Internal Stress Based Investigations Of Low Cycle Fatigue Behavior And Life Prediction Energy Model Of Ti-6A1-4V Alloy

The rapid developments of nuclear power industry call for a higher requirement to the length of last stage long blades of steam turbines.Increasing the blade length can significantly improve the efficiency of the steam turbine,but at the same time,it raises the risk of low cycle fatigue failure.The equiaxed Ti-6A1-4V alloy is the main material of the new generation steam turbine blades in China,although a great deal of research have been conducted on the low cycle fatigue properties of the material at home and abroad,the low cycle fatigue mechanism of Ti-6Al-4V alloy,the fatigue behavior,deformation and failure mechanism of the turbine last-stage long blade at service temperature are still lack of deep systematic research work.Thus,in this paper we focuse on the service conditions of a newly developed Ti-6A1-4V alloy long blade for a nuclear power turbine,and studies its low cycle fatigue behavior and life Prediction model at room temperature and 250?,based on the relationship between the evolution of internal stress and the microstructures during the low cycle fatigue.The main purpose of this paper is to provide a more solid experimental basis and theoretical analysis model for the fatigue resistance design and life prediction of the long blades,so it has important practical significance in engineering.At first,the low cycle fatigue test of Ti-6A1-4V alloy at room temperature and 250?was carried out,the variation characteristics of internal force(back stress and friction stress)of titanium alloy with fatigue cycle at two temperatures were obtained,and it was revealed that the cyclic softening behavior of the titanium alloy at room temperature and 250? was closely related to the inner stress evolution.The cyclic softening of the material under the low strain amplitude(?pa?0.2%)is the result of the back stress cycle hardening and the frictional stress cycle softening,while the cyclic softening under the high strain amplitude(?pa>0.2%)is caused by the common softening of the back stress and the frictional stress.At the same time,it is found that the service temperature has a significant effect on the frictional stress and less on the back stress in the range of the test temperature involved in this paper.Based on the analysis of the Ti-6A1-4V alloy microstructure and the digital image correlation(DIC)of grain size deformation in the process of fatigue,the micro-mechanism of stress change in titanium alloy at room temperature and 250? was studied.The results show that the cyclic hardening/softening of back stress in the process of low cycle fatigue is closely related to the uneven degree of microstructure deformation.Under the condition of low strain amplitude(?pa?0.2%),the degree of deformation of the ap grains and the ?/?between the two phases increases with the increase of cyclic cycle,which leads to the back stress cycle hardening.Under the high strain amplitude(?pa>0.2%),the plastic deformation between the ap grains and the a/(3 two phases tends to be homogeneous,which leads to the softening of the back stress cycle.The main reasons for the bilinear coffin-manson curve of ti-6al-4v alloy under low cycle fatigue are further analyzed.According to the theory of Cottrell dislocation and the principle of fatigue energy consumption,this paper separates the cyclic plastic work Wp from the back stress plastic work WpB and the frictional stress plastic work WpF,The variation characteristics of the two in the process of fatigue are studied,and the results show that the cumulative back stress plastic work in a cycle is consistent with the evolution of energy storage in the cyclic process.Combining the classical energy storage model of Tanaka and Skelton,this paper further analyzes the correlation between the back stress plastic work and the energy storage,and probes into the physical mechanism of the reverse loading energy release.Based on the energy dissipation of the fatigue damage process and the physical meaning of back stress plastic work,this paper establishes a new fatigue crack initiation energy model and a fatigue crack growth energy model based on the back stress plastic work as the fatigue damage energy consumption parameter.The prediction accuracy of the above model is validated by the results of low cycle fatigue test and crack propagation test of Ti-6Al-4V alloy at room temperature and 250?.Compared with the existing classical fatigue life Prediction model,it is shown that the fatigue energy model established in this paper has higher prediction precision.Therefore,based on the life prediction model of back stress plastic work in the fatigue process,the low cycle fatigue design and life prediction of long blades of steam turbines can be recommended.