Research On Fatigue Crack Propagation Behaviors Of Titanium Alloy Diffusion Bonded Structure

Titanium alloy diffusion welded joints have excellent performance and can effectively reduce structural weight and reduce manufacturing costs,so it has been widely used in aerospace manufacturing.Titanium alloy diffusion welded components are often subjected to complex fatigue loads in actual work.Once there are cracks in the components,the cracks will propagate under mixed mode.It is of great significance to study the fatigue crack propagation law of crack under mixed-mode loading conditions for accurate evaluation of fatigue life of titanium alloy diffusion bonded structures.In this paper,based on fatigue crack propagation experiments of TC4 diffusion bonded joints under tensile-shear combined load,combined with theoretical calculation and numerical simulation,the fatigue crack propagation behavior under I-II mixed-mode was studied.Firstly,studies of the stress intensity factors of I-II mixed-mode crack tips were carried out.The finite element simulation calculation of I-II composite stress intensity factor under different loading angles and crack lengths was carried out based on the interaction integral method.The effects of different boundary loading methods on the simulation results were investigated and compared with the analytical solutions.The stress intensity factors of I-II mixed-mode crack under initial condition were analyzed,and the relationship between the proportion of type I?type II stress intensity factors and the loading angle along with crack length was obtained.On the basis of the above,the finite element simulation calculation results were fitted,and the normalized stress intensity factor formula of the I-II composite crack of CTS speciemen was obtained.TC4 diffusion bonded joint CTS specimens were designed and processed.The I-II mixed-mode fatigue crack propagation experiments under 5 different loading angles were carried out.It was found that when the loading angle is below 45 degrees,the cracks all propagate along the joint until fracture,when the loading angle is greater than or equal to 45 degrees,the cracks tend to expand along the base metal.a-N curves of crack propagation process were obtained by using electron microscope combined with potential method.The I-II mixed-mode crack propagation process was analyzed based on the strain energy release rate.Using the strain energy release rate range as parameter.The I-II mixed-mode fatigue crack growth rate equations of specimens under different loading angles and loads were fitted.Considering the proportion of type II crack,the mixed-mode ratio ? was introduced.Based on this,a unified model of I-II mixed-mode fatigue crack growth rate under different loading angles and loads of TC4 diffusion welded joints was established based on the Paris formula.Finally,based on the XFEM,I-II mixed-mode fatigue crack propagation simulation of TC4 diffusion bonded joints under different loading angles was carried out.The fatigue crack propagation history and propagation of CTS specimens under pure type I and I-II mixed-mode under small loading angles were predicted.The simulated a-N curves agree well with the experimental results.In addition,the I-II mixed-mode fatigue crack propagation paths under large loading angles were predicted by simulation and compared with the maximum tangential stress criterion prediction values and test results,the error is within the acceptable range.