Low Cycle Fatigue Behavior Of Ultra-fine Grained Titanium With Different Strain Ratio

Ultra-fine grained pure titanium is widely used in aerospace,power generation and human implants.It is a kind of ultra-fine grained material.In the field of aviation and power generation,materials not only need to work under high stress to bear fluctuating load,but also to ensure a certain service life,which requires its high fatigue strength.As a result,it is of great practical significance to study the low cycle fatigue life of ultra-fine grained pure titanium under asymmetric loading.Ultra-fine grained pure titanium was prepared by equal channel angular pressing(ECAP)+ rotary swaging(RS).After refining,the grain size reached nanometer level,and the tensile strength and yield strength of the material were greatly improved.At room temperature,strain controlled low cycle fatigue tests with strain ratios of-1,-0.5 and 0.5 were carried out on Ultra-fine grained pure titanium.Five strain amplitudes(0.4%,0.6%,0.9%,1.0%,1.2%)were selected for each strain ratio.The microstructure was observed by TEM.The fracture mechanism was observed and analyzed by SEM.In this paper,the effects of strain ratio on cyclic hardening softening,cyclic stress-strain relationship and fatigue life of materials are studied.The results show that the higher the strain ratio,the lower the low cycle fatigue life.The fatigue life prediction model and mean stress relaxation model under asymmetrical stress load are discussed.The experimental results show that the strain ratio has a significant effect on the low cycle fatigue performance of ultra-fine grained pure titanium,and the traditional Manson coffin model can not accurately predict the fatigue life under asymmetric stress load.Therefore,the SWT mean stress correction model and three parameter power curve model are proposed in this paper,and the experimental results are verified.The final research shows that the three parameter power surface model has better representativeness.By studying the mean stress relaxation phenomenon under the condition of R ≠-1,it is revealed that the stress ratio and the strain amplitude are the factors that significantly affect the mean stress relaxation rate,and the mean stress relaxation model with the two variables is calculated to describe the mean stress relaxation phenomenon of Ultra-fine grained pure titanium under different strain ratios.The microstructure of the material was observed by transmission electron microscope.The results show that under the condition of low cycle fatigue and high strain ratio,the size and number of sub crystalwas small and large.The fracture morphology of the sample was observed by scanning electron microscopy,and the instantaneous fracture area of the fatigue fracture of ultra-fine grain pure titanium was found to be a mixture of ductile fracture and quasi-cleavage fracture.At the same strain rate,the dimple becomes larger and deeper.It can be seen that the toughness of the material increases with the increase of strain rate.With the increase of strain ratio,the crack sources of ultra-fine grained pure titanium increase,and with the increase of strain amplitude,the crack sources of Ultra-fine grained pure titanium also change from one crack source to multi-source failure,so the increase of strain amplitude and strain ratio both lead to the increase of fatigue crack sources.