The Fatigue Life Characterization And Fracture Mechanism Of TC4-DT Titanium Alloy

Titanium and its alloys have been widely used due to their excellent properties such as low density,high specific strength and corrosion resistance.As a commonly used titanium alloy,TC4 titanium alloy has reached 50% in structural parts.Compared with the traditional TC4 titanium alloy,TC4-DT titanium alloy has attracted much attention due to lower interstitial element content,higher fracture toughness and lower crack growth rate.Fatigue failure is the most dangerous failure mode in the failure mode of materials.Therefore,it is important to study the fatigue behavior of TC4-DT titanium alloy.In this paper,the high-cycle behavior at room temperature and the low-cycle behavior at room temperature of TC4-DT titanium alloy were studied.The fatigue life was quantitatively characterized by different models.Then the fatigue fracture mechanism was analyzed.Finally,the problem of heteroscedasticity of fatigue life at different stress levels is solved by processing the small sample data of fatigue life.High cycle fatigue tests with stress concentration factors of 1,3,and 5 and stress ratios of-1 and 0.5 were performed at room temperature.The results show that the reduction of stress ratio or the increase of stress concentration factor will reduce the fatigue limit,the fatigue life S-N curve will move down,and the smaller the stress concentration coefficient,the more obvious the influence of stress ratio on S-N curve;Based on the data of-1,the fatigue life prediction model of power function,exponential,Zheng formula and three-parameter power function is established.The results show that the three-parameter power function model has better prediction performance.The three-parameter power function model is used.The data under other loading conditions were fitted,and the data under different stress ratios were normalized.The fatigue life prediction model with the stress ratio equivalent of-1 was established,and the model accuracy was good.The high-cycle fatigue fracture mechanism of T4-DT titanium alloy was studied.It was found that the crack source was induced on the surface of the sample when the stress ratio was-1,which was caused by the dislocation slip mechanism.A cracked specimen is initiated on the surface of the specimen and inside the specimen at a stress ratio of 0.5,which can be explained by a cleavage mechanism.Fatigue strips and secondary cracks in the fatigue extension zone are caused by cyclic loading and excessive loading of the part.The last instantaneous break zone has a dimple,and the inner hole gradually becomes larger,which is a microporous polymer type fracture.The low-cycle fatigue test with strain amplitude of 0.6%~1.4% was carried out at room temperature.The data showed that the cycle number decreased significantly with the increase of strain amplitude.When the strain amplitude was 0.6%,the cycle number was about 20,000 times.When the strain amplitude is 1.4%,the cycle number can still be maintained 1000 times.The fatigue life is better than that of ordinary TC4-DT titanium alloy and basket-shaped TC4-DT titanium alloy.A three-parameter Weibull distribution model is established,which is iterated by right-forced method.The position parameters,shape parameters and size parameters of the Weibull distribution are obtained.The shape parameter is 26.262,which is close to the low cycle fatigue limit.It is in good agreement with the experimental data.The correlation coefficient is 0.999.That is,the three-parameter Weibull model accurately describes the low cycle fatigue life.The Manson-coffin equation and the three-parameter power function model are established to quantitatively characterize the low-cycle fatigue life.Both models can accurately describe the low-cycle fatigue test at room temperature.The correlation coefficient of the three-parameter power function model is greater than 0.9,higher precision.The effect of strain amplitude on the low-cycle fatigue fracture mechanism is studied.As the strain amplitude increases,the crack source changes from the surface of the sample to the internal initiation of the material.When the strain amplitude is 1.4%,the crack source appears.This is due to the presence of inclusions in the second phase;there are obvious dissociation steps,fatigue bands and secondary cracks in the crack propagation zone,a large number of dimples exist in the transient zone,and the density of the dimples increases with the strain amplitude.The depth increases and belongs to typical ductile fractures.The sample capacity was expanded by Bootstrap method,and the power function model was regression analysis using the data obtained by traditional group method and self-help method.The P-S-N with reliability of 50%,84.13%,95% and 99% was obtained.Curve;analyzing the P-S-N curves obtained by the two methods,it is found that the data variance obtained by the self-help method is smaller and more stable,and the fatigue life estimated by the P-S-N curve is safer than the fatigue life estimated by the PSN curve of the traditional group method.Bootstrap method can significantly improve the problem of fatigue life data dispersion and improve the accuracy of life prediction.Based on Bootstrap method,the data is weighted by least squares method,and the covariance matrix and P-S-N under different loading conditions and different reliability are obtained.In the curve linear regression model,after the weighted least squares method is applied,the correlation coefficient R of the fitted P-S-N curve is larger,between 0.987 and 0.991,and the residual distribution is uniform,effectively eliminating heteroscedasticity.