Theoretical Approach Study And Numerical Simulation For Fatigue Life Of Titanium Alloy Welded Structures

Titanium and its alloys,as an important structural metal material,are widely used in the design and manufacture of rail vehicle structures.With their excellent properties such as low density,high specific strength and high corrosion resistance,they have made outstanding contributions to the lightweighting of trains.Fatigue failure of welded structures is one of the most dangerous failure modes during the service of components,so it is crucial to study the fatigue behavior of titanium alloy welded positions.In this paper,two assessment methods are improved and characterized for fatigue life,considering the influence of the own stress singularity of the titanium alloy welded structure and the structure dimensions on the fatigue life,respectively.Finally,the theoretical framework of the different methods is combined and a new method of fatigue life assessment is proposed,which to a certain extent solves the problem of small test samples and the existence of stress singularities in titanium alloys during the fatigue assessment modeling process.First,based on the joint type of titanium alloy welded structures and fatigue characteristics,the fatigue test data of different welded parts joints were analyzed.A fatigue life prediction model applicable to titanium alloys was obtained to characterize the fatigue life quantitatively.By calculating the notch equivalent stress at the weld position of the structure and combining it with the test data in the reverse direction,a modification of the notch stress equivalent method is completed to achieve a fatigue life assessment considering the stress singularity.Secondly,129 fatigue test data of titanium alloy welded joints with different plate thickness,materials,types and stress ratios were obtained by the group method.And the numerical simulation was applied to determine the equivalent structural stress of each welded joint,and the master S-N curves model of titanium alloy was established.On this basis,the bootstrap method combined with the equivalent structural stress method,through the bootstrap method to expand the sample size,to obtain a better fit under different survival rates of the master S-N curves.Then,the novel three-parameter power function model obtained by sample augmentation is combined with the notch stress equivalent method with the inclusion of correction factors.A new method for fatigue life assessment,namely,the augmented-reverse notch equivalent stress method,is proposed,and the correctness of this method is analyzed and verified by combining with experimental data.Finally,the fatigue life of titanium alloy welded structures was evaluated and comparative analysis by using the augmented-reverse notch equivalent stress method,the master S-N curve of titanium alloy combined with the bootstrap method and the modified notch stress equivalent method,respectively.Through the above analysis,it is demonstrated that the augmented-reverse notch equivalent stress method improves the efficiency and accuracy of fatigue life assessment.It takes into account the limitations of physical test methods(long cycle time,high cost,etc.)and simulation test methods(stress singularities,detachment from reality,etc.).A new method of fatigue life assessment is found for titanium alloy welded structures with small test samples and stress singularities.It also makes a positive contribution to the further application of titanium alloys in the field of rail transportation.