A Micromechanical Model To Study The Closure Stress Effect On Fatigue Life Of Ti6Al4V Subjected To Laser Shock Peening

With the wide application of titanium alloy in various engineering fields,the service life of Ti alloy product has drawn much more attention.It’s been proven that the laser shock peening(LSP)is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress.In this thesis,a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened(LSPed)titanium alloy.Firstly,a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V.Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior.Finally,the effect of the crack closure stress on the fatigue life can be accurately predicted,and then some related results are investigated:(1)The fatigue crack propagation processing is a behavior of deceleration and acceleration of the crack growth rate through the whole grains in crack system.The LSP induced compressive residual stress can reduce the crack growth rate.(2)The driving force at barrier zone increases with the increasing crack length in each grain during fatigue crack propagation processing.A smaller critical barrier stress to overcome the constraint in grain boundary is available for the LSPed Ti6Al4V.Besides,the critical crack length in each grain is shortened due to declined barrier stress.(3)The large and deep crack closure stress in LSPed Ti6Al4V leads to the requirement of a higher threshold stress to promote the crack propagation.(4)The cycle life in the first 10 grains increases by more than one order of magnitude which implies that the LSP induced closure stress is more effective for a shorter crack length in improving the fatigue performance.Additionally,a good endurance to 10~7cycles for Ti6Al4V can be achieved through LSP.At the same time,finite element simulation was used to establish the laser impact strengthening model and simulate the residual stress of titanium alloy laser impact strengthening.Laser impact strengthening is to generate residual compressive stress on the surface of materials through plastic deformation,so as to improve the anti-fatigue performance of materials.The plastic deformation of the material surface will inevitably change the state of the material surface.This causes the surface roughness of the material to increase.In contrast,the surface roughness of the material will be changed much less by laser impact strengthening.The depth of laser shock intensification residual stress generally shows a triangular trend,and the initial depth of residual stress increases rapidly with the change of residual stress.When the residual stress depth increases gradually and reaches a certain value,the increasing trend of the residual stress depth becomes slower.