two cracks are combined into the main cracks and the specimen fails.Then,the high temperature and low cycle fatigue tests of TP347H were carried out under different strain amplitudes(0.1%,0.15%,0.2%,0.25%,0.3%)at 550?,to explore the influence of strain amplitudes on the fatigue life of the specimens,as well as the cyclic stress response and strain cycle behavior during the fatigue cycle.It is found that with the increase of strain amplitude?_a,the total fatigue life of the material decreases gradually,showing a more obvious downward trend,With the increase of strain amplitude,the decreasing range of fatigue life of the material decreases gradually.When the strain amplitude is 0.3%,the fatigue life is basically unchanged;the material shows the characteristics of cyclic hardening and cyclic softening alternately in the whole process of high temperature fatigue test,and secondary hardening occurs;TP347H material shows better strain cyclic behavior in the process of high temperature fatigue test.Finally,the Von Mises criterion is used as yield condition,and ANSYS Workbench software is used to model,mesh and calculate the 25 mm part of notch specimen in gauge distance,to determine the dangerous point of specimen.Combined with Manson coffin formula,the equivalent strain range ?(?) and the equivalent plastic strain range ?(?) are used to evaluate the fatigue life N_f of TP347H in multi-dimensional stress state.The equation of high temperature and low cycle life of TP347H in multi-dimensional stress state is fitted in MATLAB.The results show that the equivalent strain range?(?) and the equivalent plastic strain range?(?) in double logarithmic coordinate system has a good linear relationship with the fatigue life of TP347H material,so that the evaluation of high temperature and low cycle fatigue life of TP347H material under multi-dimensional stress state can be realized. |
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