Effect of Strain Ratio on Fatigue Model of Ultra-fine Grained Pure Titanium

The ultra-fine grained(UFG) pure titanium was prepared by equal channel angular pressing(ECAP) and rotary swaging(RS). The strain controlled low cycle fatigue(LCF) test was carried out at room temperature. The fatigue life prediction model and mean stress relaxation model under asymmetrical stress load were discussed. The results show that the strain ratio has a significant effect on the low cycle fatigue performance of the UFG 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, and the test results are verified. The final research shows that the threeparameter power surface model has better representation. 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 afiect 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 the UFG pure titanium under different strain ratios. The fracture morphology of the samples was observed by SEM, and it was concluded that the final fracture zone of the fatigue fracture of the UFG pure titanium was a mixture of ductile fracture and quasi cleavage fracture. The toughness of the material increases with the increase of strain ratio at the same strain amplitude.